Abstract
A polarization-diversified loop structure (PDLS) formed by a four-port polarizing beam splitter has been employed to establish the input polarization independence of optical devices. Here we report a PDLS-based optical fiber birefringence filter, which can provide continuous frequency tunability in passband-squeezed comb spectra by harnessing quarter-wave polarization conversion. The birefringence filter is composed of a four-port PBS, two high birefringence fiber (HBF) segments whose lengths are equal, the first polarization controller (PC) comprised of a quarter-wave retarder (QWR) and a half-wave retarder (HWR), which is located in front of the first HBF segment, and a set of two QWRs as the second PC that lies before the second HBF segment. The dual QWRs, chosen for the second PC, are the simplest form of a PC that can convert arbitrary input polarization into desired polarization. The slow axis of the second HBF segment is oriented at 22.5° with respect to the horizontal axis of the PBS. Through the Jones matrix formulation, we derived the sinusoidal passband-squeezed transmittance function of the filter, whose absolute phase (ϕ) should be continuously varied for the contiguous wavelength tuning of filter spectra. Then, using this filter transmittance, we found the possible loci of the orientation angles (OAs) of the four wave retarders as the functions of ϕ. By utilizing the OA loci of the wave retarders, the theoretical passband-squeezed transmission spectra of the filter were calculated for eight equally spaced values of ϕ from 0° to 315° (with an increment of 45°). This theoretical calculation was also verified by measuring the wavelength-tuned passband-squeezed spectra of the fabricated filter. We theoretically and experimentally confirmed that our filter could provide the wavelength tunability of the polarization-independent passband-squeezed comb spectrum by appropriately controlling the OAs of the wave retarders.
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