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Yb:YAG Laser Crystal

Ytterbium-doped YAG (Yb:YAG) is regarded as the preferred option for the design Of high-power, diode-pumped solid state lasers, owning to Yb:YAG's broad 18nm wide absorption band near 940 nm and 1ms long lifetime of its emission state. Yb:YAG can also be pumped by reliable InGaAs laser diodes at 969 nm, however, the bandwidth in this case is significantly narrower compared to that at 940 nm.

The Yb³⁺ ion has simple electronic structure[xe]4f¹³. There exists only ground state ²F_7/2 and excited state ²F_5/2. The lack of higher 4f excited state leads to Yb:YAG being devoid of excited state absorption, upconversion or low-concentration quenching even at high doping concentrations of Yb³⁺ ions. Furthuermore, Yb:YAG has large fluorescence lifetime linked to a lower emission cross section, and high quantum efficiency linked to a small spectral separation between the lower laser level and the ground level.

The closeness of the lower laser energy level to the ground state on the opposite end significantly increase the population of lower laser energy level through thermal exitation, in contrast to Nd:YAG. Consequently, Yb:YAG is not an ideal material choice for high-power ns-pulse Q-switched lasers. Conversely, Yb:YAG is more suitable for qusi-cw operation at lower temperture, as this allows the Yb³⁺ ions sufficient time to return to the ground state, thereby alleviating the impacts of thermal excitation. Bruesselbach et al. reported that Yb:YAG lasers operating in quasi-cw mode can indeed achieve average power levels in the kilowatt range.

Properties of Yb:YAG

Chemical Formula:

Yb:Y₃Al₅O₁₂

Crystal Structure:

Cubic

Lattice Constants:

12.01Å

Melting Point:

1970°C

Density:

4.56 g/cm³

Mohs Hardness:

8.5

Specific Heat:

0.59 J/g.cm³

Modulus of Elasticity:

310 Gpa

Young's Modulus:

3.17X104 Kg/mm²

Poisson Ratio:

0.3 (est.)

Tensile Strength:

0.13~0.26 GPa

Thermal Expansion Coefficient:

7.8 x 10^-6/K [111], 0-250°C

Thermal Conductivity:

14 W/m /K @20°C, 10.5 W /m /K @100°C

Yb:YAG Optical Properties

Refractive Index:

1.82 @1064 nm

Lasing Wavelength:

1030 nm

Emission Linewidth:

9 nm

Stimulated Emission Cross Section:

2.2 x 10^-20 < cm²

Absorption Bands:

941 nm, 968.5 nm

Absorption bandwidth:

18nm for 940nm, 2nm for 969nm

Intrinsic Flouresence Lifetime (< 15% Yb doping):

967 μs

Loss Coefficient:

0.003 /cm @ 1064 nm

Thermal Optical Coeff. (dn/dT):

9x10^-6/°C

Sellmeier equation( λ in μm):

n²=2.08745+1.2081 λ²/(λ²-0.02119 )+17.2049 λ²/(λ²-1404.45)

Capability of Yb:YAG Laser Crystal

1) Yb dopant concentration:

0.2 ~ 30% atm%

2) Diameter:

2 ~ 20 mm

3) Length:

1 ~ 70 mm

4) Anti-reflection Coating:

R < 0.1% @ 1030 nm

High transmission Coating:

T>95% @ 940 nm

High reflection Coating:

R>99.8% @ 1030 nm

General Specifications of Yb:YAG Laser Crystal

1) Orientation:

<111> crystalline direction (+/-0.5°)

2) Wavefront Distortion:

λ/10 per inch

3) Surface quality:

better than 20/10 Scratch/Dig

4) Parallelism:

10 arc seconds

5) Perpendicularity:

5 arc minutes

6) Surface flatness:

λ/10 at 633 nm

7) Clear aperture:

Central 95%

8) Chamfer:

0.15mm @45°

9) Damage threshold:

over 15J/cm² (without coating), over 700 MW/cm² (coating)

i. Yb:YAG laser crystal

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