Modelling polarization mode competition in fiber DFB-lasers

Abstract

Rare earth doped fiber DFB-lasers are interesting devices both as sensors and as CW-sources for telecommunication applications. We are interested in a sensor configuration where the birefringence of a fiber laser is determined by measuring the beat frequency between two polarization modes. This requires a stable two-polarization laser operation. Other applications require single polarization laser. Both for the single and the dual polarization laser design a proper understanding of the polarization mode competition is required. We use a transfer-matrix model based on the coupled mode equations to describe the light-propagation in Bragg-gratings with birefringence, twist and gain. Effects of saturation and non-uniformity effects are included in the model. A cavity round trip gain and phase approach is used to find the lasing conditions, like mode frequencies, output intensities and field distribution in the laser cavity.
Both chirp, polarization dependent phase shifts, and polarization dependent coupling strength is shown to cause different thresholds for the polarization modes. Four saturation effects are identified that may influence the mode competition:
1) spatial holeburning related to the overall intensity distribution of the lasing modes,
2) spatial holeburning caused by the difference in phase of the modal standing wave patterns,
3) polarization holeburning caused by the anisotropy of the dopant ions,
4) gain distribution effects related to intensity dependence of pump power absorption. Effects caused polarization dependence of the pump power absorption will be discussed only briefly.

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