Wavelength-tunable fiber comb filter with wide bandpass characteristics based on a first-order polarization-diversified loop

For the first time, a novel design methodology is presented for the wide bandpass transmittance function (WBTF), enabling spectral bandwidth extension by controlling the incident state of polarization on the second polarization-maintaining fiber segment in a first-order fiber comb filter based on a polarization-diversified fiber loop configuration. An analytical expression for the final transmission spectrum is derived as a function of the design parameter, and the feasibility of continuous wavelength tunability is verified through both theoretical analysis and experimental validation. A closed-form transmittance expression for arbitrary phase shifts is also formulated, allowing direct determination of suitable waveplate orientation angles via numerical evaluation and eliminating exhaustive angular searches. The proposed angular search procedure identifies 360 distinct waveplate orientation sets, each corresponding to a 1° incremental phase shift over the full 0°–360° range, confirming the WBTF’s tunability. Eight wavelength-shifted spectra corresponding to specific phase shifts of 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° are computed, demonstrating that deliberate waveplate angle selection enables continuous wavelength tuning. Experimental measurements validate the predicted frequency tuning, confirming the practical feasibility of the mechanism. To the best of our knowledge, this work presents the first spectral bandwidth design that enhances filter performance by manipulating the polarization trajectory in a first-order filter based on polarization-diversified fiber loop, offering a versatile approach for advanced optical filtering applications.