Optimal optoacoustic detector design

Recent Progress in Modulated Photothermal Radiometry

In this review, the emerging work using a technique known as modulated photothermal radiometry (MPTR) is evaluated. As MPTR has matured, the previous discussions on theory and modeling have become increasingly limited in their applicability to the current state of the art. After a brief history of the technique, the currently used thermodynamic theory is explained, highlighting the commonly applied simplifications. The validity of the simplifications is explored via modeling. Various experimental designs are compared, and the differences are explored. New applications, as well as emerging analysis techniques, are presented to emphasize the trajectory of MPTR.

Development of photoacoustic sensing platforms at the Army Research Laboratory

Traditionally, chemical sensing platforms have been hampered by the opposing concerns of increasing sensor capability while maintaining a minimal package size. Current sensors, although reasonably sized, are geared to more classical chemical threats, and the ability to expand their capabilities to a broader range of emerging threats is uncertain. Recently, photoacoustic spectroscopy, employed in a sensor format, has shown enormous potential to address these ever-changing threats. Photoacoustic spectroscopy is one of the more flexible infrared spectroscopy variants, and that flexibility allows for the construction of sensors that are designed for specific tasks. The Army Research Laboratory has, for the past 14 years, engaged in research into the development of photoacoustic sensing platforms with the goal of sensor miniaturization and the detection of a variety of chemical targets both proximally and at range. This paper reviews this work.

Photoacoustic cell for ultrasound contrast agent characterization

Photoacoustics has emerged as a tool for the study of liquid gel suspension behavior and has been recently employed in a number of new biomedical applications. In this paper, a photoacoustic sensor is presented which was designed and realized for analyzing photothermal signals from solutions filled with microbubbles, commonly used as ultrasound contrast agents in echographic imaging techniques. It is a closed cell device, where photothermal volume variation of an aqueous solution produces the periodic deflection of a thin membrane closing the cell at the end of a short pipe. The cell then acts as a Helmholtz resonator, where the displacement of the membrane is measured through a laser probe interferometer, whereas photoacoustic signal is generated by a laser chopped light beam impinging onto the solution through a glass window. Particularly, the microbubble shell has been modeled through an effective surface tension parameter, which has been then evaluated from experimental data through the shift of the resonance frequencies of the photoacoustic sensor. This shift of the resonance frequencies of the photoacoustic sensor caused by microbubble solutions is high enough for making such a cell a reliable tool for testing ultrasound contrast agent, particularly for bubble shell characterization.

Performance of a photothermal detector with turbid liquids

A closed-cell photothermal detector for aqueous analytes has been evaluated at 254 and 678 nm. We used a detector with a water meniscus as a pressure sensor, whose periodic deflection was measured using a low-finesse optical fiber Fabry-Perot interferometer. Performance was compared with a commercial diode array spectrometer and found to be similar for absorption measurements in nonturbid samples, but the results were affected up to 60 times less by scattered light. Finally the photothermal cell was converted into an integrating cavity using ceramic inserts, showing freedom from scattering-related errors at 678 nm but a degradation in performance at 254 nm.

Standoff detection of high explosive materials at 50 meters in ambient light conditions using a small Raman instrument

We have designed and demonstrated a standoff Raman system for detecting high explosive materials at distances up to 50 meters in ambient light conditions. In the system, light is collected using an 8-in. Schmidt-Cassegrain telescope fiber-coupled to an f/1.8 spectrograph with a gated intensified charge-coupled device (ICCD) detector. A frequency-doubled Nd : YAG (532 nm) pulsed (10 Hz) laser is used as the excitation source for measuring remote spectra of samples containing up to 8% explosive materials. The explosives RDX, TNT, and PETN as well as nitrate- and chlorate-containing materials were used to evaluate the performance of the system with samples placed at distances of 27 and 50 meters. Laser power studies were performed to determine the effects of laser heating and photodegradation on the samples. Raman signal levels were found to increase linearly with increasing laser energy up to approximately 3 x 10(6) W/cm2 for all samples except TNT, which showed some evidence of photo- or thermal degradation at higher laser power densities. Detector gate width studies showed that Raman spectra could be acquired in high levels of ambient light using a 10 microsecond gate width.

