BRH/SWRHL 70-1
LASER FUNDAMENTALS
AND
EXPERIMENTS
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Environmental Health Service
'
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SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY
TECHNICAL REPORTS
Technical reports of the Southwestern Radiological Health Laboratory,
Bureau of Radiological Health, are available from the Clearinghouse
for Federal Scientific and Technical Information, Springfield, Va.
22151. Price is $3.00 for paper copy and $0.65 for microfiche. The
PB number, if indicated, should be cited when ordering.
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BRH/SWRHL 70-1
LASER FUNDAMENTALS
AND
EXPERIMENTS
W.F. Van Pelt
H.F. Stewart
R.W. Peterson
A.M. Roberts
J.K. Worst
Radiation Medicine Program
Southwestern Radiological Health Laboratory
Las Vegas, Nevada 89114
May 1970
S
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Environmental Health Service
Bureau of Radiological Health
Roclcville, Maryland 20852
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FOREWORD
The Bureau of Radio log tea I Health continues to carry out a national
program designed to reduce the exposure of man to hazardous ionizing
and nonionizing radiation.
Within the Bureau, the Division of Electronic Products and programs
tn each of the regional laboratories CD develop and admini'ster
performance standards for radiati'on emissions from electronic prod-
ucts, C2) study and evaluate emissions of and conditions of exposure
to electronic product radiation and intense magnetic ffelds, (3)
conduct or support research, training, development and inspections
to control and mini'mize such hazards, and C4) test and evaluate the
effectiveness of procedures and techniques for minimizing such expo-
sures. The Southwestern Radiological Health Laboratory has the
responsible tty for these activities fn the laser area.
The Bureau publishes Tts findings in Radiological Health Data and
Reports Ca monthly publication), Public Health Service numbered
reports, appropriate scientific journals, and Division and Laboratory
technical reports.
The technical reports of the Southwestern Radiological Health Labora-
tory (SWRHL) allow comprehensive and rapid publishing of the results
of intramural and contractor projects. The reports are distributed
to State and local radiological health Program personnel, Bureau
technical staff, Bureau advisory committee members, university radia-
tion safety officers, libraries and information services, industry,
hospitals, laboratories, schools, the press, and other interested
individuals. These reports are also included in the collections of
the Library of Congress and the Clearinghouse for Federal Scientific
and Technical information.
t encourage the readers of these reports to inform the Bureau of any
omissions or errors. Your additional comments or requests for further
information are also solicited.
John C. VilI forth
Di rector
Bureau of Radiological Health
i 11
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PREFACE
Since the first laser was made operational in I960, the laser has grown
from a laboratory curiosity to a useful, flexible tool, justifying its
use in many applications. One of the most common uses is that of a
demonstration tool for teaching optics and wave mechanics. The laser
has proven itself to be of great value in a high school or college course
in basic physics and optics.
A laser can, however, be hazardous. When improperly used, for example,
it can cause serious and irreversible eye damage.
The Southwestern Radiological Health Laboratory, a field laboratory of
the Bureau of Radiological Health, U.S. Public Health Service, has been
given the responsibility for the technical implementation of Public
Law 90-602, the Radiation Control for Health and Safety Act, with
respect to lasers.
This manual is the result of some of our work under the law and was
prepared as a response to the increasing use of lasers for demonstra-
tion purposes in high schools and colleges, where potential exposure
of large groups of unknowledgeable people is great. It is intended to
serve as the text for a short course in laser fundamentals and use and
is directed primarily toward the high school instructor who may use
the laser in the classroom. The text is written in such a manner as
to give an intuitive understanding of the device and its inherent
properties. The instructor is expected to be conversant with certain
of the classical elementary theories of light.
Dr. Melvin W. Carter, Director
Southwestern Radiological Health Laboratory
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CONTENTS
Page
FOREWORD i i I
PREFACE v
UST OF FIGURES ix
ACKNOWLEDGEMENTS X
ABSTRACT xl
WHAT IS A LASER? 1
A. LIGHT I
B. ELECTRON ENERGY LEVELS 3
C. RADtATtVE TRANS tTtONS 5
D. POPULATION INVERSION 8
HOW DOES A LASER OPERATE? 13
A. THE LASING MEDIUM 13
B. PUMPING METHODS 14
C. OPTICAL CAVITIES 14
D. THE RUBY LASER 18
E. THE HE-NE LASER 21
F. OTHER LASERS ?4
PROPERTIES OF LASER LIGHT 25
A. DIVERGENCE 25
B. MONOCHROMATICITY 25
C. COHERENCE 25
D. HIGH INTENSITY 26
BIOLOGICAL EFFECTS OF LASER LIGHT 27
A. INTRODUCTION 27
B. DAMAGE MECHANISMS 27
C. THE EYE HAZARD 28
D. THE SKIN HAZARD 35
LASER APPLICATIONS 37
A. INTRODUCTION 37
B. ENGINEERING APPLICATIONS 37
C. BIOLOGICAL APPLICATIONS 39
VI I
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Page
SAFETY IN CLASSROOM LASER USE 43
A. INTRODUCTION 43
B. HAZARD CALCULATION 43
C. SAFETY AIDS 48
D. GENERAL SAFETY RULES 55
EXPERIMENT SECTION 58
EQUIPMENT NECESSARY FOR EXPERIMENTS 58
I. SCATTERING OF LIGHT 60
2. ABSORPTION OF LIGHT 62
3. REFLECTION OF LIGHT 64
4. REFRACTION OF LIGHT 69
5. POLARIZATION OF LIGHT 78
6. COHERENCE 94
7. DIFFRACTION 105
8. HOLOGRAM 114
REFERENCES 116
v i i I
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LIST OF FIGURES
Figure Page
I. Electromagnetic spectrum 2
2. Representation of photon 4
3. Typical energy level diagram 6
4. Photon multiplication by stimulated emission 9
5. Photon cascade 1°
6. Illustration of population inversion 12
7. Optical cavities 15
8. Q switch and pulse output diagram 17
9. Energy levels of chromium 20
10. Energy levels of neon 22
II. Energy levels of gas laser (detail) 23
12. Schematic of human eye 29
13. Transmission through ocular media 32
14. Possible sites of laser hit on eye 34
15. Beam divergence 46
16. Beam shutter 49
17. Demonstration box 51
18. Display tank 52
19. Beam expander 54
20. Law of reflection 65
21. Specular and diffuse reflection setup 66
22. Comparing angles of incidence and reflection 68
23. Law of refraction 70
24. Observing law of refraction 73
25. Determining index of refraction 74
26. Prism experiments 76
27. Orientation of fields 79
28. Schematic drawings 79
29. Polarizer/analyzer combination 81
30. Polarizing angle 83
31 . Polarization by scatter 84
32. Elliptical polarization 86
33. Effect of quarter-wave plate 88
34. Observing polarization by scatter 90
35. Brewster's angle 92
36. Sine wave 95
37. Temporal coherence, no phase difference 95
38. Temporal coherence, constant phase difference 96
39. Interference 98
40. Lateral coherence 99
41. Spatial coherence 101
42. Single slit diffraction 108
43. Double slit diffraction I 10
44. Diffraction by a straight edge Ill
45. Diffraction by a circular aperture |13
46. Viewing a hologram 115
ix
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ACKNOWLEDGEMENTS
We would Ifke to acknowledge the assistance of the following people in the
preparation of this manual:
Dr. Mason Cox, President, Laser Industry Association
Mr. David SI tney; Physicist, Laser-Microwave Division, U.S. Army
Environmental Hygiene Agency, Edgewood Arsenal, Maryland
Dr. William T. Ham, Jr., Department of Biophysics, Medical College
of Virginia
Dr. Alexander M. Clarke, Department of Biophysics, Medfcal College
of Virginia
Dr. Albert Sheppard, Microwave Engineering Division, Georgia
Institute of Technology
Dr. Robert G. Shackelford, Microwave Engineering Dfvfsion, Georgia
Institute of Technology
Dr. Lon D. Spight, Physics Department, University of Nevada, Las Vegas
Mr. Oliver Lynch, U.S. Atomic Energy Commission, Las Vegas, Nevada
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ABSTRACT
As a result of work performed at the Southwestern Radiological Health
Laboratory with respect to lasers, this manual was prepared in response
to the increasing use of lasers in high schools and colleges. It is
directed primarily toward the high school instructor who may use the
text for a short course in laser fundamentals.
The definition of the laser, laser operation, properties of laser light,
biological effects of laser light, laser applications, safety in class-
room laser use, and experiment section (equipment necessary for experiments)
are included in this manual.
This manual is written in a manner to give an intuitive understanding
of the device and its inherent properties. The instructor is expected
to be conversant with certain of the classical elementary theories of
light.
Representative products and manufacturers are named for identifica-
tion only and listing does not imply endorsement by the Public Health
Service and the U.S. Department of Health, Education, and Welfare.
x i
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WHAT IS A LASER?
The term "laser" is an acronym. It stands for "Light Amplification by
Stimulated Emission of Radiation." Thus the laser is a device which
produces and amplifies light. The mechanism by which this is accom-
plished, stimulated emission, was postulated by Einstein in 1917 but
has only recently been applied. The light which the laser produces is
unique, for it is characterized by properties which are very desirable
but almost impossible to obtain by any means other than the laser.
To gain a better understanding of what a laser is and what it can do,
we shall start with a short review of some of the phenomena involved
in laser action. A good subject with which to start is light.
A. LIGHT
Light is a form of electromagnetic energy. It occupies that portion of
the electromagnetic spectrum with which man first dealt because it was
visible to the human eye. Originally, the term "light" included only
the visible frequencies. About 1800, however, the British-German
astronomer W. Herschel placed a thermometer just beyond the blue portion
of a spectrum produced by a prism using sunlight and found its tempera-
ture was raised. Later, invisible light was found on the other side of
the visible spectrum. Thus it was that frequencies outside the visible
range were lumped with the visible frequencies under the term "light."
Later, when x rays, radio waves and other discoveries were made, light
was found to be part of a spectrum of electromagnetic radiations. The
distinction between the various radiations is primarily energy which is
proportional to frequency. Light is considered to be that portion of
the electromagnetic spectrum having wavelengths between 100 and 10,000
nanometers (nm = I0~9 meters) as shown in Figure I.
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Figure 1
WAVELENGTH
.00001 nm
.0001
.001
.01
.1
1.0
10
100
1000
10000
.01 cm
.1
1.0
10
100
1000
10000
1400 nmlViolet
Cosmic Rays
Gamma Rays
X Rays
*tv/v/ nm*
Ğ
Ultraviolet .* ^ j Blue
. 500 nm ...
'VISIBLE-*** ..Green
, VISIBLE Ğ.% : Yellow
Infrared * 600 nm .
£ range
Millimeter Waves ___ _ .
700 nm* Red
Microwaves
TV and FM
Shortwave
AM Broadcast
ELECTROMAGNETIC SPECTRUM
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From a classical point of view, electromagnetic radiations simultane-
ously display two seemingly contradictory properties. Electromagnetic
rad iations
I. propagate through space as waves, and
2. possess a definite particulate nature, since a discrete
energy and momentum are associated with them.
Each of these properties is important to the complete understanding of
the behavior of all electromagnetic radiations. Both properties are
combined in the current concept of light as described by quantum
mechani cs.
Frequently, for aid in visualizing wave behavior, light is said to move
in much the same fashion as waves on a body of water. While this is
not entirely true, certain characteristics are common to both types of
wave motions.
The fact that a definite energy is associated with the radiation is often
considered a particulate property. It is therefore difficult to
visualize electromagnetic radiations as continuous waves, propagating
continuously through space. One means of partially relieving this
conceptual difficulty is thinking of the radiations as consisting of a
limited "wave packet" which we call a "photon"(see Figure 2). The
packet, or photon., is thought to move through space, thus satisfying a
human need to visualize what truly cannot be visualized.
B. ELECTRON ENERGY LEVELS
Light can be produced by atomic processes, and it is these processes which
are responsible for the generation of laser light. Let's look first at
atomic energy levels and then see how changes in these energy levels can
lead to the production of laser light.
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Figure 2
REPRESENTATION OF PHOTON
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A number of simplifications can be made regarding the concept of the
atom. We can assume, for purposes of this discussion, that the atom
consists of a small dense nucleus and one or more electrons in motion
about the nucleus.
The relationship between the electrons and the nucleus is described in
terms of energy levels. Quantum mechanics predicts that these energy
levels are discrete. A simplified energy level diagram for a one
electron atom is shown in Figure 3.
C. RADIATIVE TRANSITIONS
The electrons normally occupy the lowest available energy levels. When
this is the case, the atom is said to be in its ground state. However,
electrons can occupy higher energy levels, leaving some of the lower
levels vacant. The electrons change from one energy level to another
by the absorption or emission of energy. One of the ways in which an
atom can change its energy state is through what is called a radiative
trans ition.
There are three types of radiative transitions. Two of these, absorp-
tion and spontaneous emission, are quite familiar, but the third,
stimulated emission, is relatively unfamiliar. It is this third type
type of radiative transition that forms the basis for laser action.
Each form of transition is described below.
I. Absorption
An electron can absorb energy from a variety of external sources. From
the point of view of laser action, two modes of supplying energy to the
electrons are of prime importance. The first of these is the transfer
of all of the energy of a photon to an orbital electron. The increase
in the energy of the electron causes it to "jump" to a higher energy
level; the atom is then said to be in an "excited" state. It is important
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Figure 3
A Ionized
-T State
First Excited
State
Ground
State
TYPICAL ENERGY LEVEL DIAGRAM
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to note that an electron accepts only the precise amount of energy that
will move it from one allowable energy level to another. Hence only
those photons of the energy or wavelength acceptable to the electron
will be absorbed.
The second means often used to excite electrons is an electrical dis-
charge. In this technique the energy is supplied by collisions with
electrons accelerated by the electric field. The result of either type
of excitation is thai through the absorption of energy, an electron has
been placed in a higher energy level than that in which it had been
residing, and the atom of which it is a part is also said to be excited.
