Power Enhancement of a Rubidium Vapor Laser with a Master Oscillator Power Amplifier

Power enhancement of an alkali laser is achieved using an amplifier.

The concept of alkali lasers was first suggested by Schalow and Townes in the late 1950s. In the 1970s, photo-dissociation of several of the alkali salts produced lasers with wavelengths ranging from the visible to the far infrared. Thirty years later, diode-pumped alkali lasers (DPAL) started rapidly gaining attention as highly efficient lasers as well as brightness converters. These systems partly owe their high efficiencies to the very small energy differences between the pump and lasing levels. Due to recent technological advances in the field of solid-state lasers, direct-diode pumping has provided the efficient, yet compact method for excitation. To date, to increase the power of alkali laser systems, higher intensity pumping of the gain medium has been used. However, there is a critical limit that can be achieved on these smaller scale systems due to decomposition of the gain medium due to excess thermal loading. One way to mitigate this problem is to use an amplifier or an amplifier chain. As demonstrated with solid-state lasers, this is a possible way to increase system power loading while alleviating the thermal management issues of the laser by spreading the heat out over several pieces of gain medium.

A master oscillator power amplifier (MOPA) with variable amplifier gain lengths was built to demonstrate power enhancement of an alkali vapor laser. A maximum intensity of ~25W/cm² of 795-nm oscillator light was used. The maximum extraction efficiency (power extracted/power in) for the amplifier experiments was 14%. This is lower than the 54% optical-to-optical efficiency that was achieved when all the radiation was directed into the oscillator. This result is not unexpected because the intensity of the master oscillator was purposely left low as to not cause saturation of the amplifier.

Power enhancement of a DPAL system utilizing an amplifier has been demonstrated. Relatively high gains with small gain lengths have been achieved. A small signal gain of 0.91/cm for two different gain lengths was observed. For a 2-cm-long amplifier gain length, an amplification of 7.9 dB was observed. More importantly, the gain is not so high as to have parasitics, such as amplified spontaneous emission, become detrimental to laser performance.

This work was done by David A. Hostutler and Wade L. Klennert of the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. AFRL-0148