EPA-R2-72-136
Septe
Environmental Protection Technology
Laser Ex
aust Measurement Program
rt
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1.0 INTRODUCTION
The Final Report of the Laser Exhaust Measurement (LEM)
program is intended to serve as an introduction to the theory of
LEM system operation, a detailed description of the operating
characteristics of the equipment, and as a general operating manual
for the entire LEM system. The LEM apparatus is composed of
a number of subsystems and individual components constructed
by a variety of manufacturers. The original operating manuals,
warranties, etc., associated with each LEM subsystem or component
are included as detached appendices of this Report for detailed
reference and possible trouble-shooting purposes.
The construction and operation of each LEM subsystem and/or
component is briefly described in the sections below, and the proper
operating procedures are clearly detailed to prevent malfunctions or
possible damage to the LEM apparatus. Certain minimal risks
to operating personnel are present in the LEM equipment, and
hazardous locations and/or poor operational procedures are both
labeled and described in the text below. These risks are principally
associated with the relatively high-pressure cooling water for the
magnet, the moderately high laser excitation voltage, and the large
magnetic fields present near the ends of the magnet solenoid. In
each case, care has been taken to reduce or eliminate potential
dangers from the LEM equipment.
A complete theoretical description of noble gas laser operation
is not included here since excellent treatments are available in the
literature (1--4), and only a cursory analysis of LEM operation will
be presented. The Final Report is divided into two major sections:
Chapters Z through 5 comprise a detailed description of each of
the major components of the LEM system, Chapter 6 contains
a detailed, step-by-step LEM Operation Manual, Chapter 7 covers the
details of xenon laser operation and adjustment, and Chapter 8 describes
the necessary steps involved in the initial LEM assembly at the EPA.
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EPA-R2-72-136
Laser Exhaust Measurement Program
Final Report
by
Gary J. Linford
Laser Technology
Hughes
Culver City, California 90230
Contract No. 68-02-0203
Program Element No. All010
Project Officer: Phillip Hanst
Division of Chemistry and Physics
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D. C. 20460
September 1972
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EPA REVIEW NOTICE
This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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TABLE OF CONTENTS
1.0 INTRODUCTION
2. 0 LEM SYSTEM DESCRIPTION '
2. 1 Introduction
2.2 Description of LEM Operation
2. 2. 1 Xenon Laser
2. 2. 2 Calibration of Xenon Laser Spectral Splitting
2. 2. 3 Detection of Aldehyde Concentrations
2.2.4 Electronic System Detection Problems
2.2.5 Xenon Laser Pressure Control System
2. 2. 6 Performance of LEM Xenon Laser in a Strong
Magnetic Field
2. 3 LEM Magnet Design
2.4 LEM Electronic Data-Processing System
3.0 ZEEMAN MAGNET DESCRIPTION
3. 1 Magnet Cooling System
3.2 Magnet Shutdown Procedure Precautions
3. 3 Potential Magnet Safety Hazards
4.0 ELECTRONIC DATA PROCESSING SYSTEM
5. 0 ALDEHYDE SAMPLE CELL
6. 0 OPERATION INSTRUCTIONS FOR LEM APPARATUS
6. 1 Xenon Laser Operation
6. 2 Magnet Operation Procedure
6. 2. 1 Introduction
6. 2. 2 Operation Instructions
6.2.3 Selecting Magnet Current Range
6.3 Electronic Data Processing System Operating Procedure
7. 0 DETAILS OF LEM XENON LASER OPERATION AND ADJUSTMENT
7. 1 Physics of Xenon Laser Oscillation
7. 1. 1 Establishment of Laser Oscillation
7. 1. 2 Saturation of Laser Amplifier Gain
7.2 Optimization of Xenon Laser Lines
7. 2. 1 Spectroscopic Details of Xenon Laser Transitions
111
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7.2.2 Effects of Xenon Pressure on LEM Laser Output Power
7.3 Laser Cavity Properties and Alignment Techniques
»
7. 3. 1 Laser Cavity Alignment Procedure
7.3.2 Future LEM Cavity Options
7.4 Coincidence of CW Xenon Laser Lines with Formaldehyde
Absorption Lines
8.0 LEM ASSEMBLY INSTRUCTIONS
8. 1 Assembly of LEM Xenon Laser Cavity
8.2 Assembly of Zeeman Magnet
8.3 LEM Cable Connections
REFERENCES
DETACHED APPENDICES (Individual Operating Manuals)
IV
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LIST OF FIGURES
Figure 2-1 Diagram of LEM Apparatus 2-9
Figure 3-1 Axial Magnetic Field vs. Solenoid Current 3-2
Figure 3-2 Variation of Magnetic Flux Intensity
Along Solenoid Axis 3-3
Figure 3-3 Schematic of Magnet Cooling System 3-5
Figure 4-1 Block Diagram of Electronic Data-Processing
System 4-2
Figure 7-1 Pressure Dependence of Xenon Laser
Discharge Current 7-10
Figure 7-2 Xenon Laser Output Power at 3.51 Microns
vs. Discharge Current 7-12
Figure 8-1 LEM Assembly Schematic 8-3
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LIST OF TABLES
Table 2-1 CW Xenon Laser Lines Obtainable with
LEM Apparatus 2-3
Table 7-1 CW Xenon Laser'Lines Near Formaldehyde
Absorption Bands 7-17
VI
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2. 0 LEM SYSTEM DESCRIPTION
2. 1 Introduction
The Laser Exhaust Measurement (LEM) apparatus utilizes
a Zeeman-split, neutral xenon laser capable of oscillation at a
variety of wavelengths in the middle infrared. The xenon laser
was initially designed to oscillate principally at 3.508 and 3.679
microns. Both of these xenon laser lines are relatively near
spectral components of formaldehyde absorption bands (5). The
spectral details of the formaldehyde absorption bands were not
known at the beginning of the LEM program, and thus an unusually
large five kiloGauss solenoid was incorporated into the LEM system
design at an early stage in order to insure adequate splitting of the
xenon laser line(s). The Zeeman splitting of the a components of
the 5d(7/2),--6p(5/2)_ transition in xenon (corresponding to the
3.508 micron laser line) is quite complex, but in general, a splitting
of some 2 MHz/Gauss was expected from theoretical considerations.
The magnitude of the Zeeman splitting at maximum magnetic flux
was measured directly using a 3/4 meter Czery-Turner spectrometer.
In addition, spectral measurements were also made at relatively
small magnetic fields (several hundred Gauss) using homodyne
detection in conjunction with a microwave spectrum analyzer.
A maximum splitting of 1.7 x 10 Hz (7 A) of the 3.508 micron
xenon laser line was measured with an axial magnetic field of
5500 Gauss.
In operation, the axial magnetic field is increased until the
spectral splitting of the two oppositely-directed circularly polarized
laser beams is sufficient to permit the observation of differential
absorption (due to the presence of HCHO in the sample cell) between
the two Zeeman components. For greater spectral splittings, the
corresponding differential absorption should be proportionally greater.
In order to achieve relatively equal laser intensities for the
oppositely-directed circularly polarized Zeeman components, v , and
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v , it is necessary either to
1. mechanically tune the longitudinal modes in the laser
t
cavity so that the cavity Q is approximately the same for
both v.and v _, or
2. adjust the angular orientation of the Brewster-angle
polarization compensation plate to equalize the relative
intensities of the two circularly-polarized waves.
In the case of relatively low laser amplifier optical gain, it may
be necessary to perform both of the above operations.
2. 2 Description of LEM Operation
2.2.1 Xenon Laser
The LEM xenon laser is capable of stable CW laser oscillation
at 11 wavelengths in the middle infrared (see Table 2-1, below).
The relative strengths of these laser lines can be dramatically
affected by varying the xenon pressure, laser discharge current,
output mirror reflectivity, and laser cavity mirror separation.
To permit the laboratory-grade LEM xenon laser to possess the
necessary flexibility to fulfill all the necessary optimum conditions
for most of these 11 laser lines, the following laser subsystems
are furnished:
1. A cryogenic pressure-control system with built-in
"fail-safe" features.
2. A variable dc excitation power supply.
3. A piezoelectric mirror translator assembly together with
its associated electronics.
4. A thermally-compensated laser cavity with quartz rod
mirror separators.
5. Easily interchangeable laser cavity output mirror assemblies.
Details regarding the optimum operating conditions for each laser
line are given below in the general operating instructions for the
LEM apparatus (see Section 7. 2).
The xenon laser discharge tube itself is terminated with
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TABLE 2-1
CW Xenon Laser' Lines Obtainable with LEM Apparatus
'
1
2
3
4
5
6
7
8
9
10
11
Wavelength
(Vacuum),
Microns •
3. 108
3.275
3.368
3.508
3.622
3.652
3.680
3.870
3.997
4. 153
5.575
Transition
5d(5/2)°---6p(3/2)2
5d(3/2)° — 6p(l/2)1
5d(5/2)°---6p(3/2)1
5d(7/2)°---6p(5/2)2
5d'(3/2)°— 7p(3/2)2
7p(l/2)r--7s(3/2)2
5d(l/2)°— 6p(l/2)1
5d'(5/2)°---6p'(3/2)2
5d(l/2)°---6p(l/2)
5d'(5/2)°---7p(3/2)1
5d(7/2)°T--6p(5/2)3
Optimum
Pressure
Low to
Moderate
Low to
Moderate
Low to
Moderate
Low to
Moderate
Low
Low
Moderate
Low
Low to
Moderate
Low to
Moderate
Low to
Moderate
Observed
Strength
0. 1*
6.0
5.0
10.0
0.4
0. 1
4.0
0.4
3.0
1.0
0.8
Theoretical
Strength
(15)
2.94
1.25
1-. 89
4.0
0.0
0.0
1.0
5.0
0.5
0.0
5.4
* The output power of this line increases drastically if a higher reflectivity output mirror is used.
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slightly canted, narrow-band "V" anti-reflection-coated sapphire
windows optimized for laser oscillation at 3.5 microns. Laser
oscillation at substantially different wavelengths is generally
difficult owing to the relatively high optical losses introduced by
these laser tube windows. In particular, argon and krypton neutral
gas lasers oscillate at shorter wavelengths than xenon due to their
higher energy level structures, and hence the LEM xenon laser tube
is not well adapted to the use of the laser tube for CW argon and
krypton laser action. With the exception of five new argon and
krypton laser lines recently discovered by the author (6), there is a
dearth of argon and krypton laser lines in the 3. 51 micron spectral
region, and hence it was decided that in order to prolong the life
of the xenon laser, the original vacuum valve would be removed and
the laser tube sealed off. Before the vacuum valve was removed,
however, several unsuccessful attempts were made to obtain CW
laser oscillation at any argon or krypton wavelengths.
The hot cathode assembly in the LEM xenon laser discharge
tube is poisoned by accidental exposure to oxygen, but if the
oxygen exposure occurs when the cathode is cool, the cathode can
be reprocessed. Since the LEM xenon laser already has the
capability of oscillation at ten infrared laser lines, the removal
of the vacuum valve is not regarded as detrimental to the overall
performance of the LEM laser.
In the absence of an externally-applied magnetic field
(ignoring the relatively small terrestrial field), the inhomogenously-
broadened xenon laser will oscillate primarily at a number of laser
frequencies corresponding to discrete longitudinal modes separated
in frequency by
where L is the optical path length between the two laser cavity
mirrors and Af is the intracavity mode spacing frequency. The
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central frequency of this group of longitudinal modes is deter-
mined by the atomic laser transition itself, which in the case of
13
the 3.508 micron xenon laser line corresponds to v = 8.55 x 10 Hz.