2. Spontaneous Emission
The entire atomic structure tends to exist in the lowest energy state
possible. An excited electron in a higher energy level will thus
attempt to "de-excite" itself by any of several means. Some of the
energy may be converted to heat. Another means of de-excitation is
the spontaneous emission of a photon. The photon released by an atom
as it is de-excited will have a total energy exactly equal to the dif-
ference in energy between the excited and lower energy levels. This
release of a photon is called spontaneous emission. One example of
spontaneous emission (and absorption) is seen in phosphorescent mater-
ials. The atoms are excited by photons of appropriate energy from the
sun or a lamp. Later, in the dark, they de-excite themselves by spon-
taneously emitting photons of visible light. A second example is the
common neon sign. Atoms of neon are excited by an electrical discharge
through the tube. They de-excite themselves by the emission of photons
of visible light. Note that in both of these examples the exciting
force is not of a unique energy, so that the electrons may be excited
to any one of several energy levels. The photons released in de-
excitation may have any of these several discrete frequencies. If
enough discrete frequencies are present in the appropriate distribution,
the emissions may appear to the eye as "white" light.
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Now let us look at the third, and probably the least familiar, type of
radiative transition.
3. Stimulated Emission
In 1917, Einstein postulated that a photon released from an excited atom
could, upon interacting with a second, similarly excited atom, trigger
the second atom into de-exciting itself with the release of a photon.
The photon released by the second atom would be identical in frequency,
energy, direction, and phase with the triggering photon, AND the trigger-
ing photon would continue on its way, unchanged. Where there was one,
now there are two. This is i I Iustrated in Figure 4. These two photons
could then proceed to trigger more atoms through stimulated emission.
If an appropriate medium contains a great many excited atoms and
de-excitation occurs only by spontaneous emission, the light output
will be random and approximately equal in all directions as shown in
Figure 5A.
The process of stimulated emission, however, can cause an amplification
of the number of photons traveling in a particular direction a photon
cascade as illustrated in Figure 5B. A preferential direction is
established by placing mirrors at the ends of an optical cavity. Photons
not normal (perpendicular) to the mirrors will escape. Thus the number
of photons traveling along the axis of the two mirrors increases greatly
and light amplification by the stimulated emission of radiation occurs.
D. POPULATION INVERSION
Practically speaking, the process of stimulated emission will not produce
a very efficient or even noticeable amplification of light unless a
condition called "population inversion" occurs. If only two of several
million atoms are in an excited state, the chances of stimulated emission
occurring are infinitely small. The greater the percentage of atoms in
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Figure 4
PHOTON MULTIPLICATION
BY STIMULATED EMISSION
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Figure 5
o
B.
PHOTON CASCADE
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in an excited state, the greater the probability of stimulated emission.
In the normal state of matter, the population of electrons will be such
that most of the electrons reside in the ground or lowest energy levels,
leaving the upper levels somewhat depopulated. When electrons are
excited and fill these upper levels to the extent that there are more
atoms excited than not excited, the population is said to be inverted.
This is illustrated in Figure 6.
I I
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Figure 6
E, --
E,
r\j
Normal
Population Distribution
Inverted
Population Distribution
ILLUSTRATION OF POPULATION INVERSION
,
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HOW DOES THE LASER OPERATE?
Now that some of the phenomena have been discussed, let us see how a
laser is constructed and how it operates. Three components are neces-
sary: (I) an active lasing medium; (2) an input energy source (called
the "pump"), and (3) an optical cavity.
A. THE LASING MEDIUM
Lasers can be classified according to the state of their lasing media.
Four common families of lasers are presently recognized.
Solid state lasers employ a lasing material distributed in a solid
matrix. One example is the ruby laser, using a precise amount of
chromium impurity distributed uniformly in a rod of crystalline aluminum
oxide. The output of the ruby is primarily at a wavelength of 694.3 nm,
which is deep red in color.
Gas lasers use a gas or a mixture of gases within a glass tube. Common
gas lasers include the He-Ne laser, with a primary output of 632.8 nm
and the C0? laser, which radiates at 10,600 nm, in the infrared. Argon
and krypton lasers, with outputs in the blue and green regions, are
becoming quite common. Even water vapor can be made to yield a laser
output in the infrared.
Liquid lasers are relatively new, and the lasing medium is usually a
complex organic dye. The most striking feature of the liquid lasers is
their "tunabiIity". Proper choice of the dye and its concentration
allows light production at almost any wavelength in or near the visible
spectrum.
Semi-conductor lasers are not to be confused with solid state lasers.
Semi-conductor devices consist of two layers of semi-conductor material
sandwiched together. One material consists of an element with a
13
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surplus of electrons, the other with an electron deficit. Two outstand-
ing characteristics of the semi-conductor laser are its high efficiency
and small size. Typical semi-conductor lasers produce light in the red
and infrared regions.
B. PUMPING METHODS
Laser action can occur only when a population inversion has been estab-
lished in the lasing medium. This population inversion can be established
by pumping energy into the lasing medium. Several methods of pumping are
commonly used. Optical pumping is employed in solid state and liquid
lasers. A bright source of light is focused on the lasing medium. Those
incident photons of correct energy are absorbed by the electrons of the
lasing material and cause the latter to jump to a higher level. Xenon
flashtubes similar to strobe lights used in photography, but more powerful,
are commonly used as optical pumps for solid state lasers. Liquid lasers
are usually pumped by a beam from a solid state laser.
Electron collision pumping is utilized in gas lasers. An electrical
discharge issent through the gas-filled tube. The electrons of the
discharge lose energy through collisions with gas atoms or molecules
and the atoms or molecules that receive energy are excited. Electron
collision pumping can be done continuously and can therefore lead to a
continuous laser output.
C. OPTICAL CAVITIES
Once the lasing medium has been pumped and a population inversion ob-
tained, lasing action may begin. If, however, no control were placed
over the direction of beam propagation, photon beams would be produced
in all directions. This is called superradiant lasing.
The direction of beam propagation can be controlled by placing the lasing
medium in an optical cavity formed by two reflectors facing each other
along a central axis (Figure 7).
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Figure 7
4-4
*ğ**'**
ğ Ğ^Ğ ğ
OPTICAL CAVITIES
15
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Photon beams which are produced along the cavity axis are reflected 180
at each reflection and travel once more through the lasing medium causing
more stimulated emission. Thus, the beam grows in magnitude with each
traverse of the lasing medium.
Since the reflectors are not 100 percent reflective some photons are lost by
transmission through the mirrors with each passage. If the pumping is
continuous, a state of equilibrium will soon be reached between the
number of photons produced by atoms raised to the excited state and the
number of photons emitted and lost. This results in a continuous laser
output and is usually used only with low power input levels. Higher
power inputs usually are achieved in the form of a pulse, and the output
is also in pulse form. One of the mirrors in the system is usually made
more transparent than the other and the output, pulsed or continuous, is
obtained through this reflector.
Q-switching (or Q-spoiling) is used to produce an exceptionally high-
power output pulse. The term "Q" as applied to lasers is derived from
the more familiar Q of electrical circuits. Lasers are resonant
cavities and in a similar way, many electrical devices are resonant.
The Q is a numerical index of the ability of the resonant cavity to
store energy at the output frequency. The higher the Q, the more effec-
tive the power concentration at the resonant frequency. Q-switching in
lasers refers to the method of laser operation in which the power of the
laser is concentrated into a short burst of coherent radiation. A
Q-switch is a device which interrupts the optical cavity for a short
period of time during pumping. A schematic of a Q-switched solid state
laser is shown in Figure 8.
Lasing action normally begins as soon as a population inversion is
achieved and continues as long as pumping action maintains the inversion.
The Q-switch interrupts the optical cavity and reduces the losses due to
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Figure 8
A. PULSED RUBY
B. Q-SWITCHED RUBY
Time
Q SWITCH AND PULSE OUTPUT DIAGRAM
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lasing until pumping can achieve a greater population inversion, say
70 to 80 percent, the Q switch then suddenly restores the cavity and
the resulting pulse is much shorter and more powerful than would nor-
mally be ach i eved.
One example of a Q switch is the Pockel's cell, made of a crystal of
ammonium or potassium dihydrogen phosphate (ADP or KDP) sandwiched be-
tween two crossed polarizers. In its de-energized state the crystal
will not affect polarized light. When an electric field is applied
across the crystal, however, the plane of polarization of the incident
light is rotated by 90°, allowing it to pass the second crossed polar-
izer. This completes the optical cavity and results in a "giant pulse".
Reflectors may consist of plane mirrors, curved mirrors, or prisms, as
shown in Figures 7 and 8. The mirror coating may be of si Iver, if
laser output power is low, but higher powers may require dichroic
material. A dichroic material is a crystalline substance in which two
preferred states of polarization of light may be propagated with dif-
ferent velocities and, more important, with different absorption. By
appropriate choice of material and thickness, the light impinging upon
the dichroic coating may be either totally absorbed or totally reflected.
The first ruby lasers were constructed with the crystal ends polished
optically flat and silvered. Semi-conductor lasers use a similar tech-
nique. Gas lasers may use mirrors as seals for the ends of the gas tube
or may utilize exterior mirrors. In the latter case, the tubes use end
windows of glass or quartz set at Brewster's angle (see Experiment 5,
Polarization), and the output is polarized light.
D. THE RUBY LASER
The laser first successfully operated was a ruby laser. It was constructed
and operated by Dr. T. H. Maiman in I960. Ruby is a crystal form of alumi-
numi oxide with about 0.05 percent by weight chromium as an impurity. The
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chromium gives the ruby its red color and is responsible for the lasing.
Chromium exhibits a 3-level energy system, as represented in Figure 9.
In a ruby laser, the electrons of chromium atoms are pumped to an ex-
cited energy level by means of a xenon flash I amp placed beside or around
the ruby rod. The chromium electrons absorb photons in a band centered
around 545.1 nm and are raised from their ground level to excited level
E2. From here they drop almost immediately to level E3 by means of a
phonon (radiationless) transition. The small amount of energy lost here
is through heat and vibration. The electrons will reside in level E3
for a considerable length of time much less than a second but for
an electron that is a relatively long time. Thus, since the flashiamp
operates in a period of microseconds, a population inversion can be ob-
ta i ned.
The excited atoms begin to de-excite spontaneously, dropping from level
E3 to El, and since a population inversion is in effect, stimulated
emission may begin. In any lasing medium, stimulated emission may occur
in all directions and no particular direction of propagation is favored.
As stated earlier, to gain control of the emission direction and increase
the amount of energy within the pulse, the lasing medium is placed within
an optical cavity. Photons not emitted along the axis of the cavity will
pass out of the system and be lost. If, however, a photon cascade is
aligned with the cavity axis, it wiI I encounter one of the mirrors and
be reflected back upon itself, pass once more through the lasing medium
and trigger more excited atoms to undergo stimulated emissions. The pulse
thus grows in size and on each encounter with the less reflective mirror,
part of it emerges from the laser as high intensity coherent light.
The pulse from a typical ruby laser lasts only a few microseconds, since
the pumping is not continuous. The flashiamp is run by a charge stored
in capacitor banks, and once the lamp has flashed, the capacitors must
be recharged. Pumping occurs over a few hundred microseconds and contin-
ues as long as the flashiamp is discharging.
19
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Figure 9
O)
t
E2
545.1 nm
Phonon Transition
E3
Photon Transition
694.3 nm
ENERGY LEVELS OF CHROMIUM
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E. THE HE-NE LASER
The most common laser used today in both industry and education is the
He-Ne laser. It was first operated in 1961 by AM Javan and has proved
to be the forerunner of a whole family of gas lasers. Since gas lasers
are all quite similar in construction and behavior, we shall discuss the
He-Ne as representative of the group.
The lasing medium in the He-Ne laser is a mixture of about 90 percent helium
and 10 percent neon, with neon providing the lasing action. An energy level
diagram for neon is shown in Figure 10.
The 4-level system of a gas laser differs from the three-level system of
chromium in that the emission of a photon does not return the atom to a
ground level. Transitions from level E3 to E4 and E4 to El are accom-
plished through a phonen transition in which energy is transferred mainly
through heat.
Pumping of neon to an excited state is not done directly by the energy
source. Rather, indirect pumping is accomplished by exciting atoms of
helium which then transfer energy to the new atoms by way of electron
collision. These two gases are picked because they have electron excita-
tion levels which are almost identical, thus facilitating the necessary
energy transfer. Additionally, in the mixture of gases used, one does
not need to affect a population inversion in helium in order to obtain a
population inversion in neon. A more complete energy level scheme for
He-Ne is shown in Figure II.
The He-Ne gas mixture is contained in a sealed tube. Excitation of the
helium is accomplished by a discharge of electricity through the tube,
similar to a neon sign. The mirrors may be enclosed within the tube or
may form the end caps of the tube containing the He-Ne mixture. This is
a rather solid geometrical configuration and results in a stable light
output.
21
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Figure 10
O)
(5
c
E2
Photon Transition
E3
Phonon Transition
E4
Phonon Transition
El
ENERGY LEVELS OF NEON
I
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Figure 11
(H)E2'
(H)E2
N:
O)
HELIUM
NEON
Collision
Collision
E4
El
ENERGY LEVELS OF GAS LASER (DETAIL)
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Since the alignment of the mirrors is a delicate procedure, one common
method is to mount the mirrors separate from the laser tube. When this
is done, the ends of the laser tube are made of pyrex or quartz set at
Brewster's angle to the axis of the laser, and the output is polarized
light. (See Experiment 5, page 78 for an explanation of Brewster's angle.)
F. OTHER LASERS
Other lasers operate in similar but more complicated ways. Changes in
molecular energy levels may be used rather than changes in electron
energy levels, but output is still obtained through the stimulated
emission of radiation.
24
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PROPERTIES OF LASER LIGHT
The light output of a laser differs from the output of ordinary light
sources. Four properties characterize the laser's output: small
divergence, monochromaticity, coherence, and high intensity. These
four properties are what make the laser so valuable and account for
the ever-lengthen!ng list of laser applications.
A. DIVERGENCE
When light emerges from the laser, it does not diverge (spread) very
much at all. Thus the energy is not greatly dissipated as the beam
travels. Laser beam divergence is measured in mi I Iiradians or I x ICT3
radians. There are 2ir radians in a circle so one milliradian equals
about 3 minutes of arc. A typical He-Ne laser has a rated divergence
of 0.5 - 1.5 mi I Iiradians.
