Potassium titanyl phosphate
Encyclopedia
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 laser
s. 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. 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 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
walkoff 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 is also used as an electrooptic modulator, optical waveguide
material, and in directional couplers.
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.
Crack-free fibers of KTP can be prepared from suitable organometallic compounds (usually ethyl alkoxide
s of potassium and titanium, and butyl ester
of phosphoric acid
).
KTP crystals coupled with Nd:YAG
or Nd:YVO4 crystals are commonly found in green laser pointer
s.
Other such materials include KTiOAsO4.
, 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.
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
material which is commonly used for frequency doubling
Second harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
diode pumped solid-state lasers such as Nd:YAG and other neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
-doped laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s. 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. 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 also frequently used as an optical parametric oscillator
Optical parametric oscillator
An optical parametric oscillator is a parametric oscillator which oscillates at optical frequencies. It converts an input laser wave into two output waves of lower frequency by means of second order nonlinear optical interaction. The sum of the output waves frequencies is equal to the input wave...
for near IR
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
walkoff 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 is also used as an electrooptic modulator, optical waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
material, and in directional couplers.
KTP has orthorhombic crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
. 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
Second harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
(SHG) coefficient is about three times higher than KDP. It has a Mohs hardness
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
of about 5.
Crack-free fibers of KTP can be prepared from suitable organometallic compounds (usually ethyl alkoxide
Alkoxide
An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands...
s of potassium and titanium, and butyl ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
of phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...
).
KTP crystals coupled with Nd:YAG
Nd:YAG laser
Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
or Nd:YVO4 crystals are commonly found in green laser pointer
Laser pointer
A laser pointer or laser pen is a small portable device with a power source and a laser emitting a very narrow coherent low-powered beam of visible light, intended to be used to highlight something of interest by illuminating it with a small bright spot of colored light...
s.
Other such materials include KTiOAsO4.
Periodically poled potassium titanyl phosphate (PPKTP)
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 generationSecond harmonic generation
An optical frequency multiplier is a nonlinear optical device, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency...
, sum frequency generation
Sum frequency generation
Sum-frequency generation is a non-linear optical process. This phenomenon is based on the annihilation of two input photons at angular frequencies \omega_1 and \omega_2 while, simultaneously, one photon at frequency \omega_3 is generated...
, and difference frequency generation. The interactions in PPKTP are based upon quasi-phase-matching
Quasi-phase-matching
Quasi-phase-matching is a technique in nonlinear optics which allows a positive net flow of energy from the pump frequency to the signal and idler frequencies by creating a periodic structure in the nonlinear medium. Momentum is conserved, as is necessary for phase-matching, through an additional...
, achieved by periodic poling
Periodic poling
Periodic poling is a formation of layers with alternate orientation in a birefringent material. The domains are regularly spaced, with period in a multiple of the desired wavelength of operation. The structure is desired to achieve quasi-phase-matching in the material.Periodically poled crystals...
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
Lithium niobate
Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...
(resulting in periodically poled lithium niobate, PPLN), lithium tantalate
Lithium tantalate
Lithium tantalate , is a crystalline solid which possesses unique optical, piezoelectric and pyroelectric properties which make it valuable for nonlinear optics, passive infrared sensors such as motion detectors, terahertz generation and detection, surface acoustic wave applications, cell phones...
, and some organic materials.
See also
Other materials used for laser frequency doubling are- Lithium triborateLithium triborateLithium triborate LBO is a non-linear optics crystal. It has a wide transparency range, moderately high nonlinear coupling, high damage threshold and desirable chemical and mechanical properties. This crystal is often used for second harmonic generation of Nd:YAG lasers...
(LBO), used for high output power green DPSS lasers - Beta barium borateBeta Barium BorateBeta barium borate is a crystal frequently used for frequency mixing and other nonlinear optics applications...
(BBO), used for high output power DPSS blue laserBlue laserA so-called blue laser is a laser that emits electromagnetic radiation at a wavelength of between 360 and 480 nanometres, which the human eye sees as blue or violet. Diode lasers which emit light at 445 nm are becoming popular as handheld lasers. Light of a shorter wavelength than 400 nm is...
s