Abstract

A method for the remote detection and identification of liquid chemicals at ranges of tens of meters is presented. The technique uses pulsed indirect photoacoustic spectroscopy in the 10-µm wavelength region. Enhanced sensitivity is brought about by three main system developments: (1) increased laser-pulse energy (150 µJ/pulse), leading to increased strength of the generated photoacoustic signal; (2) increased microphone sensitivity and improved directionality by the use of a 60-cm-diameter parabolic dish; and (3) signal processing that allows improved discrimination of the signal from noise levels through prior knowledge of the pulse shape and pulse-repetition frequency. The practical aspects of applying the technique in a field environment are briefly examined, and possible applications of this technique are discussed.

© 2003 Optical Society of America

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References

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  1. A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980).
  2. A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
    [CrossRef]
  3. M. Harris, G. N. Pearson, D. V. Willetts, K. Ridley, P. R. Tapster, B. Perrett, “Pulsed indirect photoacoustic spectroscopy: application to remote detection of condensed phases,” Appl. Opt. 39, 1032–1041 (2000).
    [CrossRef]
  4. G. N. Pearson, M. Harris, D. V. Willetts, P. R. Tapster, P. J. Roberts, “Differential laser absorption and thermal emission for remote identification of opaque surface coatings,” Appl. Opt. 36, 2713–2720 (1997).
    [CrossRef] [PubMed]
  5. D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
    [CrossRef]
  6. “DPA Microphones Catalogue” at www.dpamicrophones.com .
  7. A. Wood, Acoustics (Blackie, London, 1960).

2000 (1)

1997 (1)

1986 (1)

A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
[CrossRef]

1982 (1)

D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
[CrossRef]

Brassington, D. J.

D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
[CrossRef]

Harris, M.

Pearson, G. N.

Perrett, B.

Ridley, K.

Roberts, P. J.

Rosencwaig, A.

A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980).

Tam, A. C.

A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
[CrossRef]

Tapster, P. R.

Willetts, D. V.

Wood, A.

A. Wood, Acoustics (Blackie, London, 1960).

Appl. Opt. (2)

J. Phys. D (1)

D. J. Brassington, “Photo-acoustic detection and ranging—a new technique for the remote detection of gases,” J. Phys. D 15, 219–228 (1982).
[CrossRef]

Rev. Mod. Phys. (1)

A. C. Tam, “Applications of photoacoustic sensing techniques,” Rev. Mod. Phys. 58, 381–431 (1986).
[CrossRef]

Other (3)

A. Rosencwaig, Photoacoustics and Photoacoustic Spectroscopy (Wiley, New York, 1980).

“DPA Microphones Catalogue” at www.dpamicrophones.com .

A. Wood, Acoustics (Blackie, London, 1960).

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Figures (9)

Fig. 1
Fig. 1

Measured photoacoustic pulse profiles. The two pulses shown here are separated by 1.04 ms, corresponding to a PRF of 961 Hz, and are the result of averaging 256 measurements. The PIPAS pulse is detected 6.6 µs after the laser pulse, corresponding to the acoustic time of flight over the 2.2-cm range. We define the PIPAS signal as the height of the initial acoustic peak, shown here for the second on-resonance pulse. Note that there is still an observable signal for the off-resonance illumination. PA, photoacoustic.

Fig. 2
Fig. 2

Schematic of the arrangement for the remote PIPAS experiment. IR pulses from the CO2 laser are used to heat the skin of the liquid; this causes heating of the air immediately adjacent, producing acoustic pulses. These pressure waves radiate from the surface of the liquid, and the microphone detects them remotely.

Fig. 3
Fig. 3

Modeled PIPAS pulse and its power spectrum for a laser-pulse length of τl = 3 µs and a microphone response time of τm = 20 µs.

Fig. 4
Fig. 4

Transmitted PIPAS pulse and power spectrum, modeled assuming a faster response time for the microphone. (τl = 3 µs, τm → 0).

Fig. 5
Fig. 5

Detected power spectra for a parabolic reflector with finite-sized (left plot) and point (right plot) microphones (τm = 20 µs). This example is for a 1/2-in.-diameter microphone (1 in. = 2.54 cm). The dashed curve shows the gain from the parabola, and the solid curve describes the normalized signal after gain as a function of frequency.

Fig. 6
Fig. 6

Graph on the left shows the effects of atmospheric attenuation on the high-frequency components of the PIPAS signal (τm = 20 µs). Shown on the right is the variation in atmospheric attenuation with frequency. This shows the attenuation measured in decibels/10 m for an acoustic signal at a given frequency for a pressure of 1 atm, temperature of 20 °C, and relative humidity of 70% [refer to American National Standards Institute standard S1.26].

Fig. 7
Fig. 7

Acoustic system response spectra. “Microphone” represents the noise detected in an anechoic chamber and should be representative of the microphone’s electrical noise floor. “Calibration” shows the signal obtained by using a known calibration device; smaller spikes are caused by ringing at harmonics. “Ambient” is a graph of the ambient noise in a typical outside environment.

Fig. 8
Fig. 8

This shows how the technique of averaging the PIPAS signal, then differentiating, and finally exploiting correlation at the laser PRF can significantly increase the SNR. In this case, the time series is already averaged before the signal is processed in software. PA, photoacoustic.

Fig. 9
Fig. 9

Further example of signal processing. In this case the SNR has been examined for each data series, and the large improvements in the SNR can easily be seen. The top left graph (raw data from ethanol) shows an unaltered time series from the microphone containing two photoacoustic pulses obtained by use of a pulsed CO2 laser at 200 Hz with approximately 150 µJ/pulse. This signal was obtained on resonance (run on a highly absorbing line). The top right graph shows improvement in the SNR obtained simply by differentiating the raw time series. The bottom graph has had the correlation process described in Subsection 3.D applied to the differentiated time series.

Tables (1)

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Table 1 Parameters Used to Calculate the Acoustic Pressure Generated by the PIPAS Effect

Equations (4)

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d=2.44fλ/D,
S2t=VtVt-T0.5,
Snt=VtVt-TVt-2TVt-nT1/n,
Att=Eρ0σ3/2ακaκl4π2rT0ClCaκaCa+κlCl fσ, t.

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