Study On The Key Technology Of Extrinsic Fabry-Perot Interferometer-based Miniature Fiber Underwater Acoustic Sensing

The applications in some underwater mobile platforms,such as swift UUVs,raise the requirement for the miniaturization of fiber-optic underwater acoustic sensors.Miniature sensors have small volume,and can be conveniently installed,which will increase the array elements in a fixed installation volume and correspondingly improve the detection capacity.The miniaturization of the traditional long-arm fiber interferometers is in the state of bottleneck at present,whereas new generation of the MEMS technique brings a light path to the sensor miniaturization.In this thesis,we focus on the key technologies of the sensor miniaturization.The fiber-optic extrinsic Fabry-Perot(F-P)interferometry is adopted,together with the micromachining technology,to fabricate miniature hydrophones and accelerometers.The main achievements and innovations of this paper are listed as follows:(1).The geometry optics method,Gaussian beams method and dual-step diffraction method are separately employed to analyze the transmission characteristics of the fiberend-and-sensitive-reflector-based extrinsic F-P interferometer(EFPI).In the range of 50?m to 300 ?m,the EFPI can obtain relatively high visibility.When the F-P cavity is filled with water,the visibility will not decrease significantly.What's more,the range of the gap length in which the EFPI can obtain high visibility is enlarged.We also analyzed the influence of reflection by the rear surface of the reflector and put forward some suggestions to avoid or eliminate this effect.(2).The theoretical models of both the acoustic pressure sensing structure and the acceleration sensing structure are established.It points out that the central embossment plays the most important role in maintaining the shape of the reflection area,which helps to keep the transmission characteristics of the EFPI under huge surrounding hydrostatic pressure.Besides,the natural frequency of the acoustic pressure sensor falls to several kilohertz due to the additional mass effect in water.For acceleration sensing structure,the quality coefficient is introduced to figure out the relationship between the sensitivity and the natural frequency.The quality coefficient is independent of the material properties and the dimensions,also is nearly irrelevant to the shape of the sensing structure.The feature can greatly simplify the analyzing procedure of the acceleration sensing characteristic.By coating high-reflective thin golden film in the frontier surface of the mass reflector using the thermal evaporation deposition method,the reflection by the rear surface is effectively eliminated.The miniature assembling structure of the sensors are designed.The assembled sensors have the dimensions of ?5 mm×10 mm and show preferable optical performance.(3).The tunable-wavelength-based workpoint control demodulation method is studied.It is proved that stable workpoint can not be obtained simply by micromachining and fabrication.The implementation and the procedure of the workpoint control method based on tunable laser are described.The stabilization characteristic of three sensors are tested both in air and in water.These sensors have the same sensing structure but different outer encapsulation structures.It is shown that the instability of the workpoint mainly results from the external disturbance to the sensitive area.The workpoint control method is easily implemented but limited to laboratory calibration?(4).The white light path-matched differential interferometric(WL-PMDI)phase demodulation is researched in detail.The PGC modulation and phase demodulation technique are introduced to interrogate the EFPI sensors by applying the sinusoid voltage to the phase modulator in one arm of the path-matched interferometer(PMI).What's more,we proposed an improved system construction scheme based on detecting the phase of the PMI fringe.According to the phase information of the PMI,the common path noise is effectively eliminated.The proposed scheme has potential advantages in path matching assistance,OPD mismatch monitoring,visibility control,noise reduction and so on.(5).The characteristic of the phase noise of the white light interferometry,which is dominated by the intensity noise of the light source,is explored.The conversion procedure of the intensity noise to phase noise is derived using the differential and cross-multiplying algorithm as an example.It indicates that the phase noise is proportional to the RIN but inversely proportional to the visibility.The theoretical limit of the phase noise is RIN+12.5dB.Besides,the effect of the quantization noise to system noise performance is analyzed.It can be concluded that the data acquisition system,which is of 12-bits or higher,is nearly immune to the quantization noise.Moreover,the phase noise of the WL-PMDI system is measured using three types of broadband sources under different visibility.It is shown that the measured results coincide well with the theoretical prediction.The derived noise model is also valid for the traditional interferometric phase measurement system.Furthermore,the noise suppression method is researched both passively and actively.We put forward a multi subband NLMS adaptive noise canceling algorithm based on the correlations of the two outputs at different frequency ranges for the improved WL-PMDI system.A noise level as low as ?-97.2 dB re.rad/?Hz@ 10 Hz is gained,which is much smaller than that of the traditional long-arm interferometers.The sensing system in this paper shows great potentials in low-frequency applications.(6).The fabricated acoustic sensors,accelerometers and one-dimensional vector sensors of co-vibrating type were tested,respectively.The acoustic sensor with bare outer encapsulation,of which the sensitivity is-154.6 dB and the natural frequency is ?4 kHz,bears hydrostatic pressure problem.Whereas the sensors with rubber membrane protector suffers frequency-related transmission loss of the acoustic filed.Hence,we further designed an advanced structure which is nearly sound-transparent and hydrostatic-pressureisolated.The encapsulated sensor with the senior structure has the dimensions of ?12mm×14 mm.As a result,we acquired a flat frequency response up to 500 Hz.The sensitivity of the flat band is about-152.4 dB re.1/?Pa.The novel structure can operatively withstand the hydrostatic pressure.The sensitivity of the accelerometers is about-10 dB re.rad/g,while the frequency is about 2500 Hz.The minimum detectable acceleration is about 31.6 ?g/?Hz,which is close to that of the state-of-the-art technology,?10 ?g/?Hz.The sensitivities of the medium-low-finesse EFPI accelerometer with different reflective orders are also tested.It is proven that the sensitivity can be enhanced by optical means on the condition that the natural frequency keeps the same.However,the high-order path matching makes the visibility decrease dramatically,the detection capacity for weak signals decreases correspondingly.The sensitivity of the vector sensor based on this accelerometer is about-196.0 dB @ 1 kHz.As is expected,the frequency response nearly changes 6 dB every octave.Although aimed at the underwater acoustic sensing technology,the sensing system in this paper shows some unique features,such as small volume of the sensor head,flexible design and manufacture,low noise at low frequency,immune to electromagnetic interference,low cost,high accuracy and wideband of the demodulator.Therefore,the related research findings may find potential applications in various fields,for example,air acoustic sensing,explosion sensing,vibration measurement under strong electromagnetic field and seismic wave survey,etc..