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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

Topics:
Amplifiers Electronic equipment Hazardous materials Hazards and emergency operations Lasers Optics Photonics Radiation Thermal management Thermodynamics
This Brief includes a Technical Support Package (TSP).
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Power Enhancement of a Rubidium Vapor Laser with a Master Oscillator Power Amplifier

(reference AFRL-0148) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the April, 2010 issue of Defense Tech Briefs Magazine (Vol. 4 No. 2).

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    Overview

    The document titled "Power Enhancement of a Rubidium Vapor Laser with a Master Oscillator Power Amplifier" by David A. Hostutler and Wade L. Klennert presents research on improving the power output of rubidium (Rb) vapor lasers using a Master Oscillator Power Amplifier (MOPA) configuration. Conducted by the Air Force Research Laboratory, the study aims to enhance the efficiency and performance of diode-pumped alkali lasers (DPALs), which are known for their high radiance and potential applications in directed energy systems.

    The paper begins by discussing the background of DPAL technology, highlighting previous advancements in the field, including the use of multimode diode arrays and the development of Rb and cesium (Cs) lasers. It notes that while higher intensity pumping can increase power, it also poses challenges such as thermal loading and gain medium decomposition. To address these issues, the authors propose the use of an amplifier or amplifier chain to distribute thermal load and enhance overall system power.

    The experimental setup involved a Ti:Sapphire laser that provided the necessary pump light for the Rb vapor laser. The authors describe the configuration of the oscillator and amplifier, detailing the use of a curved high reflector and a flat output coupler to create an effective lasing environment. The gain medium consisted of a vapor cell containing Rb metal and ethane, with the number density of Rb controlled by temperature.

    Results from the experiments demonstrated a small signal gain of 0.91 cm⁻¹ for two different amplifier gain lengths, with a notable amplification of 7.9 dB observed for a 2 cm long gain length. The experiments also measured the intensity of the output light, showing that the MOPA configuration successfully enhanced the power output of the Rb vapor laser.

    The conclusions drawn from the study emphasize the effectiveness of the MOPA approach in achieving significant power enhancement without introducing detrimental effects such as amplified spontaneous emission. The authors acknowledge the support received during the research and highlight the potential implications of their findings for future developments in laser technology.

    Overall, this document contributes valuable insights into the optimization of alkali vapor lasers, paving the way for advancements in high-energy laser systems and their applications in various fields.

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