| Synthesis and implementation of a fiber Bragg grating (FBG) based all-optical microwave filter are presented in this thesis. The all-optical microwave filter is synthesized using genetic algorithms, and is implemented using fiber Bragg gratings, and other photonic components.; To design an FBG-based all-optical microwave filter with given specifications, the tapping coefficients (or the reflectivities of the FBGs) and tapping intervals should be carefully determined. Standard digital filter design algorithms cannot be applied readily to all-optical microwave filter synthesis; because incoherent interference prevents the inclusion of negative coefficients. In this thesis, genetic algorithm (GA) is applied to the optimum design of finite impulse response (FIR) filters. As numerical techniques from nature evolution, genetic algorithms are well suited for simultaneous optimization of multivariable problems and can also be used to find families of solutions.; To implement the FBG-based all-optical microwave filters, some practical issues must be addressed carefully. First, it is difficult to fabricate an FBG with peak reflectivity less than 0.1. With genetic algorithms, one may simply specify a low bound to the reflectivity and this problem can be solved easily. Another important fact is that the power spectrum of the modulated broadband light is not as flat as we expect, using an un-flat light source will lead to a microwave filter with poorer performance. To solve this problem, in the algorithms a variable is introduced to calibrate the filter coefficients according to the power spectrum of the incident light.; Experiments are carried out using two FBG arrays with the reflectivities obtained using the genetic algorithms. It is found that the experimental results agree well with the theoretical results if the filter coefficients are calibrated by considering both the un-flat power spectrum of the light source and the power reflectivities of the FBGs. (Abstract shortened by UMI.)... |
