Beta Barium Borate

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Beta Barium Borate (β-BaB₂O₄)

Barium borate (BBO) is a nonhydroscopic and chemically stable material. Crystallization takes place in two phases, alpha and beta, and the phase transition temperature equals to 925±5°C. The higher temperature phase-alpha is centric symmetrical within its crystal structure, whose square nonlinearity tensor components are identically equal to zero. In contrast to the alpha-phase, the Beta-phase of BBO is non-centric symmetrical and the crystals grown in this phase possess nonlinear optical properties.

Beta-BBO was discovered and developed in 1984 by a group of Chinese investigators at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science. It is a crystal frequently used for frequency mixing and other nonlinear optics applications.

Features:

Wide transmission region from 190 nm to 3500 nm
Broad phase-matchable range from 409.6 nm to 3500 nm
Large effective second-harmonic-generation (SHG) coefficient
High damage threshold
High Optical homogeneity
Excellent mechanical and physical properties

Applications:

Second, third, fourth and fifth harmonic generation of Nd:YAG and Nd:YLF lasers
Frequency-doubling, -tripling and -mixing of Dye lasers
Second, third and fourth harmonic generation of Ti:Sapphire and Alexandrite lasers
Optical parametric amplifier (OPA) and optical parametric oscillators (OPO)
Frequency-doubling of Argon ion, Cu-vapor and Ruby lasers.

Phase Matching Angles:

For both Type I and Type II processes, phase matching condition is critical to improve the conversion efficiency and it can be achieved by angle tuning. Figure 1 shows the phase matching angles of SHG. The effective SHG coefficients are given by:

Type I: d_eff=d₃₁sin(θ)+(d₁₁cos(3φ)-d₂₂sin(3φ))cos(θ)

Type II: d_eff=(d₁₁sin(3φ)+d₂₂cos(3φ))cos²(θ)

where θ and φ are polar coordinates referring to z=c and x=a, respectively.


	Figure 1: BBO type I and type II phase-matching angles of SHG with fundamental wavelengths form 0.5um to 2.8um

For both type I and Type II THG processes, phase matching condition to improve the conversion efficiency is different from the SHG. Taking the wavelength 1064nm as an example, the phase matching angles of both type I (ooo->e) and type II (ooe->e) confirmations are 37.33° and 46.93° respectively. However, the conversion efficiency is much higher when the phase matching angles of both type I and type II are 31.3° and 38.6° respectively. It is evident that the cascaded, second-order interactions (SHG and sum of frequencies) can contribute strongly to THG, so does 4HG. Figure 2 shows the phase matching angles of THG.


	Figure 2: BBO type I and type II phase-matching angles of THG with fundamental wavelengths form 0.7um to 2.6um

Optical parametric oscillator (OPO)is a powerful tool to generate widely tunable coherent radiation. By combining OPO and harmonics (532nm, 355nm and 266nm) of Nd:YAG lasers, the new tunable wavelength range can be extended from 200nm to 3000nm. Figure 3 shows the phase matching angles of OPO.


Figure 2: BBO type I and type II phase-matching angles of OPO with wavelength range form 0.7um to 2.6um

	Crystal Structure		Trigonal, space group R3c
	lattice constant (Å)		a = b = 12.532, c = 12.717
	Melting Point (°C)		1095±5
	Transition Temperature (°C)		925±5
	Optical Homogeneity (/cm)		≈10^-6
	Mohs Hardness		4.5
	Density (g/cm^3)		3.85
	Absorption Coefficient (/cm)		α<0.001 at 1064nm α<0.01 at 532nm
	Hygroscopic Susceptibility		Low
	Relative Dielectric Constant		ε_^T11/ε₀=6.7 ε_^T33/ε₀=8.1
	Resistivity (ohm/cm)		>10¹¹
	Linear Coefficient of Thermal Expansion (25°C~900°C) ( /K)		\|\|C: 36X10^-6 ⊥C: 4X10^-6
	Thermal Conductivity (W/mK)		\|\|C: 1.6 ⊥C: 1.2
	Transmission Range (nm)		189~3500
	Damage Threshold (GW/cm²)		10 at 1064nm (10ns) 1at 532nm (10ns)
	Constant of Dispersion (dn/dT) (/°C)		dno/dT=-9.3X10^-6 dne/dT=-16.6X10^-6
	Refractive Indices		ne=1.6146, no=1.7571 at 266nm ne=1.5555, no=1.6749 at 532nm ne=1.5425, no=1.6551 at 1064nm
	Sellmeier Equations (λ in um)		no²=2.7359+0.01878/(λ²-0.01822)-0.01354×λ² ne²=2.3753+0.01224/(λ²-0.01667)-0.01516×λ²
	Nonlinear Optical Coefficients (pm/V)		d₁₁=2.3 d₃₁=0.05×d₁₁
	Table 1: ß-BaB₂O₄ Properties

Application Examples :

Harmonic generations of Nd:YAG lasers:

1064nm SHG --> 532nm:Type I, 4X4X7mm, 22.8° cut

1064nm THG --> 355nm:Type I, 4X4X7mm, 31.3° cut

1064nm THG --> 355nm:Type II, 4X4X7mm, 38.6° cut

1064nm 4HG --> 266nm:Type I, 4X4X7mm, 47.6° cut

1064nm 5HG --> 532nm:Type I, 4X4X7mm, 51.1° cut

OPO and OPA pumped by harmonics of Nd:YAG lasers

532nm Pump --> 680-2600nm:Type I , 4X4X12mm, 21° cut

355nm Pump --> 410-2600nm:Type I, 6X4X12mm, 30° cut

355nm Pump --> 410-2600nm:Type II, 6X4X12mm, 37° cut

266nm Pump --> 295-2600nm:Type II, 6X4X12mm, 39° cut

Frequency doubling of dye lasers

670~530nm SHG --> 335-265nm:Type I, 8X4X7mm, 36.3° cut

600~440nm SHG --> 300-220nm:Type I, 8X4X7mm, 55° cut

444~410nm SHG --> 222-205nm:Type I, 8X4X7mm, 80° cut

Harmonic generations of Ti:Sapphire lasers

700~1000nm SHG --> 350-500nm:Type I, 7X4X7mm, 28° cut

700~1000nm THG --> 240-333nm:Type I, 8X4X7mm, 42° cut

700~1000nm 4HG --> 210-240nm:Type I, 8X4X7mm, 66° cut

Frequency doubling and tripling of alexandrite lasers

720~800nm SHG --> 360-400nm:Type I, 4X4X7mm, 31° cut

720~800nm THG --> 240-265nm:Type I, 4X4X7mm, 48° cut

Intracavity SHG of Ar+ laser with brewster angle cut BBO

514nm SHG --> 257nm:Type I, 4X4X7mm, 51° B-cut

488nm SHG --> 244nm:Type I, 4X4X7mm, 55° B-cut