B. MONOCHROMATIC ITY
Laser light is very close to being monochromatic. The term "monochro-
matic" means one color, or one wavelength, of light. Actually, very
few lasers produce only one wavelength of light. A typical He-Ne laser
emits light at 632.8 nm, which is orange-red, and at 1,150 nm and 3,390
nm in the near and middle infrared regions. The He-Ne laser is usually
designed to emit only one of the three wavelengths of light and the
variation in this wavelength is slight.
C. COHERENCE
Coherence is a term used to describe particular relationships between two
wave forms. Two waves with the same frequency, phase, amplitude, and
direction are termed spatially coherent. For a fuller discussion of
coherence, see Experiment 6, page 94.
No source of perfectly spatial coherent light is yet known; however, laser
light comes so close that for most practical purposes it can be considered
25
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perfectly coherent. Sophisticated equipment is necessary to detect the
variation from perfect spatial coherence.
D. HIGH INTENSITY
^
Laser light can be very intense. The sun emits about 7 x I05 W/cm /Sr/ym
at its surface. Lasers are presently capable of producing more than
I x I010 W/cm2/Sr/ym. (Sr = steradian) (It must be noted that a source
of light that exceeds the sun in intensity may certainly be hazardous
to vision.)
The magnitude I x 10 W/cm /Sr/ym is somewhat misleading, for it repre-
sents only a single pulse of light. Energy is a measure of capacity for
doing work and is usually classed as potential or kinetic energy. It is
commonly measured in joules (J) in the metric system. Power is the rate
at which work is being done and is measured in watts (W). The following
relationships hold:
I jouIe = I watt-second
I watt = I joule/second
Thus, a laser capable of emitting 10 joules in one second can be termed
a I0-watt laser. If those same 10 joules are emitted as a single pulse of
l/IOOth second duration, then the laser can be termed a 1,000-watt laser.
o
The output of pulsed lasers is usually indicated in terms of J/cm . The
effect of the laser pulse is strongly dependent upon the amount of time
it takes to deliver the pulse. Consequently, pulsed laser output is
sometimes referred to in terms of J/cm or W/cm2.
sec
26
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BIOLOGICAL EFFECTS OF LASER LIGHT
A. INTRODUCTION
Laser light can cause damage to living tissue. The extent of the damage
depends primarily upon the frequency of the light, the power density of
the beam, the exposure time, and the type of tissue struck by the beam.
How does the laser light damage tissue?
B. DAMAGE MECHANISMS
Damage can occur through three mechanisms of interaction: (I) a thermal
effect; (2) acoustic transients; or (3) other phenomena. The
latter two effects are only seen with high power density laser pulses.
When laser light impinges on tissue, the absorbed energy produces heat.
The resultant rapid rise in temperature can easily denature the protein
material of tissue, much as an egg white is coagulated when cooked.
Since tissue is not homogenous, light absorption is not homogenous and
the thermal stress is greatest around those portions of tissue that are
the most efficient absorbers. Rapid and localized absorption produces
high temperatures, steam, or results in explosive destruction of the
absorber. Steam production, readily evident only at high exposure levels,
can be quite dangerous if it occurs in an enclosed and completely filled
volume such as the cranial cavity or the eye.
A second interaction mechanism is an elastic or acoustic transient or
pressure wave. As the light pulse impinges on tissue, a portion of the
energy is transduced to a mechanical compression wave (acoustic energy),
and a sonic transient wave is built up. This sonic wave can rip and tear
tissue and if near the surface, can send out a plume of debris from the
impact.
Other phenomena such as free radical formation, are believed to exist
during laser Impact on biological systems, but this has not yet
been conclusively demonstrated.
27
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The laser is usually a hazard to only those tissues through which the
light beam can penetrate and which will absorb the wavelength involved.
With potential hazard evaluation and safety in mind, the concern is
primarily with two organs the eye and the skin.
C. THE EYE HAZARD
Eye damage from light exposure has been recognized for over one hundred
years. Czerny produced retinal burns in rabbits by using sunlight in
the mid-1800's. In 1916, solar eclipse burns on the retina of observers
were described by Verhoff and Bell.
The hazard to the human eye posed by the use of lasers is obvious. A
source of light energy that exceeds the sun in irradiance must surely be
considered as a hazard to the eye. To better understand this, the
anatomy of the eye will be considered. The cross-section of the human
eye is illustrated in Figure 12.
The outer surface of the eye is a tough white tissue called the sclera.
The anterior portion of the sclera is specialized into the cornea which
is transparent to light. The cornea is the major focusing device of the
eye.
Inside the eye are two fluid-filled cavities, both of which are under pressure
to give structural rigidity to the eye. The anterior chamber contains
a slightly viscous liquid, the aqueous humor. The rear chamber is filled
with a very viscous, collagenous suspension, the vitreous humor.
Separating the two chambers is the lens which is attached by ciliary
muscles to the sclera. These muscles alter the lens shape for fine
focusing of the incoming light beam. Overlying the lens is the pigmented
iris, a muscular structure designed to expand or contract and thus regu-
late the amount of light entering the eye.
28
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Figure 12
M
VO
Retina
Fovea
Blind Spot
Optic
Nerve
Vitreous Humor
Iris
Cornea
Aqueous Humor
Lens
SCHEMATIC OF
HUMAN EYE
-------
Lining the rear fluid filled chamber is the retina which contains the
sensory cells for light perception. The retina itself is composed of
tissue of two origins. The retinal tissue which the light encounters
first is of neuro-ectodermaI origin (about 150 ym thick) and contains
the nerve cells for light perception. The underlying tissue is the
pigment epithelium (about 10 ym thick) and contains great numbers of
melanin granules. Its functions are to stop light reflection, absorb
any scattered light, and provide support for the photoreceptor cells.
It is one of nature's quirks that light, before reaching the light
sensor cells in the primate eye, must first pass through several mem-
branes, nerve fibres, ganglion cells, bipolar cells and amacrone cells,
and then must strike the photoreceptor cells from the rear.
The photoreceptor cells of the retina are of two types: rod and cones.
Rods are quite sensitive to low light levels but cannot distinguish
color. Cones are not as light sensitive but can distinguish color.
The two types are intermixed in the retina with cones dominating near
the center of the retina and rods near the periphery.
At the focal spot of the cornea-lens system lies the macula, an area
of cones only. Within the macula is the fovea, a small region perhaps
250 to 300 ym across, in which the cones are densely packed. This is
the center for clear or critical vision. To one side of the macula is
a blind spot at which point the nerve fibres from the photoreceptors
4
exit the eye to form the optic nerve.
The retina subtends, in cross section, a visual angle of about 240°.
It is loosely bound by connective tissue to the muscular choroid, which
in turn is firmly attached to the sclera.
Light is focused by the cornea and lens onto the fovea of the retina.
In this process, the energy density of the light is concentrated by a
30
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factor of I04 to IO6 over that falling on the pupil. For this reason,
laser light may pose a serious hazard to the eye.
The human eye is relatively transparent to light in a wavelength range
of about 400 to 1400 nm. This includes not only the visible range of
400 to 700 nm, but also a portion of the infrared which is not perceived.
Figure 13 is a curve of optical transmission of light through the ocular
media for both human and rabbit. As can be seen, the greatest trans-
mission occurs in the visible range.
The portion of the eye affected by the laser is dependent upon the wave-
length of the light. The ruby laser, for example, emits at 694.3 nm.
From the curve of transmission, one can see that greater than 90 percent
of the light is transmitted through the ocular media to the retina. Of
the light reaching the retina, about 60 percent is absorbed in the neuro-
ectodermal coat. Almost all of the rest of the light, 40 percent, is
absorbed in the pigment epithelium. Since the pigment epithelium is only
10 ym thick, the greatest absorption per unit volume of energy occurs
here, and this layer is the most susceptible to damage. Lesions may be
produced here without the receptor cells being damaged.
Helium-neon, krypton, argon, and xenon lasers all operate in the visible
range, and all affect the eye in a manner similar to the ruby laser.
Neodymium laser light at 1060 nm is absorbed to a greater extent in the
ocular media with less of its energy reaching the retina than in the case
of visible light. Thus there is a greater chance of damage by means of
steam production than from other laser types. The aqueous and vitreous
bodies are colloidal suspensions in water, and the absorption character-
istics of the media are similar to those of water.
Carbon dioxide lasers produce light at 10,600 nm. The eye is not very
transparent to this frequency range and danger at low power densities
comes from lesions produced on the cornea.
31
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Figure 13
100
Mean Values through Ocular Media
Rabbit
Human
350 i i I
400 SOO 600 700 SOO 900 1000 1100 '200 1300 1400
Wave Length
TRANSMISSION THROUGH OCULAR MEDIA
-------
Research on eye damage has been under way for some time. The search for
laser effects began shortly after the invention of the laser and is
continuing. Most of the work has been aimed at determining the minimum
amount of irradiation necessary to produce a visibly detectable retinal
lesion from an acute exposure.
Current studies have been conducted with monkey, rabbit, and human eyes.
The latter were eyes with some medical problems. Work is hampered by
the fact that suitable human subjects are few and far between, and that
the human eye is such a unique organ.
In truth, power density at the retina cannot be measured but must be
calculated on the basis of transmission and focusing of the beam. The
power density which can be measured is that on the cornea. On the
basis of measurement at the cornea, lesions can be theoretically caused
by as little as I0~6 J/cm2 from a pulsed ruby laser.
At present, threshold values for visible lesion production are approxi-
mately as follows:
o
Q-switched ruby laser § 0.07 J/cm2 on the retina
Q
Pulsed ruby laser § 0.8 J/cm2 on the retina
Q
Continuous white light @ 6.0 W/cm2 on the retina
C02 laser @ 0.2 W/cm2 at the cornea9
Light levels below those producing visible lesions may also produce some
permanent damage such as partial "bleaching" of the pigment for one
particular light color. Work is now under way to detect such damage by
histochemical means as well as by electroretinography.
Damage can result from laser impact on numerous eye structures. (See
Figure 14). Oblique beam entrance may cause a lesion in the retina which
goes unnoticed. A hit upon the optic nerve could result in complete
33
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Figure 14
1. Sclera 3. Macula Retina
2. Iris 4. Retina Periphery
5. Optic Nerve
POSSIBLE SITES OF LASER HIT ON EYE
-------
destruction of vision. The iris is dark colored and quite susceptible,
while the whole sclera can fall victim to high energy beams.
A special mention must be made of the C0? laser, whose infrared emission
is destructive to the cornea. Corneal opacities are produced at a level
of 0.2 W/cm2 for 30 minutes continuous irradiation. This is a rather
high intensity as compared with other threshold levels, but then CCL
lasers produce very intense beams and one is quite likely to encounter
CCL lasers with continuous kilowatt outputs.
Maximum permissible exposure levels are calculated on a "Worst Case"
analysis of the hazard. The laser beam is assumed to be aimed directly
at the fovea, the iris is dilated to produce a large pupil diameter,
and the eye is focused at infinity. Under these circumstances, maximum
irradiation of the retina will occur.
D. THE SKIN HAZARD
The other area of concern besides the eye is the skin. Naturally, it is
not as sensitive as is the eye, and if damaged, most injuries are more
easily repaired. However, it too is subject to great damage from laser
impact when energy densities approach several J/cm2. Descriptions of
skin damage are supplied for general interest and are probably not appli-
cable in hazard considerations for He-Ne lasers.
The skin is not a homogeneous mixture. It is a specialized, layered
structure with numerous odd inclusions, such as blood vessels and hair
follicles. Like most other tissues, the skin is composed principally of
water, and therefore the laser beams interact as if skin were sea water
containing a number of inclusions. Consequently- the skin is relatively
transparent to laser light.
Absorption of light in the skin occurs, for the most part, in the pigment
granules and the blood vessels. The skin contains numerous pigments, the
35
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most common being melanin, which determine the color of one's skin.
Visible laser light is selectively adsorbed by the melanin granule,
causing it to rise in temperature at a rapid rate, and causing cavita-
tion around or bursting of the granule at high energy density
10
exposure.
Blood vessels are also quite susceptible to lasrr damage and are easily
occluded or cauterized by a laser hit.
Under certain circumstances, the organ of concern is not the skin, but
the underlying organs. Skin is so transparent that the visible light
nrr.y pass through it to be absorbed by an internal organ. For example,
the liver in mice is especially subject to this type of damage, lying
close beneath the skin and being dark in color.
Laser damage to the skin ranges from a mild erythema to a surface char-
ring, to a deep hole literally burned and blown into the skin. One
rather sensational aspect is the plume, a kickback of debris present
in high energy impacts. This plume may scatter tissue quite some
distance from the point of impact.
36
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LASER APPLICATIONS
A. INTRODUCTION
Soon after the invention of the laser, the device was described as
"a solution looking for a problem". Since that time, however, a long
list of problems in the areas of engineering and biology has been
found for which the laser is providing eagerly-sought solutions. These
solutions and their applications hold great promise for future work.
B. ENGINEERING APPLICATIONS
I. Communications
The higher the frequency of a carrier signal, the greater the amount of
information that can be impressed upon the carrier. One optical carrier
of He-Ne laser frequency (@ 5 x Id14 Hz) could in principle carry ten
million simultaneous phone calls or eight thousand simultaneous television
programs. This ability makes the laser very attractive to the communica-
tions industry.
Many problems await solution, however, before practical communications
applications are possible. Modulation of the carrier beam has been
accomplished, but it is a difficult process. Since the carrier is light,
transmission from point to point can be stopped by such simple things as
fog, rain, dust, or an object passing through the beam. The solution may
be in transmission through pipes with mirrors directing the light around
bends.
2. Tracking and Ranging Systems
A number of laser tracking and ranging systems are presently in use.
This application is often referred to as LADAR (Laser Detection and
Ranging), just as Radio Detection and Ranging is referred to as RADAR.
37
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Ranging systems record the time for a signal to travel to the target and
return and translates this time to distance. The small divergence of
the beam is important because it allows the operator to pinpoint the
object for which readings are taken. The Army has developed a range-
finder which utilizes this concept.
