Study On All-fiber Photoacoustic Gas Sensing With Interferometric Location

With the wide application of dangerous gases in industrial production and daily life,it is of great significance to achieve highly sensitive gas detection and to identify its location precisely for warning of gas leakage and gas emission monitoring.Photoacoustic spectroscopy（PAS）,as an important branch of spectroscopy,has been widely used for trace gas sensing with the advantages of zero background,high sensitivity,fast response and large dynamic range.However,it still remains an unsolved challenge of locating the target gas in practical applications.Among the many distance detection technologies,frequency-modulated continuous-wave（FMCW）laser ranging is widely used in security monitoring and distributed sensing because of its advantages of precise distance detection and wide dynamic range.In this paper,based on the remarkable characteristics of PAS and FMCW,a method of combining the PAS and FMCW is proposed for simultaneous measurement of gas concentration and location.This study designed an all-fiber PAS sensing system with an FMCW interferometer.The system has the advantages of compact device,simple operation and low cost.We chose methane（CH₄）as the example gas for experimental verification.The main contents of this paper are as follows:（1）This paper studied the principle of PAS and FMCW.The expression of distance was derived based on Mach-Zehnder（M-Z）interferometer and a sawtooth driven laser.In view of laser tuning nonlinear response,a M-Z interferometer with known length,as an auxiliary interferometer,was introduced to form a dual-interferometer ranging system.The distance expression was corrected by frequency-sampling method,which provided theoretical guidance for the subsequent gas location.（2）Starting from the principle of PAS and FMCW as well as the generation mechanism of photoacoustic signal and interference signal,the all-fiber photoacoustic gas sensing system with interferometric location was constructed.And the time-division multiplexing method was determined to measure the gas concentration and location synchronously.In this system,a reflector was spliced to the other end of the PAS cell to mark the cell location and the sensitivity was improved by amplifying the photoacoustic signal.（3）In the experiment,CH₄ was chose as the example gas.The spectral absorption simulations were performed using HITRAN database with CH₄,CO₂ and H₂O.And the target absorption line of CH₄ was determined.Through experiments,the laser characteristics and the PAS cell parameters were calibrated.With the optimal experimental,the corresponding relationship between CH₄ concentration and PAS signal was studied.The results showed an excellent linear response,and the minimum detection limit was 28 ppm(parts per million,10^-6).（4）The influence of noise in interference signal was analyzed and the denoising methods such as the algorithms of Hanning window function digital band-pass filter and wavelet threshold were introduced in detail.The experimental results showed that the measurement accuracy and precision of laser ranging could be improved effectively by filtering and resampling,and the spatial resolution of 3.87mm could be reached over a distance of 4.914 m.In addition,the capability of distributed detection was also analyzed and verified,and the influence of sampling rate and laser linewidth on ranging range was studied.