Effect of many weak side modes on relative intensity noise of distributed feedback semiconductor lasers after dispersive propagation

Summary form only given. The performance of a lightwave communication system can be affected by the laser´s relative intensity noise (RIN) and mode partition noise (MPN). In addition, measurements of RIN after propagation in fiber can be used to obtain several important intrinsic laser parameters. Therefore an accurate and simple model of RIN after dispersive propagation is necessary. We report here an effect of MPN that can affect RIN in low-noise distributed feedback (DFB) lasers even with side-mode suppression ratios higher than 40 dB and at frequencies up to 5 GHz, and which cannot be explained by previous single-mode models. We present a multimode theory of RIN after propagation in fiber in which the effect of FM to AM conversion in the fiber has been included via a generalization of the transfer function approach to the multimode case. The additional parameters that are needed in this multimode model are experimentally obtained from a measurement of the optical spectrum. This allows us to get theoretical fits that agree well with experimental measurements of RIN.