Peak-transmittance-tunable multimode interference (MMI) coupler

In large-scale optical phased arrays (OPAs), longitudinal beam steering via wavelength tuning typically forces multimode interference (MMI) couplers to deviate from their optimal design points, inducing substantial insertion loss. While this degradation may be negligible in discrete components, the excess loss rapidly accumulates within cascaded optical splitting networks, severely bottlenecking the system's overall optical power budget. In this paper, we propose and experimentally demonstrate a thermo-optic tuning strategy to actively compensate for this excess insertion loss. By dynamically modulating the modal effective refractive index, this approach successfully realigns the spatial offset of the self-image point induced by wavelength drift and fabrication tolerances. Comprehensive multiphysics simulations indicate that this active tuning suppresses beat length variations across a broad 100-nm bandwidth. Furthermore, system-level modeling of a 4 K-channel cascaded network demonstrates that our scheme reduces broadband excess loss from > 6 dB to ∼0.37 dB, with negligible lateral thermal crosstalk. Experimental characterizations confirm that silicon devices integrated with titanium nitride (TiN) micro-heaters achieve up to 3 dB of active loss recovery, requiring a minimal power consumption of only tens of milliwatts. This simple, CMOS-compatible approach provides a highly scalable architecture for efficient broadband optical power distribution in next-generation OPAs.