Abstract

We report on the static properties of Nd-doped fiber lasers submitted to optical feedback¹ in an experimental configuration that gives access to the laser intensity both inside and outside the laser cavity (Fig. 1). We show that optical feedback increases the inner laser intensity and reduces, by up to a factor two, the slope of the output-intensity-versus-pump-level characteristic (Fig. 2). We attribute this unusual phenomenon² to the spectral selectivity of the external Fabry-Perot cavity composed of the output coupler and the feedback mirror. In the absence of feedback the laser output spectrum consists of thousands of modes, uniformly distributed within the 3000 GHz spectral linewidth. When feedback is applied, the equivalent output mirror reflectivity is modulated at the Free Spectral Range of the external cavity, 1.5 GHz in the case reported here. Because of the important homogeneous linewidth, mode competition takes places. As a consequence the modes with higher reflectivity inhibit the other modes, and non-lasing bands appear in the laser spectrum. The lasing modes undergo lower losses, and the total inner laser intensity increases. In the absence of feedback, the output intensity is directly related to the inner intensity through the transmission coefficient of the output coupler. With back-coupled reflections, this transmission coefficient is frequency dependent. Since the selected laser modes are grouped in the vicinity of the maximum of the effective-reflection-coefficient function, they suffer minimum effective transmission. Based on these physical insights, an adapted theoretical model shows that feedback divides the slope of the output intensity by a factor two, in conformity with the experiments.

© 1996 IEEE