BBO crystal in nonlinear optical crystal, is a kind of comprehensive advantage obvious, good crystal, it has a very wide light range, very low absorption coefficient, weak piezoelectric ringing effect, relative to other electrolight modulation crystal, has higher extinction ratio, larger matching Angle, high light damage threshold, broadband temperature matching and excellent optical uniformity, is beneficial to improve the laser output power stability, especially for Nd: YAG laser three times frequency has a widely application.