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Abstract
We examined the performance of a potassium diode pumped alkali laser (K DPAL) using He, Ar, CH4, C2H6 and a mixture of He and CH4 as a buffer gas to provide spin-orbit mixing of the 4P3/2 and 4P1/2 states of Potassium atoms. We found that pure helium cannot be used as an efficient buffer gas for continuous wave lasing without using a flowing system with a considerable flow speed of about 100 m/s. In contrast, using a small amount of methane (10-20 Torr) mixed with helium, continuous wave lasing can be achieved using very moderate flow speeds of about 1 m/s.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recent experiments on K DPAL high power lasing performed at AFRL [1] demonstrated operation of this DPAL with an output power of 1.5 kW and an optical-to-optical efficiency 55% when using a methane/helium mixture as a buffer gas. For the high power scaling of the K DPAL, it is preferable to use hydrocarbon free buffer gas (e.g. pure He or other noble gas) to avoid problems with possible chemical reactions in the gain medium leading to lasing degradation, gain medium contamination and damage of alkali cell windows. The possibility for efficient lasing using noble gases for spin-orbit mixing of the 4P3/2 and 4P1/2 states of Potassium atoms is inspired by very close values of mixing cross sections for noble gases and hydrocarbons as it well seen in the Table 1. On the other hand, there can be additional power limiting problems such as ionization [4], thermal effects [5], and others, which can have different contributions for each buffer gas that also have to be taken into account. All these problems require research to determine the optimal buffer gas to be used to achieve sufficient spin-orbit mixing of the P states of Potassium atoms and, at the same time, being resistant to ionization and chemical interaction with the atomic and ionized alkali vapor. Thisis currently a very important aspect of DPAL research and is the focus of several investigations.
Table 1. Cross sections for 2P1/2 ↔ 2P3/2 excitation transfer (in Å2) induced by alkali – buffer gas collisions
In this paper, we present the results of our studies on the performance of K DPAL using He, Ar, CH4, C2H6 and a mixture of He and CH4 as buffer gases.
2. Experiment and results
The diagram of the experimental setup is presented in Fig. 1. We used a standard L-shape 40 cm long laser cavity with 50% flat output coupler and highly reflective concave back mirror with 50 cm radius. The polarization beam splitting cube (PBS) was used to separate lasing and pump beams. As a pump source, we used a narrowband (12 GHz line width) diode laser stack with a maximum output power of 200W. The pump beam was focused into the gain medium by a combination of spherical and cylindrical lenses and the measured beam size in the focal plane is 0.33 mm x 0.55 mm (FWHM). Calculated TEM00 laser cavity mode size in the gain medium is 0.47 mm (FWHM). The pump power used in all experiments described below and measured at the entrance of the alkali cell was 160 W.
For these experiments we built a refillable stainless steel alkali static cell (see Fig. 2), which has gain medium length 1.5 cm and can be used for experiments with any alkali atomic vapor (cesium, rubidium or potassium). Being attached to the gas handling system, it can be easily evacuated and filled with any buffer gas or mixture of different gases at any pressure up to 4 atm. The temperature of the alkali vapor cell was kept at 190C for all experiments described below.
We studied the K DPAL performance in pulsed mode with different pump pulse durations: 30 μs, 250 μs, 50 ms and 100 ms. The DPAL output pulse waveforms in the experiments described below were registered by Silicon Photodetectors (New Focus, Model 2031) and recorded by Tektronix oscilloscope Model TDS 3052.
Figure 3 illustrates the laser performance using 30 µs pump pulses (FWHM). The motivation for using such short pulses is to minimize any parasitic effects (such as thermallensing or ionization) that can adversely affect laser performance. An examination of Fig. 3 reveals that a mixture of helium and methane produces the highest laser output compared to pure gases. Conversely, not shown in the Fig. 3, is argon, which did not lase under any conditions. It appears as though ethane, while a very good gas for spin-orbit relaxation, produces higher losses which are likely attributed to quenching from the 4P3/2 state to the ground state. The pure helium case has a significantly reduced performance compared to the mixed case, and it appears that the performance degradation of K DPAL with helium can be explained by ionization, which causes an effective reduction in neutral alkali atoms number density [6]. Also, the pure methane case has lower output than the mixed case, which is likely attributable to quenching. Finally, the mixed cases, which produced the highest output, appears to have sufficient hydrocarbon to eliminate ionization, which occurs with pure helium, but has significantly less quenching losses because of the small amount of methane (≈20 Torr) required to mitigate the ionization.
Fig. 3 Output power of K DPAL pumped by 30 µs pulses using He, ethane and methane buffer gasses at a total pressure of 500 Torr
To illustrate the impact that adding methane has on ionization, we include the Fig. 4, which shows the intensity of the fluorescence from the potassium 52P state for different partial pressures of methane in the mixture with He at a total pressure 500 torr. It is well seen that the 52P level population is significantly higher for the pure helium case providing a direct path for photo ionization. In the cases where methane is added, the magnitude of the fluorescence from the 52P state and, hence, its population, is significantly reduced. This correspondingly reduced the probability for photoionization to occur.
