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Universal Wavelength Tuning Scheme for a First-Order Optical Fiber Multiwavelength Filter Based on a Polarization-Diversified Loop Structure.

In this paper, we propose a universal wavelength tuning scheme for a first-order optical fiber multiwavelength filter based on a polarization-diversified loop structure (PDLS), which is comprised of a polarization beam splitter, two sets of a half-wave plate (HWP) and a quarter-wave plate (QWP), and two polarization-maintaining fiber (PMF) segments. First, the polarization conditions, which should be satisfied to continuously tune the absolute wavelength of a transmittance function with an arbitrary transmission spectrum, are explained based on the continuous wavelength tuning mechanism of the flat-top band transmittance, which was previously reported in the PDLS-based passband-flattened first-order multiwavelength filter. Then, by considering the effect of the four waveplates (i.e., two HWPs and two QWPs) and two PMF segments on the output polarization of each element comprising the filter, we suggest a systematic scheme to find the azimuth angles of the four waveplates for continuous wavelength tuning of the arbitrary transmittance function. Three hundred sixty sets of the four waveplate azimuth angles, which give additional phase shifts of 1-360° (with a step of 1°) to an arbitrarily chosen transmittance function, are obtained with the suggested angle finding approach. Wavelength-tuned transmission spectra are displayed at eight waveplate angle sets, which are chosen from among the above angle sets so that they can induce extra phase shifts of 0, 45, 90, 135, 180, 225, 270, and 315° to the original transmittance function. These spectra prove that the proposed scheme can be applied to the wavelength tuning of every transmittance function of the PDLS-based first-order multiwavelength filter.