WDM/SCM system is attractive for access networks since the multiplexed channels could be separated easily by using optical and electrical filters. In addition, the capacity of this system could be increased gracefully by adding new WDM and/or SCM channels. These systems typically utilize directly modulated diode lasers to minimize the cost of optical transmitters. However, the direct modulation of a laser diode results in not only intensity modulation (IM) but also frequency modulation (FM). Thus, when the chirped signals (due to direct modulation) passed through an optical filter with non-flat passband, nonlinear distortion would be generated through the FM-to-IM conversion process. The amplitude of these distortion components would be dependent upon the flatness of the optical filter, laser chirp, and wavelength offset between laser diode and optical filter.
In this thesis, we estimated the scalability of directly modulated WDM/SCM systems limited by the nonlinear distortion due to optical filter and linear crosstalks. First, we measured the laser chirp parameter of 8 commercial DFB lasers obtained from 3 different vendors. When the modulation frequencies were less than 2 GHz, the chirp parameter was measured to be 50~500 MHz/mA. Using these results, we calculated the CSO and CTB distortions caused by Fabry-Perot (FP), Gaussian, $2^nd$ Super-Gaussian, $2^nd$ Butterworth, and $3^rd$ Butterworth filter, and verified these calculations by measuring the nonlinear distortions.
When the filter’s passband were narrow, the performance of WDM/SCM system was limited by the nonlinear distortion generated by the optical filter. On the other hand, when the WDM channel spacing were narrow, the linear crosstalks from the adjacent WDM channels limited the system’s performance. Thus, there exists a tradeoff between the filter bandwidth and WDM channel spacing to maximize the capacity of WDM/SCM systems. We estimated the maximum capacity that a WDM/SCM system could support within the optical bandwidth of 30 nm. When we used Gaussian filter for demultiplexers, it was desirable to design the WDM/SCM system to have its performance to be limited by nonlinear distortion (generated by optical filter). This is mainly because the Gaussian filter has a high rejection ratio outside the passband. In case of Butterworth filters, however, the maximum capacity was achieved when both the nonlinear distortion and linear crosstalks equally contributed to the system’s performance. We also found that the WDM/SCM system using AM-VSB format could support up to 12 WDM channels if we used a Gaussian filter for demultiplexing WDM channels. However, if we used a FP filter instead, this number of WDM channels would be reduced to only one.
