Standoff chemical detection with a distance of more than 41 feet using photoacoustic effect and quantum cascade laser (QCL) operated at relatively low power, less than 40 mW, is demonstrated for the first time. The option of using QCL provides the advantages of easy tuning and modulation besides the benefit of compact size, light weight and low power consumption. The standoff detection signal can be calibrated as a function of different parameters such as laser pulse energy, gas vapor concentration and detection distance. The results yield good agreements with theoretical model. Techniques to obtain even longer detection distance and achieve outdoor operations are in the process of implementation and their projection is discussed.
© 2011 OSA
Photoacoustic (PA) effect was discovered a century ago and was first reported in 1880 . It has been known as a sensitive technique for spectroscopic studies to evaluate solids, liquids, and gases. PA spectroscopy has been applied in a wide range of applications such as trace gas detection, environmental sensing and medical diagnostics [2,3].
In recent years, with the help of the development of quantum cascade lasers (QCLs) , great progress has been made for PA chemical detection, and sensitivity has been improved significantly [5–7]. However, most of the demonstrated PA detections are localized and require the chemical samples to be contained in a PA cell [8,9]. Such kinds of measurements are not viable for explosives or toxic gases detections, where a safety standoff distance is usually required for personal protection.
One earlier study done by Brassington, who called it PADAR (photoacoustic detection and ranging), demonstrated remote gas detection and ranging using PA effect . Later Yonak presented a leak detection and localization system based on PA effect which could perform standoff gas detection . PA spectroscopy was also applied to condensed matter and liquids for remote detection and sensing by Harris and Perrett [12,13]. All these studies involved using large size, heavy-weight, mid-infrared laser sources, i.e. high power OPO or CO2 lasers etc. We report in this paper for the first time QCL based standoff photoacoustic chemical detection using a sound reflector with detection distance over 40 feet. It is worth to mention that one earlier study by C. W. Van Neste and associates demonstrated standoff detection involving a variation of PA effect . In their study, they used a quartz tuning fork as a photo-detector to detect reflected light from a surface covered with target chemicals. Fundamentally, their technique relies on good surface reflection to provide sufficient photons for photo-thermal generation and quartz tuning fork excitation. Our method, on the other hand, detects generated photoacoustic signals in the air and can be utilized for standoff chemical detection in a more general environment. We believe that, with higher power QCLs, better sensitivity microphones such as laser microphones , and multi-element acoustic beam-forming techniques , we can further increase the standoff detection distance and operate the system outdoors without worrying about ambient noises.
Our experimental system includes a 7.9 µm wavelength mid-infrared (MIR) QCL running at pulsed mode, targeted gas sample of isopropanol (IPA) vapor as a safer substitute of the explosive cyclonite (RDX), and an electrets microphone with a parabolic reflector as the acoustic detector. Our experimental study investigates the generated PA signal strength as a function of different parameters such as laser pulse energy, detection distance and target gas sample concentration. These measured results yield good agreements with the theoretical model. Possible extension of detection distance and potential applications are discussed.
2. Theoretical consideration
When a gas molecule absorbs a photon from MIR radiation, it undergoes transitions from ground state to a higher state. The molecule can then release the absorbed photon energy by colliding with other molecules and convert it to translational energy or kinetic energy, thus increase the corresponding temperature and pressure of the sample gas. If the incident MIR radiation is amplitude modulated, the modulated optical signal can then produce a periodic temperature and pressure change in the gas molecules. This process is known as PA effect. It has been studied theoretically and experimentally for cases in closed PA cells in gas, liquid and solid matters [17–19]. Here we present a simplified model for the PA detection in open air environment.
We model the PA generation from a sample gas under pulsed laser radiation. Because light travels much faster than sound, the duration of the laser pulse and the pulse propagation time can be ignored. We assume the following conditions: 1. a laser pulse starts with initial pulse energy E0; 2. the laser radiation is monochromatic; 3. the laser beam is uniformly distributed within the beam area of size S; 4. the target gas sample concentration is N molecules per unit volume; 5. the target gas absorption cross-section is σ; 6. the target gas is uniformly distributed within a tested gas cloud area; and 7. the length of the cloud on the laser beam path is Δz. According to Beer-Lambert law, the transmitted energy of the laser pulse is:
Here we ignore the laser beam divergence, diffraction and reflection. Thus the absorbed energy by the gas cloud is
As for the case of a low concentration and small size gas cloud, N and Δz are very small. Therefore, Eq. (2) can be approximated as
According to the conservation of energy, in the absence of work done, the change of internal energy per unit volume in a matter is proportional to the change of temperature . So the temperature increase of the gas cloud corresponding to the absorbed energy is21]. Therefore, if measured at a remote distance, the standoff PA signal is inversely proportional to the detection distance.
3. Experimental setup
The equipment used in this experiment differs from conventional PA spectroscopy setup due to the absence of a PA cell. It is important to note that PA signal in the open air environment is much weaker than that in a closed PA cell. Another major consideration is the transmission of PA signal from the sample to the sound wave detector (the microphone). In order to collect sound signals efficiently from a remote distance, a parabolic sound wave reflector is used in the system. The experimental setup is shown in Fig. 1 .
