Beam distortion due to photorefraction limits the usability of lithium niobate (LiNbO_3) crystals for frequency conversion applications. To prevent beam distortion in LiNbO_3, 5 mol:% magnesium-doped LiNbO _3 (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 100mW=cm~2 regime, if the crystal is heated by several degrees Celsius during or before illumination. Beam distortion does not occur in undoped congruent LiNbO_3 (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.
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