Prediction of the linewidth floor in DFB lasers

Abstract

Linewidth rebroadening, or saturation, of semiconductor lasers at high output powers is often attributed to side-mode interference, although theoretically side-mode powers as high as —20 dB with respect to the lasing mode are required,1 and practically the linewidth of DFB lasers saturates with no side modes within 40 dB of the lasing mode. Other suggestions for the linewidth floor include spatial or spectral hole burning. It has been shown theoretically2 that spatial hole burning causes a constant offset of the linewidth at all power levels, whereas spectral hole burning can cause a linewidth floor. We have measured the linewidth/power relationships of two populations of DFB lasers to substantiate this prediction.