Research On The Characteristic Optimization Problem Of Edge Spectrum Chirp Fiber Bragg Grating

The reflection spectrum of edge spectrum chirp fiber Bragg grating(ES-CFBG) has the characteristics of wide bandwidth and linear edge, which is a linear relationship between the reflectivity and the incident wavelength. Filter demodulation technology has the advantages of low cost, simple structure and high demodulation accuracy based on this kind of grating. Therefore, the spectral characteristics of ES-CFBG is important for the application of demodulation technology. However, in the existing studies, it is still a serious problem to design and process ES-CFBG with better spectral performance. In this paper, the refractive index modulation distribution is regarded as the optimization goal, and the optimization design method of ES-CFBG is studied. From its structure parameters, exposure optical path of linearly chirped phase mask is optimized. The main achievements are as follows:1) A optimization design method used in the design of ES-CFBG is proposed. The discrete layer peeling algorithm is used to get the target reflection spectrum of the refractive index modulation, which is seen as initial samples' boundary conditions of cross entropy optimization algorithm, then the refractive index modulation is optimized iteratively. Afterwards, the influences of various factors on the spectral characteristics, such as the refractive index modulation range, the weighting factor, the smoothing factor and the refractive index disturbance factor are analyzed. The simulation results show that both single channel and dual channel ES-CFBG have rapid convergence and good linear reflection spectrum edge characteristic and tolerance ability within the bandwidth of 10 nm.2) On the basis of the above research, a optimization method of exposure optical path for producing the linearly chirped phase mask is proposed. The cylindrical wave is used to eliminate the groove bending phenomenon in the vertical direction, instead of the spherical or non spherical wave in the traditional holographic exposure system. Then, according to the spectral characteristics and structural parameters of ES-CFBG, a general mathematical model for the distribution of grooving cycle is established, and genetic algorithm is used to optimize the parameters of the exposure optical path. The numerical simulation results show that incident angle and incident length are related to center period and chirp coefficient of the mask separately. Thus greatly simplify the exposure control of linear chirped phase mask and implementation method of ES-CFBG.The research work of this paper provides a theoretical guidance for the optimization design and realization method of ES-CFBG, and it has certain guiding significance to the construction and control of exposure optical path. It is easier to design and process ES-CFBG with large bandwidth, linear edge and fast convergence properties.