b-BBO Crystal (Beta Barium Borate )

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Application in Nd:YAG Lasers

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 200 mW 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.

BBO is also an efficient crystal for the intracavity SHG of high power Nd:YAG lasers. For the intracavity SHG of an acousto-optic Q-switched Nd:YAG laser, more than 15 W average power at 532 nm was generated in a CASTECH AR-coated BBO crystal. When pumped by the 600 mW SHG output of a mode-locked Nd:YLF laser, 66 mW output at 266 nm was produced from a Brewster-angle-cut BBO in an external enhanced resonant cavity.

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.

Applications in Tunable Lasers

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.

BBO is widely used in the Dye lasers. 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.

2.Ultrafast Pulse Laser
Frequency-doubling and -tripling of ultrashort-pulse lasers are the applications in which BBO shows superior properties to KDP and ADP crystals. Red Optronics can provide as thin as 0.02mm BBO for this purpose. A laser pulse as short as 10 fs 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.

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.

4. Argon Ion and Copper-Vapor lasers
By employing the intracavity frequency-doubling technique in an Argon Ion laser with all lines output power of 2W, maximum 33 mW 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 230 mW average power in the UV at 255.3 nm with maximum 8.9% conversion efficiency was achieved for the SHG of a Copper-Vapor laser at 510.6 nm.

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, with the results shown in Fig.5 and Fig.6, respectively.

1.OPO pumped at 532 nm
an 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. With a longer crystal, higher conversion efficiency is expected.

2.OPO and OPA pumped at 355 nm
In the case of Nd:YAG pumping,BBO's OPOs can generate more than 100mJ,with wavelength tunable from 400nm to 2000nm. Using CASTECH's BBO crystal, the OPO system covers a tuning range from 400nm to 3100nm which guarantees a maximum of 30% and more than 18% conversion efficiency, over the wavelength range from 430nm to 2000nm.

Type II BBO can be used to decrease linewidth near the degenerate points. A linewidth as narrow as 0.05nm and usable conversion efficiency of 12% were obtained. 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 picosecond 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.

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.

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

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