In this paper, we propose a geometric shaping (GS) strategy to design 8, 16, 32 and 64-ary modulation formats for the optical fibre channel impaired by both additive white Gaussian (AWGN) and phase noise. The constellations were optimised to maximise generalised mutual information (GMI) using a mismatched channel model. The presented formats demonstrate an enhanced signal-to-noise ratio (SNR) tolerance in high phase noise regimes when compared with their quadrature amplitude modulation (QAM) or AWGN-optimised counterparts. By putting the optimisation results in the context of the 400ZR implementation agreement, we show that GS alone can either relax the laser linewidth (LW) or carrier phase estimation (CPE) requirements of 400 Gbit/s transmission links and beyond. Following the GMI validation, the performance of the presented formats was examined in terms of post forward error correction (FEC) biterror-rate (BER) for a soft decision (SD) extended Hamming code (128, 120), implemented as per the 400ZR implementation agreement. We demonstrate gains of up to 1.2 dB when compared to the 64-ary AWGN shaped formats.
Geometrically shaped (GS) 8-, 16, and 32-ary modulation formats are investigated for use in coherent passive optical networks. The modulation formats are designed to improve receiver sensitivity when paired with a binary, soft-decision forward error correction code (SD-FEC), such as a low density parity check (LDPC). Herein we consider the LDPC code specified in the draft 50G EPON standard, and show how this type of code can be particularly advantageous in a coherent transmission system. A receiver sensitivity of −26.7 dBm is achieved at a post-FEC bit error rate below 3.8×10 −6 for a polarizationscrambled 32-ary GS modulation format at 25 GBd, received using a single-polarization, phase-diverse coherent receiver. This yielded a bit rate of 100 Gbit/s, net of coding and pilot overhead. This modulation format was shown to perform equally well in transmission, with no observed dispersion penalty after 80 km standard single mode fiber. Finally, this modulation format was received, at a reduced data rate of 50 Gbit/s, using a heterodyne coherent receiver based on a single balanced photodiode. The receiver sensitivity for this simplified receiver configuration was −28.5 dBm, yielding a power budget of 34.2 dB.
A linear regression algorithm is applied to a digitalsupermode distributed Bragg reflector laser to optimise wavelength switching times. The algorithm uses the output of a digital coherent receiver as feedback to update the pre-emphasis weights applied to the laser section currents. This permits in-situ calculation without manual weight adjustments. The application of this optimiser to a representative subsection of channels indicates this commercially available laser can rapidly reconfigure over 6.05 THz, supporting 122 channels, in less than 10 ns.
A frequency-domain multiple-input multiple-output (FD-MIMO) equalizer employing a momentum-based gradient descent update algorithm is proposed for polarization multiplexing coherent receivers. Its performance in operation with dynamically varying optical channels is investigated and the impact of filter update delays, arising from the latency of the fast Fourier transforms (FFTs) and other digital signal processing (DSP) operations in the feedback loop, is assessed. We show that the proposed momentum-based gradient descent algorithm used to control the equalizer response has significantly greater tolerance to feedback delay than the conventional gradient descent algorithm. We considered a 92 Gbaud dual-polarization 64 QAM receiver, with DSP operating at two samples per symbol, and with the equalizer operating on blocks of 512 and 1024 samples (i.e., 512/1024-point FFT). We found that at an optical signal-to-noise power ratio (OSNR) of 35 dB, the momentum-based gradient descent algorithm can successfully track state-of-polarization (SOP) rotation at frequencies of up to 50 kHz and with filter update delays of up to 14 blocks (39 ns). In comparison, using the conventional gradient descent algorithm in an otherwise identical receiver, the equalizer performance starts to deteriorate at SOP rotation frequencies above 20 kHz.
Fast (nanoseconds) optical wavelength switching is emerging as a viable solution to scaling the size and capacity of intra-data center interconnection. A key enabling technology for such systems is low-jitter optical clock synchronization, which enables sub-nanosecond clock and data recovery for optically switched frames using low-cost methods such as clock phase caching. We propose and demonstrate real-time low-latency wavelength-switched clock-synchronized intra-data center interconnection at 51.2 GBd using a fast tunable laser (with ns scale switching time) and ultra-stable-latency hollow core fiber (HCF) for optically-switched data center networks. For wavelength-switched systems, we achieve a physical layer latency below 46 ns, consisting of 28 ns transceiver latency and a 18 ns inter-packet gap. Finally, we show that by exploiting the low chromatic dispersion and thermally-stable latency features of HCF, active clock phase tracking can be entirely eliminated.
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