Beam distortion due to photorefraction limits the usability of lithium niobate (LiNbO3) crystals for frequency conversion applications. To prevent beam distortion in LiNbO3, 5 mol.% magnesium-doped LiNbO3 (MgO:LN) is usually used. However, we show that strong beam distortion of green laser light can occur within seconds in MgO:LN, starting at light intensity levels in the 100 mW/cm2 regime, if the crystal is heated by several degrees Celsius during or before illumination. Beam distortion does not occur in undoped congruent LiNbO3 (CLN) 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 even if no external heating is applied

Absorption measurements on 5 mol. % MgO-doped and undoped congruent lithium niobate (LiNbO(3)) crystals revealed absorption bands in the wavelength range of 2500-2800 nm, caused by incorporated hydrogen ions. High-temperature annealing was applied to the congruent LiNbO(3) (CLN) crystals, which decreased the absorption significantly. Then the annealed CLN crystals were periodically poled. As an application of the low-loss annealed CLN crystal, the operation of a 1550 nm pumped singly resonant CW optical parametric oscillator, resonant around 2600 nm, using a periodically poled crystal was demonstrated.

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High-quality whispering-gallery-mode resonators made of polymethylmethacrylate (PMMA) are fabricated by simple mechanical turning and polishing according to a technique used by Ilchenko et al. to produce crystalline whispering-gallery-mode resonators with high quality factors (Q-factors). The high-Q PMMA resonators are investigated in two wavelength regimes: in the near infrared between the wavelengths 1470 and 1580 nm and at the wavelength 635 nm. The Q-factor in the infrared regime is limited by material absorption to 3 x 10(5) At 635 nm the Q-factor is limited by surface scattering only and reaches 4 x 10(7), which is a new record for polymers.