Good Quantum Defects Make Good Lasers
The quantum defect -- the energy difference between a pump photon and a laser photon -- is one of the most significant parameters in selecting a potential laser material. Another one for a solid-state laser is the material's thermal ruggedness, or the amount of power it can absorb without physically rupturing. But there is a trade-off between the two: The larger a laser material's quantum defect, the more waste heat it must absorb at a given pump level, and, hence, the greater its thermal ruggedness must be.
Now researchers at Ecole Nationale Supérieure de Chimie in Paris have suggested a figure of merit that takes both factors into account. They have applied it to several laser materials and have explored a new material, Yb:CaGdAlO4, with a very low (~3.5 percent) quantum defect.
Their figure of merit -- which they call the laser power resistance parameter, or RP -- is defined as RP ∝κ2/∝η, where κ is the thermal conductivity, ∝ is the expansion coefficient and η is the quantum defect (η = 1 [λpump/λlaser]). Applying this to several common laser materials, they have concluded that the new host material, CaGdAlO4, is a good competitor for YAG for use in high-power applications.
Figure 1. Yb:CaGdAlO4 is a relatively easy crystal to grow by the Czochralski technique. The Yb dopant substitutes for either the Gd3+ or the Ca2+ in the crystal lattice. Images ©OSA.
In experiments with CaGdAlO4, the researchers grew a high-quality boule of 2-percent-atomic Yb:CaGdAlO4 by the Czochralski technique (Figure 1), from which they cut several laser-quality crystals. Pumping one of the crystals with a Ti:sapphire laser at 979 nm, they observed what they believe is the first laser emission from this crystal. The output was centered at 1016 nm and exceeded 0.5 W at 2 W of pump power (Figure 2). For the wavelengths in question, the quantum defect of the laser is 3.5 percent, significantly lower than in many other lasers.
Figure 2. Pumped with 979-nm radiation from a Ti:sapphire laser, the Yb:CaGdAlO4 laser generated more than 0.5 W at 1016 nm. The quantum defect in this case was 3.5 percent.
The researchers measured a broad emission peak for Yb:CaGdAlO4, indicating that laser action at shorter wavelengths -- and, hence, with even smaller quantum defects -- might be possible. Very recently, they obtained laser action at 994 nm, corresponding to a quantum defect of only 1.4 percent. They are enhancing that performance in the laboratory and attempting to use the broad bandwidth to generate ultrafast pulses.
- solid-state laser
- A laser using a transparent substance (crystalline or glass) as the active medium, doped to provide the energy states necessary for lasing. The pumping mechanism is the radiation from a powerful light source, such as a flash lamp. The ruby and Nd:YAG lasers are solid-state lasers.
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