Properties of Cr:LiSrAlF6 crystals for laser operation
Stephen A. Payne, Larry K. Smith, Raymond J. Beach, Bruce H. T. Chai, John H. Tassano, Laura D. DeLoach, Wayne L. Kway, Richard W. Solarz, and William F. Krupke
Stephen A. Payne,2
Larry K. Smith,2
Raymond J. Beach,2
Bruce H. T. Chai,1
John H. Tassano,2
Laura D. DeLoach,2
Wayne L. Kway,2
Richard W. Solarz,2
and William F. Krupke2
1Center for Research in Electro-Optics and Lasers, University of Central Florida, Orlando, Florida 32826 USA
2The other authors are with the Lawrence Livermore National Laboratory, University of California, P.O. Box 5508, Livermore, California 94550. USA
Stephen A. Payne, Larry K. Smith, Raymond J. Beach, Bruce H. T. Chai, John H. Tassano, Laura D. DeLoach, Wayne L. Kway, Richard W. Solarz, and William F. Krupke, "Properties of Cr:LiSrAlF6 crystals for laser operation," Appl. Opt. 33, 5526-5536 (1994)
We have performed several physical and optical measurements on the Cr:LiSAF (LiSrAlF6) laser material that are relevant to its laser performance, including thermal and mechanical properties, water durabilities, and Auger upconversion constants. The expansion coefficient, Young’s modulus, fracture toughness, thermal conductivity, and heat capacity are all used to determine an overall thermomechanical figure of merit for the crystal. An investigation of the water durability suggests that the cooling solution should be maintained at pH = 7 to ameliorate problems associated with water dissolution. The Auger constant was found to become much more significant at higher Cr doping, in which excited-state migration leads to a substantial increase in the upconversion rate. We propose a design for a 50-W Cr:LiSAF laser system that is based on a detailed knowledge of all the relevant material parameters.
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Effect of Unstirred Nonaqueous Solvents, Temperature, and pH on the Durability Dw of LiSAF
Solvent
Temperature (°C)
Dw [mg/(cm2 day)]
Deionized water (DI)
50
2.6
DI/etheylene glycol
50
0.14
FC-75 (Fluorinert)
50
< 0.02
DI
25
0.64
DI/ethylene glycol
25
0.05
pH 4 buffer
50
1.1
pH 7 buffer
50
< 0.03
pH 10 buffer
50
2.3
Table 4
Effect of pH on the Durability Dw of LiSAF at 22 °C during Vigorous Stirring
pH
Dw [mg/(cm2 day)]
4.02
4.7
5.06
3.7
6.18
< 0.03
7.10
< 0.001
7.83
< 0.05
9.91
3.5
Table 5
Properties of Cr:LiSAF crystals used in Auger Up-Conversion Studies with Oxazine 750 or Kiton Red Dye Laser Excitationa
Concentration XCr (%)
Absorption Coefficient α (mm−1)
Length l (mm)
Veff (mm3)
2
0.25
9.4
1.28
15
0.74
3.4
0.61
15
0.40
1.0
0.75
34
0.91
1.15
0.40
51
1.11
1.14
0.35
100
4.9
0.18
0.07
Calculated wiuth wx = 0.37 mm and wy = 0.21 mm for Oxazine 750, and wx = 0.28 mm and wy = 0.20 mm for Kiton red (used for XCr = 2% sample only). Note that the wavelength was varied depending on the sample.
Table 6
Parameters Corresponding to < 50 W Operation of Diode Pumped CrLiSrF6
Diode pump array
Pitch
10/cm
Array size
75 linear cm
Operating power
80 W/cm peak
Wavelength λp
770nm
Optical Conditioning
cylindrical microlenses
Pulse format
τp
200 μs
fR
250 Hz
Total pump energy/pulse Ep
1.2 J
Cr:LiSAF slab
Dimensions t × w × l
2 mm × 4 mm × 10 cm
Cr3+ number density NCr
1.4 × 1020/cm3, (1.7 at. %)
Cr3+ fluorescence lifetime τ
67 μs
Passive crystal loss
0.1%/cm
Laser characteristics
Wavelength λl
852 nm
Pump transport efficiency ηtrans
0.8
Pump absorption efficiency ηabs
0.75
Modefill efficiency ηmode
1.0
770-nm absorption cross section σabs
0.05 × 10−20 cm2
852-nm emission cross section σem
4.8 × 10−20 cm2
852 nm ESA cross section σESA
1.6 × 10−20 cm2
Output coupler reflectivity RO.C.
