Celebrating the 20th anniversary of Optics Express, this paper reviews the evolution of optical fiber communication systems, and through a look at the previous 20 years attempts to extrapolate fiber-optic technology needs and potential solution paths over the coming 20 years. Well aware that 20-year extrapolations are inherently associated with great uncertainties, we still hope that taking a significantly longer-term view than most texts in this field will provide the reader with a broader perspective and will encourage the much needed out-of-the-box thinking to solve the very significant technology scaling problems ahead of us. Focusing on the optical transport and switching layer, we cover aspects of large-scale spatial multiplexing, massive opto-electronic arrays and holistic optics-electronics-DSP integration, as well as optical node architectures for switching and multiplexing of spatial and spectral superchannels.

Fiber-optic transmission and networking: the previous 20 and the next 20 years [Invited]

Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

We report a 100-MHz continuous-variable quantum key distribution (CV-QKD) experiment over a 25-km fiber channel without sending a local oscillator (LO). We use a "locally" generated LO and implement with a 1-GHz shot-noise-limited homodyne detector to achieve high-speed quantum measurement, and we propose a secure phase compensation scheme to maintain a low level of excess noise. These make high-bit-rate CV-QKD significantly simpler for larger transmission distances compared with previous schemes in which both LO and quantum signals are transmitted through the insecure quantum channel.

High-speed continuous-variable quantum key distribution without sending a local oscillator.

This tutorial paper provides a foundation for integrating probabilistic shaping (PS) and forward error correction (FEC). A layered PS-FEC architecture consisting of a PS encoder and an FEC encoder is introduced, of which probabilistic amplitude shaping is a practical instance. Achievable PS encoding rates and achievable FEC decoding rates are derived using information-theoretic arguments. The developed tools are applied to the design and performance assessment of optical transponders based on measurements from optical transmission experiments.

Probabilistic Shaping and Forward Error Correction for Fiber-Optic Communication Systems

A set of principal modes that maintain pulse properties and spatial coherence despite propagation in 100 metres of multimode fibre are observed.

Observation of Eisenbud–Wigner–Smith states as principal modes in multimode fibre

We review the most recent advanced concepts and methods employed in the cutting-edge spectrally efficient coherent fiber-optic transoceanic transmission systems, such as probabilistic shaping, adaptive digital nonlinear compensation, rate-adaptive spatially coupled low-density parity check codes, and dual-band C+L-band transmission. Building upon all these concepts and methods, we demonstrate transmission of 179 channels carrying a record net data rate of 65 Tb/s over 6600 km, achieving a spectral efficiency of 7.3 b/s/Hz, and a record average per-channel net bit rate of 363.1 Gb/s. We provide numerical and experimental results, which serve us to comment on the pertinence of the physicist's intuitive measure of performance, the signal-to-noise ratio, versus the information theorist's more accurate measure, the generalized mutual information, for the systems under study.

Advanced C+L-Band Transoceanic Transmission Systems Based on Probabilistically Shaped PDM-64QAM

Few-mode fibers (FMFs) have found applications in optical communications and sensors with attractive features that standard single mode fiber (SSMF) do not possess. We report our recent progress on FMF based optical sensors, and show the potential of utilizing the spatial dimension for multi-parameter sensing with discrimination capability. We first show a discrete type FMF sensor based on interferometer structure with a short FMF, utilizing the modal interference between either the polarizations (x and y) or the spatial modes (LP(01) and LP(11)). We then show a distributed type FMF sensor by generating the stimulated Brillouin scattering (SBS) in a long FMF. We characterize the Brillouin gain spectrum (BGS) with a pump-probe configuration, and measure the temperature and strain coefficients for LP(01) and LP(11) modes. The proposed FMF based optical sensor can be applied to sensing a wide range of parameters.

Few-mode fiber based optical sensors

Data center interconnect has stimulated the research on the short-reach communications with data rate beyond 100G per wavelength and transmission distance of hundreds of kilometers. Aiming at the high-speed short-reach communications, we recently proposed the Stokes vector direct detection (SV-DD) that realizes a linear complex optical channel similar to the coherent detection. In SV-DD, the transmitter places the signal and the carrier onto the orthogonal polarizations, while the receiver achieves the polarization insensitive 3-D detection in the Stokes space with the digital signal processing enabled polarization acquisition. SV-DD achieves 100% spectral efficiency with reference to the single-polarization coherent detection, and simultaneously attains the receiver phase diversity and the cancellation of photo-detection nonlinearity. We experimentally demonstrate the SV-DD signal transmission over 160-km standard single-mode fiber at data rates of both 80 and 160-Gb/s. SV-DD significantly decreases both the system hardware and DSP complexity compared with the polarization multiplexed coherent detection, while increases the system spectrum efficiency compared with the conventional intensity modulation direct detection. Therefore, SV-DD offers a cost-effective solution for the 100G per wavelength and beyond metropolitan area network (MAN). It also owns the potentials to be deployed in the future high-speed passive optical network (PON).

Stokes Vector Direct Detection for Linear Complex Optical Channels

We propose the Stokes vector direct detection (SV-DD) algorithm to achieve photo-detection nonlinearity cancellation and phase diversity. A 160-Gb/s single-polarization modulated SV-DD signal is successfully received after 160-km SSMF transmission with 7.76-bit/s/Hz electrical spectrum efficiency. © 2014 OSA.

160-Gb/s stokes vector direct detection for short reach optical communication

We propose the Stokes vector direct detection (SV-DD) scheme which simultaneously achieves receiver phase diversity and the cancellation of photodetection nonlinearity. An 80  Gb/s single-polarization modulated SV-DD signal is successfully received after 160 km SSMF transmission with 11.64  bit/s/Hz electrical spectrum efficiency.

Qian Hu

Papers

Advanced C+L-Band Transoceanic Transmission Systems Based on Probabilistically Shaped PDM-64QAM

Few-mode fiber based optical sensors

Stokes Vector Direct Detection for Linear Complex Optical Channels

160-Gb/s stokes vector direct detection for short reach optical communication

Stokes vector direct detection for short-reach optical communication.