Linewidth rebroadening in DFB lasers due to weak side modes

A new mechanism for linewidth rebroadening in distributed feedback (DFB) lasers is proposed. It is shown that mode partitioning between the main mode and a weak side mode in conjunction with the difference in longitudinal distributions can lead to strong rebroadening even for side mode suppression ratios (SMRs) larger than 30 dB. An analytical expression for the new linewidth contribution is derived. The contribution increases with the third power of the photon number in the dominant side mode. This agrees with experiments on DFB lasers when one generally observes a strong increase in linewidth where the SMSR decreases below 30 dB. This work and the reported experimental results clearly show that linewidths increase if the side mode suppression goes down. However, it seems that in some DFB lasers rebroadening can occur without any decrease in SMSR, so that other mechanisms need to be considered.<>