The ability to rapidly detect and identify hazardous materials, such as explosives and hazardous gases, is a critical technology that can facilitate security screening in the global effort against terrorism. Recent R&D works focus on technologies to perform standoff sensing of hazardous materials in the open environment without any sample preparation. In this work, a long-distance laser Doppler vibrometer developed by China is applied as a sensor to detect the photoacoustic signal on solid and gaseous chemicals.

Photoacoustic/photothermal (PA/PT) techniques can provide sensitive non-contact solutions to overcome this challenge at a meaningful standoff distance.Since the last century, PA spectroscopy (PAS) has been applied in many applications for solid, liquid, and gas evaluation. Conventional PAS requires prior preparation of analyte samples, and the measurement is conducted within a controlled environment, such as a well-isolated photoacoustic cell. The response signal is captured by a highly sensitive microphone or any other form of sensor located close to or in direct contact with the sample. Hence, commercial PAS equipment limits the technique in a laboratory environment. In recent years, research focuses on improving the excitation source and the detection technology in the open air. Two trends have been observed in the research area of standoff detection of chemicals and explosives: (1) Quantum cascade laser (QCL) becomes a more practical excitation source due to its high power, broadly tunable wavelength in the mid-infrared ray (IR) range, and compact size over other light sources such as optical parametric oscillators (OPOs); (2) non-contact optical detection techniques, such as thermal imaging and laser interferometry become generally accepted methods for standoff detection of PA/PT signals in the open air. Recently, a conventional and mature interferometric system, namely a laser Doppler vibrometer (LDV), has been applied to detect the photo-vibrational signal on solid and liquid due to the PA effect. The standoff distance of the LDV will influence the received power of the reflected beam only, but it will not affect the phase change detected due to the PA/PT effect.

We apply a long-distance laser Doppler vibrometer developed by China (Fig. 3) to detect the photo-vibrational signal 200 m away. A QCL with a chopper is adopted as the excitation source. By scanning the wavelength of the QCL, we measure the amplitude of the vibration signals and obtain photoacoustic spectra. The results are compared with that of Fourier transform infrared spectroscopy (FTIR) and found to be highly consistent.

In order to detect the vibration caused by the photoacoustic effect, a long-distance laser Doppler vibrometer is fabricated. Fig. 3(a) shows the optical design. Generally, it is a typical Mach-Zender interferometer. The light source used is a narrow linewidth laser at 1550 nm from Keopsys company. A fiber-coupled acoustic-optic modulator with frequency of 40 MHz is applied in the reference beam to introduce a carrier frequency. A motorized 4-inch beam expander is designed to focus the laser beam to a distance of 20-200 m. The reflected object beam interferes with the reference beam, and the output signal from a balanced detector is a frequency-modulated (FM) signal with a central carrier frequency of 40 MHz. A field programmable gate array (FPGA)-based demodulation system developed by China is used to demodulate the FM signal and retrieve the optical path length change due to the photoacoustic effect. The output is displacement or velocity that can be selected by the user. Generally, displacement is directly proportional to the phase change, and velocity is proportional to the frequency shift of the object beam. When the signal-to-noise ratio (SNR) of the interferometric signal output from the detector is more than 45 dB, the noise floor of the displacement measurement is < 5 pm. This means an amplitude vibration of 5 nm at a certain frequency will have a peak of 30 dB in the spectrum. For long-distance LDV, the SNR of the optical signal is fluctuating from 20 dB to 50 dB.

Fig. 6(b) shows the photoacoustic spectrum of trace polytetrafluoroethylene (PTFE) powder obtained by the long-distance LDV. Compared with a standard FTIR of PTFE [Fig. 6(a)], it is found that they coincide very well. Fig. 7(b) is the photoacoustic spectrum of leaking acetone, and it coincides with FTIR [Fig. 7(a)] obtained by the PerkinElmer Frontier FT-IR/MIR spectrometer. It is worth noting the amplitude of the photo-vibrational signal obtained by LDV may fluctuate due to environmental factors. Longer integration time may stabilize the reading and eliminate the noise floor, but it may reduce detection efficiency in real applications.

We adopt a QCL as the excitation light source due to its high power, broadly tunable wavelength in the mid-IR range, and compact size over other light sources. The intensity of the QCL beam is modulated by an optical chopper, while the LDV is used to detect the vibration signals due to the photoacoustic effect. The photo-vibrational spectra obtained by plotting the normalized vibration amplitude against the QCL output wavenumber range are compared with standard FTIR spectra. The experiments demonstrate that the long-distance laser Doppler vibrometer developed by China can effectively detect photo-induced vibration signals from hazardous solid and gaseous chemicals at up to 200 m in an imperfect environment. It is a necessary first step in a series of developments to realize the proposed technology for standoff detection of hazardous materials in defense and security screening applications.