Fig. 4 Fluorescence signal from the potassium 52P states. The population of the 52P state is significantly reduced when methane is added to the buffer gas. Pump pulses duration in this experiment was 250µs
The impact adding methane has on ionization and, thus, on lasing efficiency, is also illustrated in Fig. 5, which shows the intensity of the potassium blue fluorescence line (5P→4S, 405 nm) and lasing power as a function of the partial pressure of methane in the mixture with He at total pressure of 600 torr. Figure 5(a) shows a very significant drop of the fluorescence intensity when adding only a few torr of methane, and the optimal partial pressure (≈20 torr) that provides maximum laser output power is shown in Fig. 5(b).
Fig. 5 Fluorescence intensity from the potassium 52P state (a) and lasing power (b) vs. methane partial pressure (He added to methane at a total pressure of 600 torr). Pump pulses duration in this experiment was 50 ms.
Study of the K DPAL performance using longer pulses allowed us to gain an understanding of how the different gases would behave for continuous wave (CW) operation. Figure 6 shows the lasing pulses with 250 µs pump for pure helium, pure methane and mixed gasses. Argon buffer gas was also tested in this experiment, but it again did not support lasing as was the case with the short pump pulses. Although the 250 µs pulse is long enough to reach a steady state in the laser, it is sufficiently short to minimize any thermal effects which always plague CW studies when using a static cell. The results for the 250 µs duration pulse are similar to the results for the shorter pulses. Again, the mixed gas caseproduces the highest laser output and the output power in the pure methane case is reduced, likely because of quenching. The anomaly is with helium. The output power of the DPAL with helium buffer gas degrades by 60% throughout the entire pulse. This degradation is probably caused by ionization, which effectively reduces the number density of the neutral alkali atoms available to support lasing. This degradation indicates that pure helium should not be used as the buffer gas in a K DPAL unless the flow speed is sufficient to remove the ionized atoms and replenish the neutral alkali atoms to maintain efficiency. Assuming a 1 cm flow path length and a characteristic degradation time (to 1/e level) of ~1x10−4 sec measured in this experiment, then the minimum perpendicular flow speed to eliminate the ionization problem in such a system would be ~100 m/s.
Fig. 6 K DPAL pumped by 250 µs pulses for pure helium, pure methane and mixed helium and methane buffer gasses.
Also, in this experiment we recorded a time dependence of the fluorescence intensity from the potassium 52P states for the Helium and Argon buffer gases at 200 torr (see Fig. 7), which illustrates the difference between these two gases in supporting K ionization. In this figure we can see a distinct difference in time evolution of the potassium 52P fluorescence line between the use of He and Ar. In both cases, there is an initial rapid rise inthe 52P line population due to energy pooling or photoexcitation and relaxation to the 52P level. Again, in both cases, the fluorescence peaks and begins to drop. This decrease in population is a result of direct photoionization from the 52P state by the 766 nm pump. What is interesting is the difference in the rate of decrease for the two cases: for helium, the drop is slow but is very rapid for argon. These differences can likely be attributed to the ion recombination rate for each buffer gas. Finally, both achieve a steady state level with the helium case having a higher steady state fluorescence which again is attributable to ion recombination replenishing the neutral potassium atoms.
Fig. 7 Time resolved pump and the potassium 52P fluorescence pulses using 200 Torr helium (a) and argon (b) buffer gasses.
In order to examine the thermal decay processes for the methane cases we increased the pulse length to 100 ms and the results are shown in Fig. 8. With these longer pulses,heat begins to build up inside the static cell and cause a degradation of the laser performance. Again, the mixed case using 20 Torr methane and 480 Torr helium produced the highest laser output. An examination of the mixed case reveals a 1/e degradation time of approximately 120 ms. If we again assume a flow distance of 1 cm then the flow speed necessary to mitigate this thermal degradation would be approximately 0.1 m/s. This means that by adding only 20 Torr of methane to pure helium decreases the flow speed requirements by three orders of magnitude. This significantly simplifies the design of a K DPAL flowing system.
3. Conclusion
We examined the performance of K DPAL using helium, argon, methane, ethane and a mixture of helium and methane buffer gasses. It appears as though using pure helium as the buffer gas will only support lasing if the flow speed is on the order of 100 m/s. Conversely, if methane is used, the flow requirements are significantly reduced. The ideal buffer gas composition appears to be mixtures of helium sufficient to pressure broaden the absorption line-shape to match the pump source with 20 Torr of methane to significantly reduce excited upper state populations thereby reducing ionization. Finally, we observed that pure argon is an inferior choice of buffer gas in a K DPAL.
Acknowledgement
This work was supported by High Energy Lasers Joint Technology Office under the Project with control number JTO-14-UPR-0525
References and links
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