Research On Broadband Underwater Acoustic Sensing Characteristics Based On Silicon Dioxide Waveguide Resonator

With the widespread application of underwater acoustic sensors in military and civilian fields such as underwater target detection,marine resource exploration,seismic wave detection,frogman detection,mine detection and collision avoidance sonar,the demand for highly sensitive and wideband response range underwater acoustic sensors has significantly increased.Compared to traditional electric underwater sensors,optical underwater sensors have irreplaceable advantages such as strong resistance to electromagnetic interference,high sensitivity,and light weight.The optical resonant cavity has a high quality factor and small size,acoustic sensors with optical resonantor as sensitive unit are expected to achieve high sensitivity and ultra wideband acoustic detection,providing a feasible means to improve the universality of acoustic sensor applications.In response to the problem that the current frequency range of optical underwater acoustic sensors is narrow and difficult to meet the needs of broadband acoustic wave applications,this paper proposes an optical waveguide resonator acoustic sensor（OWRAS）with no diaphragm,all solid state,high sensitivity and ultra wideband,meanwhile designs a broadband underwater acoustic detection system for the OWRAS.As a new broadband underwater acoustic sensor,the OWRAS can achieve ultra wideband and high sensitivity underwater acoustic signal measurement,meeting the application needs of the broadband underwater acoustic field.The relevant research content of this article is as follows:1.A mapping model between the sensitive unit of the optical waveguide micro ring resonant cavity and the intensity of the sound field was established based on the principle of quantum optics;Analyzed the relationship between underwater sound and the effective refractive index of optical waveguides,revealing the coupling mechanism between optical waveguide resonators and underwater sound;And theoretical analysis was conducted on the frequency response performance and temperature characteristics of the resonant cavity,providing a theoretical basis for high-sensitivity wideband underwater acoustic detection.2.Designed a high Q value slot type silicon dioxide waveguide resonant cavity.In order to maximize the resonance depth and high Q factor of the resonant cavity,the coupling state of the resonant cavity is designed as critical coupling;In order to enhance the interaction between sound waves and the coupling area of the resonant cavity,grooves were made in the coupling area of the resonant cavity,and the micro groove size was designed to be 40μm×40μm and 180μm×180μm based on simulation results;Using plasma enhanced chemical vapor deposition（PECVD）and inductively coupled plasma etching（ICP）techniques to process slot type optical waveguide resonators;Use a glass sleeve to couple and package the input and output terminals of the waveguide with the polarization maintaining fiber.After testing,the quality factor Q of the resonant cavity is as high as 2.3×10⁷.3.In response to the presence of system noise and temperature fluctuation noise in the testing environment,an OWRAS underwater acoustic testing system based on PDH（Pound-Drever-Hall）frequency locking technology was designed.The system uses feedback error signals to control the laser frequency,locking the laser frequency in real-time at the resonant point of the resonant cavity,thereby diminishing the influence of noise on acoustic signal testing and improving detection accuracy.In order to achieve better signal-to-noise ratio,the influence of modulation frequency on demodulated signals was studied,and the optimal modulation frequency of the system was determined to be 9MHz.Finally,the underwater acoustic sensing effect of the OWRAS was verified through experiments,and the effects of temperature fluctuations and micro groove size on the performance of the OWRAS were studied.The experimental results show that the OWRAS detection system can achieve underwater acoustic signal measurement at40-1250 Hz,with an average sensitivity of-173.4 d B（re 1V/μPa）,and a sensitivity response of-163.1 d B at a frequency of 1 k Hz;The minimum measurable sound pressure is 50 m Pa/Hz^1/2@1 k Hz;The system resolution is 1 Hz,and the repeatability and stability of the measurement of underwater acoustic signals are good;The sensitivity of the system is not affected by temperature fluctuations within a certain temperature range.4.In response to the current narrow operating frequency range of optical underwater acoustic sensors,an OWRAS wideband underwater acoustic detection method based on intensity demodulation is proposed.Firstly,theoretical analysis was conducted on the influence of Q factor and cavity length on sensitivity of the resonant cavity.The structure of the optical waveguide resonant cavity was optimized,further improving the Q value of the resonant cavity;Afterwards,the impact of the input power of the photodetector on the signal-to-noise ratio was analyzed;To ensure the accuracy of mid to high frequency underwater acoustic detection,a detailed explanation of mid-high frequency underwater acoustic testing method was provided,including the necessity of far-field testing distance and the number of pulses emitted by the sound source for ultrasonic signals of different frequencies.Finally,The feasibility of the OWRAS wideband underwater acoustic detection based on the intensity demodulation method has been experimentally confirmed.The experimental results show that the OWRAS can achieve high sensitivity broadband underwater acoustic signal measurement from 10 Hz to 1.2 MHz,and the average sensitivity of the OWRAS in the frequency range of 10 Hz to 2 k Hz reaches-139.1 d B（re1V/μPa）with a fluctuation range of±4 d B,the equivalent noise pressure is 67.6 d B@1k Hz,which is lower than the background noise level of seawater,and has the ability to be applied to underwater acoustic signal detection in the ocean;The average sensitivity reaches-167.1 d B in the frequency range of 10 k Hz to 1.2 MHz,with a fluctuation range of±4 d B.This method greatly expands the application scope of the OWRAS in the field of broadband underwater acoustic detection.This article proposes for the first time the application of a high Q value slot type optical waveguide resonant cavity in broadband underwater acoustic detection,which can achieve high sensitivity broadband underwater acoustic signal measurement from low frequency 10 Hz to high frequency 1.2 MHz.This study provides a new direction for broadband sound wave detection technology and lays a research foundation for the engineering application of silica optical waveguide resonant cavity acoustic sensors.