Highly Sensitive Optical Multilevel Transmission of Arbitrary Quadrature-Amplitude Modulation (QAM) Signals With Direct Detection

A proposal for the realization of highly sensitive optical direct-detection (incoherent) multilevel transmission with arbitrary signal constellation without optical chromatic dispersion (CD) compensation is reviewed in this paper. We introduce transmitter-side signal processing, ¿CD pre-equalization,¿ and ¿phase preintegration¿ techniques to realize direct-detection CD-compensation free transmission of arbitrary optical quadrature-amplitude modulation (QAM) signals. We also adopt receiver-side signal processing, ¿multi-symbol phase estimation (MSPE),¿ and ¿non-Euclidean metric-based symbol detection¿ to improve the optical SNR (OSNR) sensitivity. We have performed up to 160-km standard single-mode fiber transmission of a 40-Gbit/s 16QAM signal and achieved the OSNR sensitivities only 0.5-2.4 dB away from those of coherent detection.