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

Abstract

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.

Eight-Channel Silicon-Photonic Wavelength Division Multiplexer With 17 GHz Spacing

“…The change in cladding index is on the order of 0.05-0.1 RIU [33], raising the local effective modal index by approximately 0.01 RIU [30], [31]. At higher intensity, a mass transfer effect takes place instead, locally removing the upper cladding layer altogether [30], [31]. In this case, the local effective index of the optical mode is reduced by 0.13 RIU.…”

“…In contrast, the values of the critical phase delays were arbitrary. To allow for phase delay trimming, patches of photo-sensitive As 2 Se 3 chalcogenide glass were deposited on top of the silicon-photonic waveguides, at selected locations [30], [31]: one on the longer arm of each MZI and one within each ring resonator (see Fig. 4).…”

“…Numerical simulations suggest that the effective index of the single transverse-electric (TE) mode of the silicon waveguide at the same wavelength is 2.85 RIU and 2.72 RIU, with and without the chalcogenide cladding, respectively. The chalcogenide cladding modifies the group delays of waveguide paths as well [30], [31]. In order to retain the proper ratios among group delays along different elements, the lengths of the chalcogenide cladding patches scaled with the group delay: from 50 μm in the C-stage MZIs (unit delay τ ) to 400 μm over the ring resonator of the A-stage unit (delay of 8τ ).…”

“…When the illumination intensity is comparatively modest, the refractive index of the layer is locally increased due to a photo-darkening process [33]. The change in cladding index is on the order of 0.05-0.1 RIU [33], raising the local effective modal index by approximately 0.01 RIU [30], [31]. At higher intensity, a mass transfer effect takes place instead, locally removing the upper cladding layer altogether [30], [31].…”

“…The large number of output ports and the narrow FSR raise a formidable challenge in phase delay trimming. To that end, patches of a thin upper cladding layer of As 2 Se 3 chalcogenide glass were deposited on top of the silicon waveguides in select locations [29]- [31]. The illuminations of the upper cladding with a focused laser beam locally modifies its refractive index, and with higher exposure can even result in the removal of the cladding layer [32], [33].…”