Thus the frequency separation,Af , of the individual laser modes
is, in general, determined by the geometry of the laser cavity,
whereas the frequency centroid of laser oscillation is determined
by the active laser medium (in the present case, xenon) as well
as the kinetics of noble gas .laser excitation.
The "natural" or homogeneously-broadened linewidth of a
noble gas laser transition is inversely proportional to the life-
times of the excited energy levels, and in the case of the 3.508
micron xenon laser line, it has been determined to be approximately
2 MHz (7). The inhomogeneously-broadened character of low
pressure middle infrared noble gas lasers is due to Doppler
broadening caused by the thermal motion of the gas atoms (for
xenon, Ai>p= 150 MHz). In addition, naturally-occurring xenon
contains nine stable isotopes which induce an isotopic broadening (8)
of the laser line-width in addition to that produced by the Doppler
effect.
The number of longitudinal modes which will oscillate in a
given laser cavity is approximately equal to the ratio of the inhomo-
geneously-broadened linewidth,A>/n, to the intracavity mode spacing,
Ac . In the present xenon laser design, only a few longitudinal modes
will oscillate (in the absence of a magnetic field). Gas lasers can.
in addition, oscillate at several discrete frequencies at or near
the kth longitudinal mode frequency,Ai/ due to the presence of
relatively weak transverse modes associated individually with each
longitudinal mode. When the axial magnetic field is applied to the
laser oscillator, the resulting Zeeman splitting is not continuous
(as it is in the case of spontaneous emission), but is instead discrete,
corresponding to jumps in frequency equal toAi/ , given above by
equation (2-1). This quantized phenomenon is caused by the requirement
that the laser resonator determines the precise frequencies of laser
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oscillation, whereas the laser medium determines only the
approximate oscillation frequency (within the spectral linewidth,
*•&
In the case of the JLEM xenon laser, the cavity length, L,
is approxiinately 1Z7 cm, so thatAi^ =118 MHz (from equation
(2-1), above), and hence the relative changes in laser oscillation
frequencies tend to be quite large for magnetic fields of several
hundred Gauss. As a result, it is not a simple matter to measure
accurately the relative splitting of the L.EM laser for small magnetic
fields in the comparatively short laser cavity required for the
LEM system.
If the laser cavity mode-closing length, L, is increased
from the present 127 cm to something of the order of 150 meters
(9), thenAj/ =1.0 MHz, and the homodyiie spectrum analysis of a
Zeeman-split xenon laser becomes much more precise — this
technique is very useful when used with small magnetic fields
(B «200 Gauss).
2.2.2 Calibration of Xenon Laser Spectral Splitting
As noted above, a useful technique for calibrating the relative
Zeeman- splitting of the two oppositely-directed circularly polarized
laser beams employs a suitable "square-law" photodetector connected
directly to a high-frequency electronic spectrum analyzer. When
the Zeeman-split xenon laser beam is focused on a square-law
detector, an electronic beat frequency can be observed with an
electronic spectrum analyzer (providing it has a sufficiently high
electronic frequency response) as an rf current generated by the
"beating" of the two optical waves differing in frequency by
~ 2 MHz/Gauss
where e/m is the charge to mass ratio of the electron, g is the
reduced Lande g-factor, and B is the axial magnetic field in Gauss.
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The splitting defined in equation (2-2), above, is subject to
the additional conditon that the longitudinal modes be spaced at
intervals of c/2L, and hence in the case of relatively small
magnetic fields, the Zeeman splitting becomes
M
= K
<2-3'
where K is an integer. When the magnetic field is steadily
increased from zero, the discrete homodyne beats observed with
a high-frequency electronic spectrum analyzer permit an
approximate calibration of the Zeeman-induced spectral splitting
of the xenon laser. As discussed above, the homodyne technique
is much more accurate for large values of the laser cavity mode-
closing length, L. Direct measurements of the maximum Zeeman
splitting of a xenon laser can be made using a spectrometer, but
the accuracy of the technique is relatively low in the absence of
ancillary optical equipment such as a Fabry-Perot interferometer.
2.2.3 Detection of Aldehyde Concentrations
Once the spectral splitting of a given xenon laser line is
determined (either theoreticaly or empirically), it is then possible
to use the Zeeman-tuned xenon laser as a relatively sensitive,
extremely narrow-band differential absorption device. One of
the two Zeeman-components is absorbed more strongly than the
other by the aldehyde gas sample, and thus by comparing the ratio
of the two Zeeman component intensities at a relatively rapid rate
(in the present case, the comparison frequency is 24 Hz with a
provision, for increasing the frequency), it is possible to infer the
•
concentration of the sample aldehyde present in the sample cell.
The technique employed here first converts the oppositely-directed
circularly-polarized Zeeman components into two perpendicular
plane-polarized beams by passing them through a quarter-wave plate
constructed of^anti-reflection-coated lithium niobate. Since the laser
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is designed to oscillate at a number of infrared wavelengths
simultaneously, the assembly carrying the quarter-wave plate
also mounts a narrow-band interference filter passing only one
of the oscillating wavelengths. The two Zeeman-split laser beams
then pass through a ZnS compensating plate (see Figure 2- 1) to
equalize the intensities of the two laser beams (if necessary).
The beams then pass through the test cell containing the sample
absorbing gas. After passage through the test cell, the relative
intensities of the two Zeeman components will, in general, be
different, and an electronic data-processing system measures the
ratio of the Zeeman component intensities. The electronic data-
processing system consists of an autolock amplifier, a dc reference
amplifier, and a digital ratiometer. Display of the data output is
provided both by a digital readout and a chart recorder.
The time variation of the optical signal detected by the
photodetector is given by
I = I+ cos2(wt) + I_ sin2(wt) (2-4)
where I is the intensity of the CCW Zeeman component, I_ that
of the CW Zeeman component, and a) is the angular frequency of
the analyzer prism shown in Fig. 2-1. It is evident from equation
(2-4) that when the relative intensities of the Zeeman components
(I and I ) are equalized, the output signal from the photodetector
should be entirely dc. This situation corresponds to zero differential
absorption by the aldehyde sample cell. Unfortunately, any output
fluctuations in the laser beam(s) due to mechanical vibrations, noise,
etc., would appear as spurious noise on top of the dc signal.
When the intensities of the two Zeeman components are not
equal, or when I > I , then equation (2-4) can be written:
I = I_ + Acos2(wt) (2-5)
where we have written
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Piezoelectric Mirror
Translator Assembly
Cathode
Output Mirror
Adjustment Micrometers
Vacuum Aldehyde
Sample Cell
Quartz Rod Mirror
Spacer for Thermal Compensation.
ZnS Polarization
Compensator
Rotating Prism
nalyzer
Lens and
Photodetector
Assembly
Cavity Mirror •
Adjustment Micrometers
Water-Cooled
Electromagnet
Pressure-Control
System
Mount for 1/4 Wave
Plate and Interference
Filter
•Ealing Optical Bench
Figure 2-1 Diagram of LEM Apparatus Mounted on an Optical Bench
N3
i
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(2-6)
Making the usual trigonometric substitution, equation (2-5) can
be written:
1 = 1 + -^-(1 + cos 2wt) (2-7)
The relative Zeeman intensities (I, and I ) are not equal when a
suitable absorbing gas (such as formaldehyde) is placed in the sample
cell, and thus under these circumstances, the photodetector output
consists of both ac (at a frequency of 2 w ) and dc components. A
magnetic pickup mounted on the rotating analyzer prism assembly
provides phase information to the phase -sensitive detection section
of the autolock amplifier.
In actual operation, some undesirable beam steering by the
analyzer prism produces an ac signal at an angular frequency of w,
and it is necessary for the electronic system to ignore this spurious
signal component. It is desirable to obtain a differential absorption
fraction of at least one part in 10 , and the intial tests of the electronic
data-processing system indicate that several parts in 10 can be
measured if the electrical signals input to the system are very stable.
It is evident from equations (2-5) and (2-7) that the differential
absorption fraction, £ , is given by
Thus, the differential absorption fraction, £ , is simply the
ratio of the ac photodetector signal to the dc photodetector signal.
2.2.4 Electronic System Detection Problems
A number of problems appeared during the assembly and testing
phases of the LEM apparatus in the photodetection and electronic signal
processing areas of the LEM system. These problems are briefly
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described below.
Electronic Data-Processing System
Difficulty was initially encountered with the lock-in amplifier
circuitry when it was discovered that it was not possible to obtain
a frequency-lock on the reference input signal from the analyzer
prism magnetic pickup. A tunnel diode wired in parallel with the
magnetic pickup permitted the lock-in amplifier to match the reference
frequency (2u>), but the output from the lock-in amplifier remained
quite noisy. The noise was traced to three problem areas:
1. Ground loops,
2. beam steering by the analyzer prism, and
3. vibration introduced into the laser cavity by the
massive magnet cooling water pump.
The latter two problems are difficult to solve, and at present they
represent the limiting mechanism for achieving high sensitivity
of the LEM apparatus for detecting low concentrations of aldehydes.
Photode tec tors
There were three photode tec tors originally considered
for use on the LEM program. They were:
1. A thermoelectrically-cooled PbSe photodetector,
2. a liquid nitrogen-cooled InSb detector, and
3. an uncooled, room temperature InAs photode tec tor.
The PbSe photodetector proved to be too noisy for use as the LEM
detector, the InSb photodetector exhibited problems with moisture
condensing on the entrance window, and the InAs photodetector has
a spectral sensitivity which cuts off beyond 3.5 microns. As a
consequence of these various problems, the InSb detector was chosen
as the prime photodetector until the Zeeman-splitting magnetic field
experiments were completed. These experiments revealed that the
proposed 3.68 micron laser line would not oscillate in a strong magnetic
field, and hence the major objection to the use of the room-temperature
InAs detector was removed. Thus, the present LEM configuration uses
a InAs photodetector in the electronic data-processing system.
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Analyzer Prism
The original LEM optical design included an air-spaced lithium
niobate Glan-Thompson prism as the rotating polarization analyzer.
Considerable fabrication difficulties were experienced by the prism
vendor, and the LiNbCX prism was eventually delivered more than
four months late. Since the original LEM program schedule was
for only six months, total, the prism delay produced an overall
delay in the LEM program.
As a consequence of the slipped schedule and a diminishing
level of confidence that the vendor would ever be able to deliver a
prism of adequate quality, a backup analyzer prism of another type
(Rochon), material, and vendor was ordered. When both prisms
were eventually received (within a week of one another), it was found
that both prisms deflect the transmittc (extraordinary) ray, thereby
inducing an amplitude modulation on the detected laser beam of
angular frequency w. Two solutions to this beam-steering problem
were apparent:
1. Use a larger surface area detector in conjunction with
a short focal-length lens to minimize the effect of the beam
steering, and
2. insert a compensating wedge after the analyzer prism to
correct the deflection angle.
The first approach was selected because of the time-consuming
problems associated with fabricating and properly installing a
suitable compensating wedge.
Background Noise Generated by Water Pump Vibration
In addition to the noise introduced by the prism beam steering,
a serious background noise problem is generated by the vibrations
induced by the massive thrust of the large magnet cooling water pump.
The most obvious elimination of this source of noise is to eliminate
the use of the cooling water pump, an attractive solution if flowing
tap water is to be used to cool the magnet in any case. The Environ-
mental Protection Agency is considering using tap water for the
cooling of the magnet, at least for magnetic fields sufficiently low
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that the cooling water does not boil. Another partial solution
to the problem makes use of the narrow-band properties of the
lock-in amplifier circuitry used in the LEM electronics. This
solution involves increasing the angular velocity of the analyzer
prism until its magnitude is higher than the major (semi-random)
low frequencies induced by the vibration of the water pump. The
analyzer mechanism has been modified to accept different gear
ratios, and hence the analyzer can be set at a number of angular
speeds corresponding to the requirements of the experimenter(s).
