Ultimate capacity of a laser diode in transporting multichannel M-QAM signals

In this paper, we investigate the ultimate M-ary quadrature amplitude modulated (M-QAM) channel capacity of a laser diode which is limited by the laser clipping induced nonlinear distortions. Our study includes a spectral analysis, a complete system simulation, and an experiment which used up to 70 channels of vector arbitrary waveform synthesizer generated quadrature phase shift keyed (QPSK) or 16-QAM signals to modulate an isolated/cooled distributed-feedback (DFB) laser and two unisolated/uncooled Fabry-Perot (FP) lasers, respectively. Our analytical results show that for an upstream laser diode, over 1000 QPSK channels or 170 16-QAM channels can be delivered, even in the presence of a high relative intensity noise (RIN) of -115 dB/Hz. However, these high capacities are reduced significantly when we consider the effect of collision-based medium access control (MAC) protocols. We found that, in the worst case condition (collisions occur in all but one channels), the ultimate QPSK channel capacity of an upstream laser diode is dramatically reduced from over 1000 to 125 for eight collisions/channel. These results have important implications to systems transporting frequency-stacked return-path bands with or without collision-based MAC channels. As regard to the ultimate capacity of a down-stream laser diode with a RIN level of -135 dB/Hz, we found that as high as 600 and 128 channels of 64-QAM and 256-QAM signals (equivalent to 3600 and 1152 channels of MPEG-II live video signals) can be transported, respectively.