Phase-Noise-Canceled Differential Phase-Shift-Keying (PNC-DPSK) for Coherent Optical Communication Systems.

摘要

A major problem in using phase-shift keying (PSK) techniques in practical optical communication systems is the phase noise of the laser. Typical semiconductor lasers have phase noise induced linewidths in excess of 1 MHz. This cause a power penalty in the bit error rate (BER) performance at low signal-to-noise ratios, and a high signal-to-noise creates a floor in the BER curve that cannot be surpassed by increasing signal power. For such degradation to be negligible in PSK systems, the linewidth of the sources must be less than 0.001 times the data rate. While this criteria can be met for high data rate systems (>1 Gbit/s) using currently available lasers, low data rate systems (<1 Gbit/s) suffer severe phase noise induced degradation. In this paper, a novel modulation scheme employing a nonlinear demodulation process, phase-noise-canceled differential phase-shift keying (PNC-DPSK), is used to circumvent the effects of phase noise in a PSK system. A theoretical description of the system is given and results of an experimental test system are shown to be in good agreement with the predictions. In predictions. In particular, it is shown that the phase noise induced BER floor can be eliminated despite using lasers with linewidths comparable to the data rate. Furthermore, for an optimized PNC-DPSK system, the degradation at high signal-to-noise ratios compared to heterodyne PSK with ideal source (i.e, no phase noise) should only 3 dB. Reprints. (RRH)