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KNbO3 is a perovskite type negative biaxial crystal. It has the largest nonlinear optical coefficients among the commonly used inorganic crystals. Hence, it is normally used for frequency doubling of low and medium power lasers, such as diode lasers, diode pumped solid state lasers, Ti:Sapphire lasers and cw Nd:YAG lasers.It is well suited to SHG of near infra-red radiation. Temperature tuning over the range -40oC to 180oC provides non-critical (90o) phase matching in the spectral range of diode lasers, with phase matching for 860nm occurring at room temperature. Crystals may be cut to suit specific wavelengths at room temperature. It is also a highly efficient crystal for low threshold OPO and OPG. KNbO3 may also be prepared for photorefractive application. The photorefractive properties of KNbO3allow applications in modulators and phase conjugation processes.
Physical Properties |
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| Crystal structure/point group | orthorhombic/mm2 |
| Lattice parameters | a=5.697Å, b=3.971Å, c=5.722Å |
| Melting point | 1050 °C |
| Phase transition temperature | -50°C, 200°C, 430°C |
| Density | 4.62 gm cm-3 |
| Mohs hardness | 5 |
Optical Properties |
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| Transparency range | 400-4500 nm | ||||||||||||||||
| Nonlinear optical coefficient | d31=15.8 pm/V, d32=18.3 pm/V @ 1064 nm | ||||||||||||||||
| Effective nonlinear coefficient | d31 =26 x d 36 (KDP) | ||||||||||||||||
| Damage threshold | ~350 MW cm-2 | ||||||||||||||||
| Absorption loss | £ 1% cm at 1064nm | ||||||||||||||||
| Refractive indices |
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| Sellmeier equations l in mm |
nx2 = 4.4308 + 0.10044/(l2-0.054084) - 0.019592 l2 ny2 = 4.8355 + 0.12839/(l2-0.056342) - 0.025379 l2 nz2 = 4.9873 + 0.15149/(l2-0.064143) - 0.028775 l2 |
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Phase-matching Properties |
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| The shortest SHG wavelength | 425nm (Type I NCPM, y=cut). |
| For Type I SHG of Nd:YAG | laser 1064nm at room temperature, q=19o, f=0o. |
| For Type I NCPM of 1064nm | PM temperature @ 188oC, q=0o (z-cut) |
| Room temperature NCPM | 982nm, z-cut, and 858nm, y=cut. |
| wavelengths | |
| Acceptance angle for Type I | Dq=0.24 mrad-cm (internal). |
| SHG of 1064nm | |
| Acceptance temperature for | DT=0.3oC - cm. |
| Type I SHG of 1064nm | |
Finished Quality Indications |
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| End faces flatness | l/4 @633nm |
| (transmitting wavefront distortion) | |
| Dimension tolerance | W±0.1mm x H±0.1mm x L+0.2mm/-0.1mm |
| Clear aperture | >80% central area |
| Scratch/Dig | 20/10 |
| Parallelism | better than 20 arc sec. |
| Perpendicularity | 5 arc min. |
| Angular tolerance | Dq <0.5o, Df <0.5o |
AR-coatings are available at different wavelengths. KNbO3 crystals need to be handled gently. Shaking must be avoided to avoid domains appearing. The temperature of the crystal must be kept in the range between -50°C and 210°C.
Because of its small acceptance temperature, the use of an oven with temperature stability no less than ±0.1°C is recommended if highly stable output is required.
Fe doped KNbO3 is available as an enhanced photorefractive crystal.