3. Surveying
The collimated beam of the laser is ideal for a number of surveying
applications. One laser, operating continuously, can replace two men
and a transit.
Giant earth-boring machines are now aligned through use of the laser.
Bulldozers clearing land, graders leveling land, barges or dredges
working on dredging harbors or setting piers, pipe layers and ditch
diggers are all making use of the laser as a simple method of alignment.
4. Mechanical Measurements
The Michael son interferometer has been the center of renewed interest
since the advent of the laser. Formerly: the interferometer could be
used only to measure very small changes in length. Now the device is
useful for distances up to several hundred feet.
Applications include the following: seismology, where a stable source
of coherent light can detect very small earth movements; metaI work!ng,
which utilizes the interferometer to control the operations of a milling
machine; flow rate control; and large scale movements such as building
sway or bridge movements.
5. Welding and Cutting
The high intensity output capability of the laser was first demonstrated
by burning holes in razor blades. Presently this capability is being
utilized on production lines in cutting and welding applications.
38
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Diamonds are used as dies to make wire. Before the discovery of the
laser, drilling holes in the diamonds took days. Today, the use of
lasers has reduced the cutting time to minutes. Cutting and working
of other hard materials is also done easily with the laser.
Welding of wires in transistors and microchip circuits is also done
using lasers, and laser beams can be projected through the envelope of
a glass tube to weld broken wires inside.
6. Holography
The laser's coherent light has given new impetus to the photographic
process of holography. Three-dimensional images are being used for
display devices and as a method of spotting defects in automobile
tires, as well as in scientific research applications such as particle
size measurement. Recently, a cube of crystal material has been used
to record numerous holograms. The small size of the cube and the large
number of three-dimensional images stored may herald a new era in
information and data storage and retrieval.
C. BIOLOGICAL APPLICATIONS
I. Retinal Coagulation
The retina of the eye is loosely attached to the choroid coat. The
retina is of neurodermal origin while the choroid is ectodermal. In
the embryo, these two join and subsequently throughout the life of the
individual are held by a thin layer of connective tissue. In the
adult, any number of circumstances, including trauma, can result in
the separation of the retina from the body of the eye. This of course
leads to a loss of vision because the light cannot be properly focused
upon the detached retina.
For a number of years, retinas were reattached by using a long needle-
like probe to weld the retina to the choroid with a scar. This
worked quite well, producing one or more blind spots but allowing the
39
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proper focus to be attained once more. About 1950, the xenon photocoagu-
lator was introduced, producing this same effect by means of a pulse of
intense white light which, when focused by the lens on the retina,
resulted in reattachment by coagulated blood in a fashion similar to a
spot we Id.
More recently; retinal repair has been accomplished by using a laser as
the light source. Ruby lasers were used first, then neodymium, and
finally argon lasers. The real value of using the argon laser over the
xenon photocoagulator is the size of the spot weld. An argon laser can
produce welds much smaller than the size of a xenon weld, allowing finer
"stitching," this being of particular value around the fovea. In
addition, neither anesthesia nor hospitaIization is required with laser
photocoaguI at ion.
2. Skin - Cosmetic Repair
Much use has been made of the laser's destructive effects in treating
skin disorders. Since the laser light is preferentially absorbed by
pigmented tissue, one of the first experiments undertaken was the removal
of tattoos. Favorable results were obtained, leading to further
work, especially in the cosmetic treatment of angiomas.
An angioma is an excessive proliferation of blood and lymph vessels in
the upper skin layers. The multitude of fine blood vessels in the upper
skin layers produce a discoloration of the skin and appears as a port
wine color. The impact of a laser can occlude the blood vessels and
blanch the skin, leading to an eventual healing of the impact area and
normal coloration of the skin.
3. Skin Cancer
Skin cancers have also been experimentally treated. Since there is a
difference between normal and cancerous skin cells, a search has been
under way for a dye or pigment that is completely selective for cancer
40
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cells. Partial results have been obtained and cancer cells can now be
stained considerably darker than normal cells. The darker cancer cells
are then more susceptible to the impact of a laser beam because they
absorb more light energy and are more severely damaged than are normal
unstained eel Is.
Two problems have arisen with this treatment. First, the plume of debris
from the laser impact was found to contain viable cancer cells, posing a
possible hazard to operating room personnel. Second, the impact drove
some of the tumor cells deeper into uninfiItrated tissue, thus spreading
the cancer. The first problem has been solved by placing a cone over the
laser head which catches the plume from the impact. This cone may even
be attached to a suction device for vacuum cleaner action. The second
problem may be overcome by improved techniques.
Two types of cancer treatment have been practiced. A low energy beam has
been used to selectively disrupt tumor cells. Higher energy beams are
used to excise nodules from deeper tissues.
4. Bloodless Surgery
The possibility of bloodless surgery with a laser scalpel has given rise
to many new techniques in surgery. It facilitates surgical procedure on
organs such as the liver and kidney where blood loss is a problem. High
energy argon lasers should soon become a tool for liver operations, wrth
concurrent use of plastic adhesives to complete closure.
5. TransiI Iumination
TransiI Iumination is a technique whereby a strong light is projected
through soft tissues to aid in detecting tumors. The skin is relatively
transparent to light, as is demonstrated by putting your thumb over a
flashlight. Lasers hold promise for this type of examination allowing,
for example, an immediate examination for breast cancer without the
potential hazard or wait associated with x rays.
41
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6. Neurosurgery
Neurosurgery appears to be a promising area for laser use. Precisely
controlled cutting is extremely important and can be accomplished with
lasers. Transection and tumor treatment will benefit from the use of
lasers, and bloodless tumor removal may be within reach. A number of
operations may be done in the grey matter using the laser, thus lowering
the possibility of infection.
7. Dentistry
Some experimental work has already been done in the field of dentistry.
The glazing of teeth by a laser has been shown to reduce significantly
the demineraIization of enamel, and may also be effective against caries.
Dental caries have been exposed to laser impact with favorable results.
If the caries can be retarded or stopped by laser impact, dentistry will
have gained a valuable tool.
8. CeI I Identi fication
A new method of instant and positive identification of micro-organisms
and tissues is now being produced commercially. The sample in question
is cooled to a low temperature and irradiated with ultraviolet laser
light. Under those circumstances the sample itself produces phospho-
rescent light whose frequency and decay time are unique for the organism.
A small computer matches the frequency and decay time data with informa-
tion previously stored and can identify instantly the present of specific
micro-organisms or tissues.
42
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SAFETY IN CLASSROOM LASER USE
A. INTRODUCTION
Laser light poses a definite hazard to the eye and, to a lesser extent,
the skin. The purpose of this manual is to assist you as an instructor
in evaluating the hazard involved in classroom laser use and to suggest
precautions that may be taken to reduce this hazard.
The U.S. Public Health Service has not, at this time of writing, estab-
lished radiation protection guides for laser irradiation. However, a
number of private corporations, laboratories, and military and government
groups have formulated internal standards for safe laser use. One can
use these formulations in evaluating one's own criteria for laser safety.
(See reference list, page 116)-
One note of warning must be made regarding the use of these published
guides. Most of the guides were compiled from work performed in the
determination of the damage threshold for visible eye lesions. Because
of the importance of these lesions to sight, the term "threshold" must
be carefully scrutinized. Just what is the lowest level of biological
change one should use as the criterion for damage? No general agreement
now exists.
Visible lesions may not be true threshold lesions because more sensitive
processes may detect permanent damage at exposure levels well below those
producing visible lesions. HistochemicaI methods can be used to detect
permanent enzyme inactivation at exposure levels 10-15 percent below those
resulting in visible lesion production. Electroretinography can be used
to detect some permanent changes at exposure levels 50 percent below visible
lesion levels. Permanent damage may result at even lower levels.
B. HAZARD CALCULATIONS
Exposure standards are of little value unless used. Proper use of exposure
standards includes an estimate of the hazard presented by the beam from
43
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your laser. Let us take a brief look at how to determine what hazards
exist.
An accurate determination of the hazards posed by a laser requires meas-
urement of several beam parameters. Many methods for measuring or
defining the beam parameters are presently in common use, all requiring
the use of calibrated and costly electronic gear. However, an approximate
idea of the hazard posed by a laser can be gained by using the manufac-
turer's specifications.
One note of caution: The specifications listed by a laser manufacturer
are likely to be minimal guaranteed levels for a particular model, and an
individual laser may exceed these specifications. A nominal two milliwatt
laser will usually have an actual output greater than 2 mW. A laser with
a nominal divergence of 1.0 milliradians may have a divergence of 0.8
mi I Iiradians. These supplied figures can provide a general idea of the
power densities involved. However, it must be realized that this can lead
to a serious underestimation of the hazards involved. Use of the nominal
specifications to calculate the power density leads to a nominal calculated
value which may be considerably less than that determined by direct meas-
urement.
Exposure levels which are listed as "safe" or "tolerable" are given in
units of irradiance, mW/cm2. Therefore, the output of your laser must
be expressed in similar units. The usual information given by a laser
manufacturer, and typical values for a classroom type He-Ne laser are
given below:
Power output 1.0 mi I Iiwatts
Beam diameter at I.5 mi I Iimeters
aperture
Beam divergence 1.0 milliradian
44
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12
The irradiance at the aperture is given by the following formula:
P P _ P
a ~~*~^~~~~- - - -
where:
area ir(D /2)2
a
P = irradiance at aperture
a
P = power output
D = Beam diameter at aperture in cm
a
For the laser listed above
P _ I.0 mW
3
3.14 (0.15 cm/2)2
= 57.I mW/cm2
= 5.7 x I0~2 W/cm2
This laser, therefore, has an output irradiance of approximately
6 x IO-2 W/cm2 at the aperture. If we compare this with the "safe"
value of 5 x I0~5 W/cm2recommended by the American Conference on
o
Government Industrial Hygienists (1968) for daylight illumination,
it is approximately 1000 greater than the "safe" value.
As the laser beam travels beyond the aperture, it diverges slightly.
At a distance of r meters, the beam will have diverged to a diameter
D . This is illustrated in Figure 15.
The formula for determining the irradiance at a distance r centimeters
from the aperture is:
Pr = P = 4P
TT(D /2)2
45
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Figure 15
BEAM DIVERGENCE
46
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where:
P = irradiance at distance r
D = diameter of beam at distance r in centimeters
The divergence of the beam in radians can be used to determine the beam
diameter at distance r as follows:
D = rd> + D
r y a
where:
r ~ distance in centimeters
e seen, even a small classroom type laser can emit enough power
to be unsafe for direct viewing of either the primary beam or of specular
reflection (see Experiment 3, page 64), even though the viewer may be
seated in the rear of a classroom.
47
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C. SAFETY AIDS
The laser can be used safely in the classroom. Common sense and pre-
planning of experiments will point out the most obvious hazards. Safety
aids will prevent injury even when the unexpected does occur. Let us
look at some useful aids for classroom use.
I. The Beam Shutter
Lasers are constructed to withstand continuous use for 8 hours a day,
5 days a week. Their life span is actually shortened by intermittent
use. Turning the laser on and off at short intervals is also inconven-
ient. One way to overcome this strain and inconvenience is by the use
of a beam shutter.
The shutter can be a simple mechanical device fitted over the aperture
of the laser. It allows the operator to cut off the laser beam without
actually turning off the laser. The shutter should be made of black,
non-reflective material and should completely stop passage of the laser
beam. One example is shown in Figure 16.
Use of the beam shutter can reduce the hazard to the operator whenever
the experimental configuration in front of the laser is being changed
or altered. It eliminates the possibility of accidental reflection from
a piece of equipment during the changing process. As with all safety
aids, one must develop the habit of shuttering the beam at ALL times it
is not actually needed.
2. The Target
The laser beam will travel outward from the laser until absorbed or
reflected. To prevent accidents a target of suitable material should
be provided for the beam. This target should be made of a non-reflective
material and should also be large enough to stop the beam under a wide
variety of experimental situations. Black foam rubber material is one
example of a good beam target. Black ink on blotter paper also makes a
good target.
48
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Figure 16
Pivoting Arm
Beam Exit Hole
BEAM SHUTTER
49
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3. The Demonstration Box
Accidents can happen during setup and alignment of the laser demonstration.
Observers should be protected during this phase of the demonstration by
placing a shield between the laser and the observers. One way of doing
this is to hang a black, duI I-surfaced curtain or drape between the observ-
ers and the demonstration. Another simple and inexpensive way of accom-
plishing this task is to use a large cardboard box with holes in either
end for entrance and, if necessary, exit of the laser beam. Large panels
can be cut out of either side, one for use while setting up the experiment,
another for viewing. Doors or flaps can cover the openings when not in
use. An example is shown in Figure 17.
4. The Di spI ay Tank
Laser light is invisible from the side unless it is scattered by a medium.
One convenient and relatively safe way of viewing the beam is to use a
clear, liquid-filled display tank. The tank can be readily constructed
out of plexiglass or any clear, thick plastic. Surfaces should be flat
and square with one another to allow accurate passage of the beam through
the tank. The liquid filling can be made of a solution of a clear sub-
stance with some large molecules. Transmission oil may be acceptable, or
a soap solution with some red food coloring mixed in. (See Figure 18).
Caution must be exercised in setting up the display tank, as reflections
from the tank sides could be hazardous.
Do not use a jar or bottle as a display tank. Inexact alignment of the
beam could result in the beam being specularly reflected from the round
side of the bottle.
A second, and perhaps more convenient, display device can be made from
plastic casting resin such as is found in hobby stores. When cast into
a square or rectangular block, with one face painted flat black, the
resin will serve well to display a beam passing through it.
50
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Figure 17
Cardboard Box
Laser
DEMONSTRATION BOX
51
-------
Figure 18
DISPLAY TANK
52
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5. Black Paint
Painting the surfaces of demonstration equipment with a flat, dull,
black paint may be the most important single precaution one can take.
It is unfortunate that many pieces of optical gear are supplied with
bright plated surfaces. Chrome and stainless steel are beautiful, but
the specular reflection from these surfaces can be literally blinding.
