We review probabilistic constellation shaping (PCS), which has been a key enabler for several recent record-setting optical fiber communications experiments. PCS provides both fine-grained rate ...adaptability and energy efficiency (sensitivity) gains. We discuss the reasons for the fundamentally better performance of PCS over other constellation shaping techniques that also achieve rate adaptability, such as time-division hybrid modulation, and examine in detail the impact of sub-optimum shaping and forward error correction (FEC) on PCS systems. As performance metrics for systems with PCS, we compare information-theoretic measures such as mutual information (MI), generalized MI (GMI), and normalized GMI, which enable optimization and quantification of the information rate (IR) that can be achieved by PCS and FEC. We derive the optimal parameters of PCS and FEC that maximize the IR for both ideal and non-ideal PCS and FEC. To avoid plausible pitfalls in practice, we carefully revisit key assumptions that are typically made for ideal PCS and FEC systems.
Celebrating the 20
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.
We explore the potential benefits of digital nonlinearity compensation (NLC) techniques in fully loaded coherent wavelength-division multiplexed (WDM) transmission systems. After providing an ...overview of the various classes of nonlinear interference noise (NLIN) and digital signal processing approaches for their mitigation, we consider the two practically most relevant digital NLC methods known today: back-propagation (BP) and equalization of nonlinear phase and polarization rotation noise (PPRN). We consider a wide range of system configurations including a variety of modulation formats from quadrature phase-shift keying (QPSK) to 256-ary quadrature amplitude modulation (QAM), single-carrier and digital subcarrier multiplexed (SCM) optical superchannels, as well as both point-to-point line systems and optically routed networks (ORNs). Using theoretical predictions from the time-domain model for NLIN, we show that the gain in peak signal-to-noise ratio in fully loaded WDM systems using single-channel and three-channel joint BP is typically limited to 0.5 and 1 dB. The additional gain provided by increasing the number of jointly back-propagated channels beyond three is limited to 0.1 dB per additional back-propagated channel. The remarkably slow growth of the BP gain with the bandwidth of the back-propagated signal is shown to apply also for systems in which the receiver employs PPRN removal. We additionally explore the potential benefits of SCM across a wide range of system configurations, including the impact of BP and PPRN removal on SCM systems. We show that while the BP gain is similar to single-carrier systems, PPRN removal can have a much stronger effect, particularly for higher order QAM systems, implying that SCM can be advantageous not only for constant modulus formats such as QPSK, but also for high spectral efficiency systems. In the context of ORNs, we find that SCM not only improves system tolerance to nonlinearities, but also induces significantly smaller performance variations in various ORN scenarios.
We discuss the role of parallel transport options such as spatial multiplexing or multiband transmission in the capacity scaling of optical transport systems. Taking current experimental spectral ...efficiency records as a baseline, a system supporting 20 b/s/Hz over 1500 km can be about 100 times more energy efficient when it is built using spatial multiplexing than when it is built using a single waveguide at high capacity.
As 100-Gb/s coherent systems based on polarization- division multiplexed quadrature phase shift keying (PDM-QPSK), with aggregate wavelength-division multiplexed (WDM) capacities close to 10 Tb/s, ...are getting widely deployed, the use of high-spectral-efficiency quadrature amplitude modulation (QAM) to increase both per-channel interface rates and aggregate WDM capacities is the next evolutionary step. In this paper we review high-spectral-efficiency optical modulation formats for use in digital coherent systems. We look at fundamental as well as at technological scaling trends and highlight important trade-offs pertaining to the design and performance of coherent higher-order QAM transponders.
With wavelength-division multiplexing (WDM) rapidly nearing its scalability limits, space-division multiplexing (SDM) seems the only option to further scale the capacity of optical transport ...networks. In order for SDM systems to continue the WDM trend of reducing energy and cost per bit with system capacity, integration will be key to SDM. Since integration is likely to introduce non-negligible crosstalk between multiple parallel transmission paths, multiple-input multiple output (MIMO) signal processing techniques will have to be used. In this paper, we discuss MIMO capacities in optical SDM systems, including related outage considerations which are an important part in the design of such systems. In order to achieve the low-outage standards required for optical transport networks, SDM transponders should be capable of individually addressing, and preferably MIMO processing all modes supported by the optical SDM waveguide. We then discuss the effect of distributed optical noise in MIMO SDM systems and focus on the impact of mode-dependent loss (MDL) on system capacity and system outage. Through extensive numerical simulations, we extract scaling rules for mode-average and mode-dependent loss and show that MIMO SDM systems composed of up to 128 segments and supporting up to 128 modes can tolerate up to 1 dB of per-segment MDL at 90% of the system's full capacity at an outage probability of 10(-4).
We investigate upper bounds for single-channel and multi-channel digital back-propagation (BP) in fully loaded wavelength-division multiplexed systems. Using the time-domain model for nonlinear ...interference noise, we expand previous estimates of BP gains to accurately cover a wide range of system configurations, including a variety of modulation formats from quadrature phase-shift keying to 256-ary quadrature amplitude modulation. In typical scenarios, the potential benefit of singlechannel BP is limited to ~0.5 dB in terms of the peak signal-tonoise ratio, and to ~1 and ~1.2 dB in the case of joint threeand five-channel BP. The additional gain from increasing the number of jointly back-propagated channels beyond five is limited to ~0.1 dB per additional back-propagated channel. We also study the role of BP for receivers that separately compensate for nonlinear phase and polarization rotation noise and show that while the additional gain provided by BP does not change significantly in long-haul systems, it holds the promise of being notably higher in short-reach ultra-high-capacity systems.
Optical networks are facing fundamental capacity scalability problems owing to Shannon limits in silica fiber, frequently summarized as the "optical networks capacity crunch." We explore the ...possibility of scaling communication capacities through the use of significantly higher carrier frequencies, such as the extreme ultraviolet or the soft x-ray spectral region, in analogy to the 3-7 orders of magnitude increase in carrier frequencies when optical fiber replaced microwave transmission technologies in the late 1970s. We find that only very limited capacity gains on the order of 10 may be achieved relative to the 1550-nm telecommunications band without fundamentally increasing a system's received energy per bit, its relative bandwidth, or its degree of spatial parallelism. Conversely, we predict the required reduction of waveguide losses that would be needed to make higher carrier frequencies a viable proposition from an energy efficiency point of view. Our analyses include the use of as of yet unexplored quantum communication techniques, whose required energy per bit we find to be lower-bounded by <inline-formula><tex-math notation="LaTeX">\boldsymbol{kT}/\log _2\boldsymbol{e}</tex-math></inline-formula> at any carrier frequency, which may have implications beyond the communications applications discussed in this paper.
With joint contributions from academia and industry, this special issue provides a balanced overview of the area of fiber-optic network capacity scaling.
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