ABSTRACT
Analysis of the operation of flowing-gas low power DPALs is crucial for designing high power devices. In particular, the comparison between the measured and calculated temperature rise in the laser cell makes it possible to estimate the contribution of the quenching of the alkali atoms electronic states to the gas heating. Here we report on an experimental and theoretical study of continuous wave flowing-gas Cs DPAL with He and CH4 buffer gases, flow velocities of 1-4 m/s and pump powers of 30-65 W. In the calculations we used a 3D computational fluid dynamics model, solving the fluid mechanics and kinetics equations relevant to the laser operation. Maximum CW output power of 24 W with a slope efficiency of 48% was obtained. The experimental and theoretical values of the power and gas temperature are in good agreement. The lasing power was not affected by the flow velocity at this range of pump power and the gas temperature rise was only several degrees. It was found that the best agreement between the measured and calculated temperature rise is achieved for quenching cross-section ~0.05 Å2.
ABSTRACT
In high-power diode pumped alkali lasers with stable resonators the radius of the pump beam is usually larger than that of the fundamental laser mode and thus several high order transverse modes of the resonator can participate in the lasing. A simple optical model of multi-transverse mode operation of alkali vapor lasers is reported. The model is based on calculations of the pump and laser beam intensities in the gain medium, where the laser beam intensity is a linear combination of the azimuthally-symmetric Laguerre-Gaussian modes. It was applied to Ti:Sapphire and diode pumped cesium vapor lasers. The model predicts that for low pump power only the fundamental lasing mode oscillates. However, for higher pump powers several transverse modes participate in oscillation. The number and intensities of the oscillating modes as a function of the pump beam power and radius were found. The model predicts linear dependence of the laser power on the pump power, the values of the former being in agreement with the experimental results obtained for diode pumped cesium laser [Electron. Lett.44, 582 (2008)]. The mode-matching efficiency for the multi-transverse mode lasing is ~0.8 - 0.85 which means that in this case almost complete overlap of the laser and pump beams takes place. The laser beam quality factorM2increases with increasing pump power from 1 at the threshold power to 5-6 at maximum values of the pump power resulting in lower beam quality at high powers.
ABSTRACT
Experimental and theoretical study of the influence of the pump-to-laser beam overlap, a crucial parameter for optimization of optically pumped alkali atom lasers, is reported for Ti:Sapphire pumped Cs laser. Maximum laser power > 370 mW with an optical-to-optical efficiency of 43% and slope efficiency ~55% was obtained. The dependence of the lasing power on the pump power was found for different pump beam radii at constant laser beam radius. Non monotonic dependence of the laser power (optimized over the temperature of the Cs cell) on the pump beam radius was observed with a maximum achieved at the ratio ~0.7 between the pump and laser beam radii. The optimal temperature decreased with increasing pump beam radius. A simple optical model of the laser, where Gaussian spatial shapes of the pump and laser intensities in any cross section of the beams were assumed, was compared to the experiments. Good agreement was obtained between the measured and calculated dependence of the laser power on the pump power at different pump beam radii and also of the laser power, threshold pump power and optimal temperature on the pump beam radius. The model does not use empirical parameters such as mode overlap efficiency and can be applied to different Ti:Sapphire and diode pumped alkali lasers with arbitrary spatial distributions of the pump and laser beam widths.
ABSTRACT
A simple optical model of K DPAL, where Gaussian spatial shapes of the pump and laser intensities in any cross section of the beams are assumed, is reported. The model, applied to the recently reported highly efficient static, pulsed K DPAL [Zhdanov et al, Optics Express 22, 17266 (2014)], shows good agreement between the calculated and measured dependence of the laser power on the incident pump power. In particular, the model reproduces the observed threshold pump power, 22 W (corresponding to pump intensity of 4 kW/cm2), which is much higher than that predicted by the standard semi-analytical models of the DPAL. The reason for the large values of the threshold power is that the volume occupied by the excited K atoms contributing to the spontaneous emission is much larger than the volumes of the pump and laser beams in the laser cell, resulting in very large energy losses due to the spontaneous emission. To reduce the adverse effect of the high threshold power, high pump power is needed, and therefore gas flow with high gas velocity to avoid heating the gas has to be applied. Thus, for obtaining high power, highly efficient K DPAL, subsonic or supersonic flowing-gas device is needed.
