We have demonstrated continuous wave operation of a laser diode array pumped Rb laser with an output power of 8 Watts. A slope efficiency of 60% and a total optical efficiency of 45% were obtained with a pump power of 18 Watts. This laser can be scaled to higher powers by using multiple laser diode arrays or stacks of arrays.
©2008 Optical Society of America
Alkali-vapor lasers, although among the first kind of laser proposed , have only recently been successfully operated [2–4]. One great advantage of alkali lasers is their potential for high efficiencies and high output powers. A slope efficiency over 80% and overall optical efficiency exceeding 60% were demonstrated  in cesium. Another very important advantage of alkali lasers is their potential to efficiently convert the relatively low beam quality output of diode lasers into a high quality laser beam. Using high power diode lasers for pumping alkali atoms allows a significant increase in the output power of an alkali laser. A diode pumped continuous wave (CW) Cesium vapor laser with 10 W output was demonstrated recently , that is for now the highest output power published for an alkali laser. The best published result for a diode pumped rubidium (Rb) laser is an output peak power about 1 W in pulsed mode (5% duty cycle ). In this work we applied the technology developed in  for the Cs laser to demonstrate an efficient diode pumped CW Rb laser. Using narrowband Laser Diode Array (LDA) with power of 17 W, we obtained efficient lasing in Rb vapor with maximum output power of 8 W and slope efficiency 60%. The overall optical efficiency was 45%.
2. Experiment description
To create a population inversion and gain on the D1 transition (5P 1/2→5S 1/2, 795 nm) in Rb atomic vapor, we used a standard three-level pump scheme suggested by Konefal . A diode laser pump source with a wavelength corresponding to the D2 line (5S 1/2→5P3/2, 780 nm) excited Rb atoms to the 5P3/2 state and a buffer gas (ethane) provided population transfer from the 5P3/2 state to the 5P1/2 state, which is the upper lasing level. We used an LDA with a maximum output power of 18 W operating at 780 nm with an external cavity, similar to described in our previous publications [6, 9]. The LDA radiation linewidth was less than 10 GHz so that it matches the Rb absorption line which was broadened to about 12 GHz by the 600 torr ethane buffer gas (the typical value for pressure broadening is about 20 MHz/torr ).
In this experiment we employed the laser cavity design similar to the one developed in  for Cs laser (see Fig. 1). The 44 cm long L-shape laser cavity consisted of a high reflective 50 cm radius concave back mirror and a flat output coupler. The experimentally determined optimal reflectivity of the output coupler was 11%. The polarizing beam splitting cube was used to separate the pump and lasing beams, which had orthogonal polarizations. A 2 cm long cell with antireflection coated windows was filled with metallic Rb and 600 Torr of ethane and placed in a temperature controlled oven. The Rb cell was positioned near the curved mirror, where the cavity mode size was maximal and varied from 590 to 625 µm FWHM along the cell. The LDA pump beam with a close to rectangular shape (10 mm×40 mm at the focusing lens) was focused into the center of the Rb cell by a lens with a focal length 20 cm. The measured size of the pump beam in the center of the Rb cell was 500 µm×600 µm (FWHM) and it expanded at the cell windows to the size 940 µm×700 µm. An optimal Rb cell temperature depends on several factors, such as cell length, pump radiation linewidth, pump power. In our experiments we determined its value experimentally (see Fig. 2) for the pump power of 17.5 W. The output coupler used in this experiment had a reflectivity 33%. The optimal temperature was in the range 103–106 C (corresponding Rb vapor pressure is 3×10-4 torr ).
The experimental dependence of the Rb laser output power on the pump power was measured for the output couplers with reflectivities 33%, 21% and 11% (see Fig. 3). Corresponding measured values of the slope efficiencies for these output couplers are 40%, 49% and 60%. The maximum output power of 8 Watts was obtained with the 11% output coupler using 17.8 Watts of incident LDA pump power. The overall optical to optical efficiency was 45%. The slope and overall efficiencies obtained in these experiments for the LDA pumped CW Rb laser are slightly lower than the ones, obtained in  for the LDA pumped Cs laser, though they are almost an order of magnitude higher than the previous results obtained for LDA pumped pulsed Rb laser . The efficiency decrease in this experiment compared to Cs laser  might be explained by the mismatch between the pump beam and the laser cavity mode.
The optimal cell temperature that provided the maximum output power was 103–106 C in our experiments, which is about 40 C lower than in previous experiments (about 150 C) . The temperature optimizes at lower level for longer cell length (2 cm in our case, compared to 2.25 mm in ) and for narrower pump radiation line (10 GHz in our case, compared to 120 GHz in ). The lower cell operation temperature is much preferable because at higher temperatures there is a high probability of the cell contamination by products of chemical reaction between the alkali atoms and a buffer gas [6, 12]. In our experiments we didn’t perform the laser lifetime studies, but, during several hours of operation at highest power we didn’t observe any output power decrease and window contamination.
Alkali lasers, as it was mentioned above, can be considered as efficient converters of spatially incoherent multimode radiation of laser diode arrays into highly coherent laser light providing significant radiance enhancement (see also [6, 7]). We have measured a beam quality of he developed Rb laser using a ModeMaster M2 Beam Propagation Analyzer (Coherent). The propagation factor measured at a 0.5 m distance from the output coupler was M2=1.1 that is very close to a TEM00 Gaussian beam value.
In conclusion, we have demonstrated a diode array pumped continuous wave Rb vapor laser with an 8 W output power. This is about an order of magnitude lager than previously demonstrated and shows that a rubidium laser has similar performance to a cesium laser. Using more powerful diode laser arrays or multiple arrays for pumping alkali vapors promises the possibility of scaling these lasers to higher powers.
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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