GAMDAN Optics β-BBO
GAMDAN Optics BBO
beh buh buh Oh? No, beta BBO!
Beta barium borate is an exceptional nonlinear crystal. Its chemical formula is BaB₂O₄. BBO is a negative uniaxial crystal, which is capable of phase matching for second-order photonic interactions over almost its entire transparency range (from the UV, 185 nm to the Near Infrared, 3.5 µm).
What Makes It Outstanding?
It is a birefringent material, meaning the refractive index of the extraordinary axis is different, i.e., smaller, than that of the ordinary axis. The indices depend on the frequency of the light being transmitted, as for any transparent material. They also vary with the angle between propagation and the optical axis. The wide transparency is joined with a large phase-matching range. BBO has excellent optical homogeneity, so it doesn’t introduce transmitted wavefront distortion. The thermal acceptance bandwidth is large. Properly polished, it possesses high laser damage threshold.
It is well suited for high peak power or high average power lasers. BBO can yield second, third, fourth, and even fifth harmonic generation of neodymium-doped lasers. The high effective nonlinearity makes BBO the preferred material for tunable OPO’s and OPA’s (optical parametric oscillators and optical parametric amplifiers). Adjustable OPO’s can replace dye lasers in many cases.
If there is a beta barium borate, there must be an alpha version as well (sorry, gamma fans). The α-BBO forms at 925 degrees C. It maintains the same chemical formula but has a different crystal form. The α phase has centric symmetry. It is also birefringent but lacks the nonlinear properties that make β-BBO so desirable. It is especially suitable for high-power UV polarization optics. Barium borate is generally flux-grown, but can also be grown by the CZ (Czochralski) method. The alpha phase is used in electro-optic cells.
Barium borate has generally good stable physical and chemical properties, although it is somewhat hygroscopic. A protective coating or an AR coating or a crystal housing is usually recommended.
Other Uses
Non-crystal barium borate can act as a bactericide and fungicide. It is added to paints, coatings, adhesives, plastics, and paper products. Because barium borate is resistant to ultraviolet radiation, it is used as a UV stabilizer for PVC plastic.
Crystal structure of BBO viewed nearly perpendicular to the c-axis. Source: Wikipedia
Source: https://en.wikipedia.org/wiki/Barium_borate
Laser Applications for BBO
In addition to frequency multiplying Nd doped lasers, beta BBO is also excellent for frequency conversion of shorter wavelength lasers, although it can’t reach fifth harmonics for those. Examples are dye lasers, Alexandrite lasers, argon ion lasers, Ti:sapphire lasers, and copper vapor lasers. These are generally frequency doubled. In an electro-optic device, β BBO can be a Q-switch, a pulse picker, a cavity dumper, or a laser shutter, among others. BBO crystals allow generating high peak power at wavelengths that cannot be otherwise generated. They can fill in the gaps of directly available laser wavelengths.
For nonlinear frequency conversion to be efficient sufficiently high optical intensities are necessary. This can be the result of a high power laser, or high peak power through pulse amplification. BBO can be used intracavity or outside the laser head.
GAMDAN can manufacture BBO crystals with a thickness and aperture and crystal orientation that meets your specific requirements.
DENNIS J. GARRITY, AUTHOR
Dennis is an engineer with over 45 years of experience in fabrication, testing, and material evaluation for high precision optics, with extensive hands-on experience. More on the Author can be found here.