2. 2. 5 Xenon Laser Pressure Control System
Noble gas lasers in general exhibit a serious gas adsorption
phenomenon often referred to as gas "clean-up. " If, for example,
a noble gas laser is filled to an optimum pressure for laser operation,
gas adsorption into the laser tube walls occurs quite rapidly, and the
laser oscillation will often cease after only one hour of operation.
If an operating noble gas laser tube is repeatedly filled at the
optimum pressure after many hours of operation, the long term
pressures remain unpredictable.
The solution to this serious problem of gas "cleanup" employed
for the LEM xenon laser was to grossly overfill the laser tube with
xenon, and then trap cryogenically the excess gas with a tube
appendage immersed in a suitable cryogen. Continuous pressure
control is afforded by electrically heating the cold finger while
it is immersed in a bath of liquid nitrogen at a temperature of 77 K.
When operated without the cold finger heater, the xenon pressure
can be controlled by either
1. Adjusting the height of the liquid nitrogen on the cold finger,
2. inserting an insulator around the cold finger, or
3. using a mixture of three parts liquid nitrogen to one
part liquid oxygen (such as liquid air).
The optimum pressure (6) for CW xenon laser oscillation at 3.5 microns
is approximately five milliTorr of xenon, slightly higher than the
partial pressure of xenon in equilibrium with solid xenon at 77 K.
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The corresponding optimum cryogenic temperature for operating
the cold finger (to obtain the five milliTorr optimum pressure) is
approximately 79 K, which explains why a mixture of liquid nitrogen
(at 77 K) and liquid oxygen (83 K) performs better than pure liquid
nitrogen (1 0).
2. 2. 6 Performance of LEM Xenon Laser in a Strong Magnetic Field
Although five of the 11 continuous wave (CW) xenon laser lines
obtainable with the LEM xenon laser exhibited such high optical
gain (when the xenon pressure and the discharge current were optimized)
that they could be characterized as "super-radiant, " (see Chapter 7
for a definition of "super-radiant") only three CW xenon laser lines
would oscillate in a strong magnetic field (B « 2000 Gauss). These
three lines were the 3.37, 3.51, and 3.99 micron xenon laser transi-
tions. The 3.37 and 3.508 micron xenon lines split into two spectral
components each, separated by approximately 7 A for B —5500 Gauss.
The 3.99 micron line splitinto two unequal components with both upper
and lower frequency components displaced some 5 A from the zero
magnetic field laser oscillation wavelength (B — 5500 Gauss). This
unusual behavior was unexpected, and to date no completely satisfactory
explanation has been made. The splitting of the 3.37 and 3.51 micron
laser lines should be adequate to permit each of these lines to be used
in the intended differential absorption application.
There are a number of alternative Zeeman-splitting techniques
which might also be worthwhile developing. Some of these are:
1. Obtain higher peak magnetic fields by pulsing the magnet.
2. Obtain higher laser amplifier gains by pulsing the laser.
3. Pulse both the Zeeman magnet and the xenon laser in a
synchronous fashion.
The suggested pulsed operation of the LEM Zeeman magnet would
permit sweeping the oscillating laser wavelength through a range
of wavelengths, AX, and also permit much higher magnetic fluxes
to be obtained. A synchronized rotating combination quarter-wave
plate and analyzer could double the spectral range of the Zeeman
sweep while permitting instantaneous single-frequency laser operation.
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The maximum pulsed magnetic field which can be generated by the
present LEM solenoid is probably limited only by the inductance of
the coil, the size of the pulsed current source, and the required (short)
magnet pulse length. There are assoicated problems (such as back-
EMF and the mechanical stresses placed on the magnet structure by
the varying magnetic field), but the addition of a pulsed magnet cap-
ability to the LEM solenoid appears to be a worthwhile future addition
to the experimental flexibility of the LEM system.
A pulsed noble gas laser design would likely necessitate a
cold cathode construction to avoid damaging the present CW hot
cathode structure. The pulsed laser design would have higher (peak)
optical gain, more (peak) power, many more available laser lines (6),
and would be much less sensitive to the potentially serious back-EMF
associated with strong pulsed magnetic fields.
Pulsed and CW noble gas lasers geometrically similar to the
LEM xenon laser have been operated by the author at more than 50
neutral laser lines in the near and middle IR regions (6). When operated
in the pulsed mode, most of the observed laser lines were super-
radiant (using electric discharges of He/Ne, Ar, Kr, and Xe) and may
be good candidates for a Zeeman-split laser application.
Additional possible LEM system modifications are described
briefly below in section 7.3.2.
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2. 3 Magnet Design
The LEM dc Zeeman magnet is a water-cooled custom-designed
solenoid subcontracted by Hughes to Tamarack Scientific Co. of Orange,
California. Although it was originally intended to operate only up to
fields of 5000 Gauss, the LEM magnet will briefly develop magnetic
fields nearly as great as 6000 Gauss if operated at the maximum input
current for a relatively short period of time (to avoid boiling the cooling
water). In its present configuration, the water-cooled magnet system .
is connected to a closed-cycle heat exchanger cooled, in turn, by
tower water available in the HAC laser laboratories. The magnet cooling
water is circulated under high pressure by a large 400 psi water pump
using a small 5 gallon tank as a water reservoir. The largest heat
exchanger available here, a 20 kilowatt unit, is grossly overloaded
when the magnet is operated at maximum field, and the cooling water
boils after only a few minutes of operation. The power removed by
the cooling water at maximum magnetic field is more than 80 kilowatts
(approximately four times the heat exchanger capacity), and since we
are only using a 5 gallon reservoir, we anticipate that the magnet can
be operated at maximum field for longer periods of time if a larger
heat exchanger and larger reservoir were available.
The very large welding-type air-cooled magnet power supply must
be hard-wired in a permanent installation using either a 480 V 3 0
145 A (per phase) or a 240 V 3 0 -145 A (per phase) power source.
The cooling water source and sink (if a heat exchanger--closed cycle
system is not used) should supply up to a maximum of four gallons
per minute for cooling the magnet. The water pump can still be used
to provide a pressure boost function when operated in a continuously-
flowing mode.
Further details regarding the design and operation of the magnet
can be found in sections 3. 0 and 6. 2 below. Assembly of the magnet
system is described in section 8.2.
2-16
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2. 4 LEM Electronic Data-Processing System
The rack-mounted LEM Electrpnic Data-Processing System
processes the signal output from the LEM system photodetector in
order to provide direct differential abosrption data for test (absorbing)
gases in the LEM Nielson-type sample cell. Two differential absorption
data outputs are provided:
1. A visual readout on the dial of the digital ratiometer
(at a rate of three readings per second).
2. A printed output on a chart recorder mounted in the
LEM electronics rack.
The location and arrangement of the Electronic Data-Processing
System components are sufficiently flexible that other connections
and arrangements of the components desired by the user(s) of the
LEM equipment are readily obtainable. The Electronic Data-Processing
System is composed of four productized, individually rack-mounted
components:
1. A high-gain dc reference amplifier mounted above the
LEM Control Panel.
2. A Keithley Model 840 Autolock lock-in amplifier.
3. A Dana Model 4800 digital ratiometer and voltmeter.
4. A Keithley chart recorder.
At the present time, the lock-in circuitry operates at a frequency
of 24 Hz with a provision for increasing the speed of rotation to
96 Hz by changing the drive gear ratio. The analyzer motor is a
hysteresis-synchronous design locking into the 60 Hz power source.
The Autolock (Keithley Model 840) has a variable bias feature
which permits it to compensate for the gross effects of non-signal
ac modulation occurring at the lock-in frequency. Since the rotating
prism has a component at 12 Hz due to beam steering effects, the
bias feature can eliminate 2nd harmonic effects attributable to the
prism beam steering.
Additional information regarding the design and operation of
the Electronic Data-Processing system can be found below in sections
4/0 and 6. 3.
2-17
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3.0 ZEEMAN MAGNET DESCRIPTION
The large five kiloGauss water-cooled LEM solenoid is
constructed of copper tubing wound on a massive aluminum bobbin.
Filtered cooling water is forced through four parallel cooling channels
to carry away as much as 80 kilowatts of heat generated when maxi-
mum magnetic fields are required. The cooling water is circulated
(in the present design) by a massive 400 psi water pump. The magnet
vendor was unaware of the actual size of the required water pump
in the original design negotations with HAC, and the cost of the
large water pump was not included in the magnet subcontract.
The original magnet design called for ordinary tap water (at a
pressure w 60 psi) to be used for magnet cooling, and this original
goal is still realizeable providing that the magnet is operated below
the boiling point of the circulating water. As noted in the previous
section, the high-pressure water pump vibration problem may be a
serious noise problem for the LEM data-processing system, and the
successful elimination of this problem is essential if the ultimate
sensitivity of the LEM Zeeman absorption apparatus is to be realized.
The large electromagnet and its massive air-cooled power supply
are protected by four interlock features. They are:
1. A key-lock start control,
2. Four cooling water flow switches located in each cooling
water channel are interlocked, preventing or shutting-down
magnet operation if insufficient water flow occurs.
3. The air-cooling fans of the magnet power supply must be
operating before the magnet will energize.
4. A: variable overcurrent relay interlock shuts down the magnet
if a (variable) preset current limit (up to 300 A) is exceeded.
The magnetic field generated by the solenoid was carefully measured
using a Hall probe, and the measured magnetic field vs. the input
current are plotted below in Fig. 3-1. The z-axis variation of the
magnetic field is shown below in Fig. 3-2.
Since the strong magnetic field extends well beyond the end bells
3-1
-------
fcj
Solenoid Current
(Amperes)
350
300
250
200
150
100
50
\
Current Limit of Power Supply
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500
"*" Magnetic Field
(Gauss)
Figure 3-1 Axial Magnetic Field vs. Solenoid Current for LEM Magnet
-------
Magnetic Field
in Gauss
3000.
2500
2000.
1500
1000
500
0
-o o-
200 Ampere Current
•End of Solenoid
i 1-
0 5 10 15 20 25 30 35 40 45 50 55
-»»- Displacement Along
Solenoid Axis (cm)
Figure 3-2 Variation of Magnetic Flux Intensity Along Solenoid Axis
co
Co
-------
of the electromagnet assembly, it is strongly recommended that all
potentially affected sensitive instruments (particularly watches) be
»
removed from the vicinity of the magnet before the large magnet is
operated.
As indicated above in Fig. 2-1, the xenon laser is mounted
directly inside the magnet solenoid. The xenon laser discharge tube
is constructed with massive walls since it is supported only at its
ends for ease of installation and removal. It has been observed that
simultaneous operation of the large magnet and the CW xenon laser
tends to "clean-up" (a gas-adsorption phenomenon) the xenon present
in the discharge tube at a relatively high rate. Fortunately, the
xenon pressure control system is able to maintain adequate xenon
pressures even in the presence of the strongest magnetic fields
obtainable with the magnet (up to 5500 Gauss).
The large magnet power supply does not supply a zero to
300 ampere capability. Instead, it operates continuously over smaller
intervals in a series of six selectable ranges which provide an over-all
current range of from 40 A to over 300 A. The output current ranges
are selected at the main magnet power supply, and the continuous
adjustment control variac is located in the remote control console.
Details concerning magnet power requirements, cooling water require-
ments, space requirements, and general operation instructions are
provided in section 6.
