Principle, Design, and Prototyping of Core Selective Switch Using Free-Space Optics for Spatial Channel Network

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.