Minimal-hardware colorless-directionless ROADM based on a single multi-input-port-WSS with joint switching

Recent advances in wavelength-selective switch (WSS) technology have significantly enhanced the functionality of reconfigurable optical add/drop multiplexers (ROADMs), particularly in core networks. However, metro and access networks face unique constraints, where cost, space, and energy efficiency are paramount. As one approach, filter-less optical networks have been proposed, in which WSSs are replaced by passive splitters and combiners operating across the entire spectrum band. While this strategy reduces equipment cost and power consumption, the lack of wavelength-selective elements prevents wavelength reuse, leading to increased planning complexity and limited scalability. To address these challenges, we propose a streamlined colorless directionless (CD)-ROADM architecture that employs a single multi-input-port (MIP)-WSS with joint switching. By consolidating ROADM functionality into a single-WSS device, the proposed architecture reduces the equipment cost, physical footprint, power consumption, and amount of intra-node fiber wiring while retaining essential CD-ROADM capabilities. The joint switching mechanism effectively mitigates wavelength contention inside the MIP-WSS, thereby preserving routing flexibility. Moreover, the reduced insertion loss achieved using this architecture extends the transmission reach and enhances network capacity. This paper makes three main contributions. First, we present the operating principle and design of a single MIP-WSS CD-ROADM while emphasizing how joint switching enables efficient resource utilization. Second, we analyze WSS characteristics and review conventional ROADM designs based on single- and multi-input-port WSSs, positioning our work within the broader design landscape. Third, we experimentally show the feasibility of the proposed architecture through a three-degree CD-ROADM implementation using a commercial liquid-crystal-on-silicon-based 1×9 WSS configured as a 4×4 MIP-WSS. The experimental results confirm that the proposed approach simplifies node architectures while delivering sufficient functionality and performance for deployment.