Beam-control systems for high-average-power lasers began in the late 1960s and early 1970s. Early systems propagated the beams across laboratories using heavy-water-cooled copper optics and open-beam trains with commercial fans to provide fresh air. They have evolved in the intervening 40-plus years to include highly sophisticated gimbaled-control systems with extremely high-reflectance uncooled optics and adaptive optics to compensate for less-than-ideal laser beams and for atmospheric distortions. An overview of that evolution is presented.
Inertial Reference Units (IRUs) are the basic reference for a precision pointing system. These units must provide an
inertially stable light source to be used as the reference to align the outgoing laser beam and to reject beam train jitter
due to vibrations. The IRU will be subjected to 6 degrees of freedom motion during operation. The correct operation of
an IRU requires it to measure the angular motion and not be affected by the linear input vibration. Testing of these units
is difficult, since the vibration input motion may be perfectly correlated between the angular inputs and the linear inputs.
This correlation makes it impossible to separate the angular and linear IRU responses during a test, even with perfect
measurements of the input vibrations. The solution to this problem is to obtain a vibration test station that can provide
linear motion without any angular motion, and angular motion without linear motion. This paper will describe the
evaluation of the test tables and show test data from an IRU that indicates how these tests can be beneficial in identifying
performance problems.
There are a number of optical systems being considered for aircraft. These include communication systems, surveillance systems, and other pointing and tracking systems. The angular vibration of the vehicle is usually the primary disturbance that degrades the optical system performance. Boeing is involved in a project to measure and analyze the angular vibration of several different aircraft. This report will show a comparison of the angular vibration spectrums of large transport aircraft and smaller fighter aircraft over a range of flight conditions.
In last years proceedings, the above authors reported a basic limitation on the maximum effective bandwidth when tracking through atmospheric turbulence. This limitation, called the optical frequency, was shown to be an upper limit on tilt detection. This paper will further expand on this fundamental limitation. Further testing at the MIT/Lincoln Laboratory has provided more insight into the optical frequency as well as other tracking limitations. It will be shown in this paper that scintillation appears to be dominant above the optical frequency and that by wisely selecting the bandwidth of the tracking system, one can exclude some of the noise of scintillation, while still performing the best possible tracking.
Tracking through a turbulent atmosphere poses several challenging problems. The authors have recently conducted a series of tracking tests at a MIT/Lincoln Laboratories facility where a complete tracking and adaptive optics system is available in a laboratory. The atmosphere is simulated using seven precision rotating phase screens. A great deal has been learned about tracking algorithms and their response under a scintillated atmosphere. Data will be shown to describe a key limitation to high bandwidth tracking. This effect, called the `Optical Frequency', appears to be an upper bound on track bandwidth when using an image based tracking system.
Tracking through a turbulent atmosphere has several challenges. If the target is extended, such as a large target being illuminated with laser energy from the tracking aperture, the problems of scintillation and anisoplanatism cause significant concerns. The 'blotches' caused by scintillation can be interpreted by the tracking algorithms as tilt and incorrectly applied to the steering mirror. Similarly, anisoplanatism imparts tilt components from separated points, that may be independent, and not act as coherent tilt that is possible to correct with one mirror. The authors have been involved in a study that compares the performance of 14 different tracking algorithms under scintillated conditions. Image data, available from Lincoln Laboratory, has been used to calculate tilt from the different algorithms and do comparisons using Power Spectral Density analysis. The results show that different algorithms have significantly different performance characteristics.
The problem of tracking of a ballistic missile during the boost phase is very challenging and is of high interest to the Airborne Laser Program (ABL). One of the components of accomplishing missile tracking is the ability to compensate for the tilt induced by the atmosphere. The ABL technology program's airborne laser extended atmospheric characterization experiment project included the task of measuring the atmospheric tilt. This effort was accomplished by propagating an optical beacon from one aircraft and receiving it at a second aircraft using a quad cell detector. This paper will describe how the Air Force obtained the atmospheric tilt calculations from quad cell data that was corrupted with noise. A comparison to ray optics tilt simulations was sued throughout to understand the data and evaluate the noise sources. The result is a comparison of simulated data and measured data with explanations of observed differences.
