Tunable guided‐wave optical polarization converters in lithium tantalate

Tunable guided‐wave TE↔TM mode converters have been produced in LiTaO3 using strain‐induced polarization mode coupling. The waveguides are formed by Zn diffusion from the vapor phase. Phase matched coupling is obtained by depositing a thick SiO2 film at an elevated temperature and delineating a spatially periodic pattern in the film at room temperature. The static shear strain resulting from the large thermal expansion mismatch between the substrate and film causes an off‐diagonal change in the refractive index via the strain‐optic effect and promotes TE↔TM coupling. Polarization conversion in excess of 98% was achieved at a wavelength of 627 nm in X‐cut LiTaO3. The conversion efficiency is highly wavelength selective, with a spectral width of 7.3 nm in the 5.4 mm long device. Thermal and electro‐optic tuning of the center wavelength was demonstrated.