Refractive-Index Changes of Titanium-Doped Near-Stoichiometric LiNbO ₃ Crystals

Titanium-doped (up to 9 mol%) near-stoichiometric LiNbO₃ (SLN) crystals were grown from lithium-rich solutions by the top-seeded solution-growth method. The ordinary and the extraordinary refractive indices (n_o and n_e) of the titanium-doped SLN (Ti:SLN) crystals were measured at room temperature by the prism-coupling technique in the wavelength range from 440 to 1550 nm. The relations between titanium concentrations and the ordinary and the extraordinary refractive-index changes (Δn_o and Δn_e) of the Ti:SLN crystals were investigated. Accordingly, it was found that both Δn_o and Δn_e linearly increase with increasing titanium concentration up to 9 mol% and that Δn_e is much larger than Δn _o over the whole wavelength range. It was also found that both Δn_o and Δn_e increase with decreasing wavelength in all the Ti:SLN crystals with different titanium concentrations. Moreover, Δn_e of Ti:SLN doped with approximately 4 mol% of titanium (approximately 1 × 10^-2) is almost the same as that of titanium-diffused SLN waveguides. In addition, a photoelastic effect on the refractive-index changes in the Ti:SLN crystal was estimated from using lattice-constant data determined by X-ray diffraction.