Ultimate capacity of a return-channel laser diode in transporting multichannel bursty QPSK/16-QAM signals

A hybrid-fiber coax (HFC) system-based bidirectional CATV communication network relies on the performance of both downstream and upstream optical fiber links. Large (AM-VSB) channel-count, high-power downstream optical fiber transmitters have been commercialized for a few years, and the fundamental capacity of a downstream DFB laser has also been extensively studied. As for the upstream links, future interactive applications such as Internet access and cable telephony will require return-channel lasers transport multiple channels of QPSK or 16-QAM channels. This interactive traffic may keep on growing, and it may become more economical to frequency-division-multiplex several return-channel bands (each occupying the conventional 5-40 MHz) before modulating a return-channel laser diode. By using this frequency-stacking technique, fewer return-channel lasers and up-link optical fibers can be used. However, until now, it is unclear what the ultimate bursty QPSK/16-QAM channel capacity of an uncooled/unisolated Fabry-Perot or DFB laser is, and how much clipping-induced nonlinear distortion due to multiple collided medium-access control channels a return-channel laser can tolerate. In this paper, we provide experimental, computer simulation, and analytical results to answer these questions.