Passive Tuning of Optical Couplers Using a Thin-Film Cladding Material

This letter reports the demonstration of a novel method to passively tune the splitting ratio of a Ti-indiffused lithium niobate (LiNbO
₃
) 3-dB directional coupler by the addition of a silicon-rich nitride cladding material. Plasma-enhanced chemical vapor deposition of such cladding layer onto a coupler alters the effective index of the odd and even modes in the coupled waveguide. By varying the thickness of the thin-film cladding layer, we are able to precisely control the coupling ratio and compensate for variations arising from fabrication tolerances that can significantly impact coupling.