ABSTRACT
This chapter describes the theoretical background of the quasi-phase matching (QPM) frequency conversion process. The QPM, which is another method of phase matching, was proposed almost simultaneously in the pioneering paper by Armstrong, Bloembergen, Ducuing and Pershan (ABDP). This involves changing the sign of the effective coupling coefficient after the waves have travelled one coherence length. QPM in periodically poled ferroelectric crystals has provided a host of efficient nonlinear optical frequency conversion devices. Several other crystals have also shown periodic poling possibility, but periodically poled lithium niobate (PPLN) is probably the most developed material. On the IR side, QPM structures based on GaAs have been made by several techniques and an OPO has been demonstrated. Mixing the output from OPO QPM also provides a promising source of terahertz radiation.
