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.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Operation of high power diode pumped alkali lasers (DPALs) is strongly influenced by the heat released to the active medium due to the pumping and lasing processes. The heat release causes significant temperature rise despite the high quantum efficiency of DPALs [1,2]. A very efficient way to remove this heat is to circulate the gas mixture through the laser cell.
Recently, several research groups reported on K and Cs flowing-gas lasers; for K lasers, output powers of 1.5 kW  and 1.9 kW  were achieved, and for Cs lasers 1 kW was obtained . Studies of lower power K and Cs flowing-gas lasers were reported as well [6,7], the powers of these systems being several Watts. In this paper we report on CW Cs DPAL with a close loop circulation of the active medium. The influence of the flow velocity on the lasing power and on the gas temperature rise along the cell was studied experimentally. The experimental results were compared to the results of a three-dimensional model which takes into account the Cs states kinetic processes in the lasing medium, the excitation of the alkali atoms to high energy levels and ionization of these levels [8,9].
In some DPALs studies it was assumed that the processes contributing to the gas heating are relaxation between the fine-structure levels of the alkali atoms (n2P3/2 → n2P1/2) and quenching of these levels to the ground state [10,11]. In the present study comparison between the measured and calculated temperature rise in the laser cell made it possible to estimate the contribution of the quenching to the gas heating in Cs DPALs. Evaluation of this contribution to the gas heating is crucial for designing high power flowing-gas laser, avoiding high temperatures in the laser cell by circulating the active medium in appropriate flow velocities.
2. Experimental setup
The flow system for the Cs DPAL gas mixture (Cs vapor, 300 Torr He and 300 Torr methane at room temperature) is depicted in Fig. 1. The gas circulation is obtained by a magnetically driven gas blower. The gas mixture flows over a storage nipple with 5 g of liquid Cs which is the alkali vapor source and enters the laser cell of 95 mm length along the flow, 28 mm width in the optical axis direction and 10 mm height. The gas flow velocity can be measured only when the system is open due to the fact that a flow-gage will interfere with the flow; we assumed that the closed loop velocity is equal to the measured open loop velocity.
Two T type thermocouple feedthroughs (Kurt Lesker) are connected to the system at the inlet and at the outlet of the laser cell in order to measure the temperature rise along the laser cell. The sensors of the thermocouples are located at the center of the pipes cross-section. The pipe walls were heated with heating tapes; the temperature of the storage nipple walls (403 K) and of the laser cell walls (424 K) were controlled separately. In order to avoid condensation of the alkali vapor on the cell windows, the cell walls were kept at a higher temperature than the temperature of the storage nipple walls.
The Optical setup is shown in Fig. 2. The gain medium was pumped longitudinally by a tunable ultra-narrow band Diode Laser (DL) system (OptiGrate, Shark Laser System). The DL radiation (λ ~852 nm) is linearly polarized with maximum power of 70 W and spectral bandwidth of 0.05 nm (~20 GHz). The pump beam was directed to the laser cell through a polarizing beam splitter (PBS) cube. The cube, along with a λ/2 waveplate (installed after a collimation lens) form an optic attenuator which makes it possible to change the pump power. The pump beam was focused by spherical lens with focal length of 100 mm. The pump waist diameter was ~2.5 mm and the maximum pump diameter in the DPAL medium was ~4 mm, whereas the diameter of the multimode laser beam was assumed to be equal to the average pump beam diameter. The diameters were measured using Spiricon SP928 beam profiler camera in free space.
Flat output coupler mirror with reflectivity of 40% at the laser wavelength was used, whereas the back mirror was a concave dichroic mirror with radius of curvature (ROC) = 0.5 m, reflectivity > 99% at λ ~894 nm and transmission of 96% at λ ~852 nm. The dichroic mirror was used in order to measure the pump absorption during the laser operation. The pump and laser output powers were measured by Ophir power meters.
3. Description of the model
Modeling of the system was performed using a 3D numerical model reported in [8,9]. Figure 3 shows the geometry which was used in the model. The buffer gas (CH4/He) flows in the x direction through a rectangular cross-section duct of 28 × 10 mm. The flow axis is perpendicular to the optical axis z. The gas pressure, temperature and velocity at the laser section inlet are pi = 1 atm, Ti = 388 K, and Vi = 0 – 4.5 m/s, respectively. Note that the gas temperature (388 K) measured at the center of the flow cross-section is lower than that at the outer surface of the laser cell (424 K) because the thermal conductivity of the gas is rather small. The Cs vapor density, NCs,i = 5.3 × 1012 cm−3 was not measured directly but was evaluated from the measurements of the pump beam absorption (~80%, see section 2) during the laser operation using our model.
