CC3

Directional couplers are a fundamental buildingblock in integrated photonics, particularly in quantum appli-cations and optimization-based design where precision is critical.Accurate functionality is crucial to ensure reliable operationwithin classical and quantum circuits. However, discrepanciesbetween simulations and measurements are frequently observed.These inaccuracies can compromise the performance and scal-ability of integrated photonic systems, underscoring the criticalneed for advanced, precise simulation methods that bridge thegap between design and implementation. In this work, we showthat this discrepancy can be mainly attributed to density changesin the oxide cladding. We conduct a systematic study involvingexperimental optical measurements, numerical simulations, anddirect electron microscopy imaging to investigate this discrepancyin directional couplers. We find that the impact of claddingdensity variations on performance increases as feature gapsshrink. By incorporating these effects into our simulations usinga novel and physically motivated Effective Trench Medium Model(ETMM), we achieve highly accurate reproduction of experimen-tal measurements. We quantify the effects of cladding densityvariations on the SU(2) symmetry parameters that govern lightpropagation in directional couplers. This insight is crucial foradvancing the precision of compact device fabrication, enablingreliable simulation of photonic integrated devices.