Effective feedback control in pump laser modules stabilized by fiber Bragg gratings

On the basis of Jones matrix calculus, we derive an effective reflector model for fiber Bragg grating (FBG) stabilized pump laser modules. This model includes a parameter that takes the effects of varying polarization upon propagation in the fiber into account. This feedback parameter is expressed as a function of the parameters describing the linear and circular birefringence in the fiber. It provides a means for control of the effective feedback received by the pump laser and, hence, for optimization of the pump laser characteristics. Several examples of different fiber arrangements are discussed both theoretically and experimentally regarding their robustness against polarization variation. In general, a combination of twist-induced circular and bend-induced linear birefringence is identified as the cause for loss of effective feedback that ultimately can lead to complete delocking of the laser from the FBG. Guidelines for optimized fiber geometries are given.

I. Introduction

The properties of an erbium-doped fiber amplifier (EDFA) depend on the characteristics of the pump laser diode. Today, fiber Bragg gratings (FBGs) are a standard passive component for wavelength and power stabilization of 980-nm pump lasers [1]–[4]. For maintaining gain flatness over a wide range of operating conditions the light reflected from the FBG locks the laser to the desired narrow wavelength interval defined within the EDFAs absorption band. The most common build technology for FBG stabilized pump laser modules is one that uses a standard single-mode fiber. This approach has proven to be practical and cost-effective in large-scale manufacturing, and several hundreds of thousands of such modules have been deployed in optical networks during the past decade by our company.