The design of ultranarrow dual-transmission-band optical FBG filter with controllable wavelength spacing

An optical bandpass filter with two ultranarrow transmission bands, based on fiber Bragg grating (FBG), is presented. The wavelength spacing can be easily changed through controlling the separate distance between two introduced π phase shifts. When it is served in a fiber ring laser, the stable dual-wavelength lasing with ultranarrow wavelength spacing is achieved. This novel device will find potential applications in the generation of various high-frequency microwave signals, fiber sensing, etc.