Principle of QPM
Due to the dispersion of refravtive index, generated optical field by
frequency conversion was quenched by the interference with the optical
field that was emitted from other position in the nonlinear materials.
The technique for the reduction of this quenching is called "phase
matching." (Ref. S. Miyazawa and S. Kurimura ed., "Fundamentals and
applications of periodically-poled devices," Optronics (2005).)
QPM (Quasi-Phase Matching) is the most convenient method for this
phase matching. Using the periodically poled (domain inverted in crystal
axes) devices with an approprioate period for aimed wavelength, phase
matching condition is automatically satisfied.