Investigation On The Multi-Channel Amplitude-Frequency And Phase-Frequency Filtering Performances Of Phase-Sampled Fiber Gratings

Fiber Bragg gratings (FBGs) are characterized by flexible filtering capability, all-fiber configuration, low insertion loss, and low cost. Therefore, FBGs have been considerder as one of hot topics around the world. The phase-sampling FBGs (PS-FBGs) are the product of continuous development of FBGs technology, which have not only multiple-channel comb spectra, but also more innovative features by modulating the phase/the amplitude of the refractive index of FBGs. Thus they have important applications in many fields such as filtering, sensing, dispersion compensation. In this thesis, the multiple-channel filters based on the PS-FBGs are designed and analyzed, providing both the amplitude-frequency and the phase-frequency responses with normal bandwidth or narrow bandwidth.Firstly, the structure of the sampled FBGs and the classic analystic model for the sampled fiber Bragg grating (i.e...Fourier series models) are presented. Then, by using the transfer matrix method, the influences of the sampling period, the duty cycle, and the index modulation’s change on the spectra of the amplitude-sampling FBGs are investigated. At the same time, the index modulation process and the analysis method for PS-FBGs are analyzed.Secondly, based on the simulated annealing algorithm, PS-FBGs based multiple-channel filters with normal bandwidth are designed. Starting from discrete and continuous phase-sampling theories, the optimized phase distributions for the discrete and the continuous phase-sampling methods are obtained respectively, with the use of the simulated annealing algorithm. The amplitude-frequency response for the designed multi-channel filter shows good consistency and high diffraction efficiency. The 3-dB bandwidth for 9-channel filter with dicrete phase sampling and 5-channel filter with continuous phase sampling are 0.15 nm and 0.18 nm, respectively. The delay profile of each channel is substantially flat. In order to expand the phase processing capabilities, a linear chirp is introduced. The resulting delay profile has a certain linear region. By using numerical fitting, the dispersion values are estimated to be 540-570ps/nm and 680～720ps/nm for the discrete and the continuous phase samplings, respectively, which might be used for dispersion compensation. In addition, the 3dB bandwidth for each channel is increased.Finally, in order to achieve narrow-band multiple-channel filters, single or multiple π phase-shift are added to the discrete and continuous phase-sampling multiple-channel FBGs. Consequently, a narrow-band tramission channels in each narrow-band reflection peak is resulted, such that multiple narrow-band tramission channels are achieved. With the increase of the used phase shifts, the 3dB bandwidth for narrowband transmission response in each channel is increased. Taking 9-channel filters for example, the 3-dB bandwidth of the amplitude-frequency responses is 1GHz when a single phase shift is inserted; such a bandwidth can be increased to 6GHz as multiple phase shifts are introduced, while the rectangular shape coefficient is improved. After the introduction of the phase shifts, fluctuations appear in the phase-frequency responses, which would be reduced by incresing the numbers of phase shifts.