Numerical Analysis of Distortions in Directly Modulated Semiconductor Lasers in the Presence of Weak Optical Feedback

Subcarrier multiplexed (SCM) optical communication systems have been studied by many researchers for their attractiveness in providing simple, low-cost and broad-band optical fibre communications. In practical SCM systems, some light unintentionally fed back into the laser diode through various discontinuities such as splices adversely affect the performance of directly modulated semiconductor lasers. Many studies have been done carried out both theoretically and experimentally[1-4]. Using small signal perturbation method, Helms[2] gave expressions for modulation response, third order two carrier intermodulation distortion and second harmonic distortion which were shown to be in reasonable agreement with experimental results, especially when the modulating signals are small. Nevertheless, when modulating signals are large, small signal analysis may lose its accuracy and validity. In this paper, we use numerical methods to study the distortion produced by a directly modulated laser diode with weak coherent optical feedback.