Research On Fiber Optic Hydrophones Based On Quantum Weak Measurement Principle

The main function of the optical fiber hydrophone system is that the optical fiber sensing probe of the system couples the underwater acoustic pressure signal though the optical fiber,which perturbs the physical parameters such as fiber length,fiber refractive index,light phase and polarization to produce signal variation.And the optical signal variation in the optical fiber is returned to the demodulation host of the hydrophone system through the optical fiber,then this system host demodulates the optical signal and reproduce the signal corresponding to the underwater acoustic pressure signal.Fiber optic hydrophones have become the core components of the new generation optical fiber based sonar systems because of its high sensitivity,large dynamic range,passive operation and ready adaptation to networked sensor arrays.With the advancement of noise reduction technology of underwater ships,plus marine oil exploration,earthquake and tsunami warning needs,current fiber hydrophones have shown inadequacy for low or very low frequency and long distance detection.Therefore,the detection of the underwater low frequency actoustic has attracted much attention in recent years.Low-frequency acoustic pressure measurements require good acoustic pressure sensitivity and equivalent noise acoustic pressure of optical fiber hydrophones.However,increasing the sound pressure sensitivity of the fiber hydrophone would increase the fiber length or the mechanical sensitization of the transduction mechanism,which also amplifies the noise.Quantum weak measurement can realize optical phase measurement accuracy,without the concurrent amplification of noise,thus an integration of quantum weak measurement technology and optical fiber hydrophone technology is expected to overcome the difficulties encountered by conventional fiber optic hydrophones for low-frequency,high-sensitivity acoustic signal detection.The theory of quantum weak measurements was first proposed in 1988,its associated weak value amplification(WVA)is achieved by pre-selection and post-selection of the quantum states of the measurement system to highlight the weak value of the system variable to be measured,which can be arbitrarily amplified,without the attendant amplification of the technical noise;the dynamical parameter to be measured is directly related to such weak value and its small variations are greatly amplified by the weak value(e.g.,in most optical phase measurements,by 3-4 orders of magnitude).In the case of the fiber optic hydrophone,the underwater acoustic pressure signal is coupled to the optical fiber,so that the optical fiber produces a small phase change that can be amplified by the weak value to amplify the tiny underwater acoustic signal.With the application of weak-value amplification(WVA)technology in the fiber optic hydrophone system,new type of fiber optic hydrophone can detect low-frequency underwater acoustic signals,owing to an innovative research scheme proposed and demonstrated in the work.The experiment used an optical fiber implementation of polarization interference for weak measurement,and employed polarization-maintaining optical fiber instead of bulky free-space optical components.The hydrophone is based on a fiber-wound mandrel structure to couple the sound pressure signal,which can measure the weak value signal to achieve high precision,and detect low-frequency underwater sound pressure.The theory and experiment of the fiber optic hydrophone based on quantum weak measurement principle are presented systematically in this Ph.D.dissertation.Specifically,the main work contents and research results are as follows.(1)The basic theory of weak measurement is discussed.Employing the von Neumann theoretical model of quantum measurement(orthogonal projection model),the weak measurement principle is deduced,and the signal amplification effect of weak measurement is explained using a birefringence crystal optical experiment.The principle of weak value amplification is also introduced.(2)Experimental measurement of optical fiber phase change based on the weak measurement principle is carried out,using all fiber optical components,and the electrically tunable phase change of an electro-optic crystal is used to fine-adjust the phase in the optical path.In the experiment,polarization-maintaining optical fiber is used instead of free-space optical components to construct a compact and robust weakly measured polarization-sensitive optical interference structure,which is used to measure the small phase variations of the fixed hydrostatic pressure on the polarization-maintaining fiber.The experimental results show that using a short segment of polarization-maintaining fiber(200 mm),when the hydrostatic pressure changes by 11 Pa,the phase change of the fiber is 30×10^-5 rad,and the minimum measurable fiber phase change is 10^-5rad,corresponding to 0.37 Pa minimum detectable pressure change.(3)Design of fiber hydrophone based on weak measurement principle is discussed.We design and fabricate a complete optical fiber hydrophone system based on the quantum weak measurement principle.This optical fiber optic hydrophone system hardware mainly consists of single frequency laser,optical fiber optic hydrophone sensor head,photodetector and data acquisition module.The fiber optic hydrophone probe consists of a pre-selection fiber-polarizer,a post-selection fiber-polarizer,a polarization-maintaining fiber-wound polycarbonate(PC)tube and a protection shell.The pre-selection and post-selection devices are integrated from space optical components,offset preserving fiber collimator and outer sleeve.The results show that the theoretical phase acoustic pressure sensitivity of the probe is-173.03 d B re rad/?Pa and the natural frequency is 47.73 Hz.Finally,a phase demodulation technique of optical fiber hydrophone systems based on the quantum weak measurement principle is demonstrated.(4)Experimental study of fiber optic hydrophone based on the weak measurement principle.Based on the parameter definition and measurement of general fiber hydrophone,we discussed the definition and measurement method of acoustic pressure linearity,phase/sound-pressure sensitivity,noise equivalent sound pressure of the developed fiber hydrophone based on weak measurement principle.Employing a self-made low-frequency underwater acoustic measurement system,the fiber hydrophone based on the weak measurement principle and a standard piezoelectric hydrophone are measured and compared.0.1 Hz-200Hz acoustic pressure signal was generated by a signal generator,and the final result showed that(a)When the frequency range is 0.1 Hz-50 Hz,acoustic pressure linearity is better than10%;(b)When the frequency range is 0.1 Hz-50 Hz,the average phase sensitivity is-173 db re rad/?Pa,and Flatness is 0.5 db re rad/?Pa,and the results is the same as those theoretically calculated;(c)At 10Hz,the noise-equivalent sound pressure for equivalent noise of the new fiber hydrophone is 1.3×10^-6 Pa/Hz^1/2;(d)The new optical fiber hydrophone can measure time-domain pressure signals of frequencies as low as 0.1Hz.