Progress in the development phases of a new optical material that exhibits very low thermal lensing and robustness against thermal shock will be reported. Material, thermal, and optical properties of the current material formulation, called OFG-04, have been determined on small- and mid-scale samples. Manufacture scale-up to full-scale has been completed and flight windows prepared. Follow-on efforts are now beginning for an optimized formulation that will exhibit even lower bulk absorptance and OPD change with temperature rise.
We begin with a brief review of prior work relating to optical windows for use with high power laser beams. A typical window must provide pressure separation between system segments, ultra-low loss, and small wavefront distortion of the many outgoing laser beams and signal returns despite heating by the high energy laser beam. Historically, two approaches have been examined to improve such windows.
Using conventional materials like fused silica and sapphire for critical window components in a high-power laser system can lead to intolerable thermal distortions and optical path difference effects. A new oxyfluoride glass is being developed which has the unique property of possessing a negative thermo-optic coefficient (dn/dT) in the near- and mid-wave infrared. Specifically, the refractive index (n) of oxyfluoride glass decreases as the temperature increases. The distortions caused by thermal expansion of the glass during laser irradiation are partly offset by the negative dn/dT. This paper specifically addresses optical properties and surface finishing of oxyfluoride glass compared to fused silica. Normarski micrographs and surface profiles were measured to inspect the surface quality since smooth surfaces are essential for suppressing surface scattering and absorption. The refractive index and thermo-optic coefficient were measured using null polarimetry near the Brewster angle. Low dn/dT is required for laser windows. Transmittance spectra were measured to deduce the extinction coefficient by comparing with the transmittance calculated from the refractive index and to screen for unwanted absorption from contaminants including hydrocarbon oils, polishing residue, and water or -OH groups. Total integrated scattering was measured for both surface and bulk scattering. All measurements were done on 1.0- and 1.5-inch-diameter witness samples.
One of the key elements of the Airborne Laser beam control system is its wavefront control subsystem. This subsystem provides compensation of local and atmospheric wavefront disturbances, which is essential in accomplishing the ABL mission. A critical performance driver for this subsystem is deformable mirror-to-wavefront sensor registration. To reduce overall sensitivity to misregistration and provide optimum wavefront control performance, the ABL system includes a built-in calibration system. This system is used in pre-flight checkout as well as during an automated calibration and alignment sequence conducted in-flight. The calibration system provides the ability to perform poke calibrations to update the estimation matrix. This paper provides a description of the methodology used for calibration of the ABL wavefront control subsystem and discusses the key features that provide this function.
An account of the recently completed, 1-1/2 year ABL risk reduction deformable mirror (DM) program will be given. In this effort, candidate sub-scale deformable mirrors (SSDM) supplied by tow vendors were tested for manufacture and performance quality. The down-selected mirror was then provided with a newly-developed, high performance, multiple wavelength, low stress, very low absorptance faceplate optical coating. The SSDM was then operated for cycle number and stroke, and under high laser flux, that exceeds the operational requirements of the ABL DMs. The successful results for these tests, carried out with the contributions of a multifaceted integrated product team, will be reported.
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.
In the conventional approach to active tracking, the target is illuminated by an illuminator laser and the backscatter is collected to produce an image for the tracker. Atmospheric turbulence, especially when it is distributed over the entire propagation path, produces intensity scintillation of the illuminator laser beam. This scintillation reduces the uniformity of target illumination and degrades tracker performance. With multibeam laser illumination the single illuminator is replaced by several, mutually incoherent illuminator beams. The multibeam approach produces a more uniform target image and improves tracker performance. In this paper, we describe the design of a multibeam illuminator capable of producing up to nine beams. We discuss characterization test performed across the 5.4-km propagation range at the Lincoln Laboratory Firepond facility.
The ABL Beam Control design incorporates Common Path/Common Mode (CP/CM) architecture because it provides many important operational features, without which the system would have difficulty performing well in the stressing military environment. These features include 1) tolerance for optical misalignment, 2) elimination of the need for boresighting, 3) graceful degradation, 4) correct accounting for all optical path disturbances, 5) relaxed tolerances of optics, and 6) compensation for thermal disturbances. The concept for CP/CM operation, both in wavefront compensation and in automatic self-alignment and pointing, will be given. Experimental and simulation data will demonstrate the effectiveness of the CP/CM ABL approach.
We have conducted active-tracking experiments in support of the Air Force's Airborne Laser program. These tests were conducted using the 5.4-km horizontal propagation range at the Lincoln Laboratory Firepond facility. Target illumination was accomplished using a multibeam illuminator laser provided by Lockheed Martin Missiles & Space; the tracking was performed using an imaging tracker provided by the Air Force Research Laboratory. Experiments were conducted using a number of different illuminator configurations, and with a variety of track algorithms. Results from these tests have quantified the performance benefits of multibeam illumination.
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