Potassium titanyl phosphate (KTP, or _KTiOPO4) is an excellent nonlinear crystal known for its high NLO coefficients. It has high optical quality, a large transparent range, relatively high SHG coefficients (3 times larger than KDP), a high damage threshold, a large acceptance angle, a small walk-off and type I and II non-critical phase matching (NCPM) is possible in a wide wavelength range. It is used for frequency doubling of Nd:YAG and other Nd-doped lasers, especially for low or medium power lasers.

The best known applications are second harmonic generation (SHG=Second Harmonic Generation) and optical parametric oscillators (OPO=Optical Parametric Oscillators). Non-critical temperature phase matching is possible for many Nd-doped active elements!