Beta-Barium Borate (b-BaB2O4)
Beta-Barium Borate, known as BBO(b-BaB2O4), is one of the best candidates for both NLO applications and E-O applications due to the combination of the nonlinear optical properties and the electro-optical properties.

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Optical Properties:

Transmitting Range:

196nm ~ 2200nm

Refractive Indices: @1064nm 1.6551(no) 1.5425(ne)
@532nm

  1.6749(no)

  1.5555(ne)

@266nm

1.75711(no)

1.6146 (ne)

Sellmeier Equations:           (l in m)

No2 = 2.7359 + 0.01878 / (l2 - 0.01822) - 0.01354l2

Ne2 = 2.3753 + 0.01224 / (l2 - 0.01667) - 0.01516l2

Therm-Optic Coefficient:(10-6/C)

dno/dT=-9.3

dne/dT=-16.6

Absorption Coefficient:

a<0.1%/cm @1064nm

Nonlinear Optical Coefficients and Equation: @1064nm

d11=5.8d36(KDP)

d31=0.05d11

d22<0.05d11

deff(I)=d31sinq + (d11cos3f - d22sin3f)cosq

deff(II)=(d11sin3f + d22cos3q)cos2q
Half-wave voltage:

48KV(at 1064nm )

Electro-Optic Coefficients: r11 = 2.7 pm/V, r22, r31< 0.1r11

Damage Threshold @1064nm and @532nm

5 GW/cm2 (10 ns); 10 GW/cm2 (1.3 ns); 1 GW/cm2 (10 ns);7 GW/cm2 (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: 926C
Mohs Hardness: 4.5
Density: 3.85g/cm3
Color: Colorless
Hygroscopic Susceptibility low
Specific Heat: 0.49 cal/gC
Thermal Conductivity: 1.2 W/m/°K (^ to C), 1.6 W/m/°K (// to C)

Applications:

In Nd:YAG and Nd:YLF laser systems:

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, and 200mW output at 213nm (5HG) have been obtained, respectively. The comparisons of BBO with KD*P in a Nd:YAG laser and the basic nonlinear optical properties from SHG to 5HG are listed in Table 1 and Table 2.

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. Relative 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

BBO is also an efficient crystal for the intra-cavity SHG of high power Nd:YAG lasers. Following are the phase matching angles for various harmonic generations.

1064nm SHG --> 532nm: Type I, q=22.8°, f=0°
1064nm THG --> 355nm: Type I, q=31.3°, f=0°; Type II q=38.6°, f=30°
1064nm 4HG --> 266nm: Type I, q=47.6°, f=0°
1064nm 5HG --> 213nm: Type I, q=51.1°, f=0°

In tunable laser systems:

1.Dye lasers

Efficient UV output (205nm-310nm) with a SHG efficiency of over 10% at wavelength of =206nm was obtained in type I BBO, and 36% conversion efficiency was achieved for a XeC1-laser pumped Dye laser with power 150KW which is about 4-6 times higher than that in ADP. The shortest SHG wavelength of 204.97 nm with efficiency of about 1% has been generated.

With type I sum-frequency of 780-950 nm and 248.5 nm (SHG output of 495 nm dye laser) in BBO, the shortest UV outputs ranging from 188.9nm to 197 nm and the pulse energy of 95 mJ at 193 nm and 8 mJ at 189 nm have been obtained, respectively.

670-530nm SHG --> 335-260nm: Type I, q=40°, f=0°

600-440nm SHG --> 300-220nm: Type I, q=55°, f=0°

444-410nm SHG --> 222-205nm: Type I, q=80°, f=0°


2.Ultrafast Pulse Laser

Frequency doubling and tripling of ultra short pulse lasers are the applications in which BBO shows superior properties to KDP and ADP crystals. We can provide as thin as 0.02mm BBO for this purpose. A laser pulse as short as 10fs can be efficiently frequency-doubled with a thin BBO, in terms of both phase-velocity and group-velocity matching.

3.Ti:Sapphire and Alexandrite lasers

UV output in the region 360nm~390nm with pulse energy of 105 mJ (31% SHG efficiency) at 378 nm, and output in the region 244nm-259nm with 7.5 mJ (24% mixing efficiency) have been obtained for type I SHG and THG of an Alexandrite laser in BBO crystal.

720-800nm SHG --> 360-400nm: Type I, q=31°, f=0°

720-800nm THG --> 240-265nm: Type I, q=48°, f=0°

More than 50% of SHG conversion efficiency in a Ti:Sapphire laser has been obtained. High conversion efficiencies have been also obtained for the THG and FHG of Ti:Sapphire lasers.

700-1000nm SHG --> 350-500nm: Type I, q=28°, f=0°

700-1000nm THG --> 240-330nm: Type I, q=42°, f=0°

700-1000nm FHG --> 210-240nm: Type I, q=66°, f=0°


4. Argon Ion and Copper-Vapor lasers

By employing the intra-cavity frequency-doubling technique in an Argon Ion laser with all lines output power of 2W, maximum 33mW at 250.4 nm and thirty-six lines of deep UV wavelengths ranging from 228.9 nm to 257.2 nm were generated in a Brewster-angle-cut BBO crystal.

Up to 230mW average power in the UV at 255.3 nm with maximum 8.9% conversion efficiency was achieved in the SHG of a Copper-Vapor laser at 510.6 nm.

514nm SHG --> 257nm: Type I, q=51°, f=0°, B-cut

488nm SHG --> 244nm: Type I, q=55°, f=0°, B-cut


In OPA, OPO Applications

The OPO and OPA of BBO are powerful tools for generating a widely tunable coherent radiation from the UV to IR. The tuning angles for type I and II of BBO OPO and OPA have been calculated, and available upon request.

1.OPO pumped at 532 nm

The OPO output ranging from 680 nm to 2400 nm with the peak power of 1.6MW and up to 30% energy conversion efficiency was obtained in a 7.2 mm long type I BBO. The input pump energy was 40 mJ at 532nm with pulse-width 75ps. The BBO crystal cut angle for this application is: Type I, q=21°, f=0°.

2.OPO and OPA pumped at 355 nm

Pumped by Nd:YAG laser, BBO’s OPO can generate wavelength tunable from 400nm to 2000nm with a maximum of 30% and more than 18% conversion efficiency.

Type II BBO can be used to decrease linewidth near the degenerate points. A linewidth as narrow as 0.05nm was obtained with the usable conversion efficiency of 12%. However, a longer (>15mm) BBO should normally be used to decrease the oscillation threshold when employing the type II phase-matching scheme.

Pumping with 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'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 nm to 3500 nm can be also generated by BBO's OPO or OPA plus a BBO for SHG. The crystal cut angle for 355nm pumped OPO is: q=30° and f=0° for Type I, q=37°and f=30° for Type II.

3.Others

A tunable OPO with signal wavelengths between 422 nm and 477 nm has been generated by angle tuning in a type I BBO crystal pumped by the fourth harmonic of a Nd:YAG laser (at 266 nm) has been observed to cover the whole range of output wavelengths 330 nm-1370nm. The crystal cut angle for 355nm pumped OPO is: Type I, q=39°, f=0°.

Pumped by a 1mJ, 80fs Dye laser at 615 nm, the OPA with two BBO crystals yields more than 50µJ (maximum 130µJ), <200fs ultra short pulse, over 800nm~2000nm.

Electro-Optical Applications:

BBO crystal is also widely used as electro-optical modulators. Please reference the Pockels Cell part of this site.

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