Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Space-division multiplexing: the next frontier in optical communication

Progress in optical networking has stimulated interest in optical performance monitoring (OPM), particularly regarding signal quality measures such as optical signal-to-noise ratio (SNR), Q-factor, and dispersion. These advanced monitoring methods have the potential to extend fault management and quality-of-service (QoS) monitoring into the optical domain. This paper reviews OPM applications and techniques, while examining the role of OPM as an enabling technology for advances in high-speed and optically switched networks.

Optical performance monitoring

Communication systems: An introduction to signals and noise in electrical communication

We show, using simulations, that a combination of orthogonal frequency division multiplexing (OFDM) and optical single sideband modulation can be used to compensate for chromatic dispersion in ultralong-haul wavelength-division multiplexed (WDM) systems. OFDM provides a high spectral efficiency, does not require a reverse feedback path for compensation, and has a better sensitivity than nonreturn to zero. This paper provides design rules for 800-4000-km optical-OFDM systems. The effects of WDM channel number and spacing, fiber dispersion, and input power per channel on the received Q are studied using extensive numerical simulations. These effects are summarized as a set of design rules

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Performance of Optical OFDM in Ultralong-Haul WDM Lightwave Systems

The exponential traffic growth, demand for high speed wireless data communications, as well as incessant deployment of innovative wireless technologies, services, and applications, have put considerable pressure on the mobile network operators (MNOs). Consequently, cellular access network performance in terms of capacity, quality of service, and network coverage needs further considerations. In order to address the challenges, MNOs, as well as equipment vendors, have given significant attention to the small-cell schemes based on cloud radio access network (C-RAN). This is due to its beneficial features in terms of performance optimization, cost-effectiveness, easier infrastructure deployment, and network management. Nevertheless, the C-RAN architecture imposes stringent requirements on the fronthaul link for seamless connectivity. Digital radio over fiber-based common public radio interface (CPRI) is the fundamental means of distributing baseband samples in the C-RAN fronthaul. However, optical links which are based on CPRI have bandwidth and flexibility limitations. Therefore, these limitations might constrain or make them impractical for the next generation mobile systems which are envisaged not only to support carrier aggregation and multi-band but also envisioned to integrate technologies like millimeter-wave (mm-wave) and massive multiple-input multiple-output antennas into the base stations. In this paper, we present comprehensive tutorial on technologies, requirements, architectures, challenges, and proffer potential solutions on means of achieving an efficient C-RAN optical fronthaul for the next-generation network such as the fifth generation network and beyond. A number of viable fronthauling technologies such as mm-wave and wireless fidelity are considered and this paper mainly focuses on optical technologies such as optical fiber and free-space optical. We also present feasible means of reducing the system complexity, cost, bandwidth requirement, and latency in the fronthaul. Furthermore, means of achieving the goal of green communication networks through reduction in the power consumption by the system are considered.

Toward an Efficient C-RAN Optical Fronthaul for the Future Networks: A Tutorial on Technologies, Requirements, Challenges, and Solutions

In this paper, we investigate the design of few-mode fibers (FMFs) guiding 2 to 12 linearly polarized (LP) modes with low differential mode delay (DMD) over the C-band, suitable for long-haul transmission. Two different types of refractive index profile have been considered: a graded-core with a cladding trench (GCCT) profile and a multi-step-index (MSI) profile. The profiles parameters are optimized in order to achieve: the lowest possible DMD and macro-bend losses (MBL) lower than the ITU-T standard recommendation. The optimization results show that the MSI profiles present lower DMD than the minimum achieved with a GCCT profile. Moreover, it is shown that the optimum DMD and the MBL scale with the number of modes for both profiles. The optimum DMD obtained for 12 LP modes is lower than 3 ps/km using a GCCT profile and lower than 2.5 ps/km using a MSI profile. The optimization results reveal that the most preponderant parameter of the GCCT profile is the refractive index relative difference at the core center, Δnco. Reducing Δnco, the DMD is reduced at the expense of increasing the MBL. Regarding the MSI profiles, it is shown that 64 steps are required to obtain a DMD improvement considering 12 LP modes. Finally, the impact of the fabrication margins on the optimum DMD is analyzed. The probability of having a manufactured FMF with 12 LP modes and DMD lower than 12 ps/km is approximately 68% using a GCCT profile and 16% using a MSI profile.

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Design of Few-Mode Fibers With M-modes and Low Differential Mode Delay

Flexgrid technology has recently been presented as the most promising option for upgrading the currently operating fixed grid optical networks and extending their capacity to be able to deal with the massive traffic volumes forecast for the next decade. Although the current traffic is successfully supported on fixed grid networks, flexgrid technology brings features that are not offered by the fixed grid networks, such as transporting optical connections with a capacity beyond 100 Gb/s and elasticity against time-varying traffic. In light of this, a gradual fixed grid to flexgrid migration is generally accepted in order to add these useful features to the network. In this article, we study the migration process where flexgrid is deployed in the network progressively, and review the main drivers and open issues induced by its deployment.

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Planning fixed to flexgrid gradual migration: drivers and open issues

A highly linear optical transmitter for radio-over-fiber subcarrier-multiplexed systems is presented. An integrated dual Mach-Zehnder modulator (MZM) is utilized to combine an optical carrier suppressed signal with an optical carrier. The proposed transmitter enables more than 10-dB improvement in the carrier-to-interference ratio compared to a quadrature biased MZM, and similar results to low biased MZM considering similar insertion loss. Negligible radio-frequency power dependence with temperature-induced bias drift is reported, while the low biased MZM is penalized by more than 12 dB for a 15% bias drift. The proposed transmitter reduces the minimum error vector magnitude from 4.2% to 3.5%, when compared to quadrature biased MZM, for a 54-Mb/s orthogonal frequency-division-multiplexed signal.

Highly Linear Integrated Optical Transmitter for Subcarrier Multiplexed Systems

This letter proposes a new power equalization scheme based on cascaded semiconductor optical amplifiers, for 10-Gb/s ethernet passive optical networks with uneven topology. For networks with a large number of subscribers, the proposed equalization scheme enables a 16-dB reduction of the receiver dynamic range.

All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10-Gb/s EPONs

A simple high-speed data transmitter to generate optical single-sideband (OSSB) signals using different electrical signaling formats is presented. The OSSB signal is generated by combining the information signal with the Hilbert transform of that signal, obtained by means of electrical processing. A detailed mathematical model is introduced to evaluate the transmitter performance, considering the generation of OSSB signals with different electrical signaling formats. Two signaling formats are evaluated and compared: nonreturn-to-zero (NRZ) and alternate mark inversion. The optimum transmitter operation conditions, namely bias and drive voltages, are derived according to the signaling format used and confirmed experimentally. Transmission tests conducted using 10-Gb/s OSSB-NRZ signals to assess the improvement obtained by electrical dispersion compensation show a significant mitigation of the dispersion distortion.

Daniel Fonseca

Papers

Design of Few-Mode Fibers With M-modes and Low Differential Mode Delay

Planning fixed to flexgrid gradual migration: drivers and open issues

Highly Linear Integrated Optical Transmitter for Subcarrier Multiplexed Systems

All-Optical Burst-Mode Power Equalizer Based on Cascaded SOAs for 10-Gb/s EPONs

Optical single-sideband transmitter for various electrical signaling formats