In recent years we have seen a growing number of designs for lasers based on alkali vapors. Their use as a laser medium has long appealed to researchers because they offer several desirable features compared to other media, such as solid-state and fiber. For example, the quantum efficiency can be high—95.3% for cesium, 98.1% for rubidium, and 99.6% for potassium—compared to 76% for a 1.06 μm Nd YAG laser. This is very important, not only for increasing the overall laser efficiency, but also for minimizing heating problems. The gas gain medium also has excellent optical quality that can yield a beam with diffraction-limited divergence.
An optically pumped potassium-vapor laser was first proposed in 1958, by Schawlow and Townes.1 Laser action at 7.18μm in Cs vapor was observed in 1962 using an RF-powered helium lamp as a pump,2 though the output power did not exceed 50μW. Considerably more efficient lasing has been observed in Rb and Cs vapors using Ar+-pumped dye lasers3 and Ti:sapphire lasers4,5 for optical pumping.
We have recently demonstrated high-efficiency Cs lasers6 and diode-pumped Cs lasers,7 and have achieved a maximum slope efficiency of 81% and 63% overall optical efficiency for a Cs laser operating in continuous-wave mode.6 We have also demonstrated a Ti:sapphire-pumped potassium-vapor laser operating in single transverse and longitudinal modes.8Figure 1 shows a potassium laser beam-intensity profile of the transverse TEM00 mode, together with an inset image of the beam. Figure 2 plots the spectrum of the single longitudinal mode, determined with a 25GHz free-spectral-range scanning interferometer. The measured line width is less than both the 900MHz laser-cavity mode spacing and the 500MHz instrument-limited resolution, so the data show lasing in a single longitudinal mode.
Figure 1. This intensity profile scan of a potassium laser output beam shows the TEM00 mode (inset) operation. Intensity is given in arbitrary units.
Figure 2. The spectrum of the K laser output is narrower than the scanning interferometer's 500MHz resolution, and the 900MHz laser cavity mode spacing, indicating the presence of a single mode. The instrument's free spectral range is 25GHz.
The use of high-power and efficient diode lasers and laser diode arrays to optically pump alkali lasers has opened a new avenue of high-power laser development. While the beam quality of these diode-based lasers is very poor, the alkali medium acts as a beam combiner that creates a high-quality, spectrally-and-spatially-coherent laser beam that is ideal for directed-energy applications.
We are pursuing further work on developing narrowband diode pump sources for alkali lasers. Although pumping with off-the-shelf models is possible,9 it is preferable to use a technique that provides narrow linewidths (see, for example, Reference10), because it allows alkali laser operations at lower temperatures and buffer-gas pressures. This, in turn, significantly decreases the rate of chemical reactions between alkali atoms and buffer gases. Narrowband output could be attained with minimal power loss by exploiting diode-laser sensitivity to optical feedback, using a device such as a grating, a hologram, or an optical cavity.