Study on Continuous Wavelength Tuning of Targeted Transmission Spectrum in Polarization-Diversified Fiber Loop Multiwavelength Filter Based on Quarter-Wave Polarization Conversion

In this study, we investigated the continuous wavelength tuning capability of a targeted transmission spectrum (TTS) in a polarization-diversified fiber loop multiwavelength filter based on quarter-wave polarization conversion. The multiwavelength filter consists of a four-port polarization beam splitter, two polarization-maintaining fiber segments of equal length, four quarter-wave retarders (QWRs), and one half-wave retarder (HWR). Within the polarization-diversified fiber loop structure, the HWR optimizes the multiwavelength transmission spectrum by maximizing its extinction ratio. We mathematically modeled the optical components within the filter using the Jones matrix formulation, and derived the general transmittance function of the filter from this model. Using this transmittance function, we selected a TTS at specific azimuth angles of the wave retarders. Theoretically, it was confirmed that, by adjusting the azimuth angles of the QWRs appropriately, an additional phase shift φ could be introduced into the filter’s transmittance function, which defines the TTS. This enables continuous wavelength tuning of the TTS. To verify this experimentally, the filter was constructed, and eight transmission spectra exhibiting a 0.1nm red shift between spectra were successfully obtained. This validates the continuous wavelength tuning capability of the TTS in our multiwavelength filter.