Quasi-phase-matching is a phase-matching method in which a nonlinear material is structured to contain a spatially periodic coefficient. An example of this is a crystal that is structured in such a way that the optical axis of its lattice periodically flips its orientation. This flipping in turn causes the nonlinear coefficient of the material to invert periodically its sign. The flipping causes the waves that are being mixed and the new wave to maintain their relative phase (match phase) along the length of the crystal. Although phase matching can be achieved by other means (birefringence, for example), QPM provides distinct advantages over other methods, including the ability to:
These advantages of QPM produce extremely efficient, tunable, and compact laser sources that enabled Sandia to develop two portable gas imaging systems. Broad tuning permitted us to make very capable IR cavity-ringdown spectrometers. The first spectrometer operated in the mid-IR range, using the nonlinear material, periodically poled lithium niobate (PPLN). The second spectrometer used PPLN and orientation-patterned gallium arsenide (OP-GaAs) obtained through collaboration with Stanford University.