All-optical bandpass microwave filter based on a superstructured fiber Bragg grating with equivalent chirp

Abstract

An all-optical bandpass microwave filter that is implemented using an optical phase modulator and equivalent-chirped superstructured fiber Bragg gratings (SFBGs) to generate negative coefficients is presented in this paper. It is well known that SFBGs, also called sampled FBGs, contain many Fourier orders in their spectrum. By appropriately chirping the period of the sampling function of the SFBG, it is possible to achieve an equivalent chirp in the +1 and the -1 orders. This method allows the fabrication of FBGs with different equivalent chirp rates by using a single uniform phase mask. While other methods to create chirped FBGs require multiple phase masks or variable tension on the optical fiber during the FBG writing process, the use of SFBGs eases the requirements for the fabrication of specific phase and amplitude responses. This is achieved by tailoring the sampling function of the SFBG instead of the Bragg period of the phase mask. In this paper, a two-tap all-optical bandpass microwave filter is demonstrated by using an equivalent-chirped SFBG. The negative coefficients of the filter are realized by exploiting the characteristics of the phase-modulation-to-intensity-modulation (PM-IM) conversion in the CFBG.