The problem of active tracking of a ballistic missile during the boost phase is a very challenging task. The Airborne Laser (ABL) program is interested in this problem in that the ABL may use this technique. The Phillips Laboratory in response to this technical requirement embarked on a project to verify the feasibility of active tracking over a horizontal path through the atmosphere. One of the main efforts in this project was to demonstrate active tracking from a ground site, using the Army's Sea Lite Beam Director at the High Energy Laser System Test Facility on White Sands Missile Range. The project proved very successful, both in verifying technical predictions and in gaining hardware experience in accomplishing the active tracking tasks. This paper will present some of the data and review the results of the active tracking missions. The problems encountered included the power of the illuminator beam, the scintillation on the image of the target, noise due to the imaging camera, and jitter in the atmosphere.
This paper discusses the application of MOEM technology to adaptive optics. An experiment is described in which a micromachined mirror array is used in a closed loop adaptive optic demonstration. An interferometer wavefront sensor is used for wavefront sensing. Parallel analog electronics are used for the wavefront reconstruction. Parallel operational amplifiers are used to drive the micromirrors. The actuators utilize a novel silicon design developed by SY Technology, Inc. The actuators have a measured frequency response of 15kHz, and a maximum usable stroke of 4 microns. The entire adaptive optic demonstration has a bandwidth exceeding 10kHz. Measured performance is described. The experiments conducted are designed to explore the feasibility of creating a single chip adaptive optic system, also described in this paper. This chip would combine all on a single VLSI chip aspects of a complete adaptive optics system, wavefront sensing, wavefront reconstruction, and wavefront correction. The wavefront sensing would be accomplished with a novel compact shearing interferometer design. The analog refractive and diffractive micro optics will be fabricated using a new single step analog mask technology. The reconstruction circuit would use an analog resistive grid solver. The resistive grid would be fabricated in polysilicon. The drive circuits and micromirror actuators would use standard CMOS silicon fabrication methods.
Active tracking of a ballistic missile during the boost phase is a very challenging problem. The airborne laser (ABL) is one of several directed energy weapon programs that is interested in active track since the ABL design may use this technique. The Phillips Laboratory in response to this technical challenge has embarked on a project to verify the feasibility of active tracking over a long horizontal path through the atmosphere. The project is composed of two independent phases. The first phase is investigating tracking through a turbulent atmosphere using a scaled range with a static target at Lincoln Laboratory. The second phase of the project will demonstrate active tracking of boosting theater ballistic missiles using the SeaLite Beam Director at the High Energy Laser System Test Facility at White Sands Missile Range. This paper will present some of the tracking data and review the progress of the tests at both sites.
A high priority Air Force interest is to understand the tilt (or jitter) induced by atmospheric turbulence on a laser beam propagated over a high altitude long horizontal path. This will aide in the design of the pointing and tracking systems such as used for communication systems and laser beam propagation. The Phillips Laboratory has undertaken a technology program to simulate, measure, and evaluate the implications of this high altitude tilt. This paper includes 3 related efforts; a simulation that predicts high altitude tilt using present knowledge of the atmosphere, a flight measurements program that is gathering data at high altitude, and a simulation effort to understand the implications on a pointing and tracking system of this high altitude tilt. The simulation is a ray trace code that has been developed at the Phillips Laboratory over the past 15 years and has been anchored using available data. Until recently this simulation had not include high altitude horizontal propagation data. However, the Phillips Laboratory has recently completed a high altitude measurement of scintillation and a temperature probe measurement effort to measure the atmospheric structure. This information has been included in the simulations. A controls simulation is used to evaluate the requirements for bandwidths and controls design and utilizes the disturbance data produced by the atmospheric simulation.
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