Research On High Resolution Dual-frequency Fiber Laser Accelerometer

Accelerometers have been playing an important role in numerous fields including geological exploration, inertial navigation, structure health monitoring and so on. High-resolution accelerometer has important significance especially for the resource exploration, guidance systems and seismic wave exploration fields. These applications require small sensor sizes for installment convenience and the ability to work under harsh environments. Compact electric seismometers have been developed with minimal detectable accelerations of ng order but lack multiplexing ability and suffering from electromagnetic noise. Among optical accelerometers,fiber Bragg grating based accelerometer owes improved performance while suffers from low signal to noise ratio and resolution, which is difficult to demodulate at the same time. Fiber laser accelerometer can be divided into wavelength modulation and beat frequency modulation according to working principle. Dual frequency fiber laser accelerometer based on beat frequency modulation, transforming vibration signal to beat frequency signal in RF domain, not only significantly reduce the cost of demodulation but also has intrinsic wavelength multiplexing ability which can be easily cascaded in array.In this thesis, based on the spring- mass system transducer structure, using dual frequency fiber laser as sensing primitives, the acceleration signal can be effectively transformed into the beat frequency signal output. A high resolution dual frequency fiber laser accelerometer is demonstrated.The main content of the thesis is as follows:（1） The sensing principle and preparation is elaborated. The dual frequency fiber laser sensing principle is discussed with its impact from outside stress being analyzed. The theoretical analysis of the dual frequency fiber laser generating mechanism is discussed hereafter.（2） The structure is designed in accordance with the theory. According to the sensing mechanism of the dual frequency fiber laser, we design transducer structure based on the spring- mass system. In addition, theoretical analysis of the system detection capability change caused by dual-frequency fiber laser with different cavity lengths is discussed from the perspective of sensitivity and noise.（3） We discuss the detection capability of dual frequency fiber laser accelerometer. Experimental results show that the resolution can reach 175 ng/Hz^1/2 at 1 kHz, with working bandwidth over 1 kHz, based on dual frequency fiber laser with cavity length of 5 mm. An accelerometer with higher resolution is proposed by using dual frequency fiber laser with cavity length of 2 mm. The accelerometer obtains a resolution of 107 ng/Hz^1/2 at 200 Hz with a working bandwidth of about 600 Hz. With the pressure loading on the laser cavity being unified, the theory that dual frequency fiber laser with shorter cavity length can have better detection capability is confirmed by the experimental data.