6. Reduction of Beam Power Intensity
Many lasers available for educational use furnish far MORE light than
is needed to conduct a demonstration adequately. The power density of
the beam should be reduced to a level commensurate with the level of
light actually required. This reduces the potential for accidental eye
damage. Two methods work well. The first is the simple expedient of
inserting an absorbing filter in the path of the beam. This filter may
be a neutral density type. Any other material which effectively absorbs
the light while at the same time does not scatter the beam, will also
serve. The second method is to increase the diameter of the beam by
means of a pair of lenses. The first lens should spread the beam to an
overall diameter of approximately one centimeter. The second lens is
then used to recollimate the beam to near its original divergence.
Expanding the beam decreases the power density and lessens the hazard
posed by direct or indirect viewing of the beam. If possible, permanent
attachment of the lenses to the laser is recommended. It should be noted
that expanding the beam will not destroy the coherent properties of laser
light. (Figure 19).
7. Key Lock
If possible, the classroom laser should be purchased with or have installed
a key lock for the power supply. When so constructed, the unauthorized
use of the laser can be readily prevented by the instructor.
8. The Dry Run
All experiments should be dry-run before presenting them to a class. This
allows the operator to ascertain where the hazards are or may be and lets
him eliminate them before they can cause damage.
53
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Figure 19
Expanded Beam
(exaggerated)
Original Beam
I * * * I *
Lens
BEAM EXPANDER
54
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D. GENERAL SAFETY RULES
Overexposure of the eye and skin are of primary concern in laser work,
but other hazards also exist: (I) electrical shock; (2) toxic chemicals;
(3) exploding components, such as flash tubes or optics; (4) cryogens
for cooling; and (5) noise or bright flash levels which may startle
personnel and thereby cause an accident. Items 2, 3, 4 and 5 are not
usually associated with He-Ne lasers.
The best safety rule is to know that a potential hazard exists with any
laser and to combine this knowledge with good common sense. A listing
of some general rules is given below. These guides should be followed
whenever the laser is used in either a classroom or a laboratory.
I. Work Area Controls
a. The laser should be used away from areas where the uninformed and
curious would be attracted by its operation.
b. The illumination in the area should be as bright as possible in
order to constrict the pupi Is of the observers.
c. The laser should be set up so that the beam path is not at normal
eye level, i.e., so it is below 3 feet or above 6-1/2 feet.
d. Shields should be used to prevent both strong reflections and the
direct beam from going beyond the area needed for the demonstration
or experiment.
e. The target of the beam should be a diffuse, absorbing material to
prevent reflection.
f. Remove all watches and rings before changing or altering the
experimental setup. Shiny jewelry could cause hazardous reflection.
g. All exposed wiring and glass on the laser should be covered with a
shield to prevent shock and contain any explosions of the laser
materials. All non-energized parts of the equipment should be
grounded.
55
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h. Signs indicating the laser is in operation and that it may be
hazardous should be placed in conspicuous locations both inside
and outside the work area and on doors giving access to the area.
i. Whenever possible, the door(s) should be locked to keep out
unwanted onlookers during laser use.
j. The laser should never be left unattended.
k. Good housekeeping should be practiced to insure that no device,
tool, or other reflective material is left in the path of the beam.
I. A detailed operating procedure should be outlined beforehand for
use during laser operation.
m. Whenever a laser is operated outside the visible range (such as
a CCL laser), some warning device must be installed to indicate
its operation.
n. A key switch to lock the high voltage supply should be installed.
2. Personnel Control.
a. Avoid looking into the primary beam at a I I ti mes.
b. Do not aim the laser with the eye: direct reflection could cause
eye damage.
c. Do not look at reflections of the beam: these too could cause
retinal burns.
d. Avoid looking at the pump source at all times.
e. Clear all personnel from the anticipated path of the beam.
f. Do not depend on sunglasses to protect the eyes. If laser safety
goggles are used, be certain they are designed to be used with
the laser being used.
g. Report any afterimage to a doctor, preferably an ophthalmologist
who has had experience with retinal burns, as damage may have
occurred.
56
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h. Be very cautious around lasers which operate in invisible
Iight frequencies.
i. Before operation, warn all personnel and visitors of the
potential hazard. Remind them that they have only one
set of eyes.
57
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EXPERIMENT SECTION
The following experiments are provided to assist the instructor in
demonstrating properties of light and other electromagnetic radiation
using the laser. The instructor is expected to be familiar with the
classical elementary theory of light; therefore, explanations will be
kept to a minimum. The experiments are so designed as to produce an
effective demonstration with minimum equipment and maximum safety.
Equipment necessary for these demonstrations:
I. Laser
2. Display tank
3. Support for display tank
4. Mi Ik
5. Aerosol room deodorizer or smoke source
6. Liquid detergent
7. Ink
8. Boiled or distilled water
9. Detector (CdS light meter with red filter)* see note below
10. Mi rror
II. Pivot mount for mirror
12. Protractor
13. Ruler
14. Thick slab of glass
15. Prism (45° - 45° - 90°)
16. Polarizing filters (3)
17. SmalI test tubes
18. Quarter wave plates (2) doubly refracting, red
19. AgN03
20. Single-slit diffraction aperture
21. Double-slit diffraction aperture
22. Circular diffraction aperture
23. Divergent lens
58
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24. Hologram
25. White paper
26. Transmission grating
27. Razor blades
NOTE: Light detection and intensity measurement can be accomplished
by use of a photographic light meter, preferably employing a
CdS detector. The light levels from a 2.5 milliwatt laser
will not overdrive the meter and used meters can be purchased
in camera stores. The meter's response to light is not linear,
however, and response must be calibrated against a more accurate
standard.
59
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EXPERIMENT I SCATTERING OF LIGHT
ExpIanation:
When light passes through the atmosphere, it is scattered by the large
number of gas molecules and particles that make up the atmosphere.
Objects are visible only because of the light they scatter toward the
viewers' eyes. It is for this reason (i.e., the lack of light scattered
toward them) that astronauts are largely in the dark when they travel in
orbit beyond the earth's atmosphere. For this same reason, an observer
may not see a laser beam headed across his path. On the other hand, if
smoke is blown into the path of a laser beam, it immediately becomes
visible.
This mechanism of optical scattering varies with the size of the scatter-
ing particles. Particles such as smoke may be considered "large" if
their radii approach the wavelength of the incident light. The scattering
from such particles is referred to as large particle (i.e., Mie)
scattering. In this type of scattering the particles may be considered
as opaque spheres which scatter according to the principles of the
diffraction theory. It is this type of scattering that can pose a
potential hazard when high-powered lasers are used in the atmosphere.
Particles whose radii are much smaller than the wavelength of the incident
light (radius < .05 X), scatter by a different mechanism called Rayleigh
scattering. In this type of scattering, each microscopic particle acts as
an electric dipole, reradiating the incident wave by electrically coupling
into resonance with the electric field of the incident light. This type
of scattering can be seen by observing different regions of the daylight
sky through a polarizing filter. An experiment using AgNO-,, a solution
which exhibits Rayleigh scattering, is given in the section on polari-
zation (Experiment 4, part B).
60
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Materia Is:
Laser
Display tank
Boiled or distiI led water
Mi Ik
Smoke source or aerosol can
EXPERIMENTAL PROCEDURE
Large Particle or Diffraction Scattering
Direct the laser beam so that it passes through the clean display tank
fiI led with boiled or distiI led water. The path of the beam wiI I probably
not be visible in the water. Add a small amount of homogenized milk to
make the water turbid. The path will then become visible. Instead of
milk, a concentrated solution of colloidal silica solution can be added
to the water to make a permanent display solution. Large particle scat-
tering can also be demonstrated by blowing smoke or the spray from an
aerosol can into the path of the laser beam.
61
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EXPERIMENT 2 ABSORPTION OF LIGHT
ExpIanation:
In passing through a material, laser light, like all electromagnetic
radiation, undergoes absorption which can be expressed by the exponential
u x
relationship I = I e , where u is a function of the absorbing material
and the wavelength of the light, and x is the thickness of the absorbing
material. If a green piece of cellophane is placed in the path of a
helium-neon laser beam (i.e. red light), there is a substantial reduction
in the beam intensity. If, on the other hand, a red piece of cellophane
is used with the same beam, relatively little absorption occurs. This
principle of selective absorption of light from laser beams with given
wavelengths is used in some of the commercially available protective
goggles sold for use with lasers. The experiment here will demonstrate
both quantitatively and qualitatively how the absorption of light depends
upon the thickness of the absorber.
Materi a Is:
Laser
Display tank
Liquid detergent
Detector for measuring light intensity
Ink
EXPERIMENTAL PROCEDURE
Prepare a display solution by adding a few drops of a liquid detergent
in water and stir until it is uniformly mixed. Fill the display tank
with this solution and project the laser beam into the tank so that the
path of the beam is clearly visible. Now add a drop or two of blue or
black ink to the solution and stir until the solution is uniform. Notice
that this causes the beam intensity to decrease rapidly as it penetrates
further into the solution. Continue to stir in ink a drop at a time until
62
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the beam vanishes (i.e. is completely absorbed) before it reaches the
opposite end of the tank.
To obtain a quantitative measurement of the exponential absorption of
light in a material, direct the laser beam onto a detector which meas-
ures light intensity or beam power. Record this value. Using various
pieces of absorbing materials such as a semi-opaque plastic, insert
one thickness at a time so as to gradually increase the thickness of
the material through which the laser beam passes. Record the light
intensity for each value of the total thickness of the material and
plot the data on semi-log paper. What is the shape of the line
obtained? Why?
63
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EXPERIMENT 5 REFLECTION OF LIGHT
ExpIanation:
Light and the manner in which it is reflected are of prime importance in
geometrical optics. There are two types of reflection: (I) diffuse
reflection, in which light striking a rough surface is randomly scat-
tered in many directions, and (2) specular (i.e. mirror-like) reflection,
in which the incident light is reflected from a smooth surface in accord-
ance with the law of reflection (i.e., the angle of incidence equals the
angle of reflection, as shown in Figure 20). As discussed in most physics
texts, the behavior of light at an interface between two media is gov-
erned by both the law of reflection and the law of refraction.
Materi a I s:
Laser
Display tank and display fluid
Support for tank
Mirror on pivot mount
White paper
Protractor
EXPERIMENTAL PROCEDURE
A. Specular and Diffuse Reflection
Arrange the experiment as shown in Figure 21 with the display tank near
the laser and a mirror on a pivot mount arranged so as to reflect the
beam back into the tank. It will be observed that near the mirror the
reflected beam has approximately the same intensity as the incident beam.
You might, however, observe a loss of intensity as the incident and
reflected beams traverse the fluid. This is due to scattering of the beam
by the fluid molecules, the process which makes the beam visible from the
si de.
64
-------
Figure 20
Incident Photon
Flux
Normal
L/
er
Reflected Photon
Flux
LAW OF REFLECTION
65
-------
Figure 21
D-
SPECULAR AND DIFFUSE REFLECTION SETUP
66
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Hold a piece of white paper in front of the mirror. The light will now
be diffusely reflected and no beam will be seen re-entering the display
tank. The point at which the light strikes the paper will be visible
through a wide angle, because of this diffuse reflection. When the paper
is removed, the reflected beam will again be visible in the display tank.
The point at which the laser beam strikes the mirror will not be readily
apparent from the side since the light is being specularly reflected.
Whatever light is seen from the side is caused by diffuse reflection from
small random mirror imperfections and to scattering of dust on the mirror
surface.
B. Law of Reflection
The second part of this experiment illustrates the Law of Reflection which
shows that the angle of reflection always equals the angle of incidence.
Arrange the apparatus as shown in Figure 22 (A or B) so that the laser
beam enters the tank and is reflected off the upper surface of the display
fluid. Using the surface of the fluid as a reference, measure the angles
of incidence and reflection with a protractor. Now change the angle of
incidence and measure angles again. At some angle termed the critical
angle, the light will cease to be reflected. This critical angle concept
is explained in more detail in Experiment 4, page 69, in the section
on total internal reflection.
67
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A.
Figure 22
C
B.
I IĞ
COMPARING ANGLES OF
INCIDENCE AND REFLECTION
68
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EXPERIMENT 4 REFRACTION OF LIGHT
Explanation:
In the previous experiment we observed that light travels in straight
lines and is reflected at the interface between two media according to
the law of reflection. Another property of light is that its velocity
depends upon the medium in which it travels. This phenomena results
in the refraction or "bending" of the light wave front as it passes
obliquely from one substance into another. The ratio of the velocity
of light in one medium to its velocity in a second medium is defined
as the index of refraction (n).
The velocity of light in a given media also depends on its wavelength.
This dependence is related to its dispersion and we account for it by
saying the index of refraction varies with wavelength. In geometrical
optics, all the properties of lenses and mirrors can be explained by
knowing that light travels in straight lines and obeys the laws of
reflection and refraction when diffraction and interference effects
can be neglected.
Total Internal Reflection
In general, when light traveling through one substance obliquely enters a
second substance having a higher index of refraction, it is bent toward
the perpendicular, i.e. toward the normal to the surface. When it enters
a substance having a lower index of refraction, it is bent away from the
perpendicular. Thus, when a light beam passes obliquely from water or
glass into air, the refracted ray is bent away from the perpendicular as
shown in Figure 23. The relationship between the angles and indices of
refraction is given by Snell's law,
n, sin6.= n? si n 9
69
-------
Figure 23
\
\
\
\
\
medium 1
\
\
medium 2
normal
index of refraction
nl
index
of refraction
A "2
medium 1
\
\
\
index of refraction
\
\
\
normal
LAW OF REFRACTION
70
-------
where:
n. = index of refraction of medium I with respect to a vacuum
n~ = index of refraction of medium 2 with respect to a vacuum
6 . = angle of incidence
9 , = angle of refraction
As the angle of incidence 6 . increases, the angle of refraction 9 ,
increases to a value where the refracted beam just grazes the surface
of the interface between the two materials. The angle of incidence
which produces an angle of refraction 9, of 90 is called the critical
angle. For a water and air interface, the critical angle is about
49 degrees. If this critical angle is exceeded, the beam does not
leave the material at all but is instead totally reflected internally.
