Design Of A Semi-buried Optical Waveguide Resonator And Verification Of Its Acoustic Sensing Effect

With the wide application of acoustic sensors in natural disaster prediction,oil field exploration,structural non-destructive testing,medical diagnosis and underwater ultrasonic imaging,the sensitivity and frequency band range of acoustic sensors are of great demand.The traditional fiber optic acoustic sensor with moving parts(such as diaphragm)as sensing element has the weaknesses of low sensitivity and narrow frequency response range of detectable sound pressure due to the limitation of mechanical resonance frequency of moving parts.Optical micro-cavity is widely used because of its high quality factor,strong anti-interference ability,miniaturization and integration.Accordingly,the acoustic sensor based on optical micro-cavity is expected to be an important development direction for high sensitivity and wide frequency band acoustic sensor.As a kind of basic optical functional unit fabricated by micro-and nano-fabrication technology,Semi-buried Optical Waveguide Resonator(SOWR)has strong local enhancement and frequency selection in time and space and it provides a feasible means to improve the sensitivity and widen the frequency response range of acoustic sensors.The main work of this paper is to deduce the theory of photo-acoustic coupling sensing based on the evanescent field excitation of the SOWR,to simulate the optical field distribution of the SOWR,to design the structure of the SOWR,to study the optical lithography process of the SOWR and to validate the acoustic sensing effect based on SOWR.The thesis focus on the following four aspects:(1)Deduction of photo-acoustic coupling theory based on evanescent field excitation of SOWRA novel structure of the acoustic sensor based on the evanescent field excitation of SOWR is proposed.The principle of the optical-acoustic sensor based on the evanescent field excitation of the SOWR is studied by combining the acoustic principle,the Effective Reflective-Index Method(ERIM)and the transmission characteristics of the SOWR,the feasibility of acoustic sensing using the evanescent field excitation of the SOWR is preliminarily verified.(2)Simulation analysis and structure design of the SOWRThe mode field distribution,the effective index and the group index of the waveguide are simulated by FDTD software,and the effective index and the group index of the waveguide at1550 nm wavelength are obtained.The characteristic parameters of the SOWR,including the Quality factor(Q-factor),Full-Width at Half-Maxim?m(FWHM),Free Spectr?m Range(FSR)and the resonant depth,are calculated by using MATLAB software.Rsoft software is used to simulate the transmission of straight waveguide and ring waveguide in SOWR at different coupling state to determine optimal coupling spacing.RSOFT software is also used to simulate the transmissivity of the waveguide under the action of different width air groove,and a reasonable groove scheme is designed.The characteristic frequency,the frequency response to the sound signal and the dynamic range of the SOWR are designed by using COMSOL simulation.(3)Analysis of manufacturing technology of the SOWRThe fabrication process of the acoustic sensor based on the evanescent field effect of the SOWR is studied and analyzed,and the whole fabrication process is introduced in detail,in order to reduce the fabrication error and guarantee the dimensional accuracy of the processed device,the fabrication process which has larger error with the actual dimension is improved.(4)Acoustic sensor effect verification based on SOWR evanescent field excitationAn acoustic sensor test system based on the evanescent field excitation of the SOWR is built.The test system can realize signal modulation,demodulation and the locking function of the SOWR resonance frequency.After adding the acoustic signal,the locked demodulation curve can demodulate the acoustic waves.In the experiment,the frequency response characteristics of the SOWR acoustic sensor based on the evanescent field excitation were obtained by testing the responses of the SOWR under the same amplitude and different frequencies of acoustic signals.The sensitivity of the SOWR acoustic sensor based on the evanescent field excitation was obtained by testing the responses of the SOWR acoustic signals with the same frequency and different amplitude.The signal to noise ratio,minim?m detectable sound pressure and standard uncertainty of the sensor are calculated from the test data.