KTP Crystals

Potassium Titanyl Phosphate (KTiOPO₄,KTP)

Potassium Titanyl Phosphate (KTiOPO₃ or KTP) is widely used in both commercial and military lasers including laboratory and medical systems, range-finders, lidar, optical communication and industrial systems.

KTP is featured by

•Large nonlinear optical coefficient

•Wide angular bandwidth and small walk-off angle

•Broad temperature and spectral bandwidth

•High electro-optic coefficient and low dielectric constant

•Large figure of merit

Nonhydroscopic, chemically and mechanically stable

Table 1. Chemical and Structural properties

Crystal Structure	Orthorhombic, space group Pna2₁,point group mm²
Lattice Parameter	a=6.404?, b=10.616?, c=12.814?, Z=8
Melting Point	About 1172°C
Mohs Hardness	5
Density	3.01 g/cm³
Thermal Conductivity	13W/m/K
Thermal Expansion Coefficient	α_x=11x10^-6/°C, α_y=9x10^-6/°C, α_z=0.6x10^-6/°C

Table 2. Optical and Nonlinear Optical Properties

Transparency Range	350～4500nm
SHG Phase Matchable Range	497～1800nm (Type II)
Therm-optic Coefficients (/°C)	dnx/dT=1.1X10^-5 dny/dT=1.3X10^-5 dnz/dT=1.6X10^-5
Absorption Coefficients	<0.1%/cm at 1064nm <1%/cm at 532nm
For Type II SHG of a Nd:YAG laser at 1064nm	Temperature Acceptance: 24°C-cm Spectral Acceptance: 0.56nm-cm Angular Acceptance: 14.2mrad-cm (φ);55.3mrad-cm (θ) Walk-off Angle: 0.55°
NLO Coefficients	d_eff(II)≈(d₂₄-d₁₅)sin2φsin2θ-(d₁₅sin²φ + d₂₄cos²φ)sinθ
Non-vanished NLO susceptibilities	d₃₁=6.5 pm/V d₂₄=7.6 pm/V d₃₂=5 pm/V d₁₅=6.1 pm/V d₃₃=13.7 pm/V
Sellmeier Equations (λ in μm)	n_x²=3.0065+0.03901/(λ²-0.04251)-0.01327λ ² n_y²=3.0333+0.04154/(λ ²-0.04547)-0.01408λ² n_z²=3.3134+0.05694/(λ ²-0.05658)-0.01682λ²

Electro-optic coefficients:	Low frequency(pm/V)	High frequency(pm/V)
R₁₃	935	8.8
R₂₃	15.7	13.8
R₃₃	36.3	35.0
R₅₁	7.3	6.9
R₄₂	9.3	8.8
Dielectric constantq	Εeff =13

Applications for SHG and SFG of Nd: lasers

KTP is the most commonly used material for frequency doubling of Nd:YAG and other Nd-doped lasers, particularly when the power density is at a low or medium level. To date, extra- and intra-cavity frequency doubled Nd:lasers using KTP have become a preferred pumping source for visible dye lasers and tunable Ti:Sapphire lasers as well as their amplifiers. They are also useful green sources for many research and industry applications.

* More than 80% conversion efficiency and 700mJ green laser were obtained with a 900mJ
   injection-seeded Q-switch Nd:YAG lasers by using extra-cavity KTP.
* 8W green laser was generated from a 15W LD pumped Nd:YVO₄ with intra-cavity KTP.
* 200mW green outputs are generated from 1 W LD pumped Nd:YVO₄ lasers by        using CASTECH ’s 2x2x5mm KTP and 3x3x1mm Nd:YVO₄.
* 2-5mw green outputs are generated from 180mw LD pumped Nd:YVO₄ and KTP glued    crystals.For more details, please refer to Glued Crystals.

KTP is also being used for intracavity mixing of 0.81μm diode and 1.064μm Nd:YAG laser to generate blue light and intracavity SHG of Nd:YAG or Nd:YAP lasers at 1.3μm to produce red light.

Applications for OPG, OPA and OPO

As an efficient OPO crystal pumped by a Nd:laser and its second harmonics, KTP plays an important role for parametric sources for tunable outputs from visible (600nm) to mid-IR (4500nm), as shown in Fig. 3 and Fig. 4.
Generally, KTP’s OPOs provide stable and continuous pulse outputs (signal and idler) in fs, with 108 Hz repetition rate and a mW average power level. A KTP’s OPO that are pumped by a 1064nm Nd:YAG laser has generated as high as above 66% efficiency for degenerately converting to 2120nm.

