Study on Continuous Wavelength Tuning of Flat-Top Comb Spectrum in Polarization-Diversity Fiber Multiwavelength Filter Based on Variable Waveplates

In this paper, we theoretically and experimentally demonstrate the continuous wavelength tuning of flat-top comb spectra in a polarization-diversity loop fiber multiwavelength filter based on variable waveplates (VWs). The multiwavelength filter consists of a four-port polarization beam splitter (PBS), two high birefringence fiber (HBF) segments of the same length, a half-wave plate (HWP), and two VWs, with each VW placed in front of an HBF segment. In the polarization-diversity loop formed by the PBS, the HWP is used to set the effective orientation angle of the second HBF segment to 22.5° with respect to the horizontal axis of the PBS to maximize the visibility of the filter output spectrum. The transfer matrix of the filter was theoretically derived using the Jones matrix formulation, and its transmittance function was obtained from the derived transfer matrix. Based on the filter transmittance, we determined the combinations of the orientation angle (OA) and phase delay (PD) of the VW, which can provide an additional phase difference ranging from 0° to 360° with a 1° increment. Some selected combinations of OAs and PDs were used to calculate flat-top comb spectra, which could be wavelength-tuned in 0.1nm increments. The theoretical analysis was experimentally validated using the fabricated multiwavelength filter. The theoretical and experimental results show that precise wavelength tuning of flat-top comb spectra in this multiwavelength filter can be achieved through simple adjustment of the OAs and PDs of the VWs.