为进一步提升多组分痕量气体检测灵敏度，设计了一套光纤光声传感系统。系统主要集成了2个近红外DFB激光器、近红外宽带光源、高速光谱模块、现场可编程逻辑门阵列信号采集与处理电路，具有激光调制控制、光声信号解调和数字锁相放大等功能。利用声学共振腔和干涉型光纤声波传感器对光声信号进行激发增强和探测增强，实现了乙炔和甲烷气体的高灵敏度检测。光纤声波传感器中以微机电系统悬臂梁作为声学敏感元件，设计了光纤法布里-珀罗干涉结构，将悬臂梁偏转位移转换为F-P腔长的变化。采用高分辨率光谱解调技术，实现了基于光纤F-P传感器的超高灵敏度光声信号检测。系统对乙炔和甲烷的检测极限分别达到 2×10-9 和 3×10-9，归一化噪声等效吸收系数为 8×10-10 cm-1 W Hz-1/2。
Multicomponent Trace Gas Detection Technology Based on Fiber Optic Photoacoustic Sensing
In order to further improve the detection sensitivity of multi-component trace gases,an acoustic resonant cavity and an interferometric fiber optic acoustic wave sensor were used to enhance the excitation and detection of the Photoacoustic(PA)signal.A fiber optic PA sensing system was designed to achieve high-sensitivity detection of C2H2 and CH4 gases.To increase the amplitude of the PA response and reduce the cross-interference of other gases,two DFB lasers with center wavelengths of 1 532.83 nm and 1 650.96 nm were selected as the excitation light sources for C2H2 and CH4.Through the digital-to-analog converter,the working parameters such as modulation frequency,bias current and modulation depth of the two DFB lasers were controlled by FPGA.Two near-infrared lasers with different wavelengths were coupled through a wavelength division multiplexer and then incident into the PA cell,achieving dual-component gas excitation of C2H2 and CH4.To further improve the detection limit of gases,a multi-pass device composed of two coaxial concave mirrors combined with a resonant PA cell formed a multi-pass resonant PA cell.The excitation light was reflected multiple times in the multi-pass resonant PA cell,which increased the interaction length between the target gas and light,increasing the effective power of the excitation light.The Micro-electro-mechanical Systems(MEMS)cantilever in the fiber optic acoustic sensor was used as the acoustic sensitive element,and a fiber optic Fabry-Perot(F-P)interference structure was designed to convert the deflection displacement of the cantilever into the change of the F-P cavity length.A superluminescent diode with a central wavelength of 1 550 nm was used as the detection light source.The emitted broadband light entered the acoustic wave sensor after passing through the optical fiber circulator.The F-P interference spectrum containing PA information was detected by the miniature optical fiber spectrum module.The signal processing circuit performed high-speed acquisition and real-time processing of the F-P interference spectrum.By using high-resolution spectral demodulation technology,ultra-high sensitivity PA signal detection based on fiber optic F-P sensor was realized.In order to obtain the highest detection sensitivity,the relationship between the PA response and the modulation parameter was measured by adjusting the modulation current of two DFB lasers.The PA response was measured under different currents,and the system obtained the best detection performance when the modulation currents of the C2H2 laser and the CH4 laser were set to 8.5 mA and 4 mA,respectively.The frequency response of the system was tested to obtain the best PA signal amplitude.In the range of 500 Hz to 1 250 Hz,the modulation frequencies of the two DFB lasers were adjusted,and the resonance peak appeared at 1 660 Hz in the two frequency response curves.The linearity of the sensing system was evaluated.The gas mixtures of C2H2/N2 and CH4/N2 with different concentrations were flushed into the PA cell,and the PA response of the system to C2H2 and CH4 was analyzed.In the concentration range of 0 ppm to 100 ppm,there was a good linear relationship between the excitation PA signal amplitude and the concentration of the two mixed gases,and the linear responsivity of the system to C2H2/N2 and CH4/N2 were 7.39 pm/ppm and 5.67 pm/ppm,respectively.Pure N2 gas was pumped into the PA cell to test the noise level of the system and evaluate the stability of the sensor.Two sets of noise data were tested repeatedly,the deviation(1σ)were 0.364 pm and 0.365 pm,and the average value was 0.36 pm.Based on sensitivities of 7.39 pm/ppm and 5.67 pm/ppm for C2H2 and CH4 gases,detection limits of 48.7 ppb and 63.4 ppb were obtained,respectively.The detection sensitivity and stability of the system were evaluated by Allan-Werle deviation analysis.When white noise dominates,the Allan-Werle variance value decreased as the averaging time increased.With an average time of 400 s,the results of Allan-Werle analysis of variance showed that the detection limits of the system for C2H2 and CH4 gases reached 2 ppb and 3 ppb,respectively.The Normalized Noise Equivalent Absorption(NNEA)coefficient normalized the absorption line intensity and the effective power of the excitation light.The output powers of C2H2 and CH4 lasers were 14.7 mW and 21.9 mW respectively.Therefore,the calculated NNEA is 8×10-10 cm-1 WHz-1/2.The designed optical fiber PA sensing system realized the high sensitivity detection of C2H2 and CH4 gas.
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