We have demonstrated a Cs vapor laser with an unstable resonator transversely pumped by 15 narrowband laser diode arrays. A slope efficiency of 43%, a total optical efficiency of 31% and a maximum output power 49 W were obtained with a pump power of 157 Watts.
©2009 Optical Society of America
Alkali vapor lasers have been under intensive development during the last several years because of their remarkable properties (gas active medium with high gain, high output power, possibility of diode laser pumping), making them good candidates for numerous applications. A small signal gain of 2.5 cm−1 was demonstrated in Cs vapor , which is higher than most solid state gain laser medium. This allows many well developed techniques and designs employed in high power solid state laser systems to be used in an alkali vapor laser system. One such technique, transverse pumping, was successfully implemented for a Cs vapor laser pumped by 15 laser diode arrays , but the optical-to-optical pumping efficiency in that experiment was only 14%, which is less than that for longitudinally pumped alkali lasers [3,4]. One reason for this discrepancy is a significant mismatch between the size of the pumped gain medium (equal to the cell size) and the smaller cavity mode size in the laser using a stable resonator operating on lowest order (fundamental) transverse mode (TEM00). Laser operation in lowest order spatial mode is important for various applications that require minimal beam divergence. Increasing the laser cavity mode volume while operating in a fundamental transverse mode is possible by using an unstable laser resonator . In this paper we present the results of our experiments on transversely pumped Cs laser with unstable resonator.
2. Experiment description
The Cs laser design is presented in Fig. 1 . The 50 mm long Cs vapor cell with an internal diameter 7 mm was filled with metallic Cs and 500 torr of ethane buffer gas at room temperature before being sealed. The cell windows had antireflection coatings on both internal and external surfaces. The cell was assembled inside a cylindrical white diffuse reflector (Labsphere), which was placed inside a temperature controlled oven. The oven and the reflector had a 2 mm x 50 mm side slit parallel to the cell axis, which was used for coupling the diode lasers pump beams into the gain medium through the side of the Cs cell. The temperature of the Cs cell was kept at about 90° C, while the cell windows had a 5° C higher temperature to protect them from condensation of metallic Cs.
To pump the gain medium we used 15 pump beams from Laser Diode Arrays (LDAs) with external cavities, which provided line narrowing of the LDAs output to a value less than 10 GHz . All fifteen LDAs were connected electrically in series and powered from one power supply. To avoid thermal effects in the gain medium, previously described in , that can reduce the laser efficiency, we used pulsed pumping of the LDAs (and, hence, the Cs vapor) with 500 μs pulses at repetition frequency of 20 Hz (duty rate 1%). All pump beams were delivered from the LDAs and coupled into the cell through the side slit with a total loss of power of about 25%. Maximum pump power coupled into the cell was 157 W. It should be noted that throughout this text, when laser power is discussed, it is the average power during the duration of the pulse (i.e. pulse energy over the pulse duration) which is being discussed. It is worth to note that the Cs laser operation with pumping by 500 μs pulses is similar to the continuous wave (CW) pumping, when the thermal effects are eliminated, because the laser turn on/off time is less than 1μs (see Fig. 3 discussed below). This method of laser pumping allows for a proof of principle of a CW Cs laser system operation where thermal contributions are better managed and hence, the efficiencies obtained in these experiments can show possible efficiencies of the CW pumped Cs laser not affected by thermal effects.
The unstable confocal laser resonator was created by two 100% reflecting 895 nm mirrors. A 2.5 cm diameter concave mirror with 50 cm radius of curvature was placed at a distance of 15 cm from the 2.8 mm diameter convex mirror with radius of curvature 20 cm. The Cs vapor cell was placed near the concave mirror where the laser beams propagating in the cavity in both directions have maximum size. The output laser beam is irradiated over the edge of the convex mirror and has a doughnut shape cross section at the output mirror with external diameter about 7 mm (the Cs cell internal diameter) and the hole diameter about 2.8 mm (convex mirror diameter).
3. Results and discussion
Figure 2 shows the experimental dependence of the Cs laser output power on total diode lasers pump power coupled into the gain medium. A linear fit to the data points gives the slope efficiency of 43%. The maximum optical-to-optical efficiency optained at 157 W pump power was 31% and the maximum output power was 49 W. The efficiencies achieved in this experiment are more than twice higher than in our previous experiment utilizing transversely pumped Cs laser with a stable resonator , but they are still lower than those for longitudinal pumping (81% for Ti:Sapphire pump  and 68% for diode laser pump ). This difference can be due to nonhomogenious pumping of the whole volume of the gain medum through the narrow slit. This assumption is supported by the observed output beam profile, which had elliptical doughnut shape instead of circular. In addition, there can be pump power losses because of multiple reflections of pump beams off the diffuse reflector. Improvements in the design of the transverse pumping system can result in an increase of the laser pumping efficiency.
As we mentioned above, the thermal effects in the gain medium can reduce the laser efficiency, especially in continuous wave operation at high power levels. Figure 3 shows the time dependence of the lasing pulses power during the pump pulse obtained for the Cs laser in our experiments. This figure shows thermal effects in the Cs laser output power. It is well seen that the output laser power decreases about 20% during the pump pulse duration of 500 μs, while the pump pulse amplitude is constant. An exponential fit to the output power pulse (dashed line on Fig. 3) gives us a decay time of about 26 msec. This decay time will depend on the system design and pump power level. These thermal effects were also observed in a longetudainally pumped Cs vapor laser  where at CW pump powers of greater than 30W, the laser efficiency decreased due to thermal lensing. The thermal effects can be eliminated or significantly reduced by flowing of the gas gain medium as it is usually done in high power gas lasers (see, for example , ).
In conclusion, we have demonstrated a Cs vapor laser transversely pumped by fifteen narrowband diode laser arrays. Using an unstable resonator allowed for an increase in the slope efficiency of the laser up to 43%, more than twice that of the same laser with a stable cavity. Additional improvements in design of the transverse pumping system can increase the efficiency of such a laser.
We acknowledge support of the Air Force Office of Scientific Research, the Joint Technology Office for High Energy Lasers, and the National Science Foundation.
References and links
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