Since the index of refraction of air with respect to a vacuum is 1.0
the index of refraction of water with respect to a vacuum is 1.33, the
relative index of refraction of air with respect to water (i.e. for a
light ray going from water into air) is about 0.75. The critical angle
for a water-air interface is therefore 48.6 as is shown in the calcu-
lation below:
where:
n sin9. = n si n 9 ,
w i w a ta
n sin9. = n sin 90°
w i w a
1.33 sin 9 . - I sin 90° =
i w
sin 9 . = 0.75
iw
9. - 48.6°
i w
n = index of refraction of water with respect to a vacuum
n = index of refraction of air with respect to a vacuum
3
9 . = angle of incidence of beam in water
9 . = angle of refraction of beam in air
Ta
71
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Materi a Is:
Laser
Display tank and fluid
Mi rror on pi vot
Protractor
Ruler
Thick slab of glass or other material
Prism (45° - 45° - 90°)
EXPERIMENTAL PROCEDURE
A. Refraction of light
In this first experiment, the relationship between the angle of incidence
and the angle of refraction will be observed. Arrange the equipment as
shown in Figure 24 using two large books or other objects to position the
display tank above the laser beam. The beam should now pass under the
tank, strike the mirror and be reflected into the display solution through
the side of the tank. You will observe that the beam is bent toward the
normal as it passes from a medium of low index of refraction to a medium
of higher index (air to fluid) and is bent away from the normal as it
passes from a higher index material to a lower index material (fluid to air)
Next, direct the laser beam into the display tank as shown in Figure 25.
Use a book to support the laser if necessary. The beam will be reflected
at the bottom and top of the fluid as it passes through the tank. Some
smalI portion of the beam wi I I escape the tank at each reflection point
and will be refracted as it passes through the fluid-air interface. Meas-
ure the separation between two adjacent emerging beams. The index of
refraction can then be calculated using the formula
2d s i n 8 . cos 9 .
i i
a =
2 - sin
72
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Figure 24
1
(
> Ğp Ğ
Supports for Display Tank
OBSERVING LAW OF REFRACTION
73
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Figure 25
I *L i
v v
4
d
{
DETERMINING INDEX OF REFRACTION
74
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where:
a = the lateral separation of the rays leaving the tank
d = the thickness or depth of the display solution
n_ = index of refraction of medium 2 with respect to a vacuum
This principle has practical application in determining the index of
refraction of a thick glass plate or other transparent material. To
perform this experiment, remove the display tank and place a thick slab
of glass in the path of the beam. Measure the separation between
emerging beams and the angle between the incident beam and the normal
to the surface. The above equation can then be used to calculate the
index of refraction of the material.
The principle of refraction is utilized in all lenses for focusing by
causing convergence or divergence of light. Refraction enables the
lens of the eye to focus light from an object on the retina. This is
a very important concept in laser safety since the energy density of
the light beam is concentrated about a million times in passing through
the lens and being focused on the retina. This effect accounts for the
eye being the most critical part of the human body as far as potential
damage from a laser beam is concerned.
B. Total Internal Reflection
Place a 45° - 45° - 90° prism as shown in Figure 26A to demonstrate
total internal reflection of light back into the display tank parallel
to the incident beam. Next place the prism as shown in Figure 26 B
and note that the light beam is bent through 90 . This effect finds
application in many optical instruments. Arrange the experimental setup
as shown in Figure 26C. Observe that the beam transmitted through the
prism rises as the incident beam is lowered. The line drawing in
Figure 26C also illustrates how a right angle prism can be used to
invert an image.
75
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A.
Figure 26
B
PRISM EXPERIMENTS
76
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The critical angle of a prism can be measured by placing the prism on
a pivot mount and rotating it so as to change the incident angle until
total internal reflection occurs.
Total internal reflection can also be illustrated by using about 1-1/2
inches of solution in the display tank. Using the setup illustrated
in Figure 24, adjust the angle of the incident beam until you obtain
total internal reflection of the laser beam in the solution. This will
occur when the incident angle is greater than the critical angle.
77
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EXPERIMENT 5 POLARIZATION OF LIGHT
ExpIanati on:
Electromagnetic radiation is, as the name implies, a combination of
electric (E) and magnetic (H) fields. These fields are perpendicular
to each other and to the direction of propagation of the radiation.
The propagation of such electromagnetic radiation, of which light is
one example, is depicted as a complex wave form as shown in Figure 27.
In discussing polarization of light, it is customary to focus attention
only upon the E field, since most common optical phenomena are due to
the interaction of the E field of the radiation with the E field of
physical structures. It is the E field, for example, that is photo-
graphically active and causes a chemical change in photographic plates.
Light from most common sources is usually unpolarized. Light in these
sources comes from an enormous number of individually oscillating atoms
or molecules. The random orientation of these atoms and molecules
results in a completely random orientation of the E fields of the vari-
ous photons of light. If, however, some control is exercised over the
orientation of the E field of a beam of light, the result is the pro-
duction of polarized light.
Linearly Polarized Light
The electric field vector of a beam of light depends upon the sum of the
individual E fields of the photons which comprise the beam. If the
vector representing the E field of a beam of light always oscillates in
a fixed plane as the beam progresses through space, the beam is said to
be linearly polarized, or plane polarized (Figure 28).
Linearly polarized light is most easily obtained through the use of a
polarizing filter or polarizer. One such device consists of dichroic
crystals of an iodine compound absorbed on a sheet of stretched polar-
ized alcohol. The polarizer preferentially transmits light beam
78
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Figure 27
direction of
propagation
ORIENTATION OF FIELDS
Figure 28
A. UNPOLARIZED LIGHT
B. POLARIZED LIGHT
SCHEMATIC DRAWINGS
79
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components whose E fields are aligned with the polarizing axis of the
polarizer. Components whose E fields are not aligned with the polarizer
are partially absorbed by the polarizer.
A second polarizer may be placed in the path of a beam which has already
been linearly polarized, as shown in Figure 29. As the second polarizer
is rotated, the intensity of the light transmitted by it will vary as
the square of the cosine of the angle between the two polarizers.
I = I cos2
o
where:
I = transmitted intensity
I = maximum intensity of light impinging on the second polarizer
<|> = angle between the axis of the two polarizers
A second means of producing polarized light is by reflection. When
unpolarized light strikes a reflecting surface, there is found to be a
preferential reflection for those photons whose E field lies in a plane
perpendicular to the plane of incidence. (The plane of incidence is the
plane determined by the incident path of the beam and the normal to the
surface). Consequently; the E field of a beam of reflected light is
strongly linearly polarized. A beam striking the surface at normal
incidence will have all polarizations reflected equally.
At one particular angle of incidence, called the polarizing angle, no
light will be reflected except that in which the E field vector is per-
pendicular to the plane of incidence. All light whose E field is not
perpendicular to the plane of incidence is refracted into the material.
At angles of incidence other than the polarizing angle, some of the
components parallel to the plane of incidence are reflected so that,
except at the polarizing angle, the reflected light is not completely
Ii nearly polarized.
80
-------
Figure 29
oo
(J %-fy
Polarizer Analyzer
POLARIZER-ANALYZER COMBINATION
-------
Should linearly polarized light be directed at a reflecting surface it
will be reflected or refracted, depending upon the orientation of the
E field with respect to the plane of incidence. If the E field of the
polarized light is aligned parallel to the plane of incidence, and the
angle of incidence is equal to the polarizing angle, no light will be
ref lected.
The polarizing angle of a material, which depends upon its index of
refraction, is that angle of incidence at which the angle between the
reflected beam and the refracted beam is equal to 90 . This is illus-
trated in Figure 30. If n. is the index of refraction of the material
in which the light is initially traveling, and n~ is the index of
refraction of the reflecting material, the polarizing angle is
determined by
tan
The polarizing angle is sometimes referred to as Brewster's angle, after
Sir David Brewster who discovered the phenomenon in 1812. Brewster's
angle can be obtained from Snell's law, given previously, by noting that
n. sin = n0 sin (ir/2 - d> )
I z
n9 sin <{> sin $
- tan <(>
n . si n (ir/2 - <|>) cos
A third method of producing linearly polarized light is by scattering.
When unpolarized light passes through a transparent material such as
glass or air, the E field of the radiation sets the electric charges of
the molecules in oscillation. The charges oscillate in every direction
perpendicular to the direction of propagation of light. The charges
re-radiate the light and the E field of the re-radiated light lies in a
plane perpendicular to the direction of propagation of the original beam.
An observer, looking perpendicularly at a beam of light passing through
a piece of glass sees linearly polarized light. This is illustrated in
F i g u re 31.
82
-------
Figure 30
Incident
Unpolarized Light
POLARIZING ANGLE
83
-------
Figure 31
Glass
oo
Observer
Scattered Linearly
Polarized Light
Observer
/N
Scattered Linearly
Polarized Light
POLARIZATION BY SCATTER
-------
Circular Polarization
Consider two plane polarized, coherent beams of light of equal frequency
and amplitude, whose planes of polarization are mutually perpendicular.
The amplitudes of these vibrations can be drawn and the resultant vector
plotted. If this is done, it wi I I be seen that the resultant vector
describes a linearly polarized beam whose plane of vibration lies at an
angle of 45 to the plane of each of the component beams. (Conversely,
a linearly polarized beam may be resolved into a pair of linearly polar-
ized beams whose planes of polarization are mutually perpendicular.)
Next consider the above situation when one component is 90° out of phase
with the other as shown in Figure 32. If the resultant vector is drawn,
it wi I I be seen that the magnitude of this vector will always be the
same but the direction will change with time. Let E represent the
magnitude of the resultant vector. If the resultant vector is drawn
through a stationary point lying along the direction of propagation, the
vector will sweep out a circle of radius E as the waves pass. This 90°
phase difference between two linearly polarized coherent equal magnitude
beams is called circular polarization.
When other than a 90° phase difference exists, this is known as ellipti-
cal polarization.
Circularly polarized light can be produced by using a doubly refracting
crystal. A doubly refracting crystal displays many unusual properties.
A beam of light passing through such a crystal is split into two compo-
nents, each linearly polarized, but with their axes of polarization
mutually perpendicular. Furthermore, the two components do not travel
at the same speeds. Thus, if coherent light is directed into the crystal
the two components are out of phase when they emerge from the crystal.
The amount of phase difference is proportional to the thickness of the
crystal.
85
-------
Figure 32
r/^\/^~~\
"^
ELIPTICAL POLARIZATION
86
-------
A crystal which causes a 90 difference in phase is called a quarter
wave plate. This phase difference can be demonstrated by using one or
more polarizing filters and the beam from a He-Ne laser. The beam
should be checked for polarization using one of the polarizing filters.
If a linearly polarized coherent beam of light is directed at a quarter
wave plate, with the plane of polarization of the beam at a 45 angle
to the optical axis of the crystal, the crystal will resolve the beam
as described above. The beam leaving the crystal will consist of two
linearly polarized components whose planes of polarization are mutually
perpendicular and whose phase difference is 90° and can be referred to
as circularly polarized light. When a second polarizing filter, the
analyzer, is slipped into the path of the beam, as shown in Figure 33,
it will be noted that the light transmitted through the filter does not
change in intensity as the analyzer is rotated.
Note: It must be noted that a given quarter wave plate is designed for
use with a specific frequency of light. A quarter wave plate
for green light is not a quarter wave plate for red light. Also
note that the quarter wave plate must be of doubly refracting
crystal, not of glass.
MateriaIs:
Laser
Polarizing filters (3)
Display tank and fluid
SmalI test tube
AgN03
Quarter wave plate (2) (Doubly
refracting) (Red)
Piece of glass
Protractor
Detector (CdS light meter
with red fiIter)
EXPERIMENTAL PROCEDURE
A. Linear Polarization with Polarizing Lenses
Project the laser beam into the display tank and insert a polarizer into
the path of the beam so that it can be rotated in the plane perpendicular
to the beam. By rotating the polarizer, find the angle for which the
87
-------
Figure 33
Result
Analyzer
Quarter Wave
Plate
Polarizer
Incident
Unpolarized
Light
EFFECT OF QUARTER WAVE PLATE
-------
beam is extinguished or is of minimum intensity. Note this angle and
rotate the polarizer 90°. Next place a second polarizer (analyzer)
between the first polarizer and the display tank. Keeping the orienta*
tion of the first polarizer fixed, rotate the analyzer until you find
an angle which extinguishes the beam. Record this angle and rotate the
analyzer until the maximum intensity is transmitted. Record this
angle. Continue to rotate the polarizer and note the angle for which
the beam fades out again. What is the angle of rotation between the
maximum and minimum intensity? Define the angle of rotation at maximum
intensity as zero. The angle for minimum intensity will then be 90°.
Place a detector in the path of the transmitted beam and record the
intensity and angle of rotation as the analyzer is rotated from the
point of minimum to maximum intensity. Plot the transmitted intensity
versus the square of the cosine of the angles of rotation from 0° to
90 . In this position the two polarizers are in a "crossed" position.
Next, place a third polarizer between the original two and rotate it,
noting that the beam is now partly transmitted. Continue to rotate the
third polarizer and note the angles of rotation between the minimum and
maximum of transmitted light as observed in the display tank. What is
the angle of rotation between minimum and maximum transmission? What
is the orientation of this third polarizer with respect to the other
two when it is rotated to the position for maximum transmission of light?
B. Polarization by Scatter
Place a few drops of AgNO, in a small test tube of tap water to obtain a
slightly "milky" solution of microscopic colloidal particles. Arrange
the experimental setup as shown in Figure 34 so that the laser beam is
directed into the top of the tube and travels down its long axis. Ob-
serve the scattered light through a polarizing filter. The fact that
this scattered light is highly polarized can be observed by holding a
polarizing filter at right angles to the direction the laser beam is
traveling and rotating it. Repeat this experiment using a few drops of
homogenized milk in a test tube of water. In this case, it will be
observed that the scattered light is not polarized.
