The ABL Lockheed Martin has prepared and validated a highly versatile adaptive optics testbed to simulate in an accurately scaled fashion all aspects of ABL laser beam propagation, including atmospheric compensation and pointing and tracking in selected atmospheres. This system allows repetitive, highly controlled, and well diagnosed experiments to be carried out that are generally impossible to do in field test where the user has little control over atmospheric and other test conditions. Testing of beam control hardware including components, assemblies, control loops and software, as well as development of methodology such as alignment and sensor techniques, determinations of system operational robustness, and finally, measurement of overall system performance under various atmospheric or other propagation and seeing conditions are routinely done. This presentation will discuss 1) the system scaling chosen to preserve diffraction, turbulence and temporal fidelity to ABL, 2) agreement of experiment results to those of other laser propagation experiments and wave optic code simulations, and 3) experiments that have demonstrated ABL beam control system robustness, compensation for jitter and turbulence, and overall performance when operating in atmospheric turbulence that emulates that measured in the real-world theater.
The Airborne Laser (ABL) program requires a large aperture, highly transparent window to allow the high energy laser beam to be focused on targets. This window presents many challenges as it is thin, large in diameter and very highly curved. Additionally, the window must be made from a material highly transparent at 1.315 micrometers, the chemical oxygen-iodine laser wavelength, have good transmission from the visible through 3 micrometers and be able to withstand the rigors of operations on a tactical aircraft. To manufacture this window, a unique partnership between two companies, Heraeus and Corning, was forged to demonstrate the process and manufacture the window blanks. Infrasil 302, a Heraeus product, is the only material with low absorption at 1.31 micrometers that can be produced in large enough quantities to make a window blank of the required size. Corning has developed the technology to flow- out and sag glass products to make highly curved optics without the need to machine them out of a cylindrical block. Using their experience and a common desire to support the ABL program, the two companies worked together to develop the processes that produce the window blanks. Contraves Brashear Systems of Pittsburgh will polish the blank in to its final form, with coatings applied by Optical Coating Laboratories, Inc. of Santa Rosa to maximize transmission.
The U.S. Air Force Airborne Laser consists of four primary subsystem segments; aircraft, battle management and C4I, laser device, and beam control segment (BCS). The BCS performs two major function, beam control and fire control, using three primary products, turret assembly, beam transfer assembly, and fire control hardware and software. The fire control sequence involves slewing the turret to the target coordinates as received from the battle management segment, centering the target in the acquisition sensor, acquiring the plume in the coarse track sensor, acquiring the hard body of the missile with the track illuminator laser, and establishing active tracking of the nose of the target theater ballistic missile. The beam control sequence begins after established active nose track by firing another beacon illuminator laser (BILL) to establish the aim point on the missile for the high energy laser. The resulting spot on the missile is imaged in the wavefront sensor and compared with the outgoing sample of the BILL. By applying the conjugate of the wavefront difference from the beacon to a deformable mirror then the atmosphere serves to correct the arriving wavefront on target. At this point the HEL is fired along the same path with similar wavefront correction, and dwells on target until the missile skin is weakened and rips open or buckles.
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