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Optics Express

Optical homodyne PSK demonstration of 1.5 photons per bit at 156 Mbps with rate-½ turbo coding

Mark L. Stevens, David O. Caplan, Bryan S. Robinson, Don M. Boroson, Alan L. Kachelmyer

Published Jun 27, 2008• 53 citations
Publisher

Abstract

We report the first demonstration of a near quantum-limited optical homodyne PSK receiver combined with powerful forward-error-correction coding, achieving 1.5 photons/bit sensitivity, within 4.5 dB of the Shannon limit. Phase-locking was achieved at 1.55 microm using an analog dither-based optical phase-locked loop with an external phase modulator. Analysis for this configuration with arbitrary loop damping is given showing a performance advantage for the overdamped case.

DOI
10.1364/oe.16.010412

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We report the first demonstration of a near quantum-limited optical homodyne PSK receiver combined with powerful forward-error-correction coding, achieving 1.5 photons/bit sensitivity, within 4.5 dB of the Shannon limit. Phase-locking was achieved at 1.55 microm using an analog dither-based optical phase-locked loop with an external phase modulator. Analysis for this configuration with arbitrary loop damping is given showing a performance advantage for the overdamped case.
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.
Because noise is inherent to all measurements, optical communication requires error identification and correction to protect and recover user data. Yet, error correction, routinely used in classical receivers, has not been applied to receivers that take advantage of quantum measurement. Here, we show how information uniquely available in a quantum measurement can be employed for efficient error correction. Our quantum-enabled forward error correction protocol operates on quadrature phase shift keying (QPSK) and achieves more than 80 dB error suppression compared to the raw symbol error rate and approximately 40 dB improvement of symbol error rates beyond the QPSK classical limit. With a symbol error rate below 10−9 for just 11 photons per bit, this approach enables reliable use of quantum receivers for ultra-low power optical communications. Limiting optical power improves the information capacity of optical links and enables scalable networks with coexisting quantum and classical channels in the same optical fiber.
We implement the cyclic quantum receiver based on the theoretical proposal of Roy Bondurant and demonstrate experimentally below the shot-noise limit (SNL) discrimination of quadrature phase-shift keying signals (PSK). We also experimentally test the receiver generalized for longer communication alphabet lengths and coherent frequency shift keying (CFSK) encoding. Using off-the-shelf components, we obtain state discrimination error rates that are 3 dB and 4.6 dB below the SNLs of ideal classical receivers for quadrature PSK and CFSK encodings, respectively. The receiver unconditionally surpasses the SNL for M=8 PSK and CFSK. This receiver can be used for the simple and robust practical implementation of quantum-enhanced optical communication.
In this paper, a decryption method for PSK quantum noise randomized cipher (QNRC) was proposed and experimentally demonstrated. By phase modulating the running key on an optical carrier provided by a local oscillator, the ciphertext was decrypted by a homodyne receiver. The experimental QNRC transmission system used in the demonstration has a length of 50 km, the ciphertext number was 1024 with a bit rate of 5Gbps. The experimental results shows that the proposed decryption method can decipher the QNRC signal successfully with promising performance.
We demonstrate a balanced-homodyne LADAR receiver employing a phase-sensitive amplifier (PSA) to raise the effective photon detection efficiency (PDE) to nearly 100%. Since typical LADAR receivers suffer from losses in the receive optical train that routinely limit overall PDE to less than 50% thus degrading SNR, PSA can provide significant improvement through amplification with noise figure near 0 dB. Receiver inefficiencies arise from sub-unity quantum efficiency, array fill factors, signal-local oscillator mixing efficiency (in coherent receivers), etc. The quantum-enhanced LADAR receiver described herein is employed in target discrimination scenarios as well as in imaging applications. We present results showing the improvement in detection performance achieved with a PSA, and discuss the performance advantage when compared to the use of a phase-insensitive amplifier, which cannot amplify noiselessly.