The system includes a QCL with emission wavelength at 7.9 µm, IPA vapor as gas sample, an electrets microphone and a sound reflector. A T-shaped piping is placed at the opening of the IPA bottle to provide a constant flow of gas vapor. Figure 2 shows the measured QCL lasing spectrum and IPA gas vapor transmission spectrum. The IPA gas vapor has a strong and broad absorption peak near the QCL lasing wavelength. The laser wavelength shift caused by modulation chirping is insignificant compared with the wavelength dependence of the spectrum. The QCL is operated under pulsed mode condition with a repetition rate of 1.5 kHz and pulse durations around 300 µs. Relatively long pulse durations are used in our experiment in order to help producing high enough laser pulse energy for photoacoustic signal generation. The maximum average output power of the QCL is 40 mW. A lock-in amplifier is used to obtain better signal to noise ratio and longer operational distance.
Our initial setup is presented in Fig. 3(a) . The microphone is placed close to the gas sample (around 1 inch), and PA signal is detected directly by the microphone without amplifiers or reflectors. Figure 3(b) shows the measurement of PA signal. As can be seen, the signal amplitude is about 1 mV. According to the microphone’s specification, its sensitivity is 7.9 mV/Pa, so the generated acoustic wave pressure is (1 mV)/(7.9 mV/Pa) = 0.13 Pa. This relatively strong acoustic signal should be able to be detected up to tens of feet detection range. Due to the ambient noise, however, the PA signal is beyond detection when microphone is placed at distance over 5 inches.
In order to increase the detection distance, a parabolic sound reflector with diameter of 1.5 feet is added to the system for better sound collection. A lock-in amplifier is used to synchronize the microphone with the QCL pulse generator. The final actual system setup with standoff detection distance up to 41 feet is shown in Fig. 4 .
4. Experimental results and comparison to theory
The PA chemical detection phenomenon is investigated as a function of different parameters, such as laser power, detection distance, temperature, and gas vapor concentration. The QCL in the experiment is operated under pulsed mode operation with pulse durations around 300 µs and the repetition rate is 1.5 kHz. In our experiments, before taking any measurements the background noise level was first measured by removing the target gas sample from the laser focusing point and keep the rest setup the same as we do PA measurements. The measured noise in our system is mainly due to background acoustic noises generated from the working environment and various running equipments in the lab. A different testing environment will certainly give a different background noise level. The measured noise level in this case is taken from the output reading of the lock-in amplifier and this background noise measurement is around 50 μV.
We change the input injecting current and measure the PA signals corresponding to different laser excitation power. Figure 5 shows the dependence of PA signal on the laser power. It is found that the PA signal is linearly proportional to the laser power, and thus the laser pulse energy. The linear relationship of PA signal and the QCL power yields good agreement with Eq. (5).
In our model we assume the generated PA signal acts as a point sound source and is propagating as a spherical wave. The acoustic power is decaying according to 1/r2 function and the acoustic wave amplitude is decaying with a 1/r dependence. It is important to note that the measured PA (electrical) signal from a microphone is usually the acoustic wave amplitude/pressure but not the acoustic power. Therefore, the detected PA signal strength should be inversely proportional to the distance. In order to verify this assumption, we study the dependence of PA signal on the detection distance. In the experiment, the output power of the QCL is kept at 40 mW. We take measurements at different detection distances. The results are illustrated in Fig. 6 . The detected PA signal is indeed inversely proportional to the detection distance, which indicates that the generated PA signal by a pulsed laser source is a point source and is propagating as a spherical wave.
In our experiment system setup, at standoff distance of 41 ft, the signal output on the lock-in amplifier still has 200 μV amplitude, giving a relatively clear signal and large enough signal to noise ratio to measure. And the system is possible to extend to even longer detection distance. For outdoor operations, ambient noises can seriously affect the measurement results and have to be eliminated. By using microphone arrays and acoustic beam-forming techniques we shall be able to eliminate these noises and achieve even longer standoff detection distance.
To monitor the concentration of the target gas vapor, we can simply use temperature to change the vapor concentration. The IPA bottle is placed in a water bath on a hot plate. Measurements are taken with temperatures ranged from 25 °C to 65 °C. The dependence of PA signal on the target sample temperature is shown in Fig. 7 (a) , which presents an exponential relationship.
Note that theoretically the IPA vapor pressure carries an exponential relationship on the temperature , thus we can calculate the IPA vapor pressure at different temperatures and plot the relation of the PA signal and the vapor pressure. The dependence of PA signal on the IPA vapor pressure is shown in Fig. 7 (b). It clearly presents a linear relationship, which yields good agreement with the theoretical model derived in Section 2.
We have demonstrated standoff chemical detection for the first time using QCLs and PA effect. The standoff PA measurements were studied as a function of laser pulse energy, detection distance and target gas sample concentration. A standoff distance over 41 feet was achieved at a low operation power of 40 mW. The detection distance can be further improved by using more sensitive microphones like laser microphones and QCLs with higher output power. To achieve outdoor operations we can further use microphone arrays and beam forming techniques to eliminate ambient noise. Remote concentration measurement can also be carried out if the system is calibrated. The demonstrated work has made PA based chemical detection systems portable and become possible for real field measurement applications.
The authors would like to acknowledge the financial support in part by MIRTHE NSF-ERC.
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