Sub-kilohertz linewidth fiber laser by using Bragg grating filters

A single-frequency (SF) fiber laser is widely applied in many fields. A superior ultra-narrow-bandwidth fiber filter, which consists of an in-series commercial common fiber Bragg grating (FBG) filters with a partial overlap in their reflection spectrum, is presented in this paper. The bandwidth of the filter with a center reflection wavelength of 1549.83 nm reaches 0.024 nm at 3 dB reflection, a remarkable reduction of approximately 300% by combination usage narrower than a single FBG used bandwidth. We also demonstrate a method to further narrowing of the filter bandwidth by increasing the usage times of filters, and the bandwidth can reach 0.0025 nm at 3 dB reflection. A long linear cavity single-frequency fiber laser with an entire cavity of 30 cm is implemented with the linewidth of 941 Hz at 3 dB, which is about 210% narrower than the previous record of SF fiber laser based on FBG filters in a linear cavity. This research may open a new door toward ultra-narrow-bandwidth filters and SF fiber lasers.