Beta – Barium Borate (b-BaB2O4 or BBO) is an efficient NLO crystal for the second, third and fourth harmonic generation of Nd:YAG lasers, and the best NLO crystal for the fifth harmonic generation at 213nm. Conversion efficiencies of more than 70% for SHG, 60% for THG and 50% for 4HG have been obtained.
Basic Properties
Crystal Structure: |
Trigonal, space group R 3c |
Cell Parameters: |
a = b = 12.532A , c = 12.717A , z = 6 |
Melting Point: |
1095 ± 5°C |
Transition Temperature: | 925 ± 5°C |
Optical Hardness: | Δn about 10-6/cm |
Mohs Hardness: | 4 |
Density: | 3.85g/cm3 |
Absorption coefficient: | < 0.1% per cm @ 1064nm |
Specific Heat: | 1.91J/cm3 K |
Hyroscopic Susceptibility: | Low |
Thermal Expansion Coefficients: | a, 4×10-6/K . c, 36×10-6/K |
Thermal Conductivity: | c, 1.2W/m/K . ||c, 1.6W/m/K |
Linear Optical Properties
Transparency Range: |
189-3500nm
|
|||
Refractive Indicies: |
@ 1064nm
|
ne = 1.5425
|
no = 1.6551
|
|
@ 532nm
|
ne = 1.5555
|
no = 1.6479
|
||
@ 266nm
|
ne = 1.6146
|
no = 1.7571
|
||
Therm-Optic Coefficients: |
dne/dT = -16.6×10-6/C
|
dno/dT = -9.3×10-6/C
|
Sellmeir Equations:
no2 = 2.7359 + 0.01878 / (λ2 – 0.01822) – 0.01354λ2
ne2 = 2.3753 + 0.01224 / (λ2 – 0.01667) – 0.01516λ2
wavelength is in µm, T = 20°C
The Features for BBO:
Applications in Nd:YAG Laser:
BBO is an efficient NLO crystal for the second, third and fourth harmonic generation of Nd:YAG lasers, and the best NLO crystal for the fifth harmonic generation @ 213nm. Conversion efficiencies of more than 70% for SHG, 60% for THG and 50% for 4HG, and 200mW output @ 213nm (SHG) have been obtained.
BBO is also an efficient crystal for intra-cavity SHG of high power Nd:YAG lasers. For the intracavity SHG of an acoust-optic Q-switched Nd:YAG laser, more than 15W average power @ 532nm was generated in an AR-coated BBO crystal. When pumped by the 600 mW SHG output of a mode-locked Nd:YLF laser, 66mW output @ 266nm was produced from a Brewster-angle-cut BBO in an external enhanced resonant cavity.
Harmonic Generations Using BBO and KD*P
Crystal
|
1 (mJ)
|
SHG (mJ)
|
THG (mJ)
|
4HG (mJ)
|
5HG (mJ)
|
BBO
|
220
|
105
|
39
|
18.5
|
5
|
600
|
350
|
140
|
70
|
20
|
|
KD*P
|
600
|
270
|
112.5
|
45
|
–
|
Relevant NLO Properties for Type I 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
|
Because of a small acceptance angle and large walk-off, good laser beam quality (small divergence, good mode condition, etc.) is the key for BBO to obtain high conversion efficiency. Tight focus of laser beam is not recommended in BBO applications.
Applications in Tunable Lasers
BBO’s OPO and OPA
The OPO and OPA of BBO 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, OPO, and OPA have been calculated.
OPO pumped at 532nm
An OPO output ranging from 680-2400nm 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 40mJ @ 532nm with a pulse-width of 75ps. With a longer crystal, higher conversion efficiency is expected.
OPO and OPA at 355nm
In the case of Nd:YAG pumping, BBO’s OPOs can generate more than 100mJ, with a wavelength tunable from 400-2000nm. Using BBO crystal, the OPO system covers a tuning range from 400-3100nm which guarantees a maximum of 30% and more than 18% conversion efficiency
Type II BBO can be used to decrease line width near the degenerate points. A line width as narrow as 0.05nm with conversion efficiency of 12% was obtained. However, a longer (> 15mm) BBO should normally be used to decrease the oscillation threshold when employing the tepy II phase-matching scheme. Pumping with a picosecond Nd:YAG @ 355nm, a narrow-band (< 0.3nm), high energy (> 200mJ), and wide tunable (400-2000nm) pulse has been produced by BBO’s OPAs. This OPA can reach as high as more than 50% conversion efficiency, and therefore is superior to common Dye lasers in many respects, including efficiency, tunable range, maintenance, and easiness in design and operation. Furthermore, coherent radiation from 205-3500nm can be also generated by BBO’s OPO or OPA plus a BBO for SHG.
Others
A tunable OPO with signal wavelengths between 422-477nm has been generated by angle tuning in a type I BBO crystal pumped with a XeCl excimer laser @ 308nm. And a BBO’s OPO pumped by the forth harmonic of a Nd:YAG laser (266nm) has been observed to cover the whole range of output wavelengths 330-1370nm. When pumped by a 1mJ, 80fs Dye laser at 615nm, the OPA with two BBO crystals yields more than 50mJ (maximum 130mJ), < 200fs ultra short pulse, over 800-2000nm.
Specifications:
Dimension Tolerance: (W ± 0.1mm) x (H ± 0.1mm) x (L ± 0.2mm)
Wavefront Distortion: < 1/8 @ 633nm
Flatness: 1/8 @ 633nm
Surface Quality: 10/5 to MIL-O-13830A
Parallelism: < 20 arc seconds
Perpendicularity: 5 arc minutes
Angle Tolerance: Dq < ± 0.5° , Df < ± 0.5°
Quality Warranty Period: one year under proper use
Coating: AR/AR on both sides for single wavelength or dual wavelength, R < 0.2% OR Protective coating (p-coating) for high damage and wide wavelength range
Size: As large as 20x20mm and as thin as 0.02mm
Mounting: it will be mounted when thickness is less than 2mm (Free!)
Note