Study on Transmission and Output Polarization Characteristics of a First-Order Lyot-Type Fiber Comb Filter Using Polarization-Diversity Loop

A first-order Lyot-type fiber comb filter was fabricated by incorporating a polarization-diversity loop structure composed of a polarization beam splitter, two half-wave plates (HWPs), and two polarization-maintaining fiber (PMF) segments. One of the two PMF segments was two times longer than the other, and they were concatenated with a 60
$^{\circ} $
offset between their principal axes. The transmission spectra and output polarization characteristics of the filter were theoretically predicted and experimentally verified. By proper control of the two HWPs, first-order transmission spectra could be obtained, including flat-top and lossy flat-top band modes, and the interleaving operation of the respective spectrum could be also achieved. This flat-top band mode spectrum showed a higher attenuation rate at the cutoff level, in comparison with that of the Solc-type filter. In particular, it was revealed that the output state of polarization of the filter was a periodical function of wavelength and that the evolution period was different, according to transmission band modes. With the help of a linear output polarizer, the specific portion of a passband could be passed or rejected by controlling input polarization. It was also theoretically predicted that the fine tuning of the passband center was possible within a specific wavelength range.