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International ICST Conference on Communications and Networking in China

Enhancing the coherent detection of optical QAM signals by exploiting the information of received optical power

Shenli Zhang, Bin Chen, Linghao Cheng, Zhaohui Li, Chao Lü, Hui Wang

Published Jan 1, 2010• 0 citations
Publisher

Abstract

In current optical coherent communication systems, balanced photo-detectors are typically employed, which preserve the product of the received optical signals and the local oscillators (LO) for detection but cancel out the power of both. Since only the product of the received optical signals and the
DOI
10.4108/chinacom.2010.135

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In optical coherent communication systems,balanced photodetectors were typically employed.Such photodetectors preserved the product of the received optical signals and the local oscillators(LO) for detection but cancelled out the power of both.A new detection scheme which aims to maintain the full information obtained from the photodetectors was proposed.Not only the product of the signals and the LO but also the power of signals were used to estimate the amplitude of the target signals.For 16-qudrature-amplitude-modulation(16-QAM) and 64-QAM signals,the simulation results show that the new scheme can outperform the conventional balanced detection method by more than 1 dB in SNR.
Recovering complex optical signals with high sensitivity is central to coherent communication and sensing. Beyond conventional homodyne and heterodyne reception, phase-conjugate twin-wave encoding and phase-sensitive joint detection offer an alternative strategy with enhanced sensitivity. Extending this approach to long-distance transmission of high-order-modulated signals is challenging because optical phase coherence must be maintained at the receiver. Here we experimentally demonstrate injection-locking-assisted phase-conjugated twin-wave reception of 200-MBaud signals with modulation formats up to 256-QAM over 100 km of optical fiber. Optical injection locking regenerates the local oscillator (LO) from a reference transmitted through the 16 dB-loss link, while pilot-assisted optical feedback stabilizes the relative signal-LO phase, enabling coherent combining of the phase-conjugated twin waves at a single balanced photodetector. At equal received signal-photon number per quadrature in each data-carrying optical mode, the measured PCTW signal-to-noise ratio (SNR) exceeds the ideal single-mode homodyne standard quantum limit (SQL) benchmark. The SNR gains over the measured homodyne and conventional heterodyne results are about 3 dB and 6 dB, respectively. Constellation and bit-error-rate measurements show that this gain improves high-order data recovery relative to heterodyne reception. An ideal Gaussian-noise model predicts a photon information efficiency of 1.54 bits per received signal photon for uniform 16-QAM. These results establish an injection-locking-assisted coherent receiver architecture that implements joint detection of phase-conjugated twin waves over 100 km of fiber with enhanced sensitivity, providing a practical route for photon-efficient coherent communication.
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