Ilya Auslender

Modeling of pulsed K diode pumped alkali laser: Analysis of the experimental results

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.

Influence of the pump-to-laser beam overlap on the performance of optically pumped cesium vapor laser.

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.

Multi-transverse mode operation of alkali vapor lasers: modeling and comparison with experiments

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.

Modeling of static and flowing-gas diode pumped alkali lasers

Modeling of static and flowing-gas subsonic, transonic and supersonic Cs and K Ti:Sapphire and diode pumped alkali lasers (DPALs) is reported. A simple optical model applied to the static K and Cs lasers shows good agreement between the calculated and measured dependence of the laser power on the incident pump power. The model reproduces the observed threshold pump power in K DPAL which is much higher than that predicted by standard models of the DPAL. Scaling up flowing-gas DPALs to megawatt class power is studied using accurate three-dimensional computational fluid dynamics model, taking into account the effects of temperature rise and losses of alkali atoms due to ionization. Both the maximum achievable power and laser beam quality are estimated for Cs and K lasers. The performance of subsonic and, in particular, supersonic DPALs is compared with that of transonic, where supersonic nozzle and diffuser are spared and high power mechanical pump (needed for recovery of the gas total pressure which strongly drops in the diffuser), is not required for continuous closed cycle operation. For pumping by beams of the same rectangular cross section, comparison between end-pumping and transverse-pumping shows that the output power is not affected by the pump geometry, however, the intensity of the output laser beam in the case of transverse-pumped DPALs is strongly non-uniform in the laser beam cross section resulting in higher brightness and better beam quality in the far field for the end-pumping geometry where the intensity of the output beam is uniform.

Parametric study of static and flowing-gas Cs DPAL

Experimental and theoretical parametric study of static and flowing-gas diode-pumped Cs lasers is reported. In the static case dependence of the output laser power and the beam quality factor M2 on the power and spatial shape of the pump beam is studied. An optical model of multi-transverse mode operation of alkali vapor lasers [Auslender et al, Opt. Express 25, 19767 (2017)] is applied to the experimental results. The values of the laser power and M2 predicted by the model are in good agreement with the experimental results for different shapes and powers of the pump beam We also report, briefly, on our recently published work [Yacoby et al, Opt. Express 26, 17814 (2018)] on flowing-gas Cs-DPAL where the output power and gas temperature rise in the laser cell at different flow velocities were studied and the results analyzed by our three-dimensional computational fluid-dynamics) model.

Optically pumped Cs vapor lasers: pump-to-laser beam overlap optimization

We present the results of an experimental study of Ti:Sapphire pumped Cs laser and theoretical modeling of these results, where we focused on the influence of the pump-to-laser beam overlap, a crucial parameter for optimizing the output laser power. The dependence of the output laser power on the incident pump power was found for varying pump beam cross-section widths and for a constant laser beam. Maximum laser power > 370 mW with an optical-to-optical efficiency of 43% and slope efficiency ~55% was obtained. Non monotonic dependence of the laser power and threshold power on the pump beam radius (at a given pump power) was observed with a maximum laser power and minimum threshold power achieved at the ratio ~0.7 between the optimal pump beam and laser 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 incident pump power at different pump beam radii and of the laser power, threshold power and optimal temperature on the pump beam radius. The model does not use empirical parameters such as mode overlap efficiency but rather the pump and laser beam spatial shapes as input parameters. This model can be applied to different optically pumped alkali lasers with arbitrary spatial distributions of the pump and laser beam widths.

Experimental studies and modeling of static Cs DPALs: dependence of the power and beam shape on different parameters

The pump-to-laser beam overlap and the cell length of static diode-pumped Cs lasers are crucial parameters for optimization of these lasers. In a previous publication we modeled the influence of the pump-to-laser beam overlap on the performance of Ti:Sapphire pumped cesium vapor laser (T. Cohen, E. Lebiush, I. Auslender, B.D. Barmashenko and S. Rosenwaks, Opt. Exp. 24, 14374 (2016)). In the present paper we report on experiments and modeling in progress on diode pumped cesium vapor laser.

Modeling of multi-transversal mode lasing in static alkali vapor lasers

In the present paper we use a simple optical model to describe multi-transverse mode operation of alkali lasers. 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. The model was applied to optically pumped cesium vapor laser studied experimentally and theoretically previously [Cohen, T., Lebiush, E., Auslender, I., Barmashenko B.D., and Rosenwaks, S., Opt. Exp. 24, 14374 (2016)]. It was found in our calculations that for low pump power and small pump beam radii, only fundamental lasing mode oscillates, just as shown experimentally in this study. However, for higher pump powers and larger pump beam diameters, several transverse modes participate in oscillation. The number and intensities of the oscillating modes as a function of the pump beam power and radius are found. In order to check the validity of the model, it was applied to pulsed static Cs DPAL [Zhdanov, B. et al, Electron. Lett. 44, 582(2008)] with the pump beam radius much larger than that of the fundamental laser mode and constant gas temperature. 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.

Experimental and theoretical study of the performance of optically pumped cesium vapor laser as a function of the pump-to-laser beam overlap

We report on the results of an experimental study of Ti:Sapphire pumped Cs laser and theoretical modeling of these results, where we focused on the influence of the pump-to-laser beam overlap, a crucial parameter for optimizing the output laser power. Non monotonic dependence of the laser power (optimized over the temperature) 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. Maximum laser power > 370 mW with an optical-to-optical efficiency of 43% and slope efficiency ~ 55% was obtained. 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 but rather the pump and laser beam spatial shapes as input parameters. The present results combined with results of the application of the model to K DPAL and Ti:Sapphire pumped Cs laser, indicate that the model can describe the operation of different optically pumped alkali lasers with arbitrary spatial distributions of the pump and laser beam widths.

Modeling of pulsed K DPAL taking into account the spatial variation of the pump and laser intensities in the transverse direction

We report on a model of highly efficient static, pulsed K DPAL [Zhdanov et al, Optics Express 22, 17266 (2014)], where Gaussian spatial shapes of the pump and laser intensities in any cross section of the beams are assumed. The model 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.