Optical Cross-Connects (OXCs) serve as fundamental components in backbone optical networks, facilitating optical signal switching, dynamic service provisioning, and rapid fault recovery. These systems constitute critical infrastructure supporting the advancement of the national digital economy. The continuous expansion of network capacity, coupled with emerging technologies such as multi-fiber links, multi-core fibers, and multi-band transmission, has substantially increased the switching degree of OXC nodes (Fig. 1), potentially extending to several hundreds of degrees. Traditional OXC architectures based on broadcast-and-select (B&S) or route-and-select (R&S) configurations face limitations due to the restricted port count of wavelength selective switches (WSSs), making them unsuitable for high-dimensional scenarios. Consequently, addressing existing technical constraints and developing innovative high-degree OXC architectures has become essential to meet future requirements for large-scale all-optical switching in backbone networks.

Two primary approaches exist for implementing high-degree OXCs. The first involves developing large-scale WSSs to extend existing OXC architectures, while the second utilizes small-scale WSS modules to construct high-degree OXCs. This paper examines three mainstream WSS fabrication technologies (Table 1), with liquid crystal on silicon (LCoS)-based WSSs emerging as the most viable solution for large-scale devices due to their inherent flexibility and scalability. Regarding high-degree OXCs constructed with small-scale WSSs, this paper presents the first systematic analysis of three representative solutions: the sparse fiber interconnection scheme, the modular OXC cascading scheme, and the multi-stage N×N WSS cascading scheme. The sparse fiber interconnection scheme, implemented on both line and add/drop sides, reduces WSS port requirements (Fig. 2 and Fig. 3). The add/drop side implementation achieves 40% cost reduction and 20% insertion loss reduction while maintaining comparable lightpath blocking performance. The modular cascading scheme enables interconnection of multiple small-scale modular OXC units through port bridging to form high-degree OXC nodes (Fig. 6). A hybrid architecture incorporating large-scale MEMS switches and WSSs, designated HMWC, enhances wavelength availability and approximates the blocking performance of traditional OXC architectures (Fig. 7). The multi-stage N×N WSS cascading scheme introduces a three-stage Clos-topology-based architecture, termed WSS-Clos OXC, implementing strictly non-blocking high-degree OXC nodes (Fig. 9). Supporting techniques including wavelength conversion (Fig. 10) and cost optimization (Fig. 11) are explored through modifications to intermediate-stage modules, addressing wavelength continuity and hardware complexity challenges.

The sparse fiber interconnection scheme approximates traditional OXC performance under specific routing and wavelength assignment (RWA) algorithms while substantially increasing network operation and maintenance complexity. The modular OXC cascading scheme provides enhanced compatibility and scalability but encounters challenges regarding wavelength contention and increased insertion loss from multiple WSS stages. The multi-stage N×N WSS cascading scheme ensures strict non-blocking performance and achieves optimal cost-performance balance through architectural optimization, indicating significant potential for future implementation. Future research directions should encompass architecture design and performance evaluation of high-degree OXCs while addressing critical aspects such as system reliability, scalability, and integration of advanced photonic technologies.