United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-070 Nov. 1983
Project Summary
An Investigation of Resonant
Optoacoustic Cells
Robert R. Plyler and Richard R. Patty
A theory describing the optoacoustic
signal is presented; dependence on
both cell and gas parameters are given
and the advantage of operating at a
resonant frequency is discussed.
Three elliptical cells with major axes
5.8, 12.7, and 15.2 cm (corresponding
minor axes 5.5,11.0, and 7.6 cm) were
utilized as resonant optoacoustic cells.
Longitudinal standing waves and stand-
ing waves analagous to radial resonances
for a cylindrical cell were driven by
passing the beam from a CO2 laser along
one focus of the ellipse. A Knowles
electret microphone (model 1754)
located at the other focus of the ellipse de-
tected the pressure variations associated
with the absorption of laser radiation by
the gas.
Plots of optoacoustic signal vs fre-
quency are presented for frequencies
up to approximately 5500 Hz; several
resonances are observed. In order to de-
termine a minimum detectable ab-
sorption coefficient, the P(14) line of
the 10.6 fjm CO2 band was used to detect
absorption by dilute ethylene samples.
Plots of optoacoustic signal vs concentra-
tion are presented for each cell; a mini-
mum detectable absorption coefficient
of about 3 x 10"8 cm ' was obtained. Re-
sults are compared with measurements
on a Helmholtz cell for which the mini-
mum detectable absorption coefficient
was about 2 x 10~7 cm'1. Windowless
operation was attempted, and the results
indicate that further improvement is
possible.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory. Research Triangle
Park. NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Over the past several years, there has
been an interest in detecting very low
concentrations of various pollutants. One
method which holds much promise inthis
area involves the use of laser illuminated
optoacoustic cells. These cells can be very
sensitive, and extremely selective.
A major advance in optoacoustic
detection occurred with the development
of lasers. Previously the only available
sources of light had been thermal sources
for which the output cannot exceed that
of an ideal blackbody at the temperature
of the source. Lasers made available very
intense sources of monochromatic light
which were not constrained by the limita-
tions of thermal sources. Lasers were
first used in optoacoustics in 1968 and
extinction coefficients as low as 1.2 x
10"7cm"1 were measured.
The use of acoustically resonant
chambers was reported in 1973. By
utilizing standing wave amplification in
the sample cell, the signal is greatly
increased. The increase in signal is
directly proportional to the acoustic
quality or Q value of the cell and Q's of up
to 1800 have been reported. For simple
cylindrical cells a Q value of 900 has been
reported. The smallest extinction coeffi-
cient that has been detected is 9 x 10~9
crrf1 using a longitudinally resonant open
cell. Elliptical cells could be more
attractive since a lower chopping frequency
can be used. [Subsequent to the comple-
tion of this work Patel and Tam published
an article suggesting the use of an
elliptical geometry: Appl. Phys. Lett., 36,7
(1980)]
Results
A carbon dioxide laser operating on the
P(14) laser line of the 10.6 jum band was
used as the source, and ethylene was
used as the sample gas in the elliptical
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cell since this gas has a strong absorption
at the wavelength of the P(14) line. The
laser beam traversed the cell along one
focus of the ellipse, and the photoacoustic
signal was produced by a small electret
microphone (Knowles model 1754)
placed at the other focus. The beam was
modulated with a mechanical chopper,
and the frequency was varied to match
the various resonant frequencies of the
cell. A typical plot of photoacoustic signal
vs chopping frequency for a cell with a
15.2 cm major axis and a 7.6 minor axis is
shown in Figure 1. Most resonances can
be identified and are compared with
calculated values; for example, the
resonance with the lowest frequency
results from a standing wave having a
wavelength equal twice the major axis.
The acoustic quality Q, determined by
dividing the resonant frequency by the
width at half maximum, is about 240.
Figure 2 shows a plot of signal vs
concentration for the above cell operating
at the lowest resonant frequency with a
laser power of 0.24 watts. A small
airborne signal emanating from the
chopper was detected by the microphone
in the cell; this signal was very stable and
could be subtracted from the total signal.
However, a background signal arising
from the windows of the cell was
observed; this signal corresponds to a
level of about 35 parts per billion of
ethylene and tends to cause the signal to
be insensitive to concentration changes
for low concentrations. By comparing
signals from cells with and without
ethylene, it was determined that approxi-
mately one part per billion could be
detected. In an effort to avoid the signal
arising from the windows, the windows
were removed and the cell was placed in
a chamber designed to isolate it from
room noise; however, since the meter
used to monitor laser power was contained
in the chamber and was the source of a
small signal, only a small improvement in
sensitivity was observed. The sensitivity
of the elliptical cell was compared with
that of a Helmholtz resonator and found
to be greater by a factor of about eight.
Improvements in both cells appear
feasible through changes in cell design
and acoustic isolation.
Conclusions
The results of this study show that the
elliptical cell has promise as an optoa-
coustic detector for use in detecting low
concentration pollutant gases. Using a
windowless cell, it has been possible to
detect slightly less than one part per
billion of ethylene; this corresponds to an
20
6 70
.งป
co
A
Frequency (kHz)
Figure 1. Plot of signal vs frequency for an elliptical cell using C2Ht in N2.
70-"
/0-3
70-2
70-'
Concentration (ppm)
Figure 2. Plot of signal vs concentration of C2Ht in N2 at lowest resonant frequency.
extinction coefficient of 2.8 x 10 8 cm
Although this is somewhat higher than
the minimum reported in the literature,
improvements can be anticipated by
limiting the laser noise (short term power
fluctuations) and by removing the laser
power meter (a source of background
signal) from the chamber that encloses
the elliptical cell.
Comparison of the elliptical cell with a
Helmholtz resonator indicated that the
elliptical cell was a factor of eight more
sensitive; improvements in both cells are
possible. Although some turbulence may
be set up, the combination of a low flow
rate and phase sensitive detection would
minimize the noise associated with
turbulence; thus, a system involving the
flow of gas through a chamber containing
a windowless cell appears feasible.
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Recommendations
Limited further study of resonant
optoacoustic cells should be undertaken
to include the following:
1. Additional measurements using
resonant windowless cells which
are better isolated from ambient
noise and power measuring devices.
2. Measurements using resonant cells
for which the laser beam enters and
leaves at a pressure node.
3. Measurements using an annular
resonant cell. An annular cell would
support a standing wave which
would be analagous to a longitudinal
standing wave, but the driving
radiation would be entirely located
at an antinode. There is an additional
advantage of low cell volume, and
windowless operation is attractive.
4. Additional studies of Helmholtz
cells. Various optical arrangements
involving two cells could reduce
window signals.
Robert R. Plyler and Richard R. Patty are with North Carolina State University,
Raleigh, NC 27650.
William A. McClenny is the EPA Project Officer (see below).
The complete report, entitled "An Investigation of Resonant Optoacoustic Cells,"
(Order No. PB 83-251 637; Cost: $8.50, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
AUS GOVERNMENT PRINTING OFFICE 1983-659-017/7231
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