Dual-wavelength Fiber Lasers Sensor Based On Beat Frequency Modulation Technology

Over the past several years, dual-wavelength fiber lasers have attracted more and more attention owing to advantages of simple structure, small size, low price, narrow linewidth, and anti-electromagnetic interference. In this thesis, a dual-wavelength FBG laser is fabricated. Employing the laser, we propose a novel dual-wavelength fiber laser sensor based on beat frequency modulation technology.First of all, we review the development of FBG(Fiber Bragg Grating) sensors and fiber laser sensors. Compared with other sensor systems, fiber laser sensors have a lot of advantages, for example, low signal-to-noise ratio and narrow linewidth, which determine that sensors have inherent high resolution. These advantages promote the application of the fiber laser, such as oil exploration, earthquake prediction and so on. Based on the discussion of different demodulation methods, it can be conclude that the beat frequency demodulation is a simple, low cost technology. However, the single-mode operation of two orthogonal polarizations laser is a little difficult to establish and the repetition may be not stable. To solve these problems, a dual-wavelength fiber laser is employed in the novel sensor system.In a dual-wavelength laser sensor system, high-performance dual-wavelength laser is the core device, so in the second section, we concentrate on production of dual-wavelength lasers. The fundamental theory of FBG is illustrated in detail, including CMT (Coupled Mode Theory) and TMM (Transmission Matrix Method). Result of simulation shows that two^ phase shifts should be inserted in the FBG in order to get a dual-wavelength laser. It is reported by X.F. Chen that Equivalent Phase Shift (EPS) technology can introduce phase shifts in FBG conveniently The EPS methods can greatly reduce the fabrication cost as the precision is decreased from the micrometer to nanometer scale. Based on the EPS technology, we obtain a stable dual-wavelength laser with the wavelength of 1556.713nm and 1556.753nm.Using the dual-wavelength laser, we design a fiber laser sensor system. Theoretically, the strain applied on the sensor can be demodulated by monitoring the beat frequency of the dual-wavelength laser. In the experiment, the beat frequency of the sensing signal varies almost linearly with the strain and the sensitivity is about-3.92kHz/με. By employing a LiNbO3 modulator, high-frequency sensing signal (more than 5 GHz) can be tuned arbitrarily to tens or hundreds of megahertz without distortion and thus a low-frequency frequency spectrum analyzer can be used. Finally, through a number of experiments, we confirm that the sensor we proposed is reliable in some point.