Research Of Fiber-gold Film Short-cavity Fabry-Perot Underwater Acoustic Sensor Unit

With the gradual development of marine resources,the application and development of underwater acoustic signals in the fields of marine oil and gas exploration,environmental monitoring,and underwater communication have received more and more attention.The current main development trend of underwater acoustic sensors is miniaturization,high sensitivity and large dynamic range.The thin-film optical fiber Fabry-Perot(F-P)sensor using the nanometer-thickness sensitive diaphragm can achieve a very small structure size while achieving high-sensitivity underwater acoustic detection,combined with the MEMS processing technology.At present,the research on the optical fiber F-P sound pressure sensor is mainly focused on improving the pressure response sensitivity of the diaphragm.The F-P cavity commonly used is tens to hundreds of microns long.The study found that short-cavity(micron or sub-micron)fiber F-P sensors composed of highly reflective mirrors can effectively improve response sensitivity and signal-to-noise ratio due to higher coupling efficiency and optical signal strength.This thesis focuses on the optical characteristics of the fiber short-cavity F-P sensor;designs and prepares the underwater acoustic sensor unit with the shortest cavity length of about 4?5?m;the underwater acoustic test system is built to test and evaluate the performance of the sensor unit.The main research work and innovations of the paper are as follows:1.Theoretical simulation and analysis of the optical characteristics,sound pressure response characteristics and demodulation method of the fiber-gold film F-P short cavity.The optical properties of the fiber-gold film FP short cavity were deduced using the transmission matrix method and the cause of the asymmetry spectrum was analyzed;the theory of the vibration response of the membrane of the sensor unit under the sound pressure is studied,and the gold film used in the paper was analyzed The deformation sensitivity and response frequency of the chip;the intensity demodulation scheme and the control of the working point are determined as the key technology.2.Design,preparation and testing of sensing unit.According to the requirements of the optical sensitivity and frequency response range of the sensing unit,design the structural parameters of the sensing unit;Explored and optimized the preparation process of the sensor unit,prepared and assembled a small underwater acoustic sensor unit with a length of 6.4mm and a diameter of 1.25mm;A static spectrum test system was built.The test results in the air showed that the cavity lengths of the prepared two optical fiber short-cavity F-P sensing units were 4.7 ?m and 7.4 ?m respectively.Among them,the 7.4?m sample maintained good airtightness under water.3.Testing and calibration of the underwater acoustic response of the sensing unit.Build a vibrating liquid column method underwater acoustic measurement system,which can meet the measurement requirements of 10Hz?2k Hz underwater acoustic signals;The output signal of the intensity demodulation system is compared and calibrated with a standard hydrophone to obtain the sensitivity,frequency response and dynamic range of the underwater acoustic sensing unit prepared by the thesis.The test results show that the prepared underwater acoustic sensor unit has good response flatness in the range of 100?800Hz,the sensitivity is-178 d B ± 0.4d B,and the dynamic range is up to 78 dB at 800 Hz.