| With the popularization of 5G communication technology,the Internet of Everything is no longer a slogan.In the entire Io T system,sensor devices play a pivotal role and have received extensive attention and research.Meanwhile,fiber optic sensors are attracting much attention,prompted by the high development of fiber optic communication technology.Optical fiber sound sensors are an important branch of optical fiber sensors.The modulation of sound signals by optical fiber sound sensors mainly includes intensity modulation,phase modulation,and wavelength modulation.This paper focuses on the phase modulation type of optical fiber acoustic sensor in the quadrature FP cavity optical fiber acoustic sensor,which has the advantages of small size,simple structure,excellent phase consistency,and high sensitivity compared with the existing electric,piezoelectric and capacitive acoustic sensors based on the traditional direct acoustic-electrical conversion principle,especially in the strong electromagnetic interference environment with irreplaceable sound signal collection It has the advantages of small size,simple structure,excellent phase consistency,and high sensitivity.Compared to single FP cavity fiber optic acoustic sensors,quadrature FP cavity fiber optic acoustic sensors have a greater dynamic range and lower temperature sensitivity than single FP cavity fiber optic acoustic sensors,which have a very high potential for application.The main research elements and conclusions obtained are as follows.1.The performance indexes of quadrature method FP cavity fiber-optic acoustic sensor based on differential cross-phase multiplication algorithm(DCM)and its influencing factors are studied.The effects of sampling rate and quadrature-phase deviation on the frequency response range and dynamic response range of the sensor,as well as the mutual constraint relationship between the upper limit of the frequency response range and the upper limit of the dynamic response range are simulated and analyzed to provide a basis for the application of the quadrature FP cavity fiber optic acoustic sensor.The results show that:(1)at a certain sampling rate,the upper limit of frequency response and the upper limit of the dynamic range of the quadrature FP cavity fiber-optic acoustic sensor based on DCM algorithm are mutually constrained,and these two performance indexes are affected by the system sampling rate at the same time.The lower the sampling rate of the system,the upper limit of frequency response and dynamic range will decrease;(2)the phase deviation of the quadrature of the two signals will affect the frequency response range and dynamic range of the quadrature FP cavity acoustic sensor based on DCM signal demodulation algorithm.The larger the quadrature-phase deviation,the lower the upper dynamic range and upper frequency response of the sensor.The quadrature-phase deviation should be less than 0.25π under the condition that the demodulated signal strength attenuation does not exceed 3 d B.2.The demodulation algorithm of the small sound pressure signal of the quadrature FP cavity fiber optic acoustic sensor is studied.For the quadrature cavity,if the operating point of one of the FP cavities(FP1 cavity)is at the quadrature operating point,the operating point of the other FP cavity(FP2 cavity)is at the peak or valley of the interference curve,and the theoretical optical sensitivity of the FP2 cavity is zero at this time,the signal-to-noise ratio of the output signal of this cavity is extremely low when collecting the small acoustic pressure signal,which leads to the increase of the error of the ellipse fitting normalization algorithm used for the quadrature signal,resulting in the distortion of the small acoustic pressure signal using the ellipse.The ellipse fitting-differential cross-phase multiplication algorithm(EF-DCM)is used to demodulate the small sound pressure signal distortion.The pre-determined ellipse parameter-differential cross-phase multiplication algorithm(PEPDCM),which determines the ellipse fitting parameters under a large sound pressure signal,is used to achieve the demodulation of small sound pressure signals.The feasibility of this algorithm to demodulate small sound pressure signals is verified in a quadrature FP-cavity fiber optic acoustic sensor system.Experimental results show that the quadrature FP-cavity fiber-optic acoustic sensor with the PEP-DCM algorithm has an improved capability of detecting small acoustic pressure signals by a factor of about 62.5.The stability of the algorithm is studied in simulation,and the results show that as the phase deviation increases,the demodulated signal strength of the improved algorithm decays,and the phase deviation of the two FP cavities of the quadrature FP cavity fiber optic acoustic sensor should not exceed 0.25π if the distortion criterion of the demodulated output signal strength decay does not exceed 3 d B.3.A "short step quadrature FP cavity" structure is proposed to suppress the quadraturephase deviation caused by the laser wavelength drift and to improve the robustness of the pre-defined elliptical parameter differential cross-multiplication algorithm.The simulation results show that the height difference ΔL of the short step pair is reduced from 500.06 μm to 0.96 μm and the permissible drift of the sensor light source wavelength is increased from0.6 nm to 313 nm.The "short step quadrature FP cavity" structure is then optically analyzed to determine the range of FP cavities.The results show that the optimum cavity length for the FP cavity is approximately 300 μm,and that the short FP cavity length should be less than 492 μm if the sensor is to avoid cross-coupling problems when the short step height of the fiber does not exceed 30 μm.4.A fabrication and assembly process for a quadrature FP-cavity optical fiber sound sensor was designed.Including the fabrication methods of key structures:(1)Fabricating micro-step structures with a height difference of 5 μm by comparing MEMS process,laser processing process,and silicon wafer assembly process methods as fabrication tooling for manufacturing composite optical fiber pair structures.(2)To design the fabrication and assembly process of the diaphragm fixing structure and design and manufacture the corresponding tooling parts.The result is a complete solution for the manufacture of quadrature FP cavity fiber optic sound sensors.5.The experiments on frequency response range,dynamic range,and sampling rate of the quadrature FP cavity fiber optic acoustic sensor based on the PEP-DCM signal demodulation algorithm were conducted,and the experimental conclusions were consistent with the simulation pattern;the experimental results showed that the sensor could maintain stable output at 5 ℃ ～ 20 ℃,which verified the strong temperature adaptability of the quadrature FP cavity fiber optic acoustic sensor.By conducting frequency test experiments,the frequency response of the quadrature FP cavity acoustic sensor was measured to be in the range of 50 Hz ～ 6 k Hz(<±1 d B),which meets the requirements of human voice acquisition and military target detection applications.The intensity of the sensor was tested and the results showed that the sensitivity of the sensor was 0.047 rad/Pa and the linearity of the intensity response curve was 0.998 in the sound pressure range of 0.14 Pa ～ 120 Pa. |
