Potassium Titanyl Phosphate (KTP)

KTiOPO₄

KTP (Potassium Titanyl Phosphate, KTiOPO₄) is a nonlinear optical material which combines excellent nonlinear and electro-optic properties. It has a relatively high damage threshold, large optical nonlinearity and excellent thermal stability.

KTP is most commonly used as an extra- or intra- cavity frequency doubler for Nd:YAG and other neodynium doped lasers. Conventional, flux grown, material is best suited to low and medium power laser systems as there is a susceptibility to grey tracking at higher powers. This can be avoided by using hydrothermally grown material – details of our low-temperature hydrothermally grown material are given below.

It is also used frequently used as an optical parametric oscillator for near IR generation up to 4 µm and is particularly suited to high power operation as an optical parametric oscillator due to its high damage threshold and large crystal aperture.

Roditi supplies standard, flux grown, KTP crystals in a variety of sizes, up to 20mm in length with apertures up to 10 x 10mm. A standard range of AR/HRcoatings are available and custom coatings are available on request. High-temperature hydrothermally grown material is also available.

 

Low-temperature hydrothermal KTP

KTP can be flux grown, the cheaper method, or hydrothermally. The latter is better for higher powers and has a greater gray tracking tolerance.

Roditi now has low-temperature hydrothermal KTP available, which has even greater advantages than high-temperature grown material. The low-temperature material has very low absorption (<0.05%/cm) and a higher damage threshold than high-temperature hydrothermal or flux grown KTP.

Hydrothermal crystal growth occurs at much lower temperatures than the high temperature, 2500˚C ‘melt methods’ traditionally used to grow crystals. These high temperatures induce defects and crystal strain in bulk crystals. In addition, since high temperature methods are performed in the open atmosphere, critical ions added for functionality can evaporate, thus limiting the yield of the as-grown bulk crystal.

With hydrothermal crystal growth, most oxide and fluoride single crystals can be grown at temperatures between 350˚C and 600˚C. At these temperatures, there are very few thermally induced defects, oxygen vacancies, or reduced metal centers. Also because the growth occurs in closed autoclaves lined with inert precious metals such as silver, platinum, or gold, there are few if any chemical impurities, and the concentration of critical ions remains constant, thus allowing for uniform yield throughout the bulk crystal.

Because of the low temperature and closed growth system, yield is greatly enhanced and therefore drives down the cost of manufacture; high purity and low defect density greatly improves the performance of the crystal; and the unique growth method allows for a wide range of options for chemically solving what have historically been technical challenges in crystal growth.

High performance green laser systems are often subject to extremes of temperature. These systems require a robust optical train with all crystals performing to their maximum capability and offer a good demonstration of why low-temperature hydrothermally grown KTP is beneficial

Below is a comparison of our low-temperature hydrothermal KTO crystal performance against the “best flux” high temperature KTP available. Test temperatures were 25C versus 45C.

Low Temperature vs High Temperature KTP

	LTHT-NC	LTHT-C	Flux-C	Flux-C
Continuous Hrs	125	65	86	60
Temperature	45°C	45°C	25°C	25°C
Input Power/mW	53	51	80	59.3
Output Power/mW	25	35.5	35	21.1
Conversion % (total)	47%	67%	44%	36%
Gray Track Damage	None	None	Damage	Damage
Input 1064nm, output 532nm LTHT = Low-temperature hydrothermally grown material. NC = Not optically coated. C = Optically coated

Gray track damage is the most serious damage to which KTP is susceptible, resulting from

• metal contamination in the crystal growth process associated with high temperatures (flux growth method)
• the electronic structure of the crystal, which is different between hydrothermal and flux growth.

Our low-temperature hydrothermally grown KTP crystals demonstrate consistent, reliable conversion efficiency in making green light while conversion efficiency of the flux material is dramatically reduced as a result of operation at higher temperatures. Even at “room temperature”, 25°C, continuous operation of flux KTP causes gray track damage. The result of gray track damage is many fold, including poor beam quality, beam walk-off and wavefront distortion.