The nature of the polarization beat frequency as a function of twist for DFB fiber lasers

Phase-shifted DFB fiber lasers (DFB-FL) can have two orthogonally polarized laser modes with a frequency offset proportional to the fiber birefringence. In order to achieve true single frequency operation one polarization mode must be extinguished. On the other hand, the existence of a polarization beat frequency (PBF) between the two states of polarization in a dual-mode DFB-FL can be utilized in a frequency-based fiber sensor. The underlying mechanisms leading to the quenching or preservation of the PBF must be controlled in either case. Twisting the DFB-FL to induce circular birefringence has been used to achieve single polarization-mode operation, though an explanation has been lacking. We present results of the reflection and transmission spectra, and the PBF, as a function of twist angle for two phase-shifted fiber gratings, and compare the results with recent theoretical calculations. We have also measured the PBF as a function of twist angle for the active case of a phase-shifted DFB-FL. Our results show that the PBF decreases monotonically as a function of twist angle for angles less than 700 degrees (over a 20 cm length), where the magnitude of the PBF signal can be suppressed by 20 dB or more. For larger twist angles, the magnitude of the PBF signal was not in general extinguished, in contrast to previously reported work. A possible explanation of such behavior will be presented in addition to a discussion of the applications of DFB-FLs as sensors.