LiCaA1F6:Cr3(Cr:LiCAF) is a new addition to a series of new tunable near-infrared laser materials. Cr:LiSAF possess suitable properties for the generation of high-energy tunable radiation in the near infrared. It exhibits small thermal lensing, a high damage threshold and a sufficiently long upper-state lifetime to ensure efficient energy storage. Furthermore, laser rods of diameters up to 25 mm can now be fabricated, which is an important consideration for the design of high-energy lasers and amplifiers. The relatively long upper-state lifetime Cr:LiCAF 170μs, makes it possible to efficiently pump these materials with flashlamps. The new chromium laser host, LiCAF, is reported to possess favorable spectroscopic and laser properties. Laser-quality LiCAF crystals appear to be moderately straightforward to produce: zone-melting, Bridgman, and Czochralski crystals have been lased. Significant scattering in these crystals, a persistent problem in early growth attempts, now appears to be under control. Recently used Bridgman crystals have scattering losses of ~0.1% cm−1, an acceptable level for most applications. The natural abundance of the constituent elements, coupled with the relatively low melting temperature (804°C) and congruent melting, forms a compelling case for the possibility of large-scale, inexpensive growth of laser-quality crystals. The laser emission of Cr:LiCAF has been tuned to between approximately 720 and 840nm and peaks at ~780nm. The radiative lifetime is ~175 μs at room temperature, and there is no evidence of concentration quenching for samples containing as many as 9 × 1020 Cr3+ ions/cm3. The intrinsic slope efficiency of LiCAF (67%) is close to that of alexandrite(65%). Recent flash-lamp-pumped Cr:LiCAF experiments demonstrated slope efficiencies of 1.6%, although passive losses in the laser rod of 3.5% cm−1 significantly affected the laser performance.
Parameter
Orientation | <2 deg (rod axis to crystal a-axis) |
Parallelism | <10〞 |
Perpendicularity | 3ˊ |
Chamfer | 0.1mm@45° |
Surface Quality | 10-5 S-D |
Wavefront Distortion | λ/8 @632.8 nm |
Surface Flatness | λ/10 @632.8 nm |
Clear Aperture | >95% |
Diameter Tolerance | +0/-0.2mm |
Length Tolerance | ±0.1mm |
Coatings | R<1%@670nm+R<0.5%@700~1100nm on both faces |
Laser Induced Damage Threshold | >15J/cm2@TEM00, 10ns, 10Hz |
Crystal Structure | Trigonal |
Space Group | P31C |
Lattice Constants | a=5.0116, c=9.9673 Å@2%Cr doping |
Density (g/cm3) | 2.988 |
Melting Point | 766°C |
Fracture Strength, σf(MPa) | 3.85±8(∥c) |
Fracture Toughness, KIC(MPa∙m1/2) | 0.39(∥a), 0.33(∥c) |
Vickers Microhardness, HV(GPa) | 1.9±0.2(∥c) |
Specific Heat(J/gK@298K) | 0.935 |
Thermal Conductivity(W·m-1·K-1) | 1.0(∥a), 1.68(∥c) |
Thermal Expansion(10-6K-1) | 22.2(∥a), -9.8(∥c) |
Young’s Modulus(GPa) | 120(∥a), 85(∥c) |
Typical Doping Level | 0.8~1.5@.% |
Absorption Peak Wavelength(nm) | 640 |
Absorption Cross-section at Peak(10-20cm2) | 3.0 |
Absorption Bandwidth at Peak Wavelength | ~100nm |
dn/dT (10-6K-1) | -4.2(no), -4.6(ne) |
Refractive Index | n=1.41 |
Laser Wavelength(nm) | 780 |
Energy-storage Lifetime(μs) | 170 |
Emission Cross-Section(10-20cm2) | 1.23@1.08mole% |
Oscillator Strength(10-6) | 48(4A2-4T2), 39(4A2-4T1a), π |
28(4A2-4T2), 51(4A2-4T1a), σ | |
Nonlinear Refractive Index(m2/W) | 4.5±0.7@5% doping, 1064nm |
Damage Threshold(J/cm2) | 25@3%doping of Cr |
- High gain cross-sections
- Very large gain bandwidths
- Relatively long fluorescence lifetimes,(~100μs)
- Offer a long energy-storage lifetime, which simplifies pumping requirements andmakes the materials suitable for Q-switching and amplifier configurations
- Have a large gain bandwidth, allowing amplification of ultrashort (femtosecond) pulses
- Have a large emission cross section for efficient extraction of stored energy
- Are tunable from 730 to 1000 nm (red to infrared), and the frequency may be doubled to 365 to 500 nm (blue to green)
- Possess favorable thermomechanical properties, allowing for ease of thermal management and material fabrication
- Are subject to very low thermal lensing, leading to good beam quality at high power levels
- Have very low nonlinear indices (n2=4 x 10-3electrostatic charge units, or esu) and very high damage thresholds (>55 J/cm2 at 10 ns), enabling transmission of undistorted high-intensity pulses through the material
- Have a uniform distribution of the chromium ions, permitting very uniform and high chromium-doping levels throughout the laser medium
- Have excellent optical quality, resulting in low loss and high output power
- Require modest crystal growth conditions (melting point < 800°C)
- Are grown from inexpensive, nontoxic starting materials
- Amenable to flashlamppumping
- Providing tunable high power laser radiation in the near IR
- Ultrashort pulse generation and amplification
- Large aperture laser rods
- Laser rangefinders and illuminators, undersea optical communications, spectroscopy, and pumping other lasers
- Amplifications of stretched femtosecond pulses
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