Fiber comb filters are widely concerned because of simple structure, low cost, strong stability, and strong compatibility with optical fiber communication systems. A common application scenario for fiber comb filters involves multiwavelength fiber lasers ( MWFLs ). Generally, an all-fiber comb filter is fixed with a constant channel interval that is challenging to meet the needs of tunability in some practical uses. In previous research, our team has conducted detailed theoretical studies and transmission characteristics tests on a double Sagnac loop and experimentally verified its viability in channel interval switching when used in MWFL. In this research, we unravel the double Sagnac loop and transform it into a novel dual Lyot filter based on parallel polarization-maintaining fibers. The findings of theoretical analysis and experimental tests demonstrate that the channel interval is switchable and the extinction ratio is tunable with the polarization controller’s adjustment. The MWFL based on the dual Lyot filter is capable of generating a multiwavelength output with two various channel intervals. This research may provide some references for scholars who engage in the study of MWFL with high flexibility.

First, the designed dual Lyot filter’s transmission function is derived based on the theoretical analysis and the transfer matrix theory. Then, the transmission spectrum is simulated using the Matlab software. The channel interval switch function is confirmed from a theoretical perspective. By replacing the polarization-maintaining fibers (PMFs), various wavelength interval combinations can be accessible, further confirming the filtering structure’s flexibility in obtaining channel interval switching. After that, the impact of the polarization controller (PC) before PMF on the extinction ratio ( ER ) of the transmission spectrum is discussed. Furthermore, a PC is added to the proposed filter to optimize the filter’s ER performance and tunability. Moreover, to confirm the theoretical conclusion, the experimental measurement of the transmission spectrum and the performance comparison of the filter before and after the optimization are conducted. Finally, a multiwavelength fiber laser based on the optimized filter is experimentally demonstrated, and its channel interval switch ability is confirmed in the laser system.

The dual Lyot filter’s transmission function (Eq. 10) is derived using a transmission matrix. The filter’s simulation transmission spectrum is drawn to simulate channel interval switching (Fig. 2) with the following parameters: the coupling coefficient k₁=k₂=0.5, the two PMFs’ birefringence B_PMF1=B_PMF2=5.1×10^-4, and the PMFs’ lengths L₁=7.2 m and L₂=13.5 m. When the polarized light’s angle to the fast axis of PMF2 is a multiple of π/2, the filter’s channel interval is produced by PMF1 with a length of 0.65 nm. Similarly, when the polarized light’s angle to the fast axis of PMF1 is a multiple of π/2, the channel interval is 0.35 nm produced by PMF2. The simulation findings are consistent with the theoretical computation. Changing the characteristics of two parallel PMFs with the following parameters: B_PMF1=4.0×10^-4, B_PMF2=5.1×10^-4, L₁=6.6 m, and L₂=10 m. Two new channel intervals of 0.91 nm and 0.47 nm can be obtained (Fig. 3). In addition to the switching of channel interval, the PC can be adjusted for the ER turning. The filter transmission spectrum’s ER varies sinusoidally as the polarized light’s angle to the PMFs’ fast axis (x-axis) increases (Fig. 4). In the transmission characteristic experiment, the same PMFs as Fig. 2 are taken to compare the filter’s maximum amplitude before and after optimization (Fig. 6). The transmission spectra with the channel intervals of 0.65 nm and 0.34 nm obtained agree with the simulation findings. The optimized filter demonstrates a better ER and tunability. An experiment is performed to confirm that the dual Lyot filter allows channel interval switching in laser applications (Fig. 7). In the experiment, the highly nonlinear fiber is 105 m long, the coupling ratio of OC3 is 10: 90, and the rest device parameters are the same as those for testing transmission characteristics. Modifying the PC gives two multiwavelength laser outputs with channel intervals of 0.65 nm and 0.34 nm, respectively (Fig. 8). The two laser outputs’ channel intervals are consistent with the channel interval measured by the filter transmission spectrum.

In this research, an all-fiber comb filter with a switchable channel interval is proposed based on theoretical derivation and experimental verification. The filter’s channel interval is determined by the PMFs connected in parallel in the dual Lyot filter. By replacing the PMFs, various wavelength interval combinations are accessible, meeting the requirements of diverse application situations. Careful adjustment of the PC allows the channel interval switching and the ER’s tuning. The experimental findings show an optimized turnability by adding a PC to the filter. Moreover, the filter’s practicality is further shown using the designed filter with a multiwavelength fiber laser. The two PMFs’ length difference in the filter is preferable in the unit of meter. Additionally, it is challenging to switch the channel intervals if they are very close, because of the challenges of observing the polarization state in the optical path. However, for the close channel intervals, the filter under this structure is better to be used as a component of a highly sensitive sensor with a vernier effect rather than a wavelength selection element of MWFL with a flexible channel interval. In conclusion, the dual Lyot filter has considerable uses in MWFLs, dense wavelength division multiplexing optical networks, and fiber optic sensors.