The semiconductor laser linewidth due to the presence of side modes

The laser linewidth is evaluated by solving the rate equations for a nearly single-mode laser with two modes. The resulting linewidth contribution due to the presence of side modes is introduced by the nonlinear gain in the laser diode. For weak side modes, the linewidth contribution is proportional to the third power of the side mode intensity. A linewidth contribution of about 20 MHz for a side-mode power of 100 mu W has been found experimentally for a 1.3- mu m buried-heterostructure laser.>