Study on Optical Fiber First-Order Comb Filter Based on Heterogeneous Wave Retarder Combination for Simple Continuous Wavelength Control of Passband-Flattened Comb Spectrum

In this paper, we propose an optical fiber first-order comb filter based on a polarization-diversity loop structure(PDLS), whose passband-flattened comb spectrum can be continuously wavelength-tuned in a convenient way through heterogeneous wave retarder combination. The proposed filter consists of a polarization beam splitter, two equal-length polarization-maintaining fiber(PMF) segments, three quarter-wave retarders(QWRs) and one half-wave retarder(HWR). For convenient wavelength control, we inserted a set of a QWR and an HWR between two PMF segments. Using the filter transmittance theoretically derived with Jones calculus, we found the azimuth angle(AA) sets of four wave retarders, which enable the additional phase difference ø to change from 0° to 360° On the basis of these AA sets, we theoretically calculated passband-flattened transmittance spectra spaced with a wavelength interval of 0.1nm. Then, we experimentally demonstrated theoretical results.