We demonstrate real-time long-haul transmission with 400G digital subcarrier-multiplexed QSFP-DD coherent pluggable in a live production network, achieving a record post-FEC-error-free reach of 1800-km with 400G-16QAM, 80% longer than all previously publicly announced records.

The throughput and reach in fiber-optic communication links are limited by in-line optical amplifier noise and the Kerr nonlinearity in the optical transmission fiber. Phase-sensitive amplifiers (PSAs) are capable of amplifying signals without adding excess noise and mitigating the impairments caused by the Kerr nonlinearity. However, the effectiveness of Kerr nonlinearity mitigation depends on the dispersion pre-compensation in each span. This paper investigates dense wavelength-division multiplexed PSA-amplified links using joint processing with a less complex digital domain Volterra nonlinear equalizer at the receiver. Both numerically and with experiments, it is shown that this significantly reduces the impact of the dispersion pre-compensation in each span. Also, with simulations, a substantial improvement in transmission reach is demonstrated for PSA links.

We report on the development of a $2\times 800$ Gbps/wave coherent module based on a monolithic InP transceiver PIC and real-time 7nm DSP ASIC capable of 800Gbps data transmission over record 1000km SMF-28 link using a 96Gbaud, PCS-64QAM modulation format.

This dataset contains measurement data and processing code for the results presented in "Phase-coherent lightwave communications with frequency combs". The program code is distributed under a GPLv3 license.

We show that the transmission reach increase with phase-sensitive amplifiers (PSAs) compared to phase-insensitive amplifiers is significantly enhanced for higher-order modulation formats in single-channel, single-span transmission experiments.

Fiber-optical networks are a crucial telecommunication infrastructure in society. Wavelength division multiplexing allows for transmitting parallel data streams over the fiber bandwidth, and coherent detection enables the use of sophisticated modulation formats and electronic compensation of signal impairments. Optical frequency combs can replace the multiple lasers used for the different wavelength channels. Beyond multiplexing, it has been suggested that the broadband phase coherence of frequency combs could simplify the receiver scheme by performing joint reception and processing of several wavelength channels, but an experimental validation in a fiber transmission experiment remains elusive. Here we demonstrate and quantify joint reception and processing of several wavelength channels in a full transmission system. We demonstrate two joint processing schemes; one that reduces the phase-tracking complexity and one that increases the transmission performance.

We quantify the maximum transmission reach for phase-insensitive amplifier (PIA) and phase-sensitive amplifier (PSA) links with different modulation formats and show that the maximum transmission reach increase (MTRI) when using PSAs compared to PIAs is enhanced for higher-order modulation formats. The higher-order modulation formats are more susceptible to smaller phase rotations from nonlinearities, and PSAs are efficient in mitigating these smaller phase distortions. Numerical simulations were performed for single- and multi-span PIA and PSA links with single and multiple wavelength channels. We obtain a significant enhancement in the MTRI with PSAs compared to PIAs when using higher-order modulation formats for both the single- and multi-channel systems in single- and multi-span links. We verify the enhancement with a single-span, single-channel system experiment. We also demonstrate, for the first time, a 64-QAM modulation format fiber transmission in phase-sensitively amplified link, with a 13.3-dB maximum allowable span loss increase compared to a phase-insensitively amplified link.

Phase-sensitive optical parametric amplifiers (PSAs) can provide low-noise optical amplification while simultaneously mitigating nonlinear distortions caused by the Kerr effect. However, nonlinearity mitigation using PSAs is affected by link parameters, and imperfect link design results in residual nonlinear distortions. In this paper, we use first-order perturbation theory to describe these residual nonlinear distortions, and develop a way to mitigate them using a modified third-order Volterra nonlinear equalizer (VNLE) in the receiver. Using numerical simulations, we show that our proposed VNLE reduces the residual nonlinear distortions in links using in-line PSAs for several combinations of symbol rates and modulation formats, and can increase the maximum transmission distance by up to 80%. We also perform a proof-of-concept experiment and confirm that our modified VNLE can mitigate the residual nonlinear distortions on a 10-Gbaud 16QAM signal after transmission through a 10×80-km link with in-line PSAs.

We experimentally investigate a wavelength-division multiplexing (WDM) system with a single span of 80 km standard single-mode fiber (SSMF) and a phase-sensitive amplifier (PSA) as a pre-amplifier. Using two channels spaced 25 GHz apart with single-polarization 10-Gbaud quadrature phase-shift keying (QPSK) signal, we show quantitatively that a PSA can mitigate self-phase modulation (SPM) and cross-phase modulation (XPM). With three single-polarization 10-Gbaud QPSK channels spaced 12.5 GHz apart, we verify that PSA can mitigate XPM and also determine an allowed span loss increase of 9.5 dB for the center channel with correlated edge channels for the PSA compared to the phase-insensitive amplifier (PIA) due to the combined improvement from low-noise amplification and nonlinearity mitigation.

