An unprecedented fiber narrow-band filter is proposed, which is composed of a phase-shifted fiber grating with an anti-symmetric refractive index distribution and a uniform fiber grating. The anti-symmetric refractive index distribution is obtained by performing a one-sided exposure on both sides of the few-mode fiber, where the two exposure positions differ by half period of the grating. By introducing an anti-symmetric refractive index distribution, coupling resonance occurs in the \(\hbox {LP}_{01}\) and \(\hbox {LP}_{11}\) modes in the two-mode fiber, and the reflected light and transmitted light are separated into \(\hbox {LP}_{11}\) mode and \(\hbox {LP}_{01}\) mode, respectively. To achieve a narrow-band reflection spectrum, the \(\pi \) phase is introduced into an anti-symmetric refractive index grating, and its transmitted light is reflected through a uniform grating. Single-mode fiber and taper-coupled fiber are introduced to select \(\hbox {LP}_{01}\) mode selection, and the \(\hbox {LP}_{11}\) mode is cutoff. Finally, the laser linewidth characteristics of the proposed structure are analyzed by co-simulation with semiconductor gain chips. Simulation results show that the proposed structure can improve the linewidth of single-longitudinal mode lasers, which has very important potential applications in the fields of Lidar, coherent communication and sensing.
