Properties of beta-BBO Single Crystal
BBO is one of the best non-linear optical crystals and the best electro-optical crystals. It is widely used in various harmonic generations and OPOs, especially in Ti:Sapphire laser systems. These days, BBO Pockels cells are getting more and more attention.
Optical Properties:
Transmitting Range: | 196nm ~ 2200nm | |||
Refractive Indices: | @1064nm | 1.6551(n_{o}) | 1.5425(n_{e}) | |
@532nm | 1.6749(n_{o}) | 1.5555(n_{e}) | ||
@266nm | 1.75711(n_{o}) | 1.6146 (n_{e}) | ||
Sellmeier Equations: (l in µm) | N_{o}^{2} = 2.7359 + 0.01878 / (l^{2} - 0.01822) - 0.01354l^{2} | |||
N_{e}^{2} = 2.3753 + 0.01224 / (l^{2} - 0.01667) - 0.01516l^{2} | ||||
Therm-Optic Coefficient:(10^{-6}/°C) | dn_{o}/dT=-9.3 | dn_{e}/dT=-16.6 | ||
Absorption Coefficient: | a<0.1%/cm @1064nm | |||
Nonlinear Optical Coefficients and Equation: | @1064nm | d_{11}=5.8d_{36}(KDP) | d_{31}=0.05d_{11} | d_{22}<0.05d_{11} |
d_{eff}(I)=d_{31}sinq + (d_{11}cos3f - d_{22}sin3f)cosq | ||||
d_{eff}(II)=(d_{11}sin3f + d_{22}cos3q)cos^{2}q | ||||
Half-wave voltage: | 48KV(at 1064nm ) | |||
Electro-Optic Coefficients: | r_{11} = 2.7 pm/V, r_{22}, r_{31}< 0.1r_{11} | |||
Damage Threshold @1064nm and @532nm | 5 GW/cm^{2} (10 ns); 10 GW/cm^{2} (1.3 ns); 1 GW/cm^{2} (10 ns);7 GW/cm^{2} (250ps) |
Physical Properties:
Crystal Structure: | Trigonal, space group R3c |
Cell Parameters: | a=b=12.532Å, c=12.717Å, Z=7 |
Melting Point: | 1095°C |
Transition Point: | 926°C |
Mohs Hardness: | »4.5 |
Density: | 3.85g/cm^{3} |
Color: | Colorless |
Hygroscopic Susceptibility | low |
Specific Heat: | 0.49 cal/g°C |
Thermal Conductivity: | 1.2 W/m/°K (^ to C), 1.6 W/m/°K (// to C) |
Non-linear Optical Applications:
In Nd:YAG and Nd:YLF laser systems:
BBO is a very efficient non-linear optical crystal for the second, third, fourth and even fifth harmonic generations of Nd:YAG lasers. In fact, BBO is the only commercially available non-linear optical crystal for Nd:YAG lasers' fifth harmonic generation (213nm). Conversion efficiencies of more than 70% for SHG, 60% for THG, 50% for 4HG, and 200mW output at 213nm (5HG) have been reported. The comparison of BBO with KD*P in the Nd:YAG laser system and its basic nonlinear optical properties are listed in Table 1 and Table 2, respectively.
Table 1. Comparison of Harmonic generations between BBO and DKDP
1064nm(mJ) | SHG (mJ) | THG (mJ) | 4HG (mJ) | 5HG (mJ) | |
BBO | 220 | 105 | 39 | 18.5 | 5 |
600 | 350 | 140 | 70 | 20 | |
DKDP | 600 | 270 | 112.5 | 45 | / |
Table 2. Non-linear optical properties for type I phase matching BBO crystal
SHG | THG | 4HG | 5HG | |
Effective NLO Coefficient (*d36 (KDP)) | 5.3 | 4.9 | 3.8 | 3.4 |
Acceptance Angle (mrad-cm) | 1.0 | 0.5 | 0.3 | 0.2 |
Walk-off Angle (degree) | 3.2 | 4.1 | 4.9 | 5.5 |
Type I phase matching BBO is also very efficient for the intra-cavity SHG of Nd:YAG lasers, especially for high power systems, where KTP can't survive.
