CC3

Directional couplers are a fundamental building

block in integrated photonics, particularly in quantum appli-

cations and optimization-based design where precision is critical.

Accurate functionality is crucial to ensure reliable operation

within classical and quantum circuits. However, discrepancies

between simulations and measurements are frequently observed.

These inaccuracies can compromise the performance and scal-

ability of integrated photonic systems, underscoring the critical

need for advanced, precise simulation methods that bridge the

gap between design and implementation. In this work, we show

that this discrepancy can be mainly attributed to density changes

in the oxide cladding. We conduct a systematic study involving

experimental optical measurements, numerical simulations, and

direct electron microscopy imaging to investigate this discrepancy

in directional couplers. We find that the impact of cladding

density variations on performance increases as feature gaps

shrink. By incorporating these effects into our simulations using

a novel and physically motivated Effective Trench Medium Model

(ETMM), we achieve highly accurate reproduction of experimen-

tal measurements. We quantify the effects of cladding density

variations on the SU(2) symmetry parameters that govern light

propagation in directional couplers. This insight is crucial for

advancing the precision of compact device fabrication, enabling

reliable simulation of photonic integrated devices.