Hydrostatic Pressure Sensor Based On Dual-polarization Fiber Grating Laser

Fiber optic sensors can measure a number of physical, chemical and biomedical parametersincluding temperature, strain, pressure, humidity, pH value, and molecules, by use of opticalfiber and fiber devices. They have presented advantages including high sensitivity and resolution,immunity to electromagnetic interference, the ability to work under harsh environments, and themultiplexing ability. In the recent years, a various kinds of fiber optic sensors have beendeveloped and applied in a variety of fields such as health monitoring for large-scale structuresand buildings. Pressure measurement is an important technique for a variety of applicationsincluding deep-sea exploration, clinic diagnose, and down-hole applications. However, thepresent sensors can not ensure high sensitivity and the multiplexing ability at the same time. Inthis paper, a fiber pressure sensor based on dual-polarization fiber grating laser is proposed anddemonstrated. The pressure measurement is carried out by monitoring the beat-frequency changeas a result of the change of intracavity birefringence induced by the applied pressure. Theproposed sensor presents high sensitivity and maintains the ability of multiplexing. It hasovercome the shortcomings of existing techniques and can find a number of potentialapplications.The main content of the thesis is as follows:(1) The fabrication of dual-polarization fiber grating laser is investigated. The fabrication isperformed by use phase-mask method with a scanning193nm-excimer laser-beam. Theinscription parameters including the exposure dosage, the lengths of the two gratings and gratingspacing are optimized to ensure single-longitudinal laser output along the two orthogonallypolarizations. The laser fabricated on an Erbium/Ytterbium co-doped fiber has a total length lessthan10mm. The output beat signal presents a signal-to-noise ratio higher than70dB.(2) The sensing mechanism of the proposed sensor is studied. We establish a multi-physicsmodel and calculate its response to applied pressure. Based on this model, the dependences of the pressure sensitivity on stress-optic coefficient, Young’s modulus, and Poisson’s ratio areinvestigated. The effect of temperature/pressure cross sensitivity can be greatly reduced byannealing the fiber grating laser because the thermal stress in the fiber is released.(3) The pressure sensitivity is greatly enhanced by embedding the fiber grating laser into acomposite structure. The structure coverts applied pressure into a non-uniform stress field andthe intra-cavity birefringence can be significantly changed. As a result, the embedded sensorspresents a pressure sensitivity of170MHz/MPa, increased by228times. The resolution isestimated as10kPa.(4) A compact pressure sensor is demonstrated by embedding the fiber grating laser into asilica U-shaped groove. The sensor has a diameter of only1.42mm and the pressure sensitivity is53.9MHz/MPa,90times higher than a bare sensor. The compact sensor can find application inautomatic industry and clinic diagnose.