Quantitative signal analysis in pulsed resonant photoacoustics

The pulsed excitation of acoustic resonances was studied by means of a high-Q photoacoustic resonator with different types of microphone. The signal strength of the first radial mode was calculated by the basic theory as well as by a modeling program, which takes into account the acoustic impedances of the resonator, the acoustic filter system, and the influence of the microphone coupling on the photoacoustic cavity. When the calculated signal strength is used, the high-Q system can be calibrated for trace-gas analysis without a certified gas mixture. The theoretical results were compared with measurements and show good agreement for different microphone configurations. From the measured pressure signal (in pascals per joule), the absorption coefficient of ethylene was calculated; it agreed within 10% with literature values. In addition, a Helmholtz configuration with a highly sensitive 1-in. (2.54-cm) microphone was realized. Although the Q factor was reduced, the sensitivity could be increased by the Helmholtz resonator in the case of pulsed experiments. A maximum sensitivity of the coupled system of 341 mV/Pa was achieved.

Analysis of isotopic CO(2) mixtures by laser photoacoustic spectroscopy

Photoacoustic spectroscopic studies on a mixture of six CO(2) isotopes ((12) C(16) O(2), (12) C(18) O(2), (13) C (16) O(2), (13) C(18) O(2), (16) O(12) C(18) O, and (16) O(13) C(18) O) in the wavelength range of 9-11 mum by use of a home-built high-pressure continuously tunable CO(2) laser with a bandwidth of 0.017 cm(-1) are discussed. The concentrations of all CO(2) isotopes present in the mixture could be determined with good accuracy. Furthermore, the previously unknown absorption cross sections of some important lines of the(12) C(18) O(2), (13) C(18) O(2), and (16)O (13) C(18) O isotopes in the 9-11-mum range are reported.

Photoacoustic spectroscopy on trace gases with continuously tunable CO(2) laser

A novel photoacoustic (PA) system that uses a continuously tunable high-pressure CO(2) laser as radiation source is presented. A minimum detectable absorption coefficient of 10(-6) cm(-1) that is limited mainly by the desorption of absorbing species from the cell walls and by residual electromagnetic perturbation of the microphone electronics has currently been achieved. Although a linear dependence of the PA signal on the gas concentration has been observed over 4 orders of magnitude, the dependence on energy exhibits a nonlinear behavior owing to saturation effects in excellent agreement with a theoretical model. The calibration of the laser wavelength is performed by PA measurements on low-pressure CO(2) gas, resulting in an absolute accuracy of ± 10(-2) cm(-1). PA spectra are presented for carbon dioxide (CO(2)), ammonia (NH(3)), ozone (O(3)), ethylene (C(2)H(4)), methanol (CH(3)OH), ethanol (C(2)H(5)OH), and toluene (C(7)H(8)) in large parts of the laser emission range. The expected improvement in detection selectivity compared with that of studies with line-tunable CO(2) lasers is demonstrated with the aid of multicomponent trace-gas mixtures prepared with a gas-mixing unit. Good agreement is obtained between the known concentrations and the concentrations calculated on the basis of a fit with calibration spectra. Finally, the perspectives of the system concerning air analyses are discussed.

Photoacoustic technique for the measurement of absorption line profiles

A spectrophone utilizing a resonant cylindrical cavity and operated by driving the first azimuthal mode of the cavity has been developed for the study of weak absorption lines of gases at pressures from 100 to 1300 Torr. Presented are the acoustic resonant amplification factor as a function of pressure, and a description of the noise sources inherent in this spectrophone. An example is given of the optical frequency resolution resulting when this spectrophone is used in conjunction with a tunable ring dye laser as a high resolution spectrometer.

Real-time in situ measurements of atmospheric optical absorption in the visible via photoacoustic spectroscopy. 1: Evaluation of photoacoustic cells

Four configurations of resonant photoacoustic cells were evaluated for maximum signal sensitivity to light absorption by the sample, with minimal noise and background. Theoretical and experimental data are discussed. Azimuthal and radial resonant modes were compared for one cell. Of the four, the best cell was a brass cylinder, 2.5-cm radius and 9.5-cm length, which was operated in the azimuthal mode. An argon-ion laser (lambda = 514.5 nm) was the light source. A continuous sample flow through the cell, required for real-time in situ atmospheric measurements, gave an acceptable noise level and time for signal response at ~500 cc/min. Linearity of the photoacoustic signal was checked in the range applicable to atmospheric absorption. At a signal-to-noise ratio (SNR) equal to 1, a light absorption detection limit of 4.7 x 10(-6) m(-1) could be achieved.

