Ultranarrow dual-transmission-band filter based on an all-fiber two-cavity structure composed of three fiber Bragg gratings

An all-fiber flexible two-cavity structure composed of three fiber Bragg gratings (FBGs) is proposed to realize a kind of ultranarrow dual transmission band filter. General condition for transmission spectra with symmetric two resonant peaks is presented and analyzed. The influences of various structure parameters on transmission responses are simulated and discussed, including index modulation depths and lengths of three FBGs and cavity lengths. By properly designing the structure parameters, two identical and symmetric transmission peaks can be produced in the stop band. The 3-dB bandwidth of each peak can be as narrow as 1 pm or even less than 1 pm. The spacing between the two transmission peaks can be flexibly designed. The fabrication error tolerances of the structures, including inaccurate cavity lengths and index modulation depths of FBGs, are investigated. Comparing with the conventional single-cavity structure, the proposed two-cavity structures provide better control on the suppression of sidebands. In applications of fiber lasers, the proposed two-cavity structure with the ultranarrow dual transmission-band can help in building the dual-wavelength single-longitudinal-mode operation at room temperature.