Research On Spatial Sensitivity Of Dual-polarization Fiber Laser Sensors

Dual-polarization fiber grating laser sensors have been exploited as high-resolution sensors for the detection of weak signals including vibration and acoustic waves. They have presented advantages including high sensitivity, low noise level, compact size and intrinsic multiplexing capability. The application of this sensing technology has been extended to homeland security, antisubmarine, biomedical science and technology and resource exploration. In our previous work, we have investigated the fabrication, noise figure, multiplexing ability and transducer design and optimization of these sensors. It has been found that the optical response is different when changing the loading position but the spatial sensitivity has not been clearly characterized and studied in detail. In order to further enhance the measurement capability, the spatial sensitivity is investigated in this thesis. Based to resonant condition, I have found that scaling law between the sensitivity and the local intracavity intensity. A load-scanning method is subsequently presented to characterize the spatial sensitivity. The effect on the sensitivity of parameters including grating coupling strengths, cavity length and fiber gain level have been investigated. The sensitivity to a point load can be greatly enhanced by shortening the laser cavity by reducing the grating separation and raising the coupling strength. I further demonstrate a fiber laser hydrophone with corrugated diaphragm transducer for the detection of acoustic waves. The acoustic sensitivity has been effectively enhanced by using laser sensors with shorter cavity length. The thesis is organized as follows:First, The relation between the sensitivity and the mode profile of the fiber grating laser sensors is studied. The resonant condition determines that the induced beat-frequency shift scales with induced birefringence change as well as the local normalized light intensity. Therefore, the sensitivity profile is mainly determined by parameters including cavity length, grating coupling strength and the gain of the Er-doped fiber.Second, a load-scanning method has been presented for the characterization of the spatial sensitivity and the profiles for different cavity lengths and different load orientations have been plotted. Based on the measured results, we have studied how to enhance the sensitivity by selecting gain fiber and optimizing parameters including grating strength and separation. We found that higher mass(or force) sensitivity can be obtained by shortening the laser cavities. A sensitivity as high as 51.1 MHz/g(or 5.192 GHz/N) has been achieved with a laser with an effective cavity length close to 1 mm. The corresponding mass resolution reaches 9.78×10-5 gram. The result offers useful guidance for transducer design for fiber laser sensors towards the detection of extremely small mass or weak perturbations.Third, photonic hydrophones based on dual polarization fiber lasers with corrugated diaphragm transducer has been implemented for acoustic detection within the range from 100 Hz to 2000 Hz. The corresponding water pressure sensitivity reaches 110 k Hz/Pa at 1k Hz by shortening the cavity length, in accordance with the theory developed from the spatial-sensitivity characterization.