Wavelength-Tunable First-Order Narrowband Fiber Comb Filter Incorporating All-Quarter-Wave Polarization Transformation

In this article, we propose a wavelength-tunable first-order narrowband fiber comb filter that incorporates all-quarter-wave polarization transformation. The proposed filter is composed of a polarization beam splitter to form a polarization-diversified fiber loop, two equally long polarization-maintaining fiber segments, and four quarter-wave plates to modify the absolute phase of the narrowband transmittance of the filter. The transmittance function of the filter was found theoretically utilizing the Jones matrix formulation. Based on this filter transmittance, we derived the azimuthal angles (AAs) of the waveplates, which can tune the spectral wavelength of the filter by continuously adjusting the transmittance phase. Narrowband comb spectra calculated at some selected sets of these four waveplate AAs showed sequential wavelength changes. In particular, the calculation result was experimentally validated.