Dispersion effects in fiber Bragg gratings based all-optical microwave filters

Abstract

Different architectures for all-optical microwave filtering using fiber Bragg gratings (FBGs) have been proposed in the last few years. Most of the research efforts have been focused on the basic structures and synthesis of wideband FBG-based all-optical microwave filters with possible reconfigurability and tunability, where all FBGs are assumed to have identical time delay for all the frequency components of the incident light. However, when the filter is proposed to work at very high frequency range, the dispersion effects induced by the FBGs have to be addressed. In this paper, a theoretical model is established to study the dispersion effects in the FBG-based all-optical microwave filter. Simulations and experiments are carried out. The results show that the effective coefficients of the filter are not constant, but a function of the modulating frequency, which will degrade the performance of the all-optical microwave filters and limit the highest operating frequency. To reduce the dispersion effects, we propose to use properly-apodized short FBGs with relatively flat reflection spectra and group delay responses.