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]

Research on space-division multiplexing (SDM) came to prominence in early 2010 being primarily proposed as a means of multiplying the information-carrying capacity of optical fibers at the same time as increasing efficiency through resource sharing. Proposed SDM transmission systems range from parallel single-mode fibers with shared amplifier pump lasers to the full spatial integration of transceiver hardware, signal processing, and amplification around a fiber with over 100 spatial channels comprising multiple cores each carrying multiple modes. In this paper, we review progress in SDM research. We first outline the main classifications and features of novel SDM fibers such as multicore fibers (MCFs), multimode fibers, few-mode MCFs, and coupled-core MCFs. We review research achievements of each fiber type before discussing digital-signal processing, amplifier technology, and milestones of transmission and networking demonstrations. Finally, we draw comparisons between fiber types before discussing the current trends and speculate on future developments and applications beyond optical data transmission.

Space-division multiplexing for optical fiber communications

As traffic volumes carried by optical networks continue to grow by tens of percent year over year, we are rapidly approaching the capacity limit of the conventional communication band within a single-mode fiber. New measures such as elastic optical networking, spectral extension to multi-bands, and spatial expansion to additional fiber overlays or new fiber types are all being considered as potential solutions, whether near term or far. In this tutorial paper, we survey the photonic switching hardware solutions in support of evolving optical networking solutions enabling capacity expansion based on the proposed approaches. We also suggest how reconfigurable add/drop multiplexing nodes will evolve under these scenarios and gauge their properties and relative cost scalings. We identify that the switching technologies continue to evolve and offer network operators the required flexibility in routing information channels in both the spectral and spatial domains. New wavelength-selective switch designs can now support greater resolution, increased functionality and packing density, as well as operation with multiple input and output ports. Various switching constraints can be applied, such as routing of complete spatial superchannels, in an effort to reduce the network cost and simplify the routing protocols and managed pathway count. However, such constraints also reduce the transport efficiency when the network is only partially loaded, and may incur fragmentation. System tradeoffs between switching granularity and implementation complexity and cost will have to be carefully considered for future high-capacity SDM–WDM optical networks. In this work, we present the first cost comparisons, to our knowledge, of the different approaches in an effort to quantify such tradeoffs.

Survey of Photonic Switching Architectures and Technologies In Support Of Spatially and Spectrally Flexible Optical Networking {[}Invited]

Survey of resource allocation schemes and algorithms in spectrally-spatially flexible optical networking

Emerging technologies such as spectrally efficient multicarrier higher order modulation and bandwidth variable wavelength selective switches have led to a change in the optical network architecture from rigid and homogeneous to flexible and heterogeneous in terms of the bit rate, center frequency spacing, modulation format, and optical reach. The elastic optical network (EON) is a network architecture toward beyond the 100-Gb/s era. This tutorial paper reviews elastic opticalnetworking technology and presents its roles and benefits in a new era, where major line rates in metro/core optical networks are 100 Gb/s and beyond. The main features of the EON are that it increases the spectral efficiency in a network manner through rate-adaptive superchannels and distance-adaptive modulation, and it provides cost- and energy-efficient traffic grooming in the optical domain. The EON concept has been widely accepted by industry and academia. Significant standardization efforts have yielded great advances evidenced by the flexible grid and the flexible OTUCn frame format recommendations. Hardware virtualization such as a sliceable multiflow transponder in cooperation with emerging flexible clients will be key to achieving more flexible and cost-effective next generation optical networks.

Elastic Optical Networking: Roles and Benefits in Beyond 100-Gb/s Era

As optical networks have evolved from point-to-point systems to ring or mesh networks, the optical devices that are needed to construct optical nodes have become more important and need to be more scalable. Hybridization of waveguide and free-space optics, or spatial and planar optical circuits (SPOCs), may provide the solutions for such needs. An SPOC platform is attractive because it can take advantage of both waveguide technology and free-space optics. Waveguide technology provides a high degree of integration of optical functionality for devices such as splitters and non-wavelength selective switches while free-space optics supplies a high degree of parallelism with two-dimensional spatial light modulators such as a liquid crystal on silicon (LCOS) devices. In this paper, we summarize the basics of SPOC technology and review its application to reconfigurable optical add-drop multiplexing (ROADM) devices. The key elements of a waveguide on an SPOC platform are an arrayed-waveguide grating and a spatial beam transformer. The latter functions as a microlens array and provides attractive features such as dense integration of switches. An LCOS device has numerous phase modulating pixels, enabling flexible manipulation of lightwaves. Using an SPOC platform, we constructed and demonstrated devices for ROADM applications including a wavelength filter, tunable optical dispersion compensators, and wavelength selective switches (WSSs). The WSSs range from an ultrahigh port count WSS to a single module wavelength cross connect.

Application of Waveguide/Free-Space Optics Hybrid to ROADM Device

We propose a compact transponder aggregator (TPA) consisting of a wavelength selective switch (WSS) array and a direction switch array. It has no intrinsic splitting loss and therefore the number of ports can exceed that of a conventional multicast switch. A waveguide-based frontend that incorporates spatial beam transformers enables us to increase the number of ports and integrate multiple WSSs in a simple optical system. The direction switch array is integrated in the frontend. We describe an 8 × 24 prototype, which is the largest port count yet achieved for a TPA with no intrinsic splitting loss.

Low-Loss Transponder Aggregator Using Spatial and Planar Optical Circuit

We propose an ultra-high port count wavelength selective switch employing a waveguide-based input/output frontend that scales the port count to the order of hundreds. We demonstrate a 1 × 95 WSS and confirm its feasibility.

Ultra-high port count wavelength selective switch employing waveguide-based I/O frontend

We propose an optimal variant of an M × N wavelength selective switch (WSS) for a transponder aggregator. A PLC frontend enables us to integrate an M array of 1 × N WSSs and port selection switches in simple optics. A high-port count (8 × 24) prototype is demonstrated.

Yuichiro Ikuma

Papers

Survey of Photonic Switching Architectures and Technologies In Support Of Spatially and Spectrally Flexible Optical Networking {[}Invited]

Application of Waveguide/Free-Space Optics Hybrid to ROADM Device

Low-Loss Transponder Aggregator Using Spatial and Planar Optical Circuit

Ultra-high port count wavelength selective switch employing waveguide-based I/O frontend

8 × 24 wavelength selective switch for low-loss transponder aggregator