Large one-time photo-induced tuning of directional couplers in chalcogenide-on-silicon platform

The stable one-time tuning of silicon-photonic directional couplers, over a broad range of coupling ratios, is achieved through the selective photo-removal of an upper cladding layer of chalcogenide glass. Analysis shows that the coupling coefficient per unit length between two parallel fully-etched silicon waveguides may be changed by 45%. The power coupling ratio of a 50 µm-long directional coupler between two such waveguides may be tuned arbitrarily between 0 and 1, with weak residual wavelength dependence. Smaller modifications in the coupling coefficient per unit length are obtained between two partially-etched ridge waveguides, on the order of 10%. The proposed procedure is demonstrated in the post-fabrication tuning of transmission notches of a race-track resonator, from over-coupling through critical coupling to weak coupling. The extinction ratio of specific resonances is varied between 4 and 40 dB. The coupling ratio of a tuned device remains stable following three months of storage.