Laser-oriented design and performance analysis of series-parallel lyot filters with cross-output

In this study, a fiber comb filter with switchable channel intervals is designed. The structure is composed of two Lyot filters connected in parallel via two polarization beam splitters (PBS), with two circulators (CIR) added to connect the two branches in series, enabling cross output. The transfer function of the filter is derived and analyzed using transfer matrix theory. Simulation results indicate that the filter can generate two small channel intervals and one large channel interval. Experimental testing confirms three channel intervals consistent with theoretical predictions, namely 0.2 nm, 4.36 nm, and 0.3 nm. Additionally, a fourth small channel interval of 0.46 nm is achieved by adjusting the angular difference between the principal axis of the PBS and the principal axis of the polarization-maintaining fiber (PMF). It is also observed that the extinction ratio (ER) and peak wavelength of the filter can be tuned by varying the rotation angle of the polarization controller (PC). A randomly distributed feedback fiber laser (RDFBFL) based on the designed filter is demonstrated to achieve switchable channel intervals across five laser channels. At room temperature, the maximum wavelength drift of the central wavelength over 30 min is measured to be 0.082 nm, while the maximum peak power fluctuation is 1.09 dB. Furthermore, the tunability and stability of a multi-wavelength fiber laser (MWFL) based on the filter are evaluated. With four laser channels, the channel intervals remain switchable, and the output power fluctuation of the new channel intervals is less than 1.07 dB within 30 min, while the frequency drift is less than 0.258 nm.