Measurements of DF laser absorption by methane using an intracavity spectrophone

Absorption coefficient measurements for trace amounts of methane are reported, in a 1-atm argon balance, at seventeen deuterium-fluoride (DF) laser line frequencies. An optoacoustic detector (spectrophone) operating within the DF laser optical cavity yielded a minimum detectable absorption of 0.007 km(-1) (7 x 10(-8) cm(-1)) for a 10:1 SNR. Results are compared with previous measurements and with calculated absorption coefficients obtained using the AFGL atmospheric absorption parameter compilations. A 2.5-fold increase in agreement results when the 1980 AFGL methane parameters are used instead of the 1978 parameters.

Helmholtz resonator enhancement of photoacoustic signals

A Helmholtz resonator attached to a nonresonant photoacoustic cell enhances the responsivity to trace gas absorption by as much as a factor of 15.3. A simple system model based on a lumped parameter approach predicts the experimentally determined resonance frequency f(0) within 6% for all five resonator volumes tried and gives a pressure amplitude response at resonance proportional to f(0)(-5/2), which is the approximate experimental dependence. Optimization of response based on the model shows a pressure amplitude dependence on a(1/2), the square root of the radius of the cylindrical tube connecting the cell and the resonator.

Characteristics of a photoacoustic air pollution detector at CO(2) laser frequencies

The characteristics of a photoacoustic detection system for measurement of ambient trace atmospheric pollutants at CO(2) laser frequencies are analyzed and described. Several photoacoustic variables are optimized in this study. An alternate-traverse acoustic cell to eliminate cell window signals is described. An ac biased 16-cm(2) microphone is used. The observed modulation frequency dependence of the acoustic signal and a theoretical analysis are also presented. Trace gas interferents limit the practical sensitivity of the system, however, even with interferents present, a minimum measurable pressure change of 2.43 x 10(-9) atm, and apparent absorptivity of 8.42 x 10(-9)/cm was obtained. Techniques to minimize effects of interferents are reported along with the methodology of the photoacoustic technique in trace gas measurements.

Optoacoustic detection using Stark modulation

Stark modulation of the absorbed laser radiation in an optoacoustic detector (or spectrophone) is reported. Measurements were made over a range of total pressure between 760 Torr and 50 Torr. Greatly enhanced molecular discrimination is suggested due to the tuning ability of the Stark-shifted absorption. The background signal obtained by operating in this mode is more than 500 times smaller than that obtained by operating the same optoacoustic detector in the conventional chopped radiation mode. The responsivity of the optoacoustic detector and the absorption coefficient of C(2)H(4) are presented as a function of total pressure.

Calorimetric measurements of weakly absorbing materials: theory

The time-dependent heat equation is solved in a cylindrical geometry of finite length with heat loss by radiation and conduction. Exact expressions are derived for the time constants of radial and longitudinal modes. The steady-state solution is obtained in longitudinal modes and used as the initial state for the decay. A simple expression is presented for the ratio of the amplitude of the first-order longitudinal order mode A(1) and the corresponding time constant tau(1)fit with the separate expressions for A(1) and tau(1). These parameters are experimentally readily accessible and directly yield the absorption coefficient alpha of rod-shaped compound glass from which optical fibers are to be drawn.

Water vapor-nitrogen absorption at CO(2) laser frequencies

In this paper we report the results of a series of pressure-broadened water vapor absorption measurements at 27 CO(2) laser frequencies between 935 cm(-1) and 1082 cm(-1). Both multiple traversal cell and optoacoustic (spectrophone) techniques were utilized together with an electronically stabilized cw CO(2) laser. Comparison of the results obtained by these two methods shows remarkable agreement, indicating a precision which has not been previously achieved in pressure-broadened studies of water vapor. The data of 10.59 microm substantiate the existence of the large (>200) self-broadening coefficients determined in an earlier study by McCoy. In this work we have treated the case of water vapor in N(2) at a total pressure of 1 atm. We have also studied water vapor in air and will report those results separately.

