Fiber Laser Sensor Based On Beat Frequency Demodulation Method

In the past few decades, fiber laser sensors have been investigated greatly due to their high sensitivity, high signal-to-noise(SNR), immunity to electromagnetic interference and so on. Many applications with fiber laser sensors have been proposed to measure strain, temperature, vibration and other parameters. In the demodulation methods of fiber laser sensor, The beat frequency demodulation method provides a simple electronical signal processing solution.The thesis focuses on Multilongitudinal Fiber laser sensor based on beat frequency demodulation method. The major results are summarized as follows:(1)The basic principle of fiber laser which includes the mechanism of erbium doped fiber, the theory of fiber Bragg grating, some fiber cavity structures and the mode theory in fiber laser.(2)An improved linear cavity fiber laser is proposed for temperature measurement. The measured temperature sensitivities are-4.22kHz/℃@1004.8MHz and-16.78kHz/℃@4041.2MHz.(3)A fiber ring laser is proposed for strain measurement. In the ring cavity, no reflector or mode selecting device is employed. As a result, there will be more longitudinal modes in the laser than those using FBGs as reflectors. The very high beat frequency can be achieved by beating two modes with large mode spacing, which has high sensitivity in the measurement. The highest sensitivity reaches-11.1kHz/με. (4)A chromatic cavity fiber laser is proposed for strain measurement. The strain is applied in the FBG. Traditionally, the wavelength shift of FBG is monitored to obtain the strain. The chromatic cavity fiber laser proposed in the thesis converts the FBG wavelength shift into beat frequency shift to demodulate the strain.(5)The sensors proposed above are sensitive to strain and temperature. So the simultaneous measurement or discrimination of the two parameters becomes a very important issue in practical application. Based on a basic linear cavity fiber laser, we utilize the sensitivity of erbium doped fiber to obtain the simultaneous measurement of strain and temperature.