Research On Demodulation System For Fiber-optic Interferometric Acoustic Sensors

Sensitive detection of acoustic wave has important applications in fields of national defense security,structural health monitoring,biomedical imaging,geophysical exploration,etc.Compared to conventional capacitive or piezoelectric sensors,fiber-optic interferometric acoustic sensors detect the phase modulation induced by acoustic waves on light transmitted in optical fiber,and has the advantages of high sensitivity,electromagnetic interference immunity,remote detection and capability of remote detection and multiplexing.These sensors have wide spread application involving acoustic wave detection.However,the phase of light transmitted in optical fiber is easily perturbed by external environment factors such as temperature and vibration,leading to sensor unstable and unable to efficiently detect the acoustic signal in a long-term.Therefore,it is of great significance to develop stable signal demodulation scheme for fiber-optic interferometric acoustic sensors.In this thesis,fiber-optic Fabry-Perot interferometers including fiber Bragg grating based Fabry-Perot interferometer?FBG-FP?and fiber-optic microbubble based Fabry-Perot interferometer was stabilized by unitizing a suitable dynamic feedback demodulation scheme.The sensors interrogated by the above demodulation scheme successfully applied to stable detection of ultrasonic signal and photoacoustic imaging.The outline of the thesis is as follows:Firstly,by servo-control of the interrogation laser wavelength,the FBG-FP interferometric acoustic sensor is locked to the quadrature point.Stable detection of a acoustic wave with frequency of 10 MHz is realized.We have investigated the immunity of the demodulation system to environmental temperature variation systematically,and experimentally verified the capability of the system for stable ultrasonic signal detection within the temperature variation range of 27°C.We further improve the performance of the demodulation system with the Pound–Drever–Hall?PDH?technology to stabilize phase-shifted fiber Bragg grating acoustic sensors featuring narrower linewidth.Secondly,The fiber-optic microbubble FP acoustic sensor is dynamic stability based on the photothermal effect by adjusting the heating laser power upon feedback signal.We have analyzed the gas diffusion dynamics during the microbubble stabilization.The laser power which needs to generate the fiber-optic microbubbles was controlled with PID algorithm based real-time feedback,The fiber-optic microbubble with the diameter range from 8?m-210?m?diameter variation less than 1 nm?was implement.The stability of fiber-optic microbubbles under different environmental perturbations such as variations in temperature,pressure,flow rate and chemical concentration was studied experimentally.The diameter fluctuations of the stabilized microbubble are less than³0 nm,which satisfies the requirements for stable acoustic wave detection.Finally,We theoretically and experimentally investigated the acoustic response of the fiber-optic microbubble based FP acoustic sensors.A 5?m diameter fiber-optic microbubble demonstrated a frequency bandwidth of 0.8 MHz and a noise equivalent pressure level of⁵ mPa/Hz^1/2.By using the fiber optic microbubble FP acoustic sensors,we built a photoacoustic tomography?PACT?system,which has an imaging resolution of 1.3 mm.This system holds the potential for human tissue imaging in the future.In addition,according to the relationship between the laser power required for photothermal stabilization of the microbubble and the liquid flow rate in the microfluidic channel,a microfluidic flow velocity sensor with a detection limit of 0.16mm/s in a flow rate range from 0 mm/s to 4 mm/s is realized.