At present , people usually use birefrigence and quasi - phase - matching methods to achieve higher effiency in the process of conversion . a significant advantage of quasi - phase - matching method is that the interaction is occurred through the largest element of the x ( 2 ) tensor ; the period of optical superlattice can be designed randomly , and any interaction can be matched without walk - off angle ; they are fitted to be used in frequency conversion of lower power especially . the quasi - phase - matching method has some advantages over birefrigence method , improving the efficiency of frequency conversion ; the range of frequency conversion covers its whole transperant wavelength 由于准相位匹配技术有其独特的优势，可以利用非线性光学晶体的最大二阶极化张量，如ppln 、 pplt 、 ppktp用的非线性极化张量都是d _ ( 33 ) ,其有效非线性系数高出普通非线性光学介质(如bbo 、 lbo 、 ktp )一个数量级，而且可以人为灵活设计调制周期，通过周期设计其频率变换范围可以覆盖准相位匹配介质的整个透明波段，选择无走离效应，解决了常规pm难以解决的问题，拓宽了非线性晶体的应用范围，极大地提高了频率转换效率，已成为非线性光学材料和固体激光器的研究热点之一。
