Potassium dihydrogen phosphate (KDP or _KH2PO4) is the most commercially used NLO material with a wide transparency range, high birefringence and a high optical damage threshold. KDP crystals are often used for frequency doubling, tripling, quadrupling of Nd:YAG lasers at room temperature. In addition, they are used as electro-optic crystals with high electro-optic coefficients so that they can be used as electro-optic modulators, Q-switches and Pockels cells.