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Beta-barium borate (BBO) is a negative uniaxial crystal grown with the flux method. It has the following exceptional properties:
•Broad phase-matching range from 410 nm to 2100 nm
•Wide optical transmission from 190 nm to 3500 nm
•Large effective second-harmonic-generation (SHG) coefficient
•High damage threshold of 10 GW/cm2 for 100ps pulse-width at 1064nm;
•High optical homogeneity with Δn≈10-6/cm;
•Wide temperature-bandwidth of about 55°C.
•Low thermo-optic coefficient
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| Structure |
Trigonal, space group R 3c |
| Cell Parameters |
a = b = 12.532Å , c = 12.717Å , Z = 6 |
| Melting point |
1095 + 5 ° C |
| Transition temperature |
925 + 5 ° C |
| Optical homogeneity |
Δn≈10-6/cm |
| Mohs hardness |
4 |
| Density |
3.85 g/cm3 |
| Absorption coefficient |
< 0.1%/cm (at 1064nm) |
| Specific heat |
1.91J/cm 3 xK |
| Hygroscopic susceptibility |
Low |
| Thermal expansion coefficients |
a, 4 x 10-6/K; c, 36 x 10-6/K |
| Thermal conductivity |
⊥ c, 1.2 W/m/K; //c, 1.6 W/m/K |
Linear Optical Properties
| Transparency range |
189-3500 nm |
| Sellmeier equations: ( l in m m) |
no2=2.7359+0.01878/(λ2-0.01822)-0.01354λ2
ne2=2.3753+0.01224/(λ2-0.01667)-0.01516λ2 |
Refractive indices:
at 1064 nm
at 532 nm
at 266 nm |
ne=1.5425, no= 1.6551
ne=1.5555, no= 1.6749
ne=1.6146, no= 1.7571
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| Therm-optic coefficients |
d no /dT = -9.3 x 10-6/ °C
dne /dT = -16.6 x 10-6/ °C |
Nonlinear Optical Properties
| Phase-matchable output wavelengths |
189 - 1750 nm |
| NLO coefficients |
d11 = 5.8 x d36(KDP)
d31 = 0.05 x d11
d22 < 0.05 x d11 |
| Effective nonlinearity expressions |
deff(I)=d31sinθ+(d11cosΦ-d22sin3Φ)cosθ
deff(II)=(d11sin3Φ+d22cos3Φ)cos2θ |
| Electro-optic coefficients |
r22=2.7pm/V
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| Half-wave voltage |
48 KV (at 1064 nm) |
Damage threshold:
at 1064nm
at 532nm |
5 GW/cm2 (10 ns); 10 GW/cm2 (1.3 ns)
1 GW/cm2 (10 ns); 7 GW/cm2 (250 ps) |
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Application
Its above properties make it the most important role in the following developments:
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Second, third, fourth harmonic generation of Nd:YAG and Nd:YLF lasers;
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efficient fifth harmonic generation of Nd:YAG lasers down to 213nm;
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broadly tunable optical parametric amplifier (OPA) and optical parametric oscillators (OPO);
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autocorrelation applications down to 0.19 μ m.
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frequency-doubling, -tripling and -mixing of Dye lasers;
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second, third and fourth harmonic generation of Ti:Sapphire and Alexandrite lasers;
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frequency-doubling of Argon ion, Cu-vapor and Ruby lasers.
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BBO is used as one of key components inside OPO and OPA lasers which are powerful tools for generating a widely tunable coherent radiation from the UV to IR. The tuning angles of type I and type II BBO for OPO and OPA have been calculated, shown in following figures, respectively.
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Specifications
| Wavefront distortion |
less than λ /8 @ 633nm |
| Dimension tolerance |
(W ± 0.1mm) x (H ± 0.1mm) x (L + 0.2mm/-0.1mm) |
| Clear aperture |
> 90% central area |
| Flatness |
λ /8 @ 633nm |
| Scratch/Dig |
10/5 to MIL-O-13830A |
| Parallelism |
better than 20 arc seconds |
| Perpendicularity |
5 arc minutes |
| Angle tolerance |
Δθ< ± 0.3 ° , Δφ < ± 0.3 ° |
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Coatings :
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P-Coating: The protective dielectric coating (P-coating) protects BBO polished surfaces from fogging due to ambient moisture. Also, P-coating reduces reflection from the polished surface.
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Anti-reflective coatings (AR-coating): Single-band, dual-band AR-coating and broad-band AR Coating on BBO surface are available.
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BBO has a low susceptibility to the moisture. Dry conditions and preservation for BBO have to be considered .
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BBO is relatively soft and therefore precautions are required to protect its polished surfaces.
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