3. 1 Magnet Cooling System
As shown below in Figure 3-3, the LEM magnet cooling water
system is composed of eight major component types. They are:
1. A large 400 psi water pump.
2. Four parallel cooling coils.
3. Four water flow interlock switches.
4. In-line water filter.
5. Flow meter (0--10 gpm capacity).
6. Water pressure gauge (0--300 psi).
3-4
-------
LEM SOLENOID
CO
I
Cn
Axis of LEM System
High Pressure Water
Inlet
Pressure
Gauge
Variable Pressure
Water Flow Meter
To Tower Water
Heat Exchanger
Water Reservoir
On/Off Switch for Pump High Pressure
Water Pump
Figure 3-3 Schematic of Closed Cycle Cooling System for LEM Magnet
-------
7. By-pass variable pressure control valve.
8. High-pressure water hoses and fittings.
It is EXTREMELY IMPORTANT to be certain that the cooling water
hoses on the high-pressure side of the magnet are attached tightly
according to the schematic depicted in Figure 3-3. The high pressure
water, if allowed to leak, can cause serious damage to some of the
optical components of the LEM apparatus (and also to certain types
of ancillary laboratory equipment present). As noted in Figure 3-3,
the inlet water pressure at the magnet can be adjusted from over
300 psi to approximately 100 psi using the screw adjustment on
the valve.
The flow meters are individually adjustable so that different
minimum flow can be selected. It will be necessary to adjust all
four of the flow interlocks in sequence since all four must be
operative in order to provide an "on" circuit indication to the magnet
power supply.
3.2 Magnet Shutdown Procedure Precautions
When, under normal operating conditions, it is desired to
shut down the LEM electromagnet, it is advisable to set the magnet
solenoid current-control variac at zero and then raise the plate
voltage (from 500 V to 600 V) on the LEM xenon laser to compensate
for the back EMF generated by the collapsing magnetic field. If
this precaution is not taken, the back EMF induced when the OFF
button on the magnet control panel is pushed can extinguish the
xenon laser discharge. If this happens, it is generally necessary to
remove the two Allen-head screws holding the pressure-control dewar
in place, and lower the dewar, thereby allowing the xenon pressure in
the discharge level to rise to a sufficiently high level to permit the
discharge to be reignited.
3. 3 Potential Magnet Safety Hazards
The LEM magnet solenoid should not be subjected to pressures
greater than 320 psi when operated with the large water pump. The
3-6
-------
solenoid is wound with relatively heavy copper tubing, but high water
pressures may cause ruptures in the water distribution manifold.
Electrical hazards in operating the LEM electromagnet are
minimal since the cables are well insulated and the high voltage
terminals are covered with protective sheet metal. It is, of
course, expected that considerable care will be exercised in the
initial wiring of the magnet power supply to prevent the possibility
of a short-circuit developing,
Separate instruction manuals for the magnet, flow rate
interlocks, and magnet power supply are included as detached
appendices of this Final Report.
3-7
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4.0 ELECTRONIC DATA-PROCESSING SYSTEM
The details of demodulating the Zeeman-split xencn laser
beams (together with their associated differential absorption data)
were presented above in section 2. 2. 3. The present section is
concerned with a general description of the component configuration
and over-all arrangement of the electronic components comprising
the LEM Electronic Data-Processing System.
A block diagram of the-Electronic Data-Processing System is
shown below in Figure 4-1. The dc and ac components present in the
output of the photodetected signal are separated and individually
amplified by the dc amplifier and lock-in amplifier respectively.
Prism phase information is obtained by the use of a magnetic pick-up
mounted on the prism analyzer mechanism and fed into the reference
channel of the lock-in amplifier circuitry. After the lock-in amplifier
is suitably synchronized with the ac (differential absorption) signal,
the amplified output from the autolock amplifier is fed into the
digital ratiometer where its amplitude is compared at a rate of 3 Hz
with the output from the dc amplifier. The output from the digital
ratiometer then drives the chart recorder.
The present HAC laser facility is limited by safety requirements
regarding the use of toxic gases, and hence the actual calibration of
the differential absorption by formaldehyde has not been possible,
and it is presumed that this calibration procedure will be carried
out at the EPA facility after delivery of the LEM equipment.
4-1
-------
Magnetic
Pic
cup
i
ISJ
Rotating Prism ^
Analyzer
Model 840 Autoloc Amplifier
Signal
Channel
Output
(rear ^^
apron)
Reference
Channel
CZI
Output
dc Voltages
/(rear apron
Lens and Photodetector
Assembly
\Phase Reference
dc Reference Amplifier
Photodetector
Output
dc
Power
Input\
dc Input
(i
dc Output
Model 4800 Digital Ratiometer
Digital Readout^
Ileference Input
Signal Input,
Chart Recorder-
(rear apron)
Output to Chart Recorder
' (rear apron)
Figure 4-1 Block Diagram of Electronic Data-Processing System
-------
5.0 ALDEHYDE SAMPLE CELL
The Aldehyde Sample Cell is basically a heated vacuum cell
constructed of stainless steel in a "T" configuration. A small
reservoir of paraformaldehyde located in the bottom of the "T"
is heated until a suitably high vapor pressure of HCHO is obtained.
The sample cell heater control is a variac located in the LEM
equipment rack.
The sample cell windows are uncoated CaF_ flats, and hence
both the 3. 5 and the 5. 6 micron absorption bands of HCHO can be
investigated (5) using the sample cell and the xenon lines at 3.508
and 5.57 microns. The filling port of the Aldehyde Sample Cell
is fitted with a vacuum valve for evacuating the cell or for filling
the cell with a suitable (absorbing) sample gas. Since the cell
is mounted on an optical bench slide, it can readily be removed
from the LEM apparatus and filled elsewhere. The vacuum line
attached to the sample cell is a standard 1/8" vacuum tubulation
for evacuation of the cell.
Vacuum sealing of the sample cell is accomplished with "O"
ring seals at the two windows and at the filling port. At present,
no provision has been made to prevent the condensation of para-
formaldehyde on the sample cell windows, and thus the heater
control must be adjusted carefully to prevent vaporizing too much
of the parent chemical.
5-1
-------
6.0 OPERATION INSTRUCTIONS FOR LEM APPARATUS
The following LEM operation instructions are given in the
order appropriate for conducting an aldehyde absorption measure-
ments. The user(s) of the LEM apparatus are strongly urged to
COMPLETELY READ these Operation Instructions before actually
attempting to operate the LEM equipment. Serious damage can
occur to the individual LEM system components if specific operational
procedures are not followed.- The following sections describe in
detail the proper step-by-step procedures to be followed for placing
in operation each major LEM system component.
6.1 Xenon Laser Operation
The following procedure must be followed in the order indicated
to prevent possible permanent damage to the LEM xenon laser.
1. SLOWLY advanced the CATHODE CURRENT control until a laser
cathode heater current between 9 and 10 amperes is indicated on
the cathode current ammeter mounted on the LEM CONTROL PANEL.
Allow approximately 5 minutes for the cathode to come up to temper-
ature. A normal setting for the LASER CATHODE CURRENT variac
is a dial reading of 44 when the cathode has reached its equilibrium
temperature. To avoid thermally shocking the heavy cathode structure,
DO NOT exceed a current of It^amperes during the warm-up phase.
The cathode must be at operating temperature (1300 K) before the dc
excitation voltage is applied to the anode in order to avoid possible
ionic bombardment (and consequent sputtering of barium from the
cathode structure). The HP dc high voltage supply.(located directly
under the Autolock amplifier in the electronics rack) should be OFF,
and the pressure-control dewar mounted under the laser cathode assembly
should either be EMPTY or removed from the cold finger assembly.
2. With the laser cathode at temperature (bright orange heat), set the
dc voltage on the HP power supply to 500V. Turn the dc voltage ON.
Press and release the LASER START button. The laser discharge
tube should ignite and draw approximately 30 mA current. This level
of discharge current is indicative of relatively high xenon pressure in
the laser tube--jCW laser oscillation will NOT occur at this pressure.
6-1
-------
3. Be certain that the xenon laser pressure-control dewar is
fully raised around the cold finger assembly located directly under
the laser cathode bottle. Pour in a few hundred milli-liters of liquid
nitrogen to cool the dewar walls down to 77 K. After the dewar walls
are'cool, add a sufficient quantity of liquid nitrogen until the equilibrium
level of the liquid nitrogen in the dewar is approximately half way up
the length of the cold finger. (It is important NOT to fill the pressure-
control dewar with liquid nitrogen before the laser cathode is hot
and the xenon discharge is ignited. If the dewar is filled first, the
xenon pressure in the discharge tube quickly drops below the mini-
mum pressure required to establish an electric discharge.) The
xenon laser can now be operated without using the pressure-control
electronics; details on the use of this control are given below in
section 7. 2.
After a period of from 5 to 10 minutes, the remaining xenon
pressure in the discharge tube should be sufficiently low to permit
CW laser oscillation to occur, and the laser discharge current should
be between 50 and 70 mA with the dc power supply voltage set at 500V.
The relative output power of the laser can be directly measured using
the output signal from the InAs photodetector. When the photodetected
output is connected to a dc microvoltmeter (such as a HP 425 micro-
voltmeter) or a dc oscilloscope, it is possible to perform the laser
cavity mirror alignment operation described below in section 7. 3.
If the xenon electrical discharge is extinguished accidentally,
and if the LASER START button does not reignite the laser tube, it
will be necessary to lower the pressure-control dewar until the cold
finger no longer is in contact with the bath of liquid nitrogen. After a
sufficient quantity of xenon sublimes from the cold finger, it will
be possible to reignite the laser tube commencing with step 2. , above.
Measurements of the laser output power, wavelengths of oscillation,
and radiance should be made after the laser cavity mirror alignment
has been optimized. As discussed below in section 7, a number of
the CW xenon laser lines obtainable with the LEM apparatus are quite
sensitive to xenon pressure and discharge current--these lines tend
to be of lower over-all optical gain, and consequently these lines also
6-2
-------
require nearly perfect laser cavity mirror alignment before they
can be obtained at all. Although a small 1/4 meter spectrometer
can be used to measure the approximate wavelengths of the xenon
laser lines present in the output spectrum, an instrument of higher
resolution will be required to clearly separate the two Zeeman
components of the several xenon lines which oscillate in very strong
(SOOO Gauss) magnetic fields. The instrument used for these
measurements at HAG was a 3/4 meter Czerny-Turner spectro-
meter. It is , of course, important also to use a suitable middle
infrared photodetector when laser lines of longer wavelength than
J.51 microns are to be examined (such as an InSb detector). The
InAs photodetector furnished with the LEM apparatus is optimized
for the 3.51 micron laser line, and has, in general, insufficient
spectral response to detect laser lines beyond 3.68 microns.
If it is desired to measure the output power of the xenon laser,
a calibrated thermopile can be inserted in the laser beam and the
output signal displayed on a microvoltmeter. C*utput powers of from
five to ten milliwatts at 3.51 microns are nominal, with proportionally
smaller output powers available at the other nine lines.
Potential Problem Areas--Cathode
The hot cathode structure of the xenon laser is a well proven,
long-lived design. It can, however, be damaged by poor operating
procedures. The most serious danger to the cathode (aside from
i ompromising the vacuum envelope of the laser tube) is from bombard-
ment by heavy ions. This bombardment can occur if:
1. the temperature of the cathode is too low when the laser
tube is conducting,
2. the discharge current is too high, or
3. the emissivity of the cathode is low.
