KTPIs one of the nonlinear optical crystal is currently the most widely used. Nd-doped laser are the most commonly used frequency doubling crystal.
Potassium Titanium Oxide Phosphate (KTiOPO4 ), or KTP, is an efficient nonlinear optical crystal in the visible to infrared spectral region with relatively low cost. It has large nonlinear coefficient and it is the most commonly used material for frequency doubling of Nd:YAG lasers and other Nd-doped lasers, particularly at the low or medium power density. Exhibits high optical quality, broad transparent range, relatively high effective SHG coefficient (about 3 times higher than that of KDP), rather high optical damage threshold, wide acceptance angle, small walk-off and type I and type II non-critical phase-matching (NCPM) in a wide wavelength range. 
Advantages of KTP Crystals 
Efficient frequency conversion and Large non-linear optical coefficientsWide transmission region from 350 nm to 4400 nmWide angular bandwidth and small walk-off angleBroad temperature and spectral bandwidthLow cost compare with BBO and LBOLarge nonlinear optical coefficients(15 times that of KDP)No decomposition below 900°C,etc.