Properties of Pump-Laser Modules Exposed to Polarization-Dependent and Wavelength-Selective Feedback From Fiber Bragg Gratings

The performance of fiber Bragg grating (FBG) stabilized pump-laser modules is strongly influenced by changes of the state of polarization (SOP) arising from intrinsic and/or induced birefringence of the fiber. Birefringence changes the SOP as the light propagates back and forth between laser and FBG, which are separated by a distance of 1 to 2 m. As a result, the effective wavelength-selective feedback provided by the FBG varies accordingly. Based on the steady-state solution of a set of multimode rate equations we present a new model allowing us to calculate the output characteristics of a grating-stabilized pump-laser module as function of effective feedback. An effective-reflector approach is applied to describe the external cavity between the front mirror and the FBG on the module's front side. The polarization- and wavelength-dependent effective feedback from the grating is taken into account by a feedback parameter in the equations for the effective reflector. We present an extended traveling-wave amplifier model for calculating the TE- and TM-polarized contributions to the power from all output ports of the module. Our model is verified by comparison of simulation results with experimental data for a typical FBG-stabilized pump-laser module. The output characteristics are analyzed as function of effective feedback for both the principal TE-polarization as well as the residual TM-polarization modes.

I. Introduction

The performance of an erbium-doped fiber amplifier (EDFA) depends critically on the output characteristics of the pump-laser module. for efficiently pumping the EDFA high fiber-coupled output power confined in a narrow wavelength band around the erbium's absorption lines is required as well as low-noise operation of the device. Fiber Bragg gratings (FBGs) are nowadays established as a passive component for wavelength stabilization of 980-nm pump-laser modules [1]–[4]. the feedback provided by the grating locks the laser to wavelengths in a narrow band around the FBG's center (Bragg) wavelength if the spectral detuning to the gain maximum is not too large [5]. in a typical configuration, the FBG has a maximum reflectivity of a few percent and it is positioned at a distance of 1–2 m from the front facet of the laser chip [6]. This forces the laser to run for all injection currents and heat-sink temperatures on multiple longitudinal modes in the coherence-collapse state of emission [7]–[9]. This is the preferred mode of operation for pumping applications since low-frequency noise (submegahertz) is strongly suppressed if several longitudinal laser modes are oscillating simultaneously [1], [2].