Dynamic Response Of External Cavity Semiconductor Laser - A Multigigabit Per Second Transmission Simulation

Abstract

The dynamic response of a single mode semiconductor laser coupled to a passive external cavity is analysed through the numerical solution of its rate equations. A complete transmission system is simulated using a model derived from the analogy with transmission lines. Although the optical feedback decreases the damping coefficient of the relaxation oscillations, it causes a considerable reduction on the frequency chirp, always present in intensity modulation. In order to analyse the performance of the external cavity laser under realistic conditions, an optical pulse is propagated through a dispersive fiber and filtered in a receiver. The investigation of pulse broadening and the eye-diagrams indicate that the system capacity can be increased by a factor of 2 by using an external resonator. Introduction A great deal of work has been done towards the enhancement of communication system capacity which has led, among other consequences, to the study and implementation of coherent systems. To implement such systems the performance of conventional semiconductor lasers has to be improved, specially regarding the spectral linewidth. DFB and DBR lasers although having small linewidth still do not achieve the necessary requirements. However, the coupling of a semiconductor laser to an external cavity can represent a 1000-fold spectral linewidth reduction [l], Another major problem always present on high bit rates intensity modulation lightwave systems is the frequency chirp. In this case the modulation of the carrier density induces a variation on the refractive index which causes a change in the emission frequency. Here, since the external cavity resonant frequency is independent on carrier flutuations the coupled configuration ECSL (external cavity semiconductor laser) reduces chirp considerably. Therefore external cavity lasers are of interest for coherent and intensity modulation optical systems. In this paper, a complete time-domain analysis of the ECSL performance is presented. The study is based on the numerical solution of the ECSL rate equations. On deriving these equations, a new approach is used, based on an equivalency with transmission lines. It consists of the analysis of the propagating field components inside the diode leading to the system rate equations [4] that are numerically solved. A similar approach was used in [2], where different assumptions led to an analytical solution. Also, the assumptions made in order to derive and solve the system of rate equations and the numerical solution itself are discussed. The last section is divided in two parts. First, the response of the ECSL