Characterisation of proton-exchange slab optical waveguides in x-cut LiNbO3

The paper reports on a systematic study of planar optical waveguides in X-cut LiNbO3 fabricated by the proton-exchange technique. The extent of the exchange process was monitored by measuring the infrared absorption peak at 3510 cm¿¿1. It was established that the waveguide profile could be accurately modelled by a step index change with ¿¿n = 0.127 at ¿¿0 = 632.8 nm and ¿¿n = 0.096 at ¿¿0 = 1150 nm. From the mode effective refractive index measurements, the effective diffusion coefficients for the exchange process were calculated. A value for the activation energy for the proton-exchange process was established. The effect of off-Y-axis propagation on the mode effective indices of the waveguide was studied. A waveguide mode cutoff at 29¿¿ from the C-axis was established. The lowest optical losses measured in single-mode planar waveguides were 1.4 dB/cm at ¿¿0 = 632.8 nm and 1.3 dB/cm at ¿¿0 = 1150 nm for light propagating normal to the C-axis.