Interaction of light with impurities in lithium niobate crystals

Congruent lithium niobate (LiNbO3 ) and 5-mol% MgO-doped LiNbO3 (MgO:LN) crystals are widely used as nonlinear-optical crystals in frequency-conversion devices due to their large nonlinear-optic coefficients. These devices usually require high optical pump powers, but absorption of photons by impurities limits their usability due to heat accumulation that leads to thermo-optic refractive index changes. These refractive index changes distort the beam shape and disturb the phase-matching condition. Furthermore pyroelectric fields can build up.

In this thesis the residual optical absorption in congruent LiNbO3 (CLN) and MgO:LN crystals is studied. Absorption spectra of CLN and MgO:LN crystals between 400 − 2000 nm reveal a residual absorption up to 0.04 cm^−1 . This absorption is mainly caused by transition metal impurities. Between 2300 − 2800 nm unknown hydrogen absorption bands in CLN and MgO:LN are revealed on the order of 0.001 cm^−1 . High-temperature annealing is applied to the CLN and MgO:LN crystals, which decreases optical absorption by up to one order of magnitude. As an application, the operation of a 1550-nm pumped singly-resonant CW optical parametric oscillator, resonant around 2600 nm, using a low-loss, periodically-poled, annealed CLN crystal is demonstrated.

Another issue that affects CLN is photorefractive damage (PRD), i.e. light-induced refractive index changes. In contrast, MgO:LN crystals do not suffer from PRD even at high optical intensities. However, it is shown in this thesis that PRD can occur within seconds in MgO:LN, using green laser light at light intensity levels as low as 100 mW/cm2 , if the crystal is heated by several degrees Celsius during or before illumination. Photorefractive damage does not occur in CLN crystals under the same conditions. We show that the pyroelectric effect together with an elevated photoconductivity compared to that of CLN causes this beam distortion and that this effect also influences frequency conversion experiments in the infrared due to beam self-heating.