Adaptive Optics Correction

Short Wave Infrared (SWIR) Cameras for Adaptive Optics Correction of Atmospheric Turbulence

Imaging through the atmosphere over very long distances is difficult due to temperature variations that, in turn, change the air density along the optical path. This results in the bending of the light rays, creating optical distortion like that seen when looking down a long road on a hot day. Adaptive optics systems correct these distortions by using one high frame rate camera behind a Shack-Hartmann optic to detect and measure the distortion. The system uses that information to drive MEMS deformable mirrors to compensate for that level of distortion and a second detector to capture the final corrected image.

When the range of visible surveillance telescopes is limited by distortion such as haze, shifting to short wave infrared (SWIR) cameras enables "seeing" through that haze. For really long distances, adding adaptive optics increases the range to even longer distances than achieved solely with a short wave IR camera. In addition, some applications require imaging in the SWIR and use InGaAs cameras with the Shack-Hartmann optics to correct for distortion. For instance, Chemical Oxygen Iodine Lasers (COIL) are used for targeting missiles at long ranges, and SWIR adaptive optical cameras are being used to image the target at the laser's wavelength, even in hazy or foggy conditions.

In addition to limiting the ability to image lasers at long distances, e.g. 10 km, atmospheric turbulence also limits the reliability and the maximum data transfer rates of free space optical communications systems (FSO). Longer eye-safe laser wavelengths and adaptive optics are being used to correct for these problems. For mobile point-to-point FSO systems, where the receiving system has to locate the transmitting laser and focus the beam on the receiving detector, Sensors Unlimited's InGaAs windowing cameras have the field of view and resolution necessary to locate the source. By then windowing down to image just a small region of interest, such as 16 x 16 pixels, frame rates of over 15,000 fps permit fast control loops to keep the light beam centered on the receiver despite movement of both the source and receiving platforms.

The Lick Observatory uses adaptive optics to correct for distortion caused by atmospheric turbulence (see image above).

Many SWIR camera capabilities are considered sensitive. If you would like to learn more about SWIR capabilities for the military, you can register at a special web site, www.swirconops.com. You must be a US Government employee or a USG contractor and be a US Citizen to access this site.

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