Post‐Fabrication Phase Correction of SOI 4 × 4 MMI‐Based 90° Optical Hybrids via Femtosecond‐Laser Trimming

Integrated 90° optical hybrids are essential passive components in coherent receivers, yet their quadrature accuracy and power balance are highly susceptible to fabrication‐induced deviations. Here, we demonstrate postfabrication phase trimming of silicon‐on‐insulator 4 × 4 multimode‐interference‐based 90° optical hybrids using localized femtosecond‐laser modification. The hybrid is implemented with a shallow‐etched rib multimode section and characterized by mapping the relative output phase onto wavelength‐dependent interference spectra. As‐fabricated devices show phase deviations exceeding the ± 5° target and exhibit device‐to‐device performance fluctuations among nominally identical structures. Raman spectroscopy and high‐resolution transmission electron microscopy confirm that femtosecond‐laser irradiation induces a localized crystalline‐to‐amorphous transformation in silicon, introducing a permanent refractive‐index perturbation. By selectively modifying the MMI region, the modal phase evolution and output phase balance can be adjusted after fabrication. After trimming, the phase deviations are reduced to within ± 5° across the full C‐band, accompanied by recovery of the quadrature response, while maintaining common‐mode rejection ratio above 20 dB. These results demonstrate a permanent and power‐free calibration route for coherent silicon photonic circuits.