• I is quite evident when the cathode is undergoing bombardment: the
trie discharge is visibly in contact with the hot cathode. Normal
operation is evidenced by a "cathode stand-off "distance of
•»' least one and preferably two centimeters. If the xenon laser cathode
6-3
-------
is observed to be thus enveloped by the xenon discharge, corrective
action must be taken promptly to avoid damaging the cathode
permanently. Corrective measures include: lowering the laser
excitation voltage (thereby reducing the discharge current), raising
the cathode heater current (thereby increasing the thermionic
output from the laser tube cathode), or if these measures are not
adequate, the high voltage supply should be shut off and the laser
allowed to operate in a stand-by mode for at least thirty minutes.
If, after re-establishing the electric discharge, the cathode does
not have adequate emission, it has already sustained damage and
will require either replacement or , at the very least, reprocessing.
If the indicated operating procedures are scrupulously followed,
however, it is expected that the hot cathode structure in the
xenon laser will give years of dependable service.
Another possible difficulty which could develop is failure
or rupture of the vacuum envelope. Obvious evidence of this
problem will be a badly discolored (generally white--gray)
cathode bottle, particularly in the vicinity of the barium getter
assembly located in the upper portion of the cathode bottle. Under
no conditions should the cathode be heated if the cathode bottle turns
white since such an indication is evidence of atmospheric oxygen
present in the cathode bottle. If the cathode is cool when such an
accidental rupture occurs, it may be possible to reprocess the cathode
after the rupture is found and corrected. Otherwise, a new cathode
and complete tube reprocessing will be required to correct the
problem. Since the LEM laser tube is equipped with expensive
"V" anti-reflection coated 0 sapphire windows, it is important to
handle the laser tube with care even if a vacuum failure occurs.
It is extremely unlikely that other problems can cause mal-
function of the xenon laser tube. The pressure-control system is
designed to be a fail-safe system, and hence it should never be
necessary to rework the cryogenic pressure-control subsystem since
it can readily be turned off and back-up techniques instead. Additional
details concerning the physics of xenon laser operation can be found
6-4
-------
in Chapter 7.
Limitations on Xenon Laser Tube
The maximum obtainable output power from the xenon laser is
approximately proportional to the plasma tube discharge current (if
the xenon pressure is near the optimum value). Restrictions, however,
are placed on the maximum discharge current to prevent premature
failure of the LEM xenon laser. Although the LEM xenon laser is
a well-proven durable design capable of many thousands of operational
hours, there are three principal causes for possible premature failure
of this type of laser tube:
1. Physical damage to the vacuum envelope due to impact or
excessive strain (particular care must be taken when the dc
magnet is operating to prevent magnetic materials from being
drawn into the fragile xenon laser tube).
2. Damage to the hot cathode structure (usually through ionic
bombardment).
3. At the lowest xenon pressure obtainable with the cryogenic
pressure control system, it is possible to generate an electron
beam of sufficient intensity to burn a hole through the vacuum
envelope of the cathode bottle. This danger can be minimized
and cathode life prolonged by operating the xenon laser at
discharge currents of 50 mA or less.
The present LEM xenon laser design uses a 5, 000 0 power load
resistor in series with the xenon plasma tube, and operation of the
xenon laser with the present load resistor at currents of 25 mA or
less may cause the electric discharge to become unstable due to
insufficiently high plate voltage. If it is desired to operate the xenon
laser at even lower discharge currents than 25 mA (to prolong the
life of the xenon laser), power load resistors of 10 or 20 kfi may be
substituted for the present load resistor.
6-5
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6. 2 Magnet Operation Procedure
6. 2. 1 Introduction
The large five kiloGauss LEM electromagnet is protected by
four interlocks which are designed to protect the magnet and its
associated power supply from damage. In order to place the magnet
in operation, it is necessary to satisfy all the interlock requirements.
The interlocks are:
1. Master control key-lock on/off switch.
2. Four cooling water flow interlocks.
3. Magnet power supply cooling fan interlocks.
4. Variable magnet overcurrent interlocks.
The key-lock prevents unauthorized operation of the magnet system.
Each of the water flow interlocks must be adjusted for the lowest
acceptable water flow level that will be used. It will be evident
if a given level of water flow is inadequate for a given input power
to the magnet solenoid; namely the cooling water will commence
boiling, and the water flow will decline, which, in turn, will cause
the water-flow interlocks to shut down the magnet power supply.
The maximum dc magnetic field developed by the LEM electro-
magnet is primarily limited by the level of water-cooling available
for magnetic fields less than five kiloGauss. Thus the approximate
upper range of the magnetic field to be generated must be decided
before the magnet is energized. The appropriate CURRENT RANGE
of the magnet power supply must also be selected at this time. Three
current ranges are available for both 460 and 230 V ac power supply
wirings. These ranges are:
1. 80--140A (460V ac) I1. 40--70A (230V ac) '
2. 90--200A (460V ac) 2'. 45--100A (230V ac)
3. 100--330A (460 V ac) 3'. 50--165A (230V ac).
If the large LEM cooling water pump (generating a water pressure
of 300+psi) is used, closed cycle cooling operation is possible for
(
maximum magnetic field generation, providing that the associated
(closed cycle) heat exchanger has adequate capacity (80--100 kilowatts).
6-6
-------
If only tap cooling water is available (and no external heat exchanger),
the exhaust water must be dumped, and the lower pressure (probably
only 60 psi) will require that the maximum magnetic field be reduced
by approximately a factor of */ 5 .
As in the case of the Operation Instructions provided above
for the Xenon Laser, the following step-by-step procedure should
be, carried out in the order indicated.
6.2.2 Operation Instructions
Step 1. Turn ON the magnet key interlock located on the remote control
console.
Step 2. Turn ON the cooling water supply to a sufficiently high flow
rate to cool the magnet adequately and also activate the water flow
interlocks. The water-flow interlock lights on the remote control
console should turn from RED to GREEN.
Step 3. Turn ON the magnet power supply cooling fans.
Step 4. Press the OVERCURRENT RESET button if the overcurrent
warning light is RED. The overcurrent GREEN light should turn on.
Step 5. If all control lights are GREEN, then press the green MAGNET
ON button to energize the LEM electromagnet.
Step 6. Adjust variac on magnet remote control console to the desired
current level. See Figure 3-1, above, for a calibrated curve of input
current vs. axial magnetic field.
Step 7. Check magnet temperature frequently at first (at least at
five minute intervals) to prevent overheating of the magnet. The
water flow interlocks will shut down the magnet power supply if
the water begins to boil, but this situation should be avoided if at all
possible.
6.2.3 Selecting Magnet Current Range
The three magnet current ranges (at 480 and 240 V 3 0) are
selectable at the main magnet power supply under the small, hinged
front door. A wiring diagram attached to the protective door clearly
indicates the proper arrangement of the current-selecting bars.
6-7
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6. 3 Electronic Data-Processing System Operating Procedure
The LEM Electronic Data-Processing System consists of four
individual units. They are:
1. High-gain dc reference amplifier.
2. Keithley Model 840 "Autoloc" amplifier.
3. Dana Model 4800 digital ratiometer.
4. Keithley Chart recorder.
Interconnection of these individual units should follow carefully the
block diagram shown above in Figure 4-1. The photodetected output
from the InAs photodetector is connected (via BNC cables of the
appropriate length) to the -i- INPUT of the Model 840 and to the
DC AMPLIFIER INPUT of the dc reference amplifier. The BNC
cable from the magnetic pickup mounted on the LEM prism analyzer
assembly should be connected to the INPUT of the Model 840 reference
channel. The OUTPUT from the Model 840 should be connected to
the INPUT of the Model 4800 located on the right side of its front panel.
The OUTPUT from the dc Reference Amplifier should be connected to
the INPUT of the Model 4800 located on the rear of its chassis (REF.).
The OUTPUT of the Model 4800 is used to drive the Chart Recorder
and should be appropriately connected.
If an aldehyde absorption measurement is desired, the CW xenon
laser should be turned on (according to the procedure described above
in section 6.1), the LEM electromagnet should be energized (following
the instructions outlined above in section 6. 2), and the vacuum sample
cell (or other test cell) should be filled with a sample absorbing gas
and placed in the optical path of the LEM system.
When the LEM apparatus has been prepared for operation, then
the Electronic Data-Processing System should have its controls turned
to the following settings:
KEITHLEY model 840:
Power: ON; AC SENSITIVITY: 10 mV; DC SENSITIVITY: xlOO;
TIME CONSTANT: 100ms; ZERO SUPPRESS: OFF; FREQ
BAND HZ: 100 Hz; TRIGGER: (-)
6-8
-------
DANA Model 4800:
POWER: ON; RANGE: lOV/dc; FRONT/REAR: Front;
Press the following control buttons: DATA OUTPUT, PERIODIC,
and RATIO.
When these units are interconnected as indicated and the individual
controls set in accordance with the above directions, then the Electronic
Data-Processing System is ready to analyze the photodetected output
of the LEM apparatus. For the purpose of illustration, we assume
that the ac (differential absorption signal) component from the photo-
detector (with a periodicity twice that of the rotational frequency of the
analyzer prism) has an amplitude only 10 that of the dc (total laser
output) signal. If we assume that the dc signal has a 50 mV level,
then the ac signal must be 50 A/V. Under these conditions, the AC
SENSITIVITY of the Model 840 can be increased to either 0. 1 or
0. 01 mV and the DC SENSITIVITY adjusted to give about a one-
half scale reading on the dc METER of the Model 840. The PHASE
control is then adjusted to maximize the signal detected on the dc
METER. It may be necessary to decrease the DC SENSITIVITY if
the METER displays an off-scale reading. In addition, the OVERLOAD
light will turn on if the METER displays an off-scale reading. When
the Autoloc amplifier is properly phase-locked, the signal on the
METER should be quite stable. The RATIO control button on the
Model 4800 can also be deactivated permitting the Model 4800 to .be
be used as a digital dc voltmeter. Under these circumstances, the
output from the Model 840 can be displayed at a 3 Hz rate.
6-9
-------
7.0 DETAILS OF LEM XENON LASER OPERATION AND ADJUSTMENT
The successful operation of the LEM apparatus is strongly
dependent upon the adequate performance of the LEM xenon laser.
This Final Report provides information and operating procedures for
optimizing xenon laser performance in a total of four locations.
A brief discussion of the spectral broadening, longitudinal mode
structure, and general oscillation characteristics in and out of an axial
magnetic field was presented above in section 2.2. The general procedure
for operating the LEM xenon laser can be found above in section 6. 1.
Details concerning the initial assembly and wiring of the LEM xenon
laser are provided in section 8. 1, below.
The present section is intended to provide an adequate amount
of information to optimize operation of the LEM xenon laser at all
of the available CW xenon laser wavelengths. Eleven CW laser lines
can oscillate simultaneously in the LEM xenon laser, and the different
requirements of each of these lines are described in this section.
Accordingly, the following areas are treated:
1. Physics of xenon laser oscillation.
2. Optimization of xenon laser lines.
3. Laser cavity properties and alignment procedures.
Each of these sections is described below.
7. 1 Physics of Xenon Laser Oscillation
The major laser lines obtainable with the LEM xenon laser
are high optical gain, low output power transitions. Although relatively
high peak output power can be obtained from pulsed TEA noble gas
lasers (particularly xenon TEA lasers) at approximately the same
transitions as the CW low power (and low pressure) noble gas lasers,
for the purposes of the present application the low power CW laser
configuration should be entirely adequate. Most treatments of CW
laser oscillation are concerned with the oscillation of laser lines
exhibiting relatively low optical power gains (of the order of only
a few dB/meter), and although, in general, the oscillation characteristics
7-1
-------
of high-gain lasers are quite similar, there are some important
differences which require special mention. In this section, the
general conditions for the establishment of laser oscillation with
a typical high-gain xenon laser line will be described and mention
will be made (where appropriate) of the weaker, low gain lines
also obtainable with the LEM xenon laser.