The novel developed application is the non-critical phase-matched(NCPM) KTP OPO/OPA pumped by the X-cut KTP crystal. As shown in Fig.5, for pumping wavelength range from 0.7μm to 1 μm, the output can cover from 1.04μm to 1.45μm(signal) and from 2.15μm to 3.2μm(idler). More than 45% conversion efficiency was obtained with narrow output bandwidth and good beam quality.

Applications for E-O Devices

In addition to unique NLO features, KTP also has promising E-O and dielectric properties that are comparable to LiNbO3These advantaged properties make KTP extremely useful to various E-O devices. Table 1 is a comparison of KTP with other E-O modulator materials commonly used:

Table 3 Electro-Optic Modulator Materials

				Phase			Amplitude
Material	ε	N	R(pm/V)	k(10^-6/°C)	N⁷r²/ε(pm/V)²	r(pm/V)	k(10^-6/°C)	n⁷r²/ε(pm/V)²
KTP LiNbO₃ KD^*P LiIO₃	15.42 27.9 48.0 5.9	1.80 2.20 1.47 1.74	35.0 8.8 24.0 6.4	31 82 9 24	6130 7410 178 335	27.0 20.1 24.0 1.2	11.7 42 8 15	3650 3500 178 124

From Table 3, clearly, KTP is expected to replace LiNbO₃ crystal in the considerable volume application of E-O modulators, when other merits of KTP are combined into account, such as high damage threshold, wide optical bandwidth (>15GHZ), thermal and mechanical stability, and low loss, etc.

Applications for Optical Waveguides

Based on the ion-exchange process on KTP substrate, low loss optical waveguides developed for KTP have created novel applications in integrated optics. Table 4 gives a comparison of KTP with other optical waveguide materials. Recently, a type II SHG conversion efficiency of 20%/W/cm2 was achieved by the balanced phase matching, in which the phase mismatch from one section was balanced against a phase mismatch in the opposite sign from the second. Furthermore, segmented KTP waveguides have been applied to the type I quasi-phase-matchable SHG of a tunable Ti:Sapphire laser in the range of 760-960mm, and directly doubled diode lasers for the 400-430nm outputs.

Table 24. Electro-Optic Waveguide Materials

Materials	r (pm/V)	n	e_eff (e₁₁e₃₃)^1/2	n³r/e_eff(pm/V)
KTP LiNbO₃ KNbO₃ BNN BN GaAs BaTiO₃	35 29 25 56 56-1340 1.2 28	1.86 2.20 2.17 2.22 2.22 3.6 2.36	13 37 30 86 119-3400 14 373	17.3 8.3 9.2 7.1 5.1-0.14 4.0 1.0

KTP Specifications:

Dimension tolerance	(W±0.1mm)x(H±0.1mm)x(L+0.5/-0.1mm) (L≥2.5mm) (W±0.1mm)x(H±0.1mm)x(L+0.1/-0.1mm) (L<2.5mm)
Clear aperture	central 90% of the diameter
Flatness	less than λ/8 @ 633nm
Transmitting wavefront distortion	less than λ/8 @ 633nm
Chamfer	≤0.2mm@45°
Chip	≤0.1mm
Scratch/Dig code	better than 10/ 5 to MIL-PRF-13830B
Parallelism	better than 20 arc seconds
Perpendicularity	≤5 arc minutes
Angle tolerance	Δθ≤0.25°, Δφ≤0.25°
Damage threshold[GW/cm ]:	>0.5 for 1064nm, TEM00, 10ns, 10HZ (AR-coated) >0.3 for 532nm, TEM00, 10ns, 10HZ (AR-coated)

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KTP Crystals

KTP Crystals

Potassium Titanyl Phosphate (KTiOPO4,KTP)

Table 1. Chemical and Structural properties

Table 2. Optical and Nonlinear Optical Properties

Applications for SHG and SFG of Nd: lasers

Applications for OPG, OPA and OPO

Applications for E-O Devices

Table 3 Electro-Optic Modulator Materials

Table 24. Electro-Optic Waveguide Materials

KTP Specifications:

Potassium Titanyl Phosphate (KTiOPO₄,KTP)