Subrat Biswal

Thermo-optical parameters measured in ytterbium-doped potassium gadolinium tungstate

Oriented absorption and fluorescence spectra, refractive indices, thermal expansion, and thermal conductivities are reported for the anisotropic laser material ytterbium-doped potassium gadolinium tungstate, Yb3+:KGd(WO4)2. Measurements of negative values for dn/dT lead to the report of several useful athermal orientations for laser propagation.

Ytterbium laser with reduced thermal loading

We report a novel design for a high-power ytterbium disk laser. This laser utilizes radial diode pumping of a back surface cooled active-mirror geometry. Wing absorption of the pump light at 0.99 /spl mu/m allows efficient laser operation at 1.05 /spl mu/m with a low quantum defect. Laser performance and thermal loading were characterized for a wide range of conditions. Optimized operation of the laser yielded 490 W in a quasi-continuous-wave mode. Electrical efficiency of the laser was found to be 9.4%, while heating of the laser disk was only 3.2% of the absorbed optical power. Fluorescence re-absorption is identified as the principal heat generation mechanism in this laser. A simplified extension to the conventional rate model is proposed for lasing in radiation-trapped systems. This model allows power flow calculations in a radiation-trapped laser system using a single parameter determined from fluorescence decay waveforms. The revised model agrees with heat load measurements.

Minimizing Heat Generation in Solid-State Lasers

Novel high-power ytterbium YAG lasers are described. These lasers incorporate the principle of anti-Stokes fluorescence cooling to reduce or eliminate detrimental heating. Lasers with net heating and net cooling are demonstrated. By balancing the spontaneous and stimulated emission, we have reduced the net thermal loading to below 0.01% of the lasers average output power. Design, testing, and analysis are reported for lasers up to 500 W average power and pulsed operation up to 30 s. Issues and limitations of this approach are discussed.

Nonradiative losses in Yb:KGd(WO4)2 and Yb:Y3Al5O12

Photothermal deflection spectroscopy was used to measure nonradiative losses in Yb:KGW and Yb:YAG (for both crystal and for the first time in ceramic form). Fluorescent quantum efficiencies for both materials were found to be greater than 98.9%. The reduction of nonradiative losses due to annealing of Yb:YAG is also presented.

Demonstration and analysis of a high power radiation balanced laser

Detrimental heating in a high power Yb:YAG laser was eliminated by incorporating anti-Stokes optical cooling.

High power ytterbium disk laser

A novel diode-pumped disk laser yielded 490 W in a quasi-cw mode. Electrical efficiency of the laser was found to be 9.4%. Heating of the laser disk was only 2.7% of the absorbed optical power.

Minimizing non-radiative losses in erbium laser systems

Very low rates of excited-state absorption and upconversion were measured from the 4I_13/2 level in Er³⁺ doped KPb₂Cl₅ crystals. Comparisons with other hosts suggest significant potential improvements in laser performance.

Radiation balanced lasers

We present progress on a new approach to the problem of laser heating. The approach here is to design a quasi-three level laser system with minimal internal heat generation. This can be accomplished by reducing the excess power left behind during the absorption/emission processes needed for lasing.