Potassium titanyl phosphate
Potassium titanyl phosphate (KTiOPO4) or KTP is a nonlinear optical material which is commonly used for frequency doubling diode pumped solid-state lasers such as Nd:YAG and other neodymium-doped lasers. The material has a relatively high optical damage threshold (~15 J/cm²), a great optical nonlinearity and excellent thermal stability in theory. In practice, KTP crystals needs to have stable temperature to operate if they are pumped with 1064 nm (infrared, to output 532 nm green). However, it is prone to photochromic damage (called grey tracking) during high-power 1064 nm second-harmonic generation which tends to limit its use to low- and mid-power systems. It is used to produce "greenlight" to perform some laser prostate surgery.
It is also frequently used as an optical parametric oscillator for near IR generation up to 4 µm. It is particularly suited to high power operation as an optical parametric oscillator due to its high damage threshold and large crystal aperture. The high degree of birefringent walk-off between the pump signal and idler beams present in this material limit its use as an optical parametric oscillator for very low power applications.
KTP has orthorhombic crystal structure. It is highly transparent for wavelengths between 350–2700 nm with a reduced transmission out to 4500 nm where the crystal is effectively opaque. Its second harmonic generation (SHG) coefficient is about three times higher than KDP. It has a Mohs hardness of about 5.
Other such materials include KTiOAsO4.
Periodically poled potassium titanyl phosphate (PPKTP)
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Periodically poled potassium titanyl phosphate (PPKTP) consists of KTP with switched domain regions within the crystal for various nonlinear optic applications and frequency conversion. It can be wavelength tailored for efficient second harmonic generation, sum frequency generation, and difference frequency generation. The interactions in PPKTP are based upon quasi-phase-matching, achieved by periodic poling of the crystal, whereby a structure of regularly spaced ferroelectric domains with alternating orientations are created in the material.
PPKTP is commonly used for Type 1 & 2 frequency conversions for pump wavelengths of 730-3500 nm.
Other materials used for periodic poling are wide band gap inorganic crystals like lithium niobate (resulting in periodically poled lithium niobate, PPLN), lithium tantalate, and some organic materials.
Other materials used for laser frequency doubling are
- Lithium triborate (LBO), used for high output power green or blue DPSS lasers
- Beta barium borate (BBO), used for high output power DPSS blue lasers