Study on Continuous Wavelength Tuning of Narrowband Multiwavelength Spectrum in Optical Fiber Multiwavelength Filter Using Contiguous Quarter-Wave Polarization Transformation

In this study, we scrutinize the continuous wavelength tuning capability of the narrowband multiwavelength spectrum in an optical fiber multiwavelength filter using contiguous quarter-wave polarization transformation. The multiwavelength filter is based on a polarization-diversity loop structure composed of a polarization beam splitter, two equal-length polarization-maintaining fiber segments, four quarter-wave plates (QWPs) used to tune the spectrum wavelength, and a half-wave plate utilized to maximize the spectrum visibility. By utilizing the transmittance function of the filter derived through the Jones matrix formulation, we theoretically found specific orientation angles of the four QWPs for the continuous wavelength tuning of the narrowband multiwavelength spectrum, which could induce an extra phase difference φ from 0° to 360° in the filter transmittance. From the spectral calculation at the eight selected orientation angle sets of the four QWPs, it was revealed that the wavelength tuning of the narrowband multiwavelength spectrum could be implemented at 0.1nm intervals by properly adjusting the orientation angles of the four QWPs and this approach could be easily extended for the continuous wavelength tuning of the spectrum. The theoretical prediction was then experimentally verified.