The optical power gain of a laser amplifier is given by
(7-1)
where o is the stimulated emission cross-section of the laser
transition, N is the inversion density, and i is the active length
of the laser amplifier. G represents the "small signal" power
gain obtained by passage of a beam of low intensity once through
the laser cavity. In actual practice, the optical power gain of
a laser amplifier "saturates" down to a level equal to the optical
losses in the laser cavity, and equation (7-1) holds in this case
also when the average inversion density,N,replaces the peak
inversion density, N, in equation (7-1). The stimulated emission
cross-section, a, is a convenient parameter which contains the
physical parameters related to the relative line strength, wavelength,
linewidth, etc. , of the laser transition:
'(")= hjM g(i-*o> (?-2)
Here T is the lifetime of the upper laser level and g( v - \> ) is the
Gaussian gain function of the Doppler and isotope-broadened linewidth:
Vi
, . ,-x in t. 4 In 2( v-
g( "- "Q) =4 '
7-2
-------
where Av is the Doppler -broadened spectral width of the laser
transition. For frequenc
equation (7-3) reduces to
transition. For frequencies nearAv , i. e. , near line center,
so that the peak stimulated emission cross-section, y , can be
written as
An alternative definition for a in terms of the "oscillator strength"
of the laser transition is given by
(7-5)
where g? and g., are the degeneracies of levels 2 and 3 of the
laser transition, e is the electronic charge, m is the mass of the
electron, F is the spectral cross-relaxation rate, and f,_ is the
oscillator strength of the laser transition.
The high-gain xenon laser transitions in the case of the LEM
laser are characterized by having large stimulated emission cross
sections (
-------
to as the "saturation flux" defined by
h«>
ft. __ 11 K * — / »
S O T
where h is Planck's constant. In practice, 0 is the input flux
s
to.a laser amplifier necessary to saturate (lower) the small signal
optical power gain, G , by 3 dB. On the average, low pressure
gas lasers have small values of 0 (corresponding to very large
values of a) and are consequently, essentially low-power devices.
Solid state lasers have large values of 0 , and are high-power
S
devices.
7.1.1 Establishment of Laser Oscillation
It is often convenient to treat the active medium of a laser
as an optical amplifier (11); i. e. , by analogy with an electronic
amplifier, an optical amplifier has a definite optical gain and a related
background optical noise level (in the case of high-gain laser ampli-
fiers). The optical power gain is defined most simply as the ratio
of the input power to the output power of the laser amplifier in the
limits of small input power and unsaturated amplifier conditions.
The "optical noise" property of the laser amplifier is due to ampli-
fied spontaneous emission (12). This "noise" property of laser
amplifiers is inherent and presents little or no problems in low-gain
lasers but assumes great importance for high-gain laser amplifiers.
Depending upon the gain of the laser amplifier, spontaneous emission
can ultimately limit the maximum single-pass gain obtained in a laser
amplifier. Thus, when a laser amplifier is adjusted in such a fashion
that the spontaneous output along the axis of the laser equals the
saturation flux of the laser medium, the laser amplifier enters the
so-called "superradiant" condition.
When operated in the superradiant limit, the high-gain laser
amplifier can emit substantial optical power without requiring an
external resonator cavity; i. e. , "oscillation" without mirrors is
possible. It is possible, using a simple geometrical model, to
7-4
-------
predict when a given high-gain laser amplifier will enter the super-
radiant limit. If the small signal amplifier gain is given as G ,
then the maximum superradiance-limited optical gain is given
by the inequality
(7-7)
In G
o
where ft is the solid angle self- sub tended by the laser amplifier
and the */ln G term is due to the spectral narrowing effected by
the passage of the spontaneously-emitted photons through the high-
gain laser amplifier
In the case of the strong xenon laser lines listed in Table 2-1,
above, nearly all of them are superradiance-limited according to
the inequality given above in expression (7-7). As a consequence,
these laser lines have output power characteristics that are not
very strongly influenced by the quality of the laser cavity mirror
alignment, and, as discussed below in section 7.3, these lines should
not be used as sensitive indicators of the final adjustments on the
LEM xenon laser cavity mirrors. A superradiant laser is relatively
easy to align, however, since it emits a considerable amount of laser
radiation even when the cavity mirrors are not well adjusted.
In general, there are three" basic conditions for a (low-gain)
laser to commence oscillation:
1. The square of the single pass small signal gain, G , must
be greater than the reciprocal of the cavity regeneration efficiency,
2 • 2 -— i'-
2. A resonant TEM cavity mode must lie within the Doppler-
broadened spectral gain distribution. The small signal gain,
G , at this frequency must satisfy the inequality (7-8).
7-5
-------
3. A means must be provided to maintain the population
inversion density, N, at a sufficient level to maintain
CW laser action (if CW oscillation is required).
The cavity regeneration efficiency, £ , is the product of all the laser
cavity mirror reflectivities, window and atmospheric transmissions,
diffraction and polarization efficiencies, etc. of the round trip optical
path in the particular laser cavity used. According to this simple
theory, if the square of the laser amplifier small signal gain is not
greater than the reciprocal of the cavity regeneration efficiency,
then laser oscillation will not commence, and the laser amplifier
emits only a relatively small quantity of essentially amplified
spontaneous emission. It is evident from expression (7-8) that
these high-gain laser lines of xenon can be used in laser cavities
of relatively low optical efficiency; this property is very useful
if high-loss, long optical paths are used to detect relatively small
molecular concentrations (9, 13).
If the optical gain of a laser amplifier is not at the superradiant
limit defined above by expression (7-7), then the onset of laser
oscillation once the "threshold" condition is met (expression (7-8),
above) is abrupt, with a large increase of circulating optical power
in the laser cavity. This "threshold" of oscillation property of
moderately-high gain lasers can be used to advantage if absorption
spectroscopic techniques are used to vary the losses in the laser
cavity on each side of laser oscillation "threshold. "
7. 1. Z Saturation of Laser Amplifier Gain
The inequality indicated in expression (7-8) above represents
a non-equilibrium situation which applies only during the build-up
of laser oscillation. This build-up of laser oscillation essentially
starts with the circulating flux in the laser cavity consisting only of
spontaneous emission. As each transit through the laser cavity
amplifies the circulating spontaneous emission by the excess of
amplifier gain over the loss coefficient (calculated from In £ ),
7-6
-------
the circulating power in the laser cavity eventually reaches the
"saturation flux" level defined above in expression (7-6). At
this level of optical flux, the small signal gain, G , of the laser
amplifier has begun to "saturate, " andwhen the circulating flux
has reached a value near 0 , the small signal gain has declined
s
by approximately 3 dB. For steady-state, continuous wave (CW)
laser oscillation, the optical power gain, G, of the laser amplifier
saturates to a level given by the optical regeneration efficiency
defined above; namely:
(7-6')
The steady- state inversion density, N , can readily be obtained
S
from the above relation. In general, however, this simple theory
is inadequate if relatively high output powers are obtained, since
the stimulated emission cross-section and the saturation flux depend
on additional parameters including the output power (and, consequently,
the stimulated emission rate),
In the case of the 3. 51 micron xenon laser transition and the
typical conditions that apply to operation of the LEM apparatus ,
the onset of laser oscillation occurs very rapidly since the laser
transition exhibits very high optical gain (as much as 30 dB/meter)
together with a relatively low saturation flux (of the order of 10 milli-
2 2
watts/cm ). G*utput power fluxes of the order of 0. 1 watts/cm can
be obtained at 3. 51 microns if all the laser amplifier and cavity parameters
are optimized (for low pressure operation), but the low pressure, CW
xenon laser is inherently an inefficient device. A large part of the
inefficiency of the xenon (and, correspondingly neon, argon, and krypton)
laser is due to the fact that it is necessary to excite a neutral xenon
atom nearly to the ionization limit in order to reach the first excited
states. The stored energy in the excited state is only partially extracted
during laser oscillation since the lower laser level itself is near the
ionization limit.
7-7
-------
7. 2 Optimization of Xenon Laser Lines
The optimization of each of the ten xenon laser lines obtainable
with the LEM xenon laser can be carried out by adjusting the xenon
pressure and discharge current to values appropriate for each laser
transition. It is possible to obtain simultaneous oscillation of all
ten xenon laser lines since these lines do not compete for the same
population inversions. In general, however, not all of these lines
will oscillate simultaneously under ordinary operating conditions
since some lines require lower pressures than others, better laser
cavity alignment, more or less discharge current, etc. The general
ranges of optimum xenon pressures to be used for each of these ten
CW laser lines are listed above in Table 2-1. In this table, "low"
xenon pressure is obtained by having no current input to the heater
of the cryogenic pressure-control cold finger. "Moderate" xenon
pressure is obtained by either applying sufficient heater current to
the cold finger heater or by lowering the level of liquid nitrogen in
contact with the cold finger. This latter pressure-control technique
can be readily accomplished by removing the two Allen cap screws
which secure the liquid nitrogen dewar under the xenon laser cold
finger assembly. A lab-jack (or similar dewar height adjus tment
device) can then be used to vary the contact height of the liquid nitrogen
bath.
If the xenon laser tube electric discharge is accidentally extinguished
with the liquid nitrogen in the pressure-control dewar, it may be
necessary to lower the dewar completely in order to increase the xenon
pressure to a level sufficient to reignite the electric discharge. In
addition, it is likely that condensation of moisture from the atmosphere
will require the emptying of the pressure-control dewar of accumulated
water, possibly as often as every day of regular operation.
7.2.1 Spectroscopic Details of Xenon Laser Transitions
The 11 xenon laser transitions listed above in Table 2-1 are
described in terms of Racah spectroscopic notation (14) applicable
to the j--l coupling scheme used to describe the energy levels in
7-8
-------
heavy noble gases. The primed energy levels in Table 2-1 refer to
the P, ,j parent ion and the unprimed energy levels refer to the P° ,,
parent ion of xenon. Theoretical calculations of the relative line
strengths of most of these laser lines have been carried out (15),
although transitions between the terms of the P. ,_ and PO/-> parent
ions are treated as forbidden. As indicated in Table 2-1, both the
3.62 and the 4. 15 micron xenon laser lines are such "forbidden"
intercombination lines. Although these lines tend to be somewhat
weaker than the "allowed" CW xenon laser lines obtained with the
LEM laser, they are not significantly weaker than the strongly
"allowed" 3.86 micron line listed in Table 2-1. Aside from these two
intercombination lines, the strong CW xenon laser lines follow the
theoretical calculations (15) quite closely, as indicated by the calculated
line strengths listed in the last column of Table 2-1. Laser lines
arising from transitions in the term scheme of the P, /_ ion tend
to have lower gain and lower output power owing to the lower
multiplicity of the parent ion.
7. 2. 2 Effects of Xenon Pressure on LEM Laser Output Power
It has been mentioned above that optimum xenon pressures and
discharge currents can be found for each of the ten CW xenon laser
lines obtainable with the LEM apparatus. These optimum xenon
pressures are obtained by varying the temperature of the cryogenic
pressure-control cold finger, and a number of techniques for effecting
this temperature variation have been suggested above in sections
6. 1 and 7.2. Although the xenon pressure control is designed to
permit continuous pressure control over the range from 2 to several
hundred milliTorr, the optimum xenon pressures for aJJ tbf CW laser
lines obtainable with the LEM apparatus lie below 20 milliTorr.,
An approximate indication of the xenon pressure present in the
LEM xenon laser discharge tube at any one time can be obtained from
the impedance characteristics of the electric discharge. As shown
below in Figure 7-1, using a (standard) voltage of 500 V on the tube
anode, the (standard) 5,000 S7 power load resistor, a pressure of
several milliTorr produces the highest current (75 mA), while a
7-9
-------
Xenon Tube
Discharge Current (mA)
70
60
50
40
30
20
10
Tube Extinguishes
\'
0.1
1.0
10
100
-»- Xenon Pressure
(milliTorr)
Figure 7-1 Pressure Dependence of Xenon Laser Discharge Current
7-10
-------
pressure of several hundred milliTorr permits a discharge current
of only 30 mA at the same anode voltage/load resistor combination.
