Analysis of laser's relative intensity noise and multichannel intermodulation distortion for subcarrier multiplexed transmission systems

Abstract

The source laser's relative intensity noise (RIN) and intermodulation distortion (IMD) are two very important system design considerations in fiber optic communication systems, especially in subcarrier multiplexed (SCM) transmission systems. It is found that the presence of the asymmetric nonlinear gain causes the longitudinal modes of Fabry-Perot lasers as well as nearly single-mode lasers to couple such that the low frequency RIN is greatly enhanced. This enhanced low frequency RIN can be translated to the signal band by beating with the modulation signal and contribute to the in-channel noise in high frequency applications. This can seriously degrade system's signal-to-noise performance. For the first time we have included the asymmetric mode competition to adequately model the enhanced low frequency noise and its translation to the signal channel in the presence of modulation and fiber dispersion. An experimental system, which provides a very sensitive means of noise measurement, was developed. The theoretical predictions were successfully verified by the experiments. Detailed investigations of system impairment caused by the enhanced in-channel noise, especially the degradation of system's signal-to-noise performance as a function of modulation depth, modulation frequency and fiber dispersion, were also performed. The results provide very important guidelines for system design and applications. Further, we adequately modeled laser's intermodulation distortion in the frequency range of 50MHz -- 10GHz. This model, which successfully explained the observed enhancement of low frequency IMD, was further generalized to include all the second-order and third-order IMDs of up to 60 channel signals, and was used to predict the impairment of multichannel lightwave systems.