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An Experimental Demonstration of Joint Linear Impairments Compensation in Coherent Optical Systems Using a Parametric Network

Alexandru Frunză, Vincent Choqueuse, Pascal Morel, Alexandru Șerbănescu, Stéphane Azou

Published Dec 1, 2022• 0 citations
Publisher

Abstract

This paper proposes a parametric network that benefits from the system knowledge and jointly compensates for the transmitter in-phase/quadrature imbalance, carrier frequency offset, and laser phase noise in coherent optical systems. The approach uses an original training technique that increases its experimentally validated effectiveness. For the considered setup, the performance of the network reaches the 3 rd Generation Partnership Project threshold of 17.5% Error Vector Magnitude for 4-QAM communications.

DOI
10.32754/jmt.2022.2.03

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This paper proposes a parametric network that benefits from the system knowledge and jointly compensates for the transmitter in-phase/quadrature imbalance, carrier frequency offset, and laser phase noise in coherent optical systems. The approach uses an original training technique that increases its experimentally validated effectiveness. For the considered setup, the performance of the network reaches the 3 rd Generation Partnership Project threshold of 17.5% Error Vector Magnitude for 4-QAM communications.
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We propose a novel scheme that uses only a single passive phase compensation device to achieve stable optical and radio frequency joint transfer. The phase noises of optical and radio frequency can be simultaneously compensated by passively embedding their phase information on the two optical carrier sidebands generated by an electro-optical modulator without using the phase discrimination and active servo controller. As a result, this scheme has many advantages, such as high spectral purity, short settling time and infinite compensation accuracy. We experimentally demonstrate the joint transfer of optical and 1 GHz RF over 120 km fiber spools. The optical frequency stability achieves 6.9 × 10 −17 at 1 s and 7.03 × 10 −19 at 10000 s, while the 1 GHz RF is 6.47 × 10 −13 at 1 s and 3.96 × 10 −16 at 10000 s.
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.
We report a technique for coherence transfer of laser light through a fiber link, where the optical phase noise induced by environmental perturbations via the fiber link is compensated by remote users with passive phase noise correction, rather than at the local site as is conventional. Neither phase discrimination nor active phase tracking is required due to the open-loop design, mitigating some technical problems such as the limited compensation speed and the finite compensation precision as conventional active phase noise cancellation. We theoretically analyze and experimentally demonstrate that the delay-limited residual fiber phase noise after phase noise compensation is a factor of seven higher than the conventional techniques. Using this technique, we demonstrate the transfer laser light through a 145-km-long, lab-based spooled fiber. After being compensated, a relative frequency instability is 1.9 × 10-15at the integration time of 1 s and scales down to the level of 10-18-range at 10,000 s. The frequency uncertainty of the light after transferring through the fiber relative to that of the input light is (-0.36 ± 2.6) × 10-18. As the transmitted optical signal remains unaltered until it reaches the remote sites, it can be transmitted simultaneously to multiple remote sites on an arbitrarily complex fiber network, paving a way to develop a multi-node optical frequency dissemination system with post automatic phase noise correction for a number of end-users.
Gordon and Mollenauer [ Opt. Lett.15, 1351 ( 1990)] have shown that the nonlinear Kerr effect limits the range of coherent communications systems using laser amplifiers. We show that parametric amplifiers avoid this limitation. Our method is novel in that we use quantum-optical master equations to model the communications systems. These are solved numerically for systems with either laser amplifiers or parametric amplifiers, with and without the nonlinear Kerr effect. Parametric amplifiers perform better because they preserve the signal-to-noise ratio and decrease the phase noise.
The Pound--Drever--Hall (PDH) technique is routinely used to stabilize the frequency of a laser to a reference cavity. Electronic sideband (ESB) locking, a PDH variant, bridges the frequency gap between the discrete cavity resonances and a desired laser frequency. Here we use quadrature amplitude modulation (QAM), a standard technique in digital communications, to generate the high-quality phase-modulated radio-frequency (rf) drive required for ESB locking. We develop a theoretical framework to analyze how in-phase/quadrature-phase (I/Q) impairments distort the ESB error signal and induce frequency offsets relevant to ultranarrow-linewidth lasers. We then design and implement a direct software-defined radio (SDR) on an UltraScale+ RFSoC platform, frequently adopted across modern quantum-computing systems, to digitally compensate QAM I/Q impairments. Using this device, we generate phase-modulated rf signals with a large phase-modulation index of 1.011.011.01 rad and root-mean-square I/Q errors below 0.3 %0.3\ \%0.3 % over a carrier-frequency range of 350 MHz350~\mathrm{MHz}350 MHz to 1.75 GHz1.75~\mathrm{GHz}1.75 GHz. Finally, we lock a laser to an ultralow expansion (ULE) reference cavity and demonstrate continuous laser-frequency tuning by ramping the carrier frequency while maintaining lock, validating the continuous tunability of our ESB locking instrument.