89
-------
Figure 34
c
D
Support
for
Laser
n
1
1
II
M
1
1
OBSERVING
POLARIZATION BY SCATTER
90
-------
C. Brewster's Angle
Arrange the equipment as shown in Figure 35 A. Place a slab of glass
on top of a thick book and align it so the laser beam strikes the glass
perpendicularly. Now rotate the glass until the angle of incidence is
about 57 . The reflected beam should be stopped by using a sheet of
paper as a screen for classroom visibility. Now insert a polarizer into
the path of the beam between the laser and the glass. Rotate the polar-
izer until the angle is found for which the beam strikes the glass and
the reflected beam disappears.
As the polarizing angle for the glass is approached, the intensity of
the reflected beam diminishes and the transmitted light increases to a
maximum. The intensity actually varies with the square of the cosine
of the angle of deviation from the polarizing (Brewster's) angle.
Rotate the glass and show that at angles other than Brewster's angle,
the light is still partially reflected (as shown in Figure 35 B).
Gas laser tubes are commonly constructed with windows at the Brewster
angle at both ends as shown in Figure 35 C. A wave propagating along
the axis of the laser with its E field in the plane of the figure is
completely transmitted without any reflection by the windows. The
light can be reflected back and forth through the cavity by using
external mirrors to establish the standing waves necessary for laser
operation. Some lasers use mirrors inside the glass tube, eliminating
the need for Brewster windows. If these internal reflectors are made
of dichroic materials, the output of the laser is polarized.
D. Circularly Polarized Light
Place a polarizing filter in the path of the beam to obtain linearly
polarized light. Insert a quarter wave plate between the polarizing
filter and the display tank so that the polarization axis of the beam
is at a 45 angle to the optical axis of the plate. Arrange the equip-
ment as shown in Figure 33.
91
-------
Figure 35
A. SETUP (TOP VIEW)
C
I
-
Screen
Polarizer
B. COMPONENTS
Reflected
Component
Normal
X
Incident Li
90
Partially
Polarized
Beam
Refracted Component
i = R - 90°
R = i + 90°
C. BREWSTER WINDOW LASER
Brewster Windows
Plane of
Polarization
Reflector
Gas Tube
BREWSTER'S ANGLE
Reflector
92
-------
Place a second polarizer (analyzer) filter between the quarter wave
plate and the tank and rotate the analyzer. There should be no change
in intensity.
Place a second quarter wave plate between the first quarter wave plate
and the analyzer. When the axes of the two plates are parallel, it
will be observed that the transmitted light is plane polarized and per-
pendicular to the original plane of polarization.
E. Unpolarized Light
Place a sheet of waxed paper into the path of the laser beam between a
pair of polarizers and rotate the second polarizing filter. The trans-
mitted intensity does not change with the angle of the second polarizer.
93
-------
EXPERIMENT 6 COHERENCE
Consider an ideal elementary sinusoidal wave form. The characteristics
of this wave are its amplitude A (its height above the zero line) and
its wavelength X (the distance from wave peak to wave peak). In this
case, the amplitude varies with time in a constant sinusoidal manner and
with the maximum remaining the same. The wave length also remains con-
stant, as the wave propagates (Figure 36).
Temp o raI Coh e re n ce
The definition of coherence involves two or more waves rather than just
one. Consider two waves, one superimposed over the other on the same
line of propagation. (Figure 37 shows the two waves with the super-
imposition modified for clarity.)
First, disregard the amplitudes of the points and consider points only
with respect to the direction of propagation.
During a time t, each of the two waves advances down the line of propa-
gation to a wave peak, point I on wave A. and point 2 on wave A-. They
are traveling at the same speed and have the same wavelength and, if
allowed to continue, the wave peaks will occur at the same places all
the way down the line. These waves are said to be in phase and coherent.
This particular coherence, that is, with respect to time, is called
temporal coherence.
With this in mind, we can now define coherence. Two or move waves are
said to be coherent if the "phase difference" between two pairs of points,
one on each wave, remains constant. In our example, where the waves are
in phase, the phase difference is zero throughout and thus constant.
Figure 38 shows a slightly different situation. In this case, one wave
"lags" the other slightly, but the waves are still temporally coherent
94
-------
Figure 36
SINE WAVE
direction of
propagation
time
(t) axis
(t)
direction of
propagation
TEMPORAL COHERENCE,
NO PHASE DIFFERENCE
95
-------
Figure 38
TEMPORAL COHERENCE,
CONSTANT PHASE DIFFERENCE
96
-------
because the sets of points remain at a constant distance d apart along
the line of propagation. They are not in phase but have a constant
phase difference.
Interference
Up to this point, the amplitude of the waves being discussed has been
ignored in order to point out other factors.
Consider two waves superimposed on each other along the same line of
propagation. It is well known that as a wave moves through a media, it
propagates a disturbance along the direction of propagation. The amount
of disturbance depends on the amplitude of the wave. When the amplitude
is positive (i.e. when a peak is formed or the wave point is moving up),
it is termed positive disturbance. Alternately, when the amplitude is
negative, it is termed negative disturbance.
If two waves are superimposed on one another along the same line of
propagation, the amplitudes add. If there are two positive amplitudes,
they total to a greater positive amplitude. Two negative amplitudes
follow the same formula, adding to a greater negative amplitude. If,
however, a negative amplitude is paired with a positive, the difference
between the two is found for the total amplitude.
The interaction of two waves as described above is called interference.
Constructive interference results from adding two wave amplitudes of
the same sign (+ or -) (see Figure 39). Destructive interference
results from adding two amplitudes of different signs (Figure 39).
Late ra I Cohe re n ce
Consider two waves traveling along two parallel lines of propagation,
with equal amplitudes, wave lengths, and in phase (Figure 40). Pick two
points, one on each wave, and note what happens as the wave propagates.
97
-------
Figure 39
A. Constructive
B. Destructive
A+B
INTERFERENCE
98
-------
Figure 40
LATERAL COHERENCE
99
-------
(It must be emphasized that only the positions of the points in a per-
pendicular direction to the line of propagation need be considered.)
As the points move from A to B to C, the distance between them remains
constant. The points are not only in phase along the path of propagation,
but also in a direction perpendicular to the paths of propagation. Since
these points are in phase perpendicular to the path of propagation, they
are said to be lateraIly coherent.
Spatial Coherence
The discussion has been limited thus far to a two-dimensional representa-
tion. However, light waves are three-dimensional, and the concept of
coherence must be expanded to cover such a system.
The transition is simple. Temporal coherence remains the same along the
direction of propagation as does lateral coherence, but now lateral
coherence is allowed to relate to any direction perpendicular to the
line of propagation.
The result of combining these two concepts, lateral and temporal coherence,
produces what is called spatial coherence in which two or more points are
in phase a) along the direction of propagation (temporal); and b) along
any direction perpendicular to the direction of propagation (lateral
(F i g u re 41).
Degree of Coherence
Since coherence has been defined, degrees of coherence can be discussed.
The question "How coherent is laser light, or any other wave form?" can
be asked.
In nature, all waves contain components with wave lengths or frequencies
both below and above the wave's primary frequency. In other words, they
have a definite bandwidth B. This bandwidth determines the phase change
that the wave can take and thus can be used as a measure of the wave's
100
-------
Figure 41
All points remain equidistant
as wave propagates
direction of
propagation
SPATIAL COHERENCE
-------
coherency. An approximation can be made to determine the time during
which a wave remains reasonably coherent and is approximately equal to
the reciprocal of the bandwidth, or T ~ I/B. The length or distance
over which a wave would remain coherent, then, would be the coherence
time T multiplied by the velocity of the wave. In the case of light,
the velocity is c = 2.99774 x I08 m/sec. The coherence length L
(_>
would then be equal to L = v/B = c/B, where v is the velocity of the
wave and c the velocity of light.
To determine the number of wavelengths n over which the wave is coherent,
we divide the coherence length by the wavelength n to obtain
n = c/BX
The number of wavelengths over which the wave train remains coherent is
the important parameter since it gives us a better picture of the quality
of coherence of a wave than does time and length. Coherence time can be
long for a low frequency of light or coherence length can be long for
long wavelengths of light. We shall use n, the number of wavelengths
over which the wave remains reasonably coherent, as our measure of the
degree of coherence of a wave or source.
Importance of Coherence
We have said that laser light is highly coherent and have subsequently
defined what is meant by coherence and degree of coherence. We shall
now investigate the consequences of the coherency of laser light, the
effects it produces and other characteristics.
The uses of ordinary light (which is not very coherent) are limited by
two physical rules known as Abbe's sine condition and the second law
of thermodynamics. Both of these were formulated well before coherent
light was really considered. These conditions define both our ability
to focus ordinary light and the amount of energy that can be transferred
from a source of ordinary light to some material at another point in space.
102
-------
Coherent light, however, is not subject to such restrictions. It can be
focused into an extremely narrow beam, and onto a spot limited only by
diffraction effects. In one form, the second law of thermodynamics
states that when temperatures of the source and sink are equal, no further
transfer of energy can take place. In this form, the law no longer
applies to laser light; the temperature at the focused image of the laser
can be made as high as the diffraction limitation allows.
Because of diffraction limited focusing, laser light can be focused down
to a minimum cross-section of about one square wavelength. This leads
to the possibility of attaining high energy density levels over extremely
smalI areas.
One of the most startling consequences of coherent light was totally
unexpected until the phenomenon was viewed for the first time during the
operation of a continuous wave laser. This phenomenon is, of course, the
speckled image produced when the highly coherent laser light is scattered
from a semi-smooth diffuse reflector. Here we see diffraction and phase
interference effects on a scale never attained before with visible light.
What we see is really a stationary diffraction pattern in space resulting
from the scattering of the coherent light by the diffuse reflector and
the subsequent interference of the coherent wave trains with each other.
The effect is enhanced by the high coherence so that the pattern extends
to such dimensions visible to the human eye.
We should note, however, that this interference destroys the light's
coherency a few wavelengths beyond the reflecting surface, just as such
interference destroys coherency in ordinary light a few wavelengths
beyond the source.
Since this phenomenon is so startling and has ramifications which carry
through to nearly all uses of coherent light, we shall explain it in
more detai I.
103
-------
First, let's look at ordinary light reflecting off a diffuse surface,
this paper for example. The waves striking the surface of the paper
are individually reflected by each tiny surface of the paper (according
to the laws of reflection) and interfere with each other. Since coher-
ence is very low, on the order of three wavelengths, the diffraction
pattern produced is not only small (again on the order of a few wave-
lengths) but changes rapidly. The dimensions are so small that the
human eye cannot resolve the phenomenon. With the highly coherent light
produced by the laser (coherence length in the order of I010 to I014
wavelengths) visible interference occurs over a much larger volume and
the resulting diffraction pattern is huge by wavelength standards. Also,
due to long coherence time, the pattern changes relatively slowly and
thus remains visible for longer periods of time. The net effect is the
granular pattern we see when the laser light is reflected.
It is interesting to note also that the pattern viewed depends substan-
tial ly upon the observer. The size of the individual grains of light
we see is directly proportional to the iris size of the eyes. Also, if
the observer remains stationary the pattern will remain stationary with
little change except that due to instability of the laser.
Even more unusual, when the viewing system is defocused the pattern still
remains sharp, although its shape changes.
The diffraction and interference effects which cause this phenomenen play
an important role in any situation where the laser beam is reflected,
transmitted, or reacts with matter in any way. Coupled with highly coher-
ent light, they make possible such applications of the laser as holography
and demonstration of many basic physical optics concepts.
104
-------
EXPERIMENT 7 DIFFRACTION
Explanation:
Under appropriate experimental conditions it can be shown that light does
not always travel in straight lines.
The term diffraction refers to phenomena in which light or other electro-
magnetic radiation is bent around an obstacle instead of exhibiting a
simple straight line propagation predicted by geometrical optics.
The wave theory explanation of the interference and diffraction of light
was first introduced by the English physician and physicist Thomas Young
(1773-1829). He performed his double-slit experiment in 1801 to explain
the waves' departure from straight line propagation. This concept was
later developed by the Franch mathematician Augustine Fresnel (1788-1827)
who gave it a strong mathematical foundation.
If light passes near the edge of an object, light and dark bands are
seen in the region of the geometric shadow. The light can thus be
"bent" around an opaque object. The light is bent (or diffracted) by
obstacles, in a fashion similar to the way waves on water are bent
around a pier in their path.
This discussion will cover only the general concepts of diffraction and
interference since most text books present calculations of the principle
characteristics of simple diffraction patterns.
According to Huygen's principle, when light emerges from an aperture or
the edge of a barrier, each point along a plane perpendicular to the
direction of propagation of the incident light may be regarded as a new
source. The amplitude of the radiation from these new sources arriving
at the viewing screen is dependent upon the distance from the new source
to the point at which they strike the screen.
105
-------
The summation of the interference of the light waves from a slit or
barrier edge produces an illumination pattern of maximum and minimum
intensity on the viewing screen. In the case of a slit, the spacing
between regions of maximum and minimum intensity is inversely propor-
tional to the width of the slit. A diffraction pattern of light from
two slits is simply the interference pattern from the two slits super-
imposed on the diffraction patterns from the individual slits. An
elaboration of the 2-slit idea includes the use of a number of slits
equally spaced. Such an array is called a diffraction grating and the
diffraction pattern obtained is the result of multiple interference of
a large number of slits so that the maximum and minimum are much
sharper than before.
NOTE:
Diffraction is an important phenomenon of light that has many applications
when considering the use of lasers and laser safety. For example it is
the diffraction phenomenon that sets a limit on the smallest spot size
that can be focused on the retina of the eye and thus the maximum concen-
tration of light or energy that can occur. Large particle scattering,
which is a diffraction phenomenon, is also important when considering the
potential hazards of high power lasers in the atmosphere.