Although each line exhibits a unique xenon pressure/discharge
current set of operational curves, Figure 7-2 illustrates a typical
example of a high-gain xenon laser line. In this case, the most
important wavelength for the purposes of the aldehyde measurements,
namely the 3.51 micron xenon laser line, is selected as the example
i
depicted in Figure 7-2. As indicated in Figure 7-2, when the xenon
pressure is too high, increases in discharge current reduce the
output power of the laser rather than increase it. Under certain
conditions of pressure and discharge current, some of the xenon
laser lines will cease oscillation due to excessive pressure and
discharge current. Those laser lines listed in Table 2-1 as requiring
"low" xenon pressures will not oscillate at moderate xenon pressures.
And, in general, those xenon laser lines requiring moderate xenon
pressure will not oscillate at "low" xenon pressure.
In practice it will probably be necessary to use an external
spectrometer of sufficient spectral resolution to separate clearly
the relatively closely-spaced xenon laser lines; that is, if the
individual optimization of each laser line is to be observed directly.
Use of such a spectrometer is also advised, as mentioned in the
following section, in the individual alignment of each laser line.
It is a good procedure to use the smallest discharge current
which will produce an adequate output power from the LEM laser.
In addition, higher pressure operation of the discharge tube tends
to reduce the amount of cathode bombardment from heavy positive
ions in the discharge. As mentioned in section 6. 2, above, it is
important to avoid operating situations in which the hot cathode is
enveloped by the xenon discharge since such a situation is indicative
of cathode bombardment (and consequently short life for the laser
tube).
7-11
-------
-0
to
Xenon Laser
Output Power
(milliwatt s)
3 milliTorr Xenon Pressure
10 milliTorr Xenon Pressure
O.
Plasma Tube
0 10 ZO 30 40 50 60 70 80 90 100 110 120 Discharge Current
Figure 7-2 Xenon Output Power at 3.51 microns vs. Discharge Current
-------
7. 3 Laser Cavity Properties and Alignment Techniques
There are two types of laser mirrors furnished with the LEM
laser. They are:
1. A plane-parallel Fabry-Perot cavity producing a well-
collimated output beam, and
2. an afocal cavity producing a slightly converging optical
beam.
Although the Fabry-Perct cavity mirrors produce a better collimated
output beam than the afocal cavity mirrors, the afocal mirrors are
considerably less susceptible to alignment variations produced by
mechanical vibration or poor mirror adjustment. Thus the afocal
laser cavity mirrors can be used to reduce the objectionable effects
of the magnet cooling water pump vibration. This vibration source
may not be a problem if the water pump is not used at the EPA, and
the source of cooling water is tap water at an adequate pressure and
flow rate.
Depending upon the particular needs of the experiment to be
performed with the LEM apparatus, the Fabry-Perot or afocal
laser cavity should be selected before the laser cavity alignment
procedure is begun. The Fabry-Perot uses a plane high-reflectivity
mirror mounted on the piezo-electric mirror translator while the
afocal cavity uses a spherical mirror having a radius of curvature
of 10 meters. The location of the piezoelectric translator is shown
above in Figure 2-1.
7.3.1 Laser Cavity Alignment Procedure
Although it is possible to align a superradiant xenon laser using
only the superfluorescent output from the laser, it is generally easier
to use a small laboratory He/Ne laser oscillating at 0. 633 microns to
complete this task. The output beam from such an alignment laser should
be directed parallel to and down the axis of the LEM xenon laser, with
particular care being taken to center the alignment beam in the center
of both LEM xenon laser tube windows. It will be useful to remove all
of the LEM optical components from the optical bench to eliminate
7-13
-------
dispersive optical wedges and opaque filters. Once the axis of
the LEM laser tube has been defined, the output mirror can be
installed in its collet with the dielectric coating toward the xenon
laser tube. The reflection from the dielectric-coated surface of
this mirror should then be brought into coincidence with the alignment
laser beam. After this has been accomplished, the reflection from the
high-reflectivity mirror should also be brought into coincidence with
the alignment laser beam. It is often a considerable convenience
to insert a beamsplitter into the alignment laser beam directly in
from of the alignment laser in order to determine as accurately
as possible the angular displacement of any reflected rays.
When both laser cavity mirrors are properly aligned, the
procedure for starting the xenon laser (section 6. 1) should be
followed carefully, and the LEM photodelector assembly positioned
in such a location that it will be able to intercept the entire xenon
laser beam. This may require a degree of searching for the output
beam, although the alignment laser often provides a relatively accurate
reference to the direction and location of the output xenon laser beam.
The output xenon laser beam should pass through the centers
of all the LEM optical component apertures without vignetting. The
LEM optical component arrangement depicted in Figure 2-1 is re-
commended when formaldehyde absorption measurements using the
furnished Nielson sample cell are desired. If a long optical path length
White cell is substituted for the LEM aldehyde sample cell, the output
beam divergence of the xenon laser (in the afocal case) may present
a problem. The focal length of the spherical mirror used in this case
is nominally 5 meters, and the optical power of this mirror can be
corrected by introducing a negative lens of suitable optical power into
the output xenon beam.
The final step in the mirror alignment procedure is to peak the
fine laser cavity mirror alignment using the magnitude of the output
signal from the LEM photodetector as a guide. As mentioned above
7-14
-------
in section 7.2, a spectrometer can be used to complete the mirror
alignment for the relatively weak, l.ow optical gain xenon laser lines
listed above in Table 2-1. Whereas the high-gain xenon laser lines
(such as the 3.37, 3.51, and 5.57 micron transitions) are relatively
easy to use in obtaining an approximate cavity mirror alignment, the
relative insensitivity of these lines to cavity losses induced by poor
mirror alignment makes it quite difficult to obtain excellent mirror
i
alignment when the experimenter monitors the output powers of these
lines only. Thus, when the relatively low-gain transitions (such as
the 3.62 and the 3.86 micron lines) are used for the "touch-up"
operation of the mirror alignment procedure, it is a relatively simple
matter to obtain optimum cavity alignment.
For the purposes of performing absorption measurements on
formaldehyde using the 3.51 micron xenon laser line, it may be ad-
visable not to have the lower-gain xenon laser lines oscillating strongly.
The 3.51 micron interference filter passes a small percentage of the
xenon laser lines at 3. 37, 3. 62, and 3. 68 microns, and if these lines
are permitted to oscillate strongly, there may be an undesirable
contribution to the detected signal. When the strong axial magnetic •
field is applied to the LEM xenon laser, however, only the 3. 37
micron line will be able to partially penetrate the narrow-band
3.51 micron interference filter (the other laser lines fall below
oscillation threshold in a strong magnetic field). The 3.37 micron
laser line splits into two components also in a strong field, and it
exhibits optical gain characteristics nearly as great as the 3.51
xenon laser line. As a consequence, it may not be possible to
entirely remove the 3. 37 micron laser radiation from the detected
signal. This should not represent a serious problem to proper LEM
system operation, particularly since such added signals can be
compensated at either the dc reference amplifier or at the ac
lock-in amplifier (see section 6.3, above).
7.3.2 Future LEM Laser Cavity Options
The utility of the stable CW xenon laser capable of laser oscillation
at 11 wavelengths in the important infrared band from 3. 1 to 5. 6 microns
7-15
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is not well exploited in the present LEM laser design. There are
several additional laser cavity options which can permit additional
flexibility in the operation of the LEM apparatus. Some of these
are:
1. Additional output mirrors optimized for specific spectral
regions: 3.1, 4.0, 5.7 microns, etc.
2. Additional (unpolarized) CW xenon laser tubes optimized
for individual spectral regions: 2.0, 3.1, 3.5, 3.9, 4.5,
5.5, 7.0, 9.0, etc.
3. Broad-band Brewster's angle-equipped xenon laser tubes
for use without a Zeeman-splitting magnet.
4. Introduction of a cavity Littrow prism to permit single
wavelength laser oscillation.
5. Replacement of the present 1/4 wave plates with an
adjustable electro-optical waveplate for use at a number of
selected high-gain wavelengths using the Zeeman magnet.
6. An option in which two different xenon laser wavelengths
are used to permit differential absorption measurements to
be made in the absence of a magnetic field. In this option,
one of the laser lines would be selected to avoid an absorption
line, while the other would be centered as closely as possible
on an absorption line. The unabsorbed line would then function
as a reference in a differential absorption measurement apparatus.
7. Modification of the present LEM magnet to permit the use
of large pulsed magnetic fields in order to achieve greater
Zeeman splitting.
8. Introduction of pulsed noble gas lasers designed to oscillate
at some 50 infrared wavelengths (from 1.2 to 9.0 microns).
9. Use of both a pulsed Zeeman magnet and a pulsed laser
designed for extended tunable spectral performance.
In addition to the above, there are additional ways in which the signal
to noise (S/N) ratio of the electronic data-processing equipment can be
improved. Further details concerning any and all of these modifications
can be obtained from the Laser Division, Hughes Aircraft Co. , Culver
City, California.
7-16
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7. 4 Coincidence of CW Xenon Laser Lines with Formaldehyde
Absorption Lines
Following the data of Nielson (5), Table 7-1 lists the most
obvious coincidences between five of the eleven CW xenon laser
lines listed above in Table 2-1 and various P and R-branch absorption
peaks in the infrared spectrum of formaldehyde.
TABLE 7-1
CW Xenon Laser Lines Near Formaldehyde Absorption
Bands
#
1
2
3
4
5
Laser
Wavelength
(air)
3.3667
3.5070
3.6209
3.6507
5.574
HCHO
Wavelength
3.3679
3.5100
3.6206
3.6512
5.577
AX
-12 A
-30 X
+ 3 X
-5 &
-30 R
Line
Identification
-2
6
-8
-19
22
Since there are six other CW xenon laser lines (several of which are
quite strong) obtainable with the LEM xenon laser, it may be worth-
while to consider using dual or multiple wavelength absorption
spectroscopic techniques to measure the relative concentrations of
HCHO in the LEM sample cell (or an ancillary White Cell). Such
dual (or multiple) wavelength absorption techniques could be employed
without requiring the use of the large Zeeman magnet, albeit at the
cost of sacrificing the 7 A tunability of the 3. 37 and 3.51 micron xenon
laser lines. The differential absorption measurements could be made
in a fashion quite similar to the present LEM techniques; namely, an
unabsorbed laser line (such as the 3.27 micron line) is alternatively
compared with an absorbed line (such as the 3.37 micron line), and
the electronic data-processing circuitry compares the relative
intensities of the two lines. The present LEM lock-in amplifier,
7-17
-------
display, and recording electronics can readily be adapted to be
used in differential absorption measurements of this type, and
only the totating prism/analyzer assembly should require extensive
modification (or replacement). A phase-locked scanning diffraction
grating either in or out of the laser cavity can perform the function
of synchronous, phase-locked wavelength selection/alternation for
this application.
As discussed in the previous section, a xenon laser oscillator
equipped with spectrally broad-band windows (such as Brewster "s
Angle CaF flats) is capable of CW or pulsed laser oscillation at
infrared wavelengths in addition to the eleven laser lines listed
above in Table 2-1. Such a laser would tend to emit linearly
polarized light, however, and thus would not be entirely suitable
for axial field Zeeman tuning.