In the calculations it was assumed that the laser and pump beams have a cylindrical shape and uniform intensity distribution at the beams cross-section. The beams enter the laser cell through the windows with transmission t. The beam diameter in the model was chosen to be 4.8 mm, i.e., about 1.5 times larger than the average pump beam diameter which was measured by the beam profiler camera. The reason is that, as shown , the volume occupied by the excited Cs atoms contributing to the losses by spontaneous emission and quenching is larger than that of the pump beam in the laser cell. Hence, in order to fit the calculated threshold pump power to the measured value, the diameter of the beam with uniform transverse intensity distribution should be about 1.5-2 times larger than that of the pump beam applied in the experiments, which has a Laguerre-Gaussian spatial profile in the transverse direction. We checked and found that the beam diameter has a very weak influence on both the slope efficiency and the temperature rise in the flow direction.
A Fabry-Perot laser resonator is assumed. The resonator consists of a reflector and an output coupler with reflectivity r1 and r2, respectively. Table 1 summarizes different parameters of the flowing-gas laser and the resonator properties. The temperature of the laser cell and storage nipple, as well as the reflectivity r2, the ROC of the dichroic mirror and the focal length of the focusing mirror were optimized experimentally to provide the highest output power at Pp = 65 W and the highest slope efficiency.
The flow properties are computed using a 3D numerical model based on an ANSYS FLUENT commercial code. The model solves the equations for the gas dynamics and kinetic processes in the active medium [8,9]. The intensity variation of the laser beams is computed using Beer-Lambert equations as described in detail in [8,9]. These references also describe in detail the transport equations and kinetics processes in Cs DPALs.
4. Results and discussion
Figure 4 shows the calculated and measured dependence of Plase on Pp for Vi = 1 and 4 m/s. The model predicts the same output power for both velocities, which is in good agreement with the measured power. Maximum CW output power of 24 W with slope efficiency of 48% and maximum optical-to-optical efficiency of 36% were obtained.
Figure 5 shows the measured and calculated values of the output power Plase for constant Pp = 65 W and for Vi = 0 – 4.5 m/s. The laser output power was constant as the flow velocity was decreased down to 1 m/s. For Vi = 0 m/s, the measured output power dropped rapidly with time due to the temperature increase caused by the gas heating by the pump beam.
Figure 6 shows the measured and calculated temperature rise along the flow direction (x-axis) at the central axis of the laser cell for Pp = 65 W and for various flow velocities. We used feedthrough T type thermocouples for measuring the gas temperature at the center of the flow pipes, both upstream and downstream of the laser cell. The gas temperature difference is affected by the heat released to the gas due to the kinetic processes and by the pipes walls heating. In order to separate and eliminate the effect of the pipes heating on the temperature measurements, we measured the temperature difference twice, with and without pumping. The measured temperature difference without pumping was then subtracted from the measured value with pumping.
Previously it was assumed that the processes contributing to the gas heating are relaxation between the fine-structure levels of the alkali atoms (n2P3/2 → n2P1/2) and quenching of these levels to the ground state [10,11]. The relaxation cross-section was measured in ; however, direct measurements of the quenching cross-section (assumed to be about the same for 2P3/2 and 2P1/2 ) was not reported in the literature but estimates have been suggested . In the present study we were able to estimate the contribution of the quenching to the temperature changes in the laser cell. In the computations three cases were examined: = 0.1 Å2, = 0.05 Å2, and = 0 Å2. As can be seen from Fig. 6, the measured and calculated values were in best agreement for = 0.05 Å2. Significantly higher value, ~1.4 Å2, was suggested in ; however, using this high value, our model predicts very high temperature rise (~70 K, much higher than the measured value) as well as strong effect of the flow velocity on the output power (contrary to the measurements presented in Figs. 4 and 5).
The paper reports on experimental and theoretical study of flowing-gas CW Cs DPAL. The theoretical study was performed using 3D CFD model, taking into account fluid dynamics and kinetic processes in the lasing medium. We examined experimentally and theoretically the effect of the flow velocity on the laser performances with maximum pumping power of 65 W. The effect of the flow velocity for Vi > 1 was found to be negligible at this range of pump powers. However, for Vi = 0 the output power decreased significantly due to the temperature increase caused by the gas heating by the pump beam. To study the effect of the flow velocity on the gas mixture heating, we measured the gas temperature at the inlet and at the outlet of the laser cell. It was shown that the contribution of the quenching processes to the gas heating is smaller than that of the relaxation. We estimate that the quenching cross-section, = 0.05 Å2, is much smaller than that estimated in  and used in the calculations of [10,11].
Air Force Office of Scientific Research (AFOSR) (FA9550-15-1-0489); Israel Science Foundation (ISF) (893/15); Office of Naval Research (ONR) (N62909-16-1-2213).
We are grateful to Mr. Yaniv Bar-Haim for his advice regarding the flow system.
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