Polarization-independent wavelength-tunable flat-top comb filter based on quarter-wave polarization transformation

Abstract

Abstract By harnessing quarter-wave polarization transformation, we propose a polarization-independent wavelength-tunable flat-top fiber comb filter composed of a polarization beam splitter, two polarization-maintaining fiber segments of equal length, a set of a half-wave plate and a quarter-wave plate (QWP), and dual QWPs, the simplest polarization controller. Using the Jones calculus, we derived the polarization-independent filter transmittance and sought the possible loci of the orientation angles (OAs) of the four waveplates as the functions of the transmittance phase (ϕ). With these OA loci, flat-top transmission spectra were calculated for eight ϕ values from 0° to 315° (increment: 45°), which manifests the frequency tunability of the filter. This theoretical prediction was also experimentally verified by using a broadband light source and optical spectrum analyzer. Eight measured flat-top transmission spectra were spaced 0.1 nm apart in wavelength as predicted, and the wavelength-tuning linearity was high enough to exhibit an adjusted R 2 value of ∼0.99925.