Enhanced inline conversion of fiber Bragg grating spectra based on increased polarization controllability

We have demonstrated enhanced spectral conversion of a fiber Bragg grating (FBG) incorporating a polarization-diversity loop configuration (PDLC) with increased degree of freedom (DOF) in polarization control through the addition of wave plates. The proposed apparatus for spectral conversion consists of a four-port polarization beam splitter for constructing the PDLC, an FBG, and three wave plates including one half-wave and two quarter-wave plates. In the previous study using only two wave plates, slight birefringence of single-mode fibers connecting optical elements within the polarization-diversity loop induced random changes of states of polarization (SOPs), resulting in the spectral degradation. In this approach, the increased number of wave plates could complement it by providing a higher DOF in controlling SOPs and significantly improve spectral performances of the fabricated filter such as an insertion loss, side-mode suppression ratio, and band-rejection ratio.