7-18
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8.0 LEM ASSEMBLY INSTRUCTIONS
The majority of the LEM components will be shipped com-
pletely assembled. Only the xenon laser and its associated hardware
need extensive assembly, and an effort is made to simplify the assembly
procedure as much as possible. Each laser cavity component is
numbered and marked for proper orientation. This is also the case
for the LEM wiring connections. It is only necessary to match the
corresponding letters on the incoming leads with the letters on the
terminal strips.
Upon receipt of the LEM equipment, each LEM system component
should be carefully inspected for damage incurred during shipment.
The laboratory site(s) and locations of the magnet power supply, Zeeman
solenoid (to be mounted on the LEM optical bench), optical bench,
electronic equipment rack, magnet cooling water pump, and magnet
remote control should be selected and the proper power and water
connections made. Both the high-pressure water pump and magnet
power supply produce a relatively high level of noise when in operation,
and it is recommended that the magnet power supply be located in an
adjacent laboratory room if possible. Placement of the magnet power
supply outside the laboratory in exposed condition is not recommended
since it is important that the magnet power supply not be exposed to
moisture since the large cooling fans would tend to draw moisture onto
the high-voltage rectifiers.
Both a source and a sink of cooling water must be provided if
a closed-cycle heat exchanger is not used with the LEM magnet. If
the cooling water is to be dumped without recirculation, the 400 psi
water pump can be used as a booster pump if a water source of from
3--4 gpm is available.
The magnet power supply is so large and heavy that a fork
lift is needed to move it, and it is recommended that the site for
the magnet power supply be completely prepared in advance of the
delivery of the LEM system. At Hughes, the 3 0 480 V input is
directed down from the laboratory ceiling through the top of the
8-1
-------
power supply cabinet. The dimensions of the power supply cabinet
are 4611 (D) x 34" (W) x 63" (H), while those of the cooling water
pump are 24" (W) x 24" (H) x 48" (L). The water pump also
requires 3 0 480 V as a power source.
The Zeeman solenoid weighs approximately 300 Ib. , and
may either be raised into place on the optical bench using a portable
hoist, or if sufficiently burly technicians are available, four strong
men can lift the solenoid onto the optical bench. As noted below,
it is very important that the special optical bench foot be properly
installed on the optical bench before the Zeeman magnet is put
on the bench.
8. 1 Assembly of the LEM Xenon Laser Cavity
All the LEM optical equipment is designed to fit on a
furnished Ealing optical bench. For additional stability of the
optical bench, an ancillary optical bench foot is furnished complete
with adjustment screws. Matching protective metal plates should
be placed under these adjustment screws to avoid marring the
laboratory table top. This foot should be mounted on the optical
bench FIRST before any other assembly is attempted. DO NOT
place the magnet solenoid on the optical bench without this foot
being in place since the torque exerted on the optical bench (in
the absence of the extra foot) by the water-cooling manifold can
cause the entire assembly to tip.
As shown above in Figure 2-1, the xenon laser cavity is
constructed around the large Zeeman magnet solenoid, and hence
the first step in the assembly of the xenon laser cavity is to locate
the solenoid at one end of the optical bench. The solenoid mount
should be flush with the end of the optical bench as shown below in
Figure 8-1. As indicated, the centimeter scale on the optical bench
should be on the opposite side of the solenoid coil from the water-
cooling manifold. After this is accomplished, the following assembly
procedure should be followed:
1. Attach the laser output mirror assembly, part #30, (using
1/4 20 Allen cap, screws) to the inboard side of the Zeeman solenoid
8-2
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Magnet Water Distributor
Cover
Plate 10
Plate 20
Laser
Cathode
\
Centimeter S cale
Optical Bench
Nylon Mount 32
•Ancillary Foot
'Adjustment Screw
oo
i
C*J
Figure 8-1 LEM Assembly Schematic
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as shown in Figure 2 -1 and Figure 8-1.
2. Insert the xenon laser discharge tube into the Zeeman
magnet solenoid with the anode lead and trigger wire deployed
along the solenoid axis. Observe Figure 2-1 for proper
orientation of the xenon laser. Loosely clamp the xenon
tube in the nylon mounting bracket on plate #30.
3. In a similar way, attach the quartz rod mounting pl;:te,
part #20, using spacers #25 and #26. Be certain that th>
xenon tube clamp is loose before attaching plate #20, an-
loosely clamp the xenon laser tube after the plate is prop-et-ly
mounted with the long 1/4 20 Allen cap screws.
4. Mount the xenon pressure-control dewar, part #27, b<
the solenoid mount and the quartz rod mounting plate, fr~O.
5. Insert the quartz stabilization rods, #1, #2, #3, and
in their respective apertures in plates #20 and #30; name/y,
rod #1 goes through aperture #31 in plate #30 and throu^
hole #21 in plate #20. Rod #2 goes through aperture #32
in plate #30 and through aperture #22 in plate #20, etc.
6. Insert nylon mounts #31 (for aperture #31 in plate £ 10\
#21 (for aperture #21 on plate #20), and #11 (for apertm e#
on plate #10) on quartz compensation rod #1. The corre.
nylon mounts should be placed on rods #2, #3, and #4. t is
important to be careful while inserting the quartz rods i;to
their respective mounts to avoid breaking these brittle ro
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The laser cavity mirrors can be inserted in their respective mounts
and installed in the micrometer-adjusted collets. As indicated above
in Figure 2-1, the high-reflectivity (spherical mirror in the afocal
cavity case) mirror is installed in the piezoelectric translator head
while the output (dielectric-coated) mirror is installed in one of the
two smaller mirror mounts.
This completes the initial assembly of the LEM laser cavity.
Optical alignment of the laser cavity can be accomplished by following
the procedure outlined above in section 7. 3. 1. It is important to be
certain that the xenon laser tube is rather accurately centered in the
Zeeman solenoid to permit the laser beam to pass unvignetted through
all the optical apertures in the LEM optical train.
8. 2 Initial Assembly of Zeeman Magnet
The installation of the LEM Zeeman solenoid was described in
the previous section. After the assembly of the laser cavity has been
completed (according to the above procedure), it is only necessary to
install the magnet cover and wire the magnet according to the
directions(presented below) to complete the assembly of the LEM
magnet system. The magnet cover is attached with the furnished
round-head machine screws and washers. The cooling system
assembly should follow closely the procedure outlined above in section
3. 1 and Figure 3-3 to permit the magnet to be adequately cooled.
The high-pressure hoses are characterized by having black cloth
covers, and these high-pressure hoses are connected (as shown in
Figure 3-3) to the magnet inlet water fitting and to the magnet water
exhaust outlet. Relatively low water-pressure hoses are used for the
remainder of the plumbing.
8. 3 LEM Cable Connections
The initial set-up and wiring of the LEM apparatus is straight-
forward, and it requires relatively little time. It is VERY IMPORTANT,
however, to insure that all wiring be doubly checked for accuracy
BEFORE power is applied to any of the LEM circuits or components.
The wiring of each major LEM subsystem is described below.
8-5
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8. 3. 1 Wiring of Xenon Laser
The electrical c ontrols of the LEM xenon laser are found in the
upper portion of the electronic equipment rack. These controls are:
1. Laser cathode current (controls cathode temperature).
2. DC Power Supply (discharge tube voltage and xenon laser
discharge current).
3. Heater controls for-cryogenic pressure-control system.
4. Laser tube starter.
In addition to the above controls, the analyzer prism motor,
cooling fan, and sample cell heater are controlled from the LEM
control panel. The interconnection of the cables from the
LEM control panel to the individual LEM components constitutes
the extent of wiring the xenon laser subassembly. Wherever
possible, non-interchangeable connectors have been used to reduce
the possibility of incorrect connections. The wiring procedure
simply consists of matching the identifying letters on the LEM
components with the corresponding letters on the cable leads
coming from the LEM control panel.
8.3.2 Wiring of LEM Magnet
A similar procedure to that used in wiring the xenon laser sub-
assembly is used for wiring the Zeeman magnet. The polarity
of the heavy, high-current leads from the large magnet power supply
must be carefully observed (see section 6.2.3, above, for information
relating to the selection of current range of the magnet power supply).
The LEM magnet control panel must have its two interlock and power
cables correctly connected before it can be tested. As was the
case with the xenon laser wiring, individual letters are used to
identify and match these cables to their respective receptacles.
Since the connectors are different, there is no chance of making
an incorrect connection.
The large magnet power supply must be hard-wired to either
a 240 V or 480 V 3 0 145 A (per phase) power source. It is strongly
recommended that a semi-permanent site be selected for the magnet
power supply before its delivery. As mentioned above, it is important
8-6
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that the magnet power supply be located in a dry area not subject
to adverse weather conditions. •
The magnet cooling water pump also requires a 480 V 3 0
source, but this unit can be soft-wired; i. e., it comes equipped
with a standard 480 V 3 0 connector at the end of a heavy cable.
8. 3. 3 Wiring of Electronic Data Processing System and LEM
Equipment Rack
All of the LEM rack-mounted electronic devices derive their
input power from the internal rack line cord (a circuit protected
by a circuit breaker). The LEM dc amplifier obtains its dc
supply voltages from the Keithley model 840 via a cable in
the back of the equipment rack--the cable is marked with an
identifying label.
The BNC and signal connections between and among the
LEM prism analyzer, photodelector, and the Electronic Data-
Processing System should carefully follow the schematic depicted
above in Figure 4-1.
8-7
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REFERENCES
1. A. Yariv, Quantum Electronics, John Wiley & Sons, N. Y. 1967.
2. C. K. N. Patel, "Gas Lasers" in Lasers , Vol. II, A. K. Levine
editor, Marcel Dekker, Inc., N. Y. 1968.
3. B. A. Lengyel, Introduction to Laser Physics, John Wiley & Sons,
N. Y., 1966.
4. A. Maitland and M. H. Dunn, Laser Physics , North Holland
Publishing Co. , Amsterdam, 1969.
5. H. A. Nielsen, Physical Review, 46_, 117(1934).
6. G. J. Linford, "High Gain Neutral Laser Lines in Pulsed Noble
Gas Discharges, " IEEE Journal of Quantum Electronics, QE-8 ,
,(1972).
7. S. C. Wang, et al. , "Observation of an Enhanced Lamb Dip in a
3.51 MicronlTenon Laser, " Applied Physics Letters, 17 , 120(1970).
8. R. Vetter, "Mesure des escarts isotopiques de 13 rais laser »
infrarouges dans le xenon, " Compt. Rend. Acad. Sci. , Series B,
265, 1414 (1967).
9. E. R. Peressini and G. J. Linford, "Effect of Cross-relaxation
on the Spectral Flux and Population Inversion Distributions in a
CW Laser Oscillator, " IEEE Journal of Quantum Electronics,
QE-4 , 657 ('1968).
10. G. J. Linford, unpublished xenon laser study, Hughes Aircraft
Company, 1969.
11. W. R. Bennett Jr., "Gaseous Optical Masers, " Applied Optics,
Suppl. #1,_24_, (1962).
12. A. Yariv, Quantum Electronics, John Wiley & Sons, Inc. , N. Y.
1967, pp. 406-418.
13. G. J. Linford, "New Oscillation Dynamics of Noble Gas Lasers,"
University of Utah, Salt Lake City, Utah, Ph.D. Thesis, 1971.
14. G. Racah, Physical Review, _6l_, 537(1942).
15. W. L. Faust & R. A. McFarlane, Journal of Applied Physics, 35,
2020, (1964).
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