Low absorption magnesium aluminate spinel windows for high energy laser applications
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Michael Hunt University Research Foundation, Greenbelt, Maryland 20770, USA
Bryan Sadowski, Fritz Miklos, and Ishwar Aggarwal Sotera Defense Solutions, Inc., Maryland 20701, USA (Received 25 April 2014; accepted 9 July 2014)
High energy laser (HEL) systems are currently being evaluated for various land, sea, and air based platforms. Some of these systems operate in or have to withstand harsh environment of sand storm, hurricane, and rain. The exit aperture on a HEL system operating in harsh environment can become the single point of failure. Current HEL systems operating in 1–2 lm wavelength use fused silica windows which are at risk of damage in the theater. Rugged window materials such as sapphire, ALON, and spinel are currently being evaluated as a potential replacement. One of the major parameters in window selection apart from its ruggedness is its absorption loss coefficient at laser wavelength. This paper reports on 3 different methods to reduce absorption loss in spinel ceramic from 100,000 ppm/cm down to 75 ppm/cm. The results are compared with ALON and sapphire.
I. INTRODUCTION
One of the most important properties of a high-energy laser (HEL) exit aperture (windows) is the absorption coefficient at the laser wavelength of interest. Absorption causes a cascade effect that negatively impacts the emitted laser power. When a high-energy beam transmits through a window material, a part of the laser energy is absorbed (usually by impurity elements that form absorption centers) and causes the window to heat. Heating, in turn, causes changes in the refractive index (n) of the window as a function of the material’s thermo-optic coefficient (dn/dT). These changes result in beam distortion and loss of output power as measured by the optical path distortion (OPD) and have a severe impact on system performance.1 In addition to low absorption, the window should also be strong and environmentally rugged. Higher strength enables thinner windows which reduces the absorption loss, improves OPD, and decreases the window weight. Environmental ruggedness prevents reactions with pollutants and sea spray. As with all systems, window material selection is a compromise. Currently, HEL systems operating in the short wavelength infrared (SWIR, 1–2 lm) region use fused silica and oxyfluoride glasses (OFG).2 Although these a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.184 2266
J. Mater. Res., Vol. 29, No. 19, Oct 14, 2014
http://journals.cambridge.org
Downloaded: 13 Mar 2015
materials have very low absorption, they are fairly weak. New options for HEL windows include transparent ceramic materials such as sapphire, ALON, and magnesium aluminate spinel, usually referred to as simply ‘spinel’. These materials are considerably stronger than glass but have higher absorption and higher dn/dT.3–8 Spinel ceramic windows are typically fabricated from commercial powder by various densification methods such as vacuum sintering and hot pressing, with the addition of LiF sinterin
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