Analysis of multichannel (2-60) intermodulation distortion in subcarrier- multiplexed systems

Abstract

Previous theories of intermodulation distortion (IMD) in semiconductor lasers are based on the single-mode rate equation and small-signal analysis and are limited to frequencies above 2 GHz. These models do not adequately describe N-channel IMD. Recent measurements show that because of longitudinal-mode competition, low-frequency (≤2 GHz) third-order IMD can be substantially increased in Fabry-Perot lasers.1 This effect, which seriously degrades system performance at low frequencies, is very important in subcarrier-multiplexed (SCM) transmission systems.2 In this paper, for the first time to our knowledge, we adequately model the laser’s IMD over the frequency range from 50 MHz to 10 GHz by performing a large-signal analysis of the general multimode rate equations.3 This analysis explains the low-frequency enhancement of IMD, including all third- order IMD’s for 60-channel signals. Additionally, we show that fiber dispersion further increases third-order IMD.