Analysis on output polarization characteristics of fiber comb filters based on polarization-diversity loop structure

The output states of polarization (SOP's) were investigated within the bandwidth of a filter channel (0.8 nm) for a variety of optical fiber comb filters based on polarization-diversity loop structure including a zeroth-order comb filter, first-order Lyot-type and Solc-type comb filters, and a second-order Solc-type comb filter. The output SOP's (SOPout's) of the filters were calculated at a flat-top band operation mode of each filter for four different input SOP's (SOPin's) except for the zeroth-order filter. It was found that the SOPout of each filter had unique dependence on wavelength. Specifically, the SOPout's of first-order and second-order comb filters vary periodically with spectral periods of a channel bandwidth (0.8 nm) and its half (0.4 nm), respectively. If an output linear polarizer is located at the output port of each filter, the specific portion of transmission spectra can be passed or rejected, and the rejected band can be continuously tuned in a limited wavelength range by controlling the SOPin. Moreover, the SOPin can be figured out by analyzing the location of the spectral dip at the output transmission spectra obtained above by using the output polarizer.