Trombay infrared pneumatic detector: theory of operational characteristics

This paper explains in a simple manner the mechanism of formation of pressure pulse inside a pneumatic detector due to incident ir radiation. The detector had been designed earlier at Trombay. Experimentally observed characteristics of the detector have been explained on the basis of the theory derived.

Laser Detection of Pollution

Spectroscopic analysis is a useful technique for identifying and quantitatively determining the presence of specific gaseous constituents. Development of high-power tunable lasers has made the spectroscopic technique for detection of trace constituents in the atmosphere very attractive for practical applications. In this article three of the currently used modes for laser detection of pollution are reviewed: (i) long-path measurements, (ii) laser Raman (differential absorption) measurements, and (iii) optoacoustic detection. Progress in the field has been extremely rapid in the last few years and very useful and reliable data on air pollution can now be obtained routinely with the techniques described.

Simple setup for single and differential photoacoustic spectroscopy

Simple cells are described for normal and differential measurements in photoacoustic spectroscopy. The differential cell allows for easy background signal correction and for comparison of related samples. The arrangement allows great flexibility in cell design for adaptation to special sample forms. The normal cell can be used for very small volumes, liquids as well as solids, and is constructed in such a way as to allow the possibility of Helmholtz resonance to occur over a range of frequencies. The two cells are compared in terms of background and maximal signal strength and examples of spectra obtained with each of them are given. The general spectrometer setup is outlined as well.

Laser optoacoustic absorption spectra for various explosive vapors

Optoacoustic absorption spectra, using a CO(2) laser source, is reported for ethylene glycol dinitrate, 2, 4 dinitrotoluene, and nitrogylcerine vapors in a nitrogen background. Strong absorption was measured at all seventy-seven laser transitions in the 9.6-microm and 10.6-microm regions. Absorption coefficient data, necessary to evaluate performance as an explosive detector, were obtained for ethylene glycol dinitrate. The strongest feature showed a value of 7.42 atm(-1) cm(-1) giving a minimum detection sensitivity of 8.26 ppb W.

Optimization of resonant cell design for optoacoustic gas spectroscopy (H-type)

The design and certain basic characteristics of a new type of optoacoustic device (cell) are described. Initial experimental data using a CO(2) laser operating in the 9-10-microm wavelength region demonstrated sufficient sensitivity to measure the absorption due to SF(6) in the air at a concentration of one part in 10(11). Acoustic Q's were demonstrated in excess of 1800. The general expression for the optoacoustic pressure variations inside the acoustical resonant cavity is given.

Spectrophone measurements of infrared laser energy absorption by atmospheric dust

A new method of quantitatively measuring the absorption of atmospheric dust or other particulate matter is described. The system uses a differential-spectrophone KBr pellet technique and should ultimately prove useful from the uv to the far ir wavelength region. Preliminary measurement data on atmospheric dust, quartz, and soot are presented at 1.06-microm, 9.6-microm, and 10.6-microm wavelengths.

Water vapor absorption of carbon dioxide laser radiation

An optoacoustic detector or spectrophone has been used to perform detailed measurements of the absorptivity of mixtures of water vapor in air. A C(12) O(2)(16) laser was used as the source, and measurements were made. at forty-nine different wavelengths from 9.2 microm to 10.7 microm. The details of the optoacoustic detector and its calibration are presented, along with a discussion of its performance characteristics. The results of the measurements of water vapor absorption show that the continuum absorption in the wavelength range covered is 5-10% lower than previous measurements.

The Calorimetric Detection of Excited States

Calorimetric techniques offer the photophysicist and photochemist the opportunity to measure a number of parameters of excited states which may be difficult to obtain by other techniques. The calorimetric strategy seeks to measure the heating of a sample resulting from radiationless decays or chemical reactions of excited states. Heating is best measured through volume and pressure transducers, and four calorimeters based on these are described. With calorimetric instrumentation one can perform measurements on samples in the gas, liquid and solid phases over a wide temperature range. Moreover time dependent processes with time constants ranging from microseconds to seconds are amenable to study. Examples of the application of calorimetric techniques to the determination of quantum yields of fluorescence, triplet formation and photochemistry are given.