ABSTRACT
The greater beam divergence brought about by tight focusing can also have a deleterious effect on efficiency depending on how it compares to the phase-matching acceptance angle for the particular non-linear material and experimental arrangement, a subject that will be discussed in the next section. In contrast, Koechner provides a more selective survey of non-linear materials but with more commentary on each. Potassium titanyl phosphate, KTiOPO4, (KTP) has a very favourable set of characteristics when compared with other birefringent materials; other materials might score more highly in particular categories but KTP has superior overall properties. Quasi-phase-matched materials will be covered in more detail in the next section. The only major weakness of BBO as a non-linear material is that it has a small angular PMB. The wavelength range of laser radiation can be greatly extended by utilizing the non-linear optical properties of birefringent crystals to frequency convert the output of a laser to other wavelengths.
