ABSTRACT
This chapter presents a strategy for phase-matching waves that does not require crystal birefringence. The technique, called quasi-phase matching (QPM), was proposed even before birefringent phase matching, but due to technical difficulties in fabricating samples, QPM was not achievable at the time. The more direct method of birefringent phase matching was the first to be widely implemented. Both techniques are now found in a wide range of applications. Stacking crystals and other more efficient fabrication techniques are discussed after a review of the basic QPM analysis. A physical reorientation of a crystal, in general, changes the linear properties as seen by a beam of light propagating through the crystal. A periodic structure results in a diffraction grating, which complicates the nonlinear mixing process. A detailed understanding of QPM is found by an analysis of the wave equation. A revolution in QPM sample fabrication occurred in the early 1990s when electric-field poling of ferroelectric crystals was applied to fabricating QPM structures.
