Microresonator frequency combs harness the nonlinear Kerr effect in an integrated optical cavity to generate a multitude of phase-locked frequency lines. The line spacing can reach values in the order of 100 GHz, making it an attractive multi-wavelength light source for applications in fiber-optic communications. Depending on the dispersion of the microresonator, different physical dynamics have been observed. A recently discovered comb state corresponds to the formation of mode-locked dark pulses in a normal-dispersion microcavity. Such dark-pulse combs are particularly compelling for advanced coherent communications since they display unusually high power-conversion efficiency. Here, we report the first coherent-transmission experiments using 64-quadrature amplitude modulation encoded onto the frequency lines of a dark-pulse comb. The high conversion efficiency of the comb enables transmitted optical signal-to-noise ratios above 33 dB, while maintaining a laser pump power level compatible with state-of-the-art hybrid silicon lasers.

We review the use of optical frequency combs in wavelength-division multiplexed (WDM) fiber optic communication systems. In particular, we focus on the unique possibilities that are opened up by the stability of the comb-line spacing and the phase coherence between the lines. We give an overview of different techniques for the generation of optical frequency combs and review their use in WDM systems. We discuss the benefits of the stable line spacing of frequency combs for creating densely-packed optical superchannels with high spectral efficiency. Additionally, we discuss practical considerations when implementing frequency-comb-based transmitters. Furthermore, we describe several techniques for comb-based superchannel receivers that enables the phase coherence between the lines to be used to simplify or increase the performance of the digital carrier recovery. The first set of receiver techniques is based on comb-regeneration from optical pilot tones, enabling low-overhead self-homodyne detection. The second set of techniques takes advantage of the phase coherence by sharing phase information between the channels through joint digital signal processing (DSP) schemes. This enables a lower DSP complexity or a higher phase-noise tolerance.

Infrared free-space optics (FSO) provide an attractive solution for ultra-high-capacity wireless communications. However, the full potential of FSO is still being hindered by the apparent random fluctuations on the received optical power, which can be triggered by external factors such as atmospheric turbulence, weather instability and pointing errors. Through the analysis of long-term experimental measurements, we identify the existence of significant time-domain memory in outdoor FSO links, which is found to be particularly strong under rainy weather conditions. Following this observation, we demonstrate that these memory effects can be effectively utilized to design accurate FSO channel estimation algorithms. Taking advantage of the arbitrary bit-rate granularity provided by probabilistic constellation shaping (PCS), and resorting to a simple moving average channel estimator, we demonstrate 400G+ transmission over a seamless fiber-FSO 55 m link with enhanced resilience towards adverse weather conditions. Comparing with unsupervised fixed modulation, we demonstrate a significant increase in average bit-rate (>35 Gbps) after continuous measurement over 3 hours, including raining periods.

We present a low-complexity fully pilot-based digital signal processing (DSP) chain designed for high spectral efficiency optical transmission systems. We study the performance of the individual pilot algorithms in simulations before demonstrating transmission of a 51 x 24 Gbaud PM-64QAM superchannel over distances reaching 1000 km. We present an overhead optimization technique using the system achievable information rate to find the optimal balance between increased performance and throughput reduction from adding additional DSP pilots. Using the optimal overhead of 2.4%, we report 9.3 (8.3) bits/s/Hz spectral efficiency, or equivalently 11.9 (10.6) Tb/s superchannel throughput, after 480 (960) km of transmission over 80 km spans with EDFA-only amplification. Moreover, we show that the optimum overhead depends only weakly on transmission distance, concluding that back-to-back optimization is sufficient for all studied distances. Our results show that pilot-based DSP combined with overhead optimization can increase the robustness and performance of systems using advanced modulation formats while still maintaining state-of-the-art spectral efficiency and multi-Tb/s throughput. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.

Microresonator-based frequency combs are strong contenders as light sources for wavelength-division multiplexing (WDM). Recent experiments have shown the potential of microresonator combs for replacing a multitude of WDM lasers with a single laser-pumped device. Previous demonstrations have however focused on short-distance few-span links reaching an impressive throughput at the expense of transmission distance. Here we report the first long-haul coherent communication demonstration using a microresonator-based comb source. We modulated polarization multiplexed (PM) quadrature phase-shift keying-data onto the comb lines allowing transmission over more than 6300 km in a single-mode fiber. In a second experiment, we reached beyond 700 km with the PM 16 quadrature amplitude modulation format. To the best of our knowledge, these results represent the longest fiber transmission ever achieved using an integrated comb source.

The plethora of services envisaged to support 5G use-cases requires the development of a radio access network (RAN) infrastructure with the capability to integrate a variety of user-demanded mobile traffic characteristics. It is therefore interesting to study a 5G deployment scenario showcasing heterogeneous transmission. In order to address this challenge, in this paper, we propose and experimentally assess a coarse-wavelength-division-multiplexing-compatible optical access network for joint provisioning of 4G/5G RAN services, supporting the coexistence of both digital and analog radio-over-fiber (ARoF) fronthauling. Resorting to an integrated testbed, we demonstrate the transport of 100 MHz 5G downlink using low-latency and bandwidth-efficient ARoF over 8 km fiber, seamlessly coexisting on the same fronthaul infrastructure with a legacy full-fledged 4G long-term evolution (LTE) network supported by Ethernet-based digital RoF with different functional splits. In addition, we experimentally demonstrate the successful operation of the 4G/5G system in a hybrid fiber–wireless scenario including a free-space optics fronthaul extension with 55 m. The proposed heterogeneous RAN architecture is demonstrated to provide a flexible and robust integration path for novel 5G technologies, enabling their coexistence with currently deployed 4G LTE systems over a shared fronthaul infrastructure.