0.85
One-way cavity transmission T
0.98
Threshold lasing density Nthr
1.74 × 1017/cm3
Pump excitation rate Rex
1.7 × 1022/cm3 s
Threshold buildup time τth
11 μs
Auger constant γ
0.55 × 10−15 cm3/s
Intracavity laser intensity I
103 kW/cm2 (peak)
Output laser intensity Iout
15.5 kW/cm2 (peak)
Output power Pout
1.24 kW (peak), 58.6 W (average)
Output energy/pulse Eout
0.235 J
Optical-to-optical efficiency
19.5%
Slab thermal characteristics
Thermal shock parameter RT
0.84 W/cm
Maximum thermal power density Pmax
252 W/cm3
Thermal power density Pth
76 W/cm3
Pth/Pmax
0.30
Surface heat flux
7.6 W/cm2
Slab center to edge temperature
12.3 °C
Tables (6)
Table 1
Thermomechanical Properties of LiSAF
Property
Value
Density
ρ = 3.45 g/cm3
Elastic constants
c11 = 117 GPa
c33 = 94 GPa
Young modulus
E = 109 GPa
Fracture toughness
K1c = 0.40 MPa m1/2
Heat capacity
Cp = 0.842 J/(g °C)
Thermal diffusivity
λc = 1.08 × 10−6 m2/s
Thermal conductivity
κc = 3.1 W/(m °C)
Thermal expansion
αc = −10 × 10−6/°C
αa = 19 × 10−6/°C
Poisson ratio
ν = 0.3
Thermal figure of merit
RT′(a) = 0.42 W/m1/2
RT′(c) = 0.80 W/m1/2
Thermal-shock parameter (for 50-μm flaw)
RT(a) = 0.84 W/cm
RT(c) = 1.60 W/cm
Table 2
Water Durability Dw of Optical Materials at 50 °C for Unstirred Conditions
Effect of Unstirred Nonaqueous Solvents, Temperature, and pH on the Durability Dw of LiSAF
Solvent
Temperature (°C)
Dw [mg/(cm2 day)]
Deionized water (DI)
50
2.6
DI/etheylene glycol
50
0.14
FC-75 (Fluorinert)
50
< 0.02
DI
25
0.64
DI/ethylene glycol
25
0.05
pH 4 buffer
50
1.1
pH 7 buffer
50
< 0.03
pH 10 buffer
50
2.3
Table 4
Effect of pH on the Durability Dw of LiSAF at 22 °C during Vigorous Stirring
pH
Dw [mg/(cm2 day)]
4.02
4.7
5.06
3.7
6.18
< 0.03
7.10
< 0.001
7.83
< 0.05
9.91
3.5
Table 5
Properties of Cr:LiSAF crystals used in Auger Up-Conversion Studies with Oxazine 750 or Kiton Red Dye Laser Excitationa
Concentration XCr (%)
Absorption Coefficient α (mm−1)
Length l (mm)
Veff (mm3)
2
0.25
9.4
1.28
15
0.74
3.4
0.61
15
0.40
1.0
0.75
34
0.91
1.15
0.40
51
1.11
1.14
0.35
100
4.9
0.18
0.07
Calculated wiuth wx = 0.37 mm and wy = 0.21 mm for Oxazine 750, and wx = 0.28 mm and wy = 0.20 mm for Kiton red (used for XCr = 2% sample only). Note that the wavelength was varied depending on the sample.
Table 6
Parameters Corresponding to < 50 W Operation of Diode Pumped CrLiSrF6