Cascaded SDM-WSS Channel Filtering under Weak and Strong Mode-Mixing Regimes

In this paper, we analyze the spectral performance of space-division multiplexing (SDM) signals traversing cascaded wavelength-selective switches (WSSs), a key component of reconfigurable optical add/drop multiplexers (ROADMs). As SDM technologies such as few-mode fiber (FMF) and multicore fiber (MCF) emerge to meet increasing data capacity demands, understanding the limitations imposed by repeated channel filtering becomes essential for system design. We investigate how the extent of spatial mixing — ranging from no mixing to weak or strong coupling — affects the MDL-limited passband and usable channel bandwidth. Our numerical analysis models full WSS transfer matrices, incorporates realistic random misalignment between switches, and evaluates the resulting MDL and effective channel bandwidth. We show that strong spatial mixing mitigates the accumulation of MDL, preserving broader passbands and improving network scalability. In the MCF case, we further demonstrate that fiber orientation plays a key role, as asymmetric supermode size result in angle-dependent filtering penalties. Our findings quantify the benefits of strong coupling and offer practical design guidelines for scalable SDM-ROADMs, maximizing bandwidth and minimizing system penalties in future optical networks.