Study on Continuous Wavelength Tuning of Flat-Top Comb Spectrum Using Contiguous Quarter-Wave Polarization Conversion

In this paper, we investigate the continuous wavelength tuning of the flat-top comb spectrum by harnessing continuous quarter-wave polarization conversion in an optical fiber filter based on a polarization-diversity loop configuration (PDLC). The fiber filter consists of two polarization-maintaining fiber segments of equal length to create comb spectra, four quarter-wave retarders (QWRs) to tune the spectrum wavelength, a polarization beam splitter to form the PDLC, and a half-wave retarder to maximize the spectrum visibility. Using the Jones calculus, we theoretically found specific orientation angles of the four QWRs for the continuous wavelength tuning of the flat-top comb spectrum. From the spectral calculation at these specific orientation angles of the four QWRs, it was confirmed that the continuous wavelength tuning of the flat-top comb spectrum could be implemented by appropriately adjusting the orientation angles of the four QWRs. The theoretical prediction was then experimentally demonstrated.