External feedback optimization by means of polarization control in fiber Bragg grating stabilized 980-nm pump lasers

Effects of polarization change in the feedback provided by fiber Bragg gratings that stabilize 980-nm pump lasers are investigated. The effective feedback in the proper transverse-electric polarization is calculated from an easily measured parameter, describing the polarization at the fiber grating. This provides a useful experimental tool to investigate the effect of feedback loss due to polarization change that can occur in nonpolarization maintaining fibers. On the basis of our calculations, we give guidelines for optimum fiber layout to minimize polarization change.

The performance of an erbium-doped fiber amplifier (EDFA) depends critically on the characteristics of the pump laser diode. For stabilization of 980-nm pump lasers, schemes with and without a fiber Bragg grating (FBG) are employed. FBGs are used where effective suppression of low-frequency (less than megahertz) power fluctuations and wavelength shift with temperature and current are critical [1], [2]. In a typical configuration, the FBG has a reflectivity of a few percent and it is positioned at a distance of 1–2 m from the laser. The feedback provided by the grating locks the laser to the wavelength defined by the FBG.