In tunable laser systems:
1.Dye lasers
Efficient UV output (205nm-310nm) with a SHG efficiency of over 10% at 206nm was obtained from a type I phase matching BBO; and the conversion efficiency of 36% was achieved at a XeC1-laser pumped Dye laser with the power of 150KW, which is about 4-6 times higher than ADP.
With type I phase matching of sum-frequency of 780-950nm and 248.5nm (the second harmonic of 495nm dye laser), the pulse energy of 95mJ at 193nm and 8mJ at 189nm have been obtained, respectively.
2.Ultrafast Pulse Laser
BBO dominates the frequency doubling and tripling applications in ultra short pulse laser systems, due to its exceptional nonlinear optical properties and its excellent mechanical strength. BBO flakes can be as thin as 5 microns (for a 5x5mm aperture).
3.Ti:Sapphire and Alexandrite lasers
With Type I phase matching BBO, the output pulse energy of 105mJ in the region of 360nm~390nm (about 31% conversion efficiency in SHG), and 7.5mJ in the region of 244nm~259nm (about 24% conversion efficiency in THG) have been obtained in an Alexandrite laser system.
4. Argon Ion and Copper-Vapor lasers
By employing the intra-cavity frequency-doubling technique in an Argon Ion laser with all line output power of 2W, maximum 33mW at 250.4nm was generated, along with thirty-six lines of deep UV wavelengths ranging from 228.9nm to 257.2nm, with a Brewster-angle-cut BBO crystal.
Up to 230mW average power at 255.3nm with the maximum 8.9% conversion efficiency was achieved in the SHG of a Copper-Vapor laser at 510.6nm.
In OPA, OPO Applications
The BBO OPO or OPA is the most powerful method used to generate a widely tunable coherent radiation range from UV to IR.
1. OPO pumped at 532nm
The OPO output from 680nm to 2400nm with the peak power of 1.6MW and up to 30% energy conversion efficiency was obtained in a 7.2mm long type I BBO. The input pump energy was 40 mJ at 532nm with the pulse-width 75ps.
2. OPO and OPA pumped at 355nm
Pumped by the third harmonic of Nd:YAG laser, BBO’s OPO can generate wavelength range tunable from 400nm to 2000nm with a maximum of 30% conversion efficiency.
Type II phase matching can help to decrease the linewidth at the desired wavelength(s). A linewidth as narrow as 0.05nm was obtained with the conversion efficiency of 12%. However, a longer (>15mm) BBO is recommended to decrease the oscillation threshold when utilizing the type II phase-matching scheme.
Pumped by a Pico second Nd:YAG at 355nm, a narrow-band(<0.3nm), high energy (>200µJ) and wide tunable (400nm to 2000nm) pulse has been produced by BBO OPA. This OPA can reach to more than 50% conversion efficiency, which is superior to common Dye lasers in many respects, including efficiency, tunable range, maintenance and easiness in operation. Furthermore, coherent radiation from 205nm to 2300nm can also be available by BBO's OPO or OPA, plus BBOs for SHG, THG etc.
3.Others
Pumped by the fourth harmonic of a Nd:YAG laser (at 266nm), a tunable OPO with signal wavelength range between 422nm and 477nm has been observed by angle-tuning a type I BBO crystal.
Pumped by a 1mJ, 80fs Dye laser at 615nm, the OPA with two BBO crystals yields more than 50µJ (maximum 130µJ), <200fs ultra short pulse, over 800nm~2000nm.
Electro-Optical Applications:
BBO crystals are also widely used as the electro-optical material in various Pockels cells and modulators. Please refer to the Pockels Cell part of this site.
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