Investigation of Partially Coherent Interaction in Fiber Bragg Grating Stabilized 980-nm Pump Modules

Partially coherent interaction of feedback light with the field in the laser cavity is affirmed with fiber Bragg grating (FBG) stabilized 980-nm pump lasers, in contrast to the normally accepted totally incoherent state of interaction. A coherence parameter $\gamma$ was defined to identify the fraction of feedback light interacting coherently. $\gamma$ was determined by fitting the measured power-difference versus pumping-rate curve to the simulation results. From the fits, it is found that coherence parameter $\gamma$ decreases when the distance between the FBG and the laser facet increases and vice versa. The relationship between $\gamma$ and the spectral bandwidth of the reflectivity of the feedback element was also investigated. This work is important to understand and improve the characteristics of FBG stabilized 980-nm pump modules.