Editing Nonlinear optics (section)

===Frequency doubling===
One of the most commonly used frequency-mixing processes is '''frequency doubling''', or second-harmonic generation. With this technique, the 1064&nbsp;nm output from [[Nd-YAG laser|Nd:YAG lasers]] or the 800&nbsp;nm output from [[Ti-sapphire laser|Ti:sapphire lasers]] can be converted to visible light, with wavelengths of 532&nbsp;nm (green) or 400&nbsp;nm (violet) respectively.<ref>{{Cite journal |last1=Bai |first1=Zhenxu |last2=Wang |first2=Yulei |last3=Lu |first3=Zhiwei |last4=Yuan |first4=Hang |last5=Jiang |first5=Li |last6=Tan |first6=Tan |last7=Liu |first7=Zhaohong |last8=Wang |first8=Hongli |last9=Cui |first9=Can |last10=Hasi |first10=Wuliji |date=2016-10-01 |title=Efficient KDP frequency doubling SBS pulse compressed 532nm hundred picosecond laser |url=https://www.sciencedirect.com/science/article/pii/S0030402616307914 |journal=Optik |volume=127 |issue=20 |pages=9201–9205 |doi=10.1016/j.ijleo.2016.07.021 |bibcode=2016Optik.127.9201B |issn=0030-4026}}</ref>

Practically, frequency doubling is carried out by placing a nonlinear medium in a laser beam. While there are many types of nonlinear media, the most common media are crystals. Commonly used crystals are BBO ([[β-barium borate]]), KDP ([[potassium dihydrogen phosphate]]), KTP ([[potassium titanyl phosphate]]), and [[lithium niobate]]. These crystals have the necessary properties of being strongly [[birefringence|birefringent]] (necessary to obtain phase matching, see below), having a specific crystal symmetry, being transparent for both the impinging laser light and the frequency-doubled wavelength, and having high damage thresholds, which makes them resistant against the high-intensity laser light.