Mid-IR Spectral Beam Combining Using Volume Gratings
ABSTRACT: Daylight Solutions proposes to develop volume Bragg gratings for the spectral beam combining of lasers in the mid infrared. High performance volume gratings exist for the visible and near infrared regions of the optical spectrum and are most easily produced by holographic methods. However, volume gratings do not currently exist as commercially available components due to a scarcity of holographic materials in the mid infrared. Daylight Solutions will evaluate the holographic potential of chalcogenide glass layers produced by inexpensive and straightforward methods. The fabrication of volume gratings will enable the demonstration of mid infrared laser combining and show the ability of these components to efficiently combine wavelengths that are close together as well as far apart in the 3 to 8 um range. BENEFIT: Military aircraft face an increasingly widespread threat from heat seeking missiles. These missiles can see and track the heat signatures of aircraft exhaust. Defeating the missiles involves confusing their tracking system with laser beams emulating the emission spectrum of the exhaust. Spectrally combining the signature wavelengths offers infrared counter measure systems a single high power and high quality laser beam to work with. This single spatial mode beam enables system architectures that side by side lasers cannot. Examples include easier beam shaping with refractive or reflective optics within a system or fiber delivery to various parts of the aircraft of a system"s output. Volume gratings operating in the mid infrared bring a high level of modularity for spectral beam combining systems not achievable with the conventional spectral combining elements of dielectric thin films and surface gratings. The practical number of layers realizable for thin films limits the minimum wavelength separation between combined beams. Surface gratings most efficiently combine lasers close in wavelength but cannot, in practice, be designed to efficiently combine wavelengths far part. With volume gratings, a single system layout can accommodate any collection of wavelengths by simply swapping out volume gratings and sources. This translates into modular, less costly combining systems. The development of holographic material to produce volume gratings will make the same material available for mid infrared holographic optical elements in general. Holograms can, for example, replace multiple lens systems, produce angle filter components, or superimpose multiple combining gratings in a material with enough dynamic range. As with their visible and near infrared counter parts, these holographic optical elements can drastically reduce the cost and size of systems built with their bulk component equivalents. With the emergence of laser sources filling the spectroscopy rich, mid infrared spectrum, these holographic optical elements will find widespread commercial uses.
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