Characterization of lithium out-diffused slab waveguides in LiNbO 3 as a function of fabrication conditions

We are interested in fabricating titanium in-diffused lithium niobate (Ti:LiNbO₃) slab waveguides that support a single TE polarized mode propagating in the direction of the crystal´s x-axis using optical grade, y-cut wafers. Titanium in-diffused waveguides are fabricated by diffusing titanium metal into a lithium niobate substrate at high temperature to yield an increased surface ordinary and extraordinary refractive index. However, lithium can diffuse from the LiNbO₃ substrate during the high temperature cycle required for titanium in-diffusion. This lithium out-diffusion creates a surface region of higher extraordinary refractive index that, by itself can support guided modes. The presence of both lithium out-diffusion and titanium in-diffusion make it difficult to fabricate single-mode slab waveguides that satisfy our particular waveguide orientation. We evaluated various fabrication methods for their ability to eliminate lithium out-diffusion in slab waveguides. These waveguides were characterized using prism out-coupling to measure mode effective index and number. From the number of guided modes and value of mode effective index we inferred the effect of a particular fabrication method on lithium out-diffusion