Materials:
Laser
Single slit diffraction aperture
Double slit diffraction aperture
Circular diffraction aperture
Transmission grating
Razor blade
106
-------
EXPERIMENTAL PROCEDURE
A. Single slit diffraction pattern
Place a single-slit aperture in the path of the laser beam. The diffrac-
tion pattern obtained with a single slit has a broad intense central
maximum with subsidiary maxima on both sides, as shown in Figure 42. To
analyze this phenomenon, imagine each point along the slit to be a source
of waves which are in phase. Consider that the slit is divided into two
zones, AB and BC. To begin with, let us consider only the waves that come
out at an angle 9 so that they alI strike the viewing screen at the point
P and are cancelled, producing a minima at that point, as shown in
Figure 42. Since the screen is far away from the slit, it can be con-
sidered that the angles is approximately equal to 9 and9?. At an
angle 9 , a wave from point B travels a distance A/2 further than a wave
from point C. Similarly, a wave going from point A to point P travels
A/2 further than a wave from point B. Thus, the intensity at point P on
the viewing screen will be zero since the waves arrive one with a crest
and the other with a trough, resulting in the cancellation of one another.
A similar cancellation will occur when the path difference of rays
point A relative to those from point C is an integral number of wave-
lengths. Thus we can deduce from Figure 42 that subsequent nodes fall
at P where
n
si n 9 = nA/d
Since sin 9 is also equal to the ratio of distance L between the slit and
the screen divided by the distance s from the central maximum to the first
minimum, then:
/, *
s/L ~- -T-
LA
This is the spacing between successive minima. Thus, the wider the slit
size is, the closer is the spacing between the nodes. Test this equation
by using single slits of different widths.
107
-------
Figure 42
c
d>
Viewing
Screen
s-*, X\ Screen
\/\/V I
\\ ,2A .3A
d" +"d" +d
Huygen's
wavelets
Monochromatic
Coherent
Wavefront
SINGLE SLIT DIFFRACTION
108
-------
B. Double slit diffraction pattern.
This experiment was originally performed by Thomas Young in order to
demonstrate the effects to be expected from a wave description of
light. In this experiment, we consider what happens when there is
more than one slit.
Insert a double slit barrier in the path of the laser beam. Alter-
nately expose first one and then both slits. When both slits are
exposed, one obtains a combined interference and diffraction pattern,
as shown in Figure 43. In this case, each of the two slits has a
diffraction pattern. The diffraction pattern of each is on top of the
other, causing an interference pattern on top of the diffraction pat-
tern. Thus, we obtain an interference pattern (because of the two
slits) which in effect modifies (modulates) the intensity distribution
pattern of the single slit. That is, one obtains a lot of spots of
equal intensity under the envelope formed by the diffraction pattern
of each individual slit.
C. Diffraction grating patterns.
Substitute a simple transmission diffraction grating for the two slits.
The grating may be considered as a general case of the double slit
barrier. The diffraction maximum and minimum intensities are very
sharp. Their spacing is given by the equation 9 = nX/d.
D. Diffraction by a straight edge.
Position a razor blade in the path of the laser beam so that the sharp
edge intersects about half of the beam. This will give rise to a
diffraction pattern which is characterized by parallel bands of maxima
and minima as shown in Figure 44. The intensity at the edge of the
geometric shadow is about 1/4 of the undiffracted intensity. Beyond
the geometric shadow, the intensity varies with position in an oscilla-
tory way, giving rise to regular variations in brightness which gradually
settle down to the undiffracted intensity.
109
-------
Figure 43
Viewing
Screen
Huygen's
wavelets
1
t
Monochromatic
Coherent
Wavefront
DOUBLE SLIT DIFFRACTION
-------
Figure 44
1
c
c
Straight Edged
Object
\
Geometric
Shadow
Viewing Screen
Monochromatic
Coherent
Wavefront
DIFFRACTION BY A STRAIGHT EDGE
111
-------
E. Circular aperture diffraction patterns.
Many optical systems have circular apertures. The diffraction pattern of
such a system appears as a set of concentric circular rings, as shown in
Figure 45. The calculation of the angles at which maxima and minima of
illumination occur is more complicated and requires the use of Bessel
functions. Since this is not a simple mathematical function, only the
results will be given. For the single slit, the points of minimal inten-
sity are given by:
sin 9 = nX/d
n
For the circular aperture with diameter d, the minimal intensity observed
on a viewing screen at a distance L from the aperture is given by the
formuI a:
sin 6 = (K )(X/d)
n n
where n is an integer (I, 2, 3...) and K is determined by using Bessel
functions. The values of K are:
= 1.22
= 2.23
= 3.24
= 4.24
If the diffraction pattern is projected on a screen at a large distance,
L, from the circular ap<
intensity are given by:
L, from the circular aperture, the radii (r ) of the circles of minimum
tan 9 = r /L.
n n
For small angles, sin 6 is a good approximation for tan 6 . Thus,
sine - r /L or r - L sin 6
n n n n.
Thus the radius of the minimum intensity is given by:
r = (K )(A/d).
n n
Therefore, the first dark ring has a radius of r. = 1.22 LA/d and a diameter
d( = 2.44 LA/d.
The diffraction of light by a circular aperture is of special importance in
laser safety because diffraction by the aperture of the eye (the iris) deter-
mines the smallest spot size that can be produced.
I 12
-------
Figure 45
Viewing
Screen
Huygen's
wavelets
Circular Pinhole
^ Monochromatic
Coherent
Wavefront
DIFFRACTION BY A CIRCULAR APERTURE
I 13
-------
EXPERIMENT 8 HOLOGRAM
ExpIanation:
The preceding experiments in diffraction and interference provide the
foundation for an undei standing of the process of holography. The word
"holography" comes from the Greek words "holos" meaning whole and
"graphics" meaning writing. Thus, a hologram would be a picture of an
object in its entirety. Holography is a technique for storing and repro-
ducing the image of a three-dimensional object. It is formed by photo-
graphing the interference pattern produced when a laser beam that is
scattered from an object interacts with a second reference laser beam.
Since the reference beam and the scattered laser light have a definite
phase relation at each point on the film, an interference pattern is
formed and recorded. The three-dimensional image of the object is
reconstructed by shining a laser beam through the film thus produced.
It is because of the monochromatic!ty and the fixed phase relationships
between the individual photons of laser light that holography is possible.
Materials: Laser
Hologram
Divergent lens
EXPERIMENTAL PROCEDURE
Place the divergent lens in front of the laser to spread the beam. Insert
the hologram into the path of the beam at a point where the beam has
diverged enough to cover the entire hologram. View from in front of the
laser but do not look directly down the beam path (Figure 46). Now cover
a portion of the hologram and note that the image is still complete, though
reduced in clarity. This illustrates that the information contained by a
hologram is recorded over the entirety of the hologram. Next, find the
real image.
An alternate and perhaps safer method of viewing the hologram is to view
from the laser side of the hologram, with the hologram held at a slight
angle to the path of the beam. Much sharper images can be obtained with
a coI Iimated beam.
I 14
-------
Figure 46
observer
.XT
expanding
collimating
lens
hologram
image
VIEWING A HOLOGRAM
-------
REFERENCES
Cited References
I. Goldman, Leon, M.D., Biomedical Aspects of the Laser, Springer-
Verlag, New York.
2. Czerny, V., Klin MblAugenhei lk,Vol. 5, p. 393, 1817, cited in
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3. Verhoff, F.H., Bell, L. and Walker, C.B., The Pathological Effect
of Radiant Energy on the Eye. Proc. Am. Acad. Art and Sci.,
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4. Gray, Henry, Anatomy of the Human Body. Lea - Feibiger, 1959,
Ph iladelphia.
5. Sheppard, J.S. Jr., Human Color Perception: A Critical Study of
the Experimental Foundation, American Elsevier Publishing Company,
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and Schmidt, F.H., The Loss of Light Energy in Retina and Choroid,
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7. Ham, W.T., Jr. et al, Effects of Laser Radiation on the Mammalian Eye,
Trans. N.Y. Acad. Sci. Vol. 28, Feb. 1966.
8. American Conference of Government Industrial Hygienists, Uni form
Industrial Hygiene Codes on Regulations for Laser Installation,
presented in Laser Focus, October 1968.
9. Feigen, L., Fin, S., Mockeen, D., and Klein, E., Hazards and
Protective Devices Associated with 10.6 p Radiation. Proc. 20th
Ann. Conf. Eng.in Med. and Bio., November 1967, Boston, Mass.
10. Hansen, W.P., and Fine, S., Melanin Granule Models for Pulsed Laser
Induced Retinal Injury, Applied Optics, Vol. 7, January 1968.
II. White, H.E., and Levutim, P., Floaters in the Eye, Sci. Amer.,
July 1962.
12. Lynch, O.D.T. Jr., Lazer Hazard Evaluation and Calculations Utilized
in the Review of Laser Safety Plans and Procedures, presented at the
Health Physics Society Topical Symposium, January 1970, Louisville, Ky
B. Additional References
13. Moore, W., J. D.V.M., Ph.D., Biological Aspects of Laser Irradiation -
A Review of Hazards. Bureau of Radiological Health, Rockville,
Maryland, JanuaryT969.
-------
l4- Lasers and Light, by the editors of Scientific American,
W.H. Freeman and Company, 1969.
15. Proceedings of the Laser Safety Conference of 1968
presented in Archives of Environmental Health, vol. 18,
No. 3, March 1969.
16. Lasers^ one of the "Understanding the Atom" series,
USAEC Division of Technical Information Extension, Oak
Ridge, Tennessee.
17. Setter, Lloyd R., et a I, Regulations, Standards, and Guides
for Microwaves. Ultraviolet Radiation, and Radiation from
Lasers and Television Receivers an Annotated Bibliography,
Public Health Service Publication No. 999-RH-35, April 1969.
117
* U. S. GOVERNMENT PRINTING OFFICE : 1910 O - 389-429
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The ABSTRACT CARDS
Coordinate Index i ng. They provide space for an
suggested keywords, bibliographic
in by the user)
below are designed to facilitate document retrieval using
accession number (to be filled
information, and an abstract.
The Coord i nate Index
concept of reference
mater i a I f i I i ng is
readily adaptable to
a var iety of f i I i ng
systems. Coordinate
Indexing is described
i n the pub Ii cat i on
"IBM Data Processing
Techniques - Index
Organ i zat ion for In-
formation Retrieval"
(C 20-8062). Copies
are available through
BM Branch Offices.
The cards are furnished
in tri pI icate to a I Iow
for flexibility in their
use (for example, author
card index, accession
number card i ndex).
W
U
W.
K.
Accession No,
F- Van Pelt, H. F. Stewart, R.
Peterson, A. M. Roberts, and J.
Worst: LASER FUNDAMENTALS and
EXPERIMENTS.
S. Department of Health, Education
Health Service, Bureau of Radiolog
tion No. BRH/SWRHL 70-1 (May 1970)
d i str i but ion ) .
ABSTRACT: As a result of work performed at the Southl-
and We I fare, Pub I i c
ca I HeaIth Pub Iica-
pp . (Ii m ited
I 17
western Radiological Health Laboratory with respect to
lasers, this manual was prepared in response to the
increasing use of lasers in high schools and colleges.
It is directed primarily toward the high school
instructor who may use the text for a short course in
laser fundamentals.
(over)
W. F. Van Pelt, H. F. Stewart, R. W.
Peterson, A. M. Roberts, and J. K.
Worst: LASER FUNDAMENTALS and
EXPERIMENTS.
U.S. Department of Health, Education
Accession No.
and We I fare, Pub I i c
Health Publ ica-
pp. (I im ited
Health Service, Bureau of Radiologica
tion No. BRH/SWRHL 70-1 (May 1970) 117
d i str i but ion ) .
ABSTRACT: As a result of work performed at the South-
western Radiological Health Laboratory with respect to
lasers, this manual was prepared in response to the
increasing use of lasers in high schools and colleges.
It is directed primarily toward the high schoo
instructor who may use the text for a short course
aser fundamentals.
(over)
n
Accession No.
W. F. Van Pelt, H. F. Stewart, R. W.
Peterson, A. M. Roberts, and J. K.
Worst: LASER FUNDAMENTALS and
EXPERIMENTS.
U.S. Department of Health, Education, and Welfare, Public
Health Service, Bureau of Radiological Health Publica-
tion No. BRH/SWRHL 70-1 (May 1970) 117 pp. (limited
d i str i but i on).
ABSTRACT: As a result of work performed at the South-
western Radiological Health Laboratory with respect to
lasers, this manual was prepared in response to the
increasing use of lasers in high schools and colleges.
It is directed primarily toward the high school
instructor who may use the text for a short course in
laser fundamentals.
(over)
-------
The definition of the laser, laser operation,
properties of laser light, biological effects of laser
light, laser applications, safety in classroom laser
use, and experiment section (equipment necessary for
experiments) are included in this manual.
This manual is written in a manner to give an intui-
tive understanding of the device and its inherent
properties. The instructor is expected to be conversant
with certain of the classical elementary theories of
I ight.
KEYWORDS: Biological Applications; Coherence; Damage
Mechanisms; Diffraction; Electron Energy
Levels; Engineering Applications; Hazards; He-
Ne Laser; Hologram; Laser; Light; Mono-
chromatic! ty; Optical Cavities; Pumping
Methods; Ruby Laser; Safety.
The definition of the laser, laser operation,
properties of laser light, biological effects of laser
light, laser applications, safety in classroom laser
use, and experiment section (equipment necessary for
experiments) are included in this manual.
This manual is written in a manner to give an intui-
tive understanding of the device and its inherent
properties. The instructor is expected to be conversant
with certain of the classical elementary theories of
I ight.
KEYWORDS: Biological Applications; Coherence; Damage
Mechanisms; Diffraction; Electron Energy
Levels; Engineering Applications; Hazards; He-
Ne Laser; Hologram; Laser; Light; Mono-
chromaticity; Optical Cavities; Pumping
Methods; Ruby Laser; Safety.
The definition of the laser, laser operation,
properties of laser light, biological effects of laser
light, laser applications, safety in classroom laser
use, and experiment section (equipment necessary for
experiments) are included in this manual.
This manual is written in a manner to give an intui-
tive understanding of the device and its inherent
properties. The instructor is expected to be conversant
with certain of the classical elementary theories of
Iight.
KEYWORDS: Biological Applications; Coherence; Damage
Mechanisms; Diffraction; Electron Energy
Levels; Engineering Applications; Hazards; He-
Ne Laser; Hologram; Laser; Light; Mono-
chromatic ity; Optical Cavities; Pumping
Methods; Ruby Laser; Safety.
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