Research On Fiber Laser Dynamic Demodulation Technology For Air Acoustic Sensing

With the gradual maturity of optoelectronic devices and the continuous promotion of engineering requirements,a new generation of fiber optic sensors using distributed feedback-fiber laser（DFB-FL）as the sensing element has emerged.Because its advantages of resistance to electromagnetic interference,small size,narrow line width,high power,high sensitivity,and other properties,it has become an important research direction in the field of optical fiber sensing in recent years.Using DFB-FL acoustic sensor can realize the detection of air acoustic signals in a wide frequency band.It has the advantages of long transmission distance,high sensitivity,good concealment,strong environmental adaptability,and so on.It has great potential for development in the fields of border security,turbing leak detection,public security system eavesdropping,medical communications,and marine detection.Based on the research status at home and abroad,this paper conducts an in-depth research on the signal demodulation technology of the DFB-FL air acoustic sensing demodulation system:1.Summarizes the research background of the DFB-FL acoustic sensing technology,as well as the research status and existing problems of wavelength demodulation technology.2.Introduces the working principle,sensing characteristics,creation method and basic performance index of DFB-FL,and the basic performance of the DFB-FL sample is tested.The test results show that,the line width of the DFB-FL sample is 6.79 k Hz.There is a linear relationship between the DFB-FL output power and the pump power,and the slope efficiency is about 0.31‰.The relative intensity noise is-102 d B/Hz@1MHz,and the relative intensity noise of relaxation oscillation peak is-81.94 d B/Hz@221.54 k Hz.The phase noise is 1×10^-14 rad²/Hz@1 k Hz.3.The commonly used DFB-FL wavelength demodulation technology is summarized,and selectes the phase generated carrier（PGC）demodulation technology based on the unbalanced Michelson interferometer to realize the wavelength demodulation through comparative analysis.4.The improved technology of the PGC demodulation algorithm is studied,and an improved PGC demodulation algorithm based on the asymmetric division and a differential-self-multiplication phase generated carrier（PGC-AD-DSM）algorithm is introduced into the DFB-FL wavelength demodulation system.Through simulation and experimental analysis,it is verified that the proposed demodulation algorithm can suppress the influence of light intensity interference on the demodulation results,and can obtain stable demodulation results when the phase modulation depth is in the range of 0.2 rad to 3.8 rad.5.A DFB-FL air acoustic sensor demodulation system is built,and a non-metallic DFB-FL air acoustic sensor packaging structure based on Polyetheretherketone（PEEK）material and polyimide（PI）sensitizing diaphragm is designed.The system is used to test the acoustic pressure sensitivity and frequency response of the sensor in the form of bare fiber and the sensor after packaged.Through experimental comparative and analysis,it proves that the package structure can improve the acoustic pressure sensitivity of the demodulation system and reduce the fluctuation of the frequency response curve.Its minimum detectable acoustic pressure is about 66.7μPa/√Hz@1k Hz,which can realize the stable demodulation of acoustic signals in the wide frequency band（200 Hz～18 k Hz）.An acoustic pressure sensitivity of about-40.39 d B re.pm/Pa is obtained,with a fluctuation range of about±3.15 d B.6.The experiment proves the cross-sensitivity problem of the unbalanced Michelson interferometer in the DFB-FL air acoustic sensor demodulation system,and introduces an anti-environmental interference technology combining the reference compensation method and DFB-FL acoustic and vibration isolation packaging technology.On this basis,an experimental analysis is carried out.The acoustic pressure sensitivity in a wider frequency band（200 Hz～18 k Hz）is about-40.64 d B re.pm/Pa,and the fluctuation is about±2.99 d B,which proves that this technology is helpful to solve the cross-sensitivity problem of optical fiber interferometer.