偏硼酸钡（BaB2O4）存在着高温相（α-BBO）和低温相（β- BBO）两种多型体，熔点为1095℃，其相变温度为925℃左右。
一、α-BBO
高温相α-BBO晶体属三方晶系，单胞参数为：a=b=0.7235nm，c=3.9192 nm。高温相α-BBO晶体为负光轴晶体，具有大的双折射率，透光范围为189～3500nm，尤其在紫外区的透过率很高（95%），而且物理化学性能稳定，机械加工性能优异。具有中心对称结构，α-BBO晶体无二阶非线性效应。
广泛应用于光偏转技术、高功率激光和超快激光应用等领域。主要用于制作紫外偏振棱镜如格兰泰勒棱镜、渥拉斯顿棱镜、高功率光隔离器以及超快激光的群速失配补偿器等。
High temperature phase BBO (alpha-BBO, a-BBO) is a negative uniaxial crystal with a large birefringence over the broad transparent range from 189 nm to 3500 nm. The physical, chemical, thermal, and optical properties of alpha-BBO crystal are similar to those of the low temperature phase beta-BBO crystal. However, there is no second order nonlinear effect in alpha-BBO crystal due to the centrosymmetry in its crystal structure and thus it has no use for second order nonlinear optical processes. Instead, alpha-BBO is widely used for fabrication of polarizers, polarizing beam displacers, phase retarders, birefringent plates, and time delay compensators especially those for UV and high power lasers.
二、β- BBO
低温相β-BBO晶体也属三方晶系，单胞参数为：a=b=12.532A,c=12.717A,Z=7。可实现相位匹配的波段范围宽（409.6—3500nm），可透过波段范围宽（190—3500nm），倍频转换效率高(相当于KDP晶体的6倍），光学均匀性好，高损伤阈值（100ps脉宽的1064nm10GW/cm^2），温度接收角宽（55℃左右）。
低温相β-BBO晶体在非线性光学上面应用广泛，在Nd:YAG激光二、三、四倍频上的性能优异，是213nm光五倍频的最佳选择。二倍频的转换效率大于70％，三倍频60％，四倍频50％。具有较小的接受角和较大的走离角，因此获得理想的转换效率的关键是使用较好质量的基频光（具有小的发散角，较好的模式条件等）。
BBO Crystal is one of the essential nonlinear optical crystals. Beta-barium borate (β-BaB2O4，β-BBO) combines many outstanding features such as its high nonlinear optical coefficients and low group-velocity dispersion, broad transparency range (189–3500 nm) high damage threshold. This unique combination ensures that β-BBO crystal is a promising candidate for nonlinear optical applications such as frequency converters and optical parametric oscillators. In quantum optics, β-BBO crystal can generate entangled photon pairs and ten-photon entanglement.
BBO nonlinear crystal is a negative uniaxial crystal. It provides phase matching for various second-order interactions almost over its entire transparency range. This transparency ranges from 185 nm to 3.3 µm, as inferred from transmittance measurements using crystal samples of a few mm thickness. It is widely used for nonlinear frequency conversion in the ultraviolet, visible, and near-infrared. The BBO crystal is the most critical nonlinear crystal for near-infrared optical parametric chirped-pulse amplifiers, which currently deliver few optical cycle pulses with high average and ultrahigh peak powers.