We demonstrate the use of in-line low-noise phase-sensitive amplifiers (PSAs) in a wavelength-division multiplexing long-haul transmission link with a recirculating loop experiment. We quantify how replacing phase-insensitive amplifiers with PSAs can improve the reach of one-, two-, and three-channel transmission systems both numerically and experimentally.

We investigate combining phase-sensitive optical amplifiers (PSAs) with a static Volterra nonlinear equalizer (VNLE) to mitigate residual nonlinear distortions. Experiments show that the VNLE reduces penalties from link design and improves QBER by 0.4 dB for a 10-GBaud 16QAM signal in a 10×80-km PSA-amplified link.

Considering the nonideal response of Mach-Zehnder modulators, uniform cross QAM constellations improve upon probabilistically shaped QAM by a factor of up to 4 in uncoded symbol error rate and also offer higher achievable information rates, at the same source entropy and optimal electrical signal powers.

We show that a simplified, single-photodiode per polarization heterodyne receiver is able to directly suppress signal-signal beat interference (SSBI), without the need for cancellation in the digital domain. We characterize performance degradation due to SSBI, and show that a strong LO in the receiver can mitigate SSBI. Transmission of 400 Gb/s-class signals is shown over single fiber spans of up to 160 km, and over field-deployed metropolitan area fiber. These results indicate that a single photodiode can be used to receive complex optical signals in high speed fiber systems without the need for SSBI cancellation in the digital domain.

We experimentally investigate a phase-sensitive amplifier (PSA) link in a WDM transmission scenario, focusing on nonlinearity mitigation. We show that a PSA effectively mitigates both cross-phase modulation (XPM) and self-phase modulation (SPM) after transmission over an 80-km span.

We present a simplified heterodyne receiver using one single ended photodiode per polarization for polarization multiplexed coherent signals. We demonstrate this receiver for the reception of PM-16QAM over field-installed metro-area fibers at distances up to 306-km.

We demonstrate long-haul transmission using a hybrid amplifier approach combining distributed Raman amplification and lumped phase-sensitive amplification. Aside from the well-known resulting SNR improvement, distributed Raman amplification is included in an effort to improve the nonlinearity mitigation capability of the phase-sensitive amplifiers. When changing from phase-insensitive operation to phase-sensitive operation in a link employing distributed Raman amplification, the transmission reach at BER = 10-3 is increased from 15 to 44 spans of length 81 km while simultaneously increasing the optimal launch power by 2 dB.

We experimentally demonstrate, for the first time, cross-phase modulation (XPM) compensation using an optical phase modulator and low-bandwidth electronics. We first show that our nonlinearity compensator suppresses the XPM distortion from a 10-Gb/s ON-OFF keyed (OOK) channel on a continuous-wave (CW) probe signal. We then replace the CW tone with a 28-Gbaud quadrature phase-shift keyed (QPSK) signal and show that the OOK power can be doubled when XPM compensation is used. This demonstrates proof-of-concept for XPM compensation using phase modulators placed along a fiber link.

We propose and experimentally demonstrate an all-optical digital-to-analog converter based on cross-phase modulation with temporal integration. The scheme is robust for driving signal noise due to the low-pass filtering feature of the temporal integrator. The proof-of-concept experiment demonstrates the generation of pulse-amplitude modulation (PAM) sequences up to eight levels. The performance of random PAM 2 and PAM 4 signals with different optical signal-to-noise ratios of the binary driving signal is also investigated. The scheme is scalable for high-speed operation with an appropriate dispersion profile of the nonlinear medium.

We exploit excess spectrum of DWDM systems, where the Nyquist bandwidth of the optical signals is narrower than the channel spacing. Digitally generated cyclic-spectrum RZ shaping is shown to enhance nonlinear tolerance. Simulations show a 26% reach increase in a 7-channel DWDM PolMux 16QAM long-haul transmission.

We propose and experimentally validate a blind phase recovery algorithm based on tracking low-frequency components of the phase noise, which we call "filtered carrier-phase estimation (F-CPE)." Tracking only the low-frequency components allows F-CPE to reduce the computational complexity by using a frequency-domain equalizer and to simplify the partitioning of a 16 quadrature amplitude modulation (16QAM) constellation. Further, this approach eliminates cycle slips by suppressing the impact of amplified spontaneous emission on phase noise estimation. The experimental results demonstrate cycle-slip-free operation for 15 and 32 GBd 16QAM signals. Additionally, the proposed method showed similar or better sensitivity compared with the blind-phase-search algorithm, near standard forward error correction thresholds of modern wavelength division multiplexing systems.