Design and analysis of an improved parallel Lyot filter based on secondary interference

We introduce a fiber filter with a switchable channel interval, featuring a polarization beam splitter (PBS) and two parallel Lyot filters. A unique modification in one branch induces secondary interference. The combined beams are output through the PBS. The transmission function, derived using Jones matrix theory, was validated via simulation. Subsequently, experimental tests on the filter, designed for channel intervals of 0.88, 0.41, and 0.2 nm, showed that new intervals can be created by replacing the polarization-maintaining fiber. Adjusting the polarization controller allows tuning of the extinction ratio and peak position, with the peak tuning range being half of the channel interval. Finally, the designed filter’s feasibility for use in a multiwavelength random distributed feedback fiber laser was experimentally confirmed. This laser can switch channel intervals and output 1 to 4 channels, demonstrating stability with a central wavelength drift under 0.47 nm and power fluctuation below 0.64 dB over 30 min at room temperature.