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Good Quantum Defects Make Good Lasers

Photonics Spectra
Jul 2005
Breck Hitz

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 ­ [λpumplaser]). 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.

Good Quantum Defects Make Good Lasers
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.

Good Quantum Defects Make Good 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.


GLOSSARY
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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