A spatial channel network (SCN) was recently proposed toward the forthcoming spatial division multiplexing (SDM) era, in which the optical layer is explicitly evolved to the hierarchical SDM and wavelength division multiplexing layers, and an optical node is decoupled into a spatial cross-connect (SXC) and wavelength cross-connect to achieve an ultrahigh-capacity optical network in a highly economical manner. In this paper, we report feasibility demonstrations of an evolution scenario regarding the SCN architecture to enhance the flexibility and functionality of spatial channel networking from a simple fixed-core-access and directional spatial channel ring network to a multidegree, any-core-access, nondirectional, and core-contentionless mesh SCN. As key building blocks of SXCs, we introduce what we believe to be novel optical devices: a 1 × 2 multicore fiber (MCF) splitter, a core selector (CS), and a core and port selector (CPS). We construct free-space optics-based prototypes of these devices using five-core MCFs. Detailed performance evaluations of the prototypes in terms of the insertion loss (IL), polarization-dependent loss (PDL), and intercore cross talk (XT) are conducted. The results show that the prototypes provide satisfactorily low levels of IL, PDL, and XT. We construct a wide variety of reconfigurable spatial add/drop multiplexers (RSADMs) and SXCs in terms of node degree, interport cross-connection architecture, and add/drop port connectivity flexibilities. Such RSADMs/SXCs include a fixed-core-access and directional RSADM using a 1 × 2 MCF splitter; an any-core-access, nondirectional SXC with core-contention using a CS; and an any-core-access, nondirectional SXC without core-contention using a CPS. Bit error rate performance measurements for SDM signals that traverse the RSADMs/SXCs confirm that there is no or a very slight optical signal-to-noise-ratio penalty from back-to-back performance. We also experimentally show that the flexibilities in the add/drop port of the SXCs allow us to recover from a single or concurrent double link failure with a wide variety of options in terms of availability and cost-effectiveness.

We design and prototype a 5-core 1 × 6 core selective switch (CSS) with an integrated input and output multi-core-fiber collimator and spatial multiplexer/demultiplexer array. Spatial bypassing and spectral grooming using a CSS-based hierarchical cross-connect are demonstrated.

We describe the principle, design, and prototyping of a free-space-optics-based core selective switch (CSS) for spatial channel (SCh) networks (SCNs) in the age of space division multiplexing (SDM). A CSS in an SCN corresponds to a conventional wavelength selective switch (WSS) in a current wavelength division multiplexing network. It incorporates functionalities for spatially demultiplexing SChs from an input SDM port and for switching and multiplexing any of them into any of output SDM ports. We discuss the design for free-space-optics-based CSSs that is applicable to CSSs supporting various numbers of cores per multicore fiber (MCF) and various numbers of MCF ports. We show an example of an implemented 5-core 1 × 6 CSS prototype that integrates an MCF collimator, spatial multiplexer/demultiplexer array, and liquid-crystal-on-silicon spatial light modulator. Although some output MCF ports exhibit relatively high insertion loss (IL), the CSS prototype shows that a CSS will potentially provide low net IL at the level of approximately 2 dB. To verify that there are no unknown deteriorating factors in the CSS prototype, we tested the prototype implemented in a hierarchical optical cross-connect configuration by measuring the pre-forward error correction bit error rate. Spatial bypassing and spectral grooming of a 900-Gb/s SCh employing a CSS-based spatial cross-connect and a conventional wavelength cross-connect are successfully demonstrated with no optical signal-to-noise ratio penalty.

This article reports on a proof of concept demonstration of a recently proposed spatial channel network (SCN) that was conducted over an SCN testbed that comprises low-loss hierarchical optical cross-connect (HOXC) prototypes and four-core multicore fiber links. The HOXC prototypes used in the testbed are based on sub-matrix-switches and core selective switches both implemented with commercially available discrete optical switches. Using these two types of HOXC prototypes, the spatial channel networking including spatial bypassing, spatial add/drop and spectral grooming, spatial-lane change, and spatial-channel (SCh) protection is successfully demonstrated for SChs carrying 100-Gb/s-900-Gb/s optical channels.

The core selective switch (CSS) is an optical spatial switch that has been recently proposed as a key building block to achieve a scalable and low-insertion-loss spatial cross-connects for use in future spatial channel networks. In this paper, we report on a novel CSS design employing a two dimensionally arranged microlens-based multicore fiber (MCF) collimator array and a micro-electromechanical systems (MEMS) mirror array. The former enables precise alignment between MCFs and collimator lenses, and the latter yields polarization-independent high reflection over a wide wavelength range and a large tilt angle. Based on the design, a compact (~50 mm) five-core 𝟏 × 𝟖 CSS prototype is fabricated. We experimentally show that the CSS prototype exhibits low insertion loss (1.2~2.7 dB), low polarization dependent loss (< 0.25 dB), and low crosstalk (< −𝟑𝟎 dB) characteristics over an ultra-wide wavelength range from 1500 nm to 1630 nm. Bit-error-rate measurements using optical signals in the C-band, S-band, and L-band show that the CSS prototype incurs no optical signal-to-noise ratio penalty in spatial channel routing over an ultra-wide wavelength band.