Tunable all-fiber birefringence comb filters

We have demonstrated a novel all-fiber comb filter with adjustable channel spacing. The filter can be used as an element in a multi-wavelength laser and provide great flexibility in laser design. Both Lyot and Sagnac-Lyot filters have been demonstrated. The Lyot filter provides a large number of wavelength spacings with only a few fiber segments. (e.g. 40 spacings for 4 segment filter) whereas the Sagnac-Lyot filter provides a tunable filter function that is independent of input polarization. Typical values of measured insertion loss were 4 dB and extinction ratio of 15 dB. The insertion loss values can be improved by using low-loss polarization controllers and improving the fusion splicing of the PM/non-PM fiber splices.