We demonstrate, for the first time to our knowledge, successful beam control of a fiber optic phased array containing a large number of polarization maintaining fibers. As many as forty-eight fibers have been coherently combined via individual all-fiber phase modulators. The residual phase error is less than 1/30th of a wave. Results with both near-field interferometric control and target-in-the-loop control have been obtained. Experimental results are compared with numerical simulations and excellent agreement has been achieved. We investigated propagation of this phased array output through a turbulent atmosphere, and used the all-fiber phase modulators for the compensation of turbulence effects on the array output. This work paves the way towards scaling such fiber optic phased arrays to very high fiber count. Eventually thousand of fibers can be controlled via such a scheme.
We report experimental results that demonstrate compensation of extended turbulence and thermal-blooming of high-energy lasers using target-in-the-loop techniques in a scaled laboratory environment. For these experiments the deformable mirror figure was controlled by an algorithm designed to maximize the target-plane intensity as measured by a camera at the transmitter. Results using this TIL configuration were compared under identical conditions to results obtained under control of a Hartmann wavefront sensor and least-squares reconstructor. Experiments were performed for a variety of propagation scenarios anticipated for tactical HEL applications and in all cases the TIL system was seen to outperform the conventional Hartmann-driven adaptive-optics system. We will discuss the details of the the target-in-the-loop algorithm, the laboratory configuration, and the experimental results.
Atmospheric turbulence and laser-induced thermal blooming effects can degrade the beam quality of a high-energy laser (HEL) weapon, and ultimately limit the amount of energy deliverable to a target. Lincoln Laboratory has built a thermal blooming laboratory capable of emulating atmospheric thermal blooming and turbulence effects for tactical HEL systems. The HEL weapon emulation hardware includes an adaptive optics beam delivery system, which utilizes a Shack-Hartman wavefront sensor and a 349 actuator deformable mirror. For this experiment, the laboratory was configured to emulate an engagement scenario consisting of sea skimming target approaching directly toward the HEL weapon at a range of 10km. The weapon utilizes a 1.5m aperture and radiates at a 1.62 micron wavelength. An adaptive optics reference beam was provided as either a point source located at the target (cooperative) or a projected point source reflected from the target (uncooperative). Performance of the adaptive optics system was then compared between reference sources. Results show that, for operating conditions with a thermal blooming distortion number of 75 and weak turbulence (Rytov of 0.02 and D/ro of 3), cooperative beacon AO correction experiences Phase Compensation Instability, resulting in lower performance than a simple, open-loop condition. The uncooperative beacon resulted in slightly better performance than the open-loop condition.
In this paper we present an overview of the laboratory configuration and provide details of the adaptive-optics and tracking hardware. Experimental results obtained using a variety of propagation scenarios are presented and compared with results from wave-optics simulations. In addition, we present results illustrating the impact of increasing beacon size and active illumination on system performance.
We have constructed a target-board platform to provide adaptive-optics and tracking performance characterization for the Airborne Laser Advanced Concepts Testbed program. The target board comprises 1536 discrete sensors distributed over a 1-meter by 2.5-meter array mounted to the side of a specially modified Cessna Caravan aircraft. The aircraft platform includes multiple beacon sources for adaptive- optics and tracking, a large-capacity data-recording system and a real-time telemetry ground-link for data display. In this paper we provide an overview of the target-board platform. We discuss the results of requirements analysis for target-board detector configuration, and describe the detailed design and capabilities of the various sub-systems.
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.
To support the Air Force's Airborne Laser (ABL) program, we have configured an adaptive-optics and tracking facility at Lincoln Laboratory's Firepond site. By using a 5.4-km horizontal propagation range and properly selecting the beam diameter and wavelength we are able to replicate important aspects of ABL propagation scenarios expected at high altitude. The experimental system was configured to conduct experiments with a point-source beacon, or using active illumination for both adaptive optics and tracking. In this paper we provide an overview of the Firepond facility and include details on the parameter scaling and experimental conditions required to replicate ABL conditions. We discuss optical configurations for various experiments and provide details of the adaptive-optics, tracking, and active illumination systems.
We have conducted atmospheric compensation experiments using active illumination for both adaptive-optics and tracking. Tests were performed in support of the Air Force's Airborne Laser program. The tests utilized the 5.4-km horizontal propagation range at the Lincoln Laboratory Firepond facility. The adaptive-optics beacon was provided by actively illuminating the target with a multibeam laser illuminator. A second multibeam laser illuminator was used to provide a beacon for an imaging tracker. Experiments were conducted using two different adaptive-optics illuminator configurations, as well as with point-source beacons. Data were collected over a range of atmospheric conditions. Results from these tests have helped to provide a performance benchmark for the Airborne Laser program.
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.
This paper addresses the problem of estimating the noise equivalent angle (NEA) of several tilt estimation algorithms resulting from including zero-mean Poisson and Gaussian noise. The Poisson noise is due to both photon shot noise and the photoelectric conversion. The readout noise of the detector is assumed to be Gaussian. There are no simple means to relate the noise on the detector to the noise in the measurement. More signal increases the signal-to-noise (SNR) and decreases the NEA. The 2D signal density profile strong influences the NEA. Three different profiles will be analyzed: a Gaussian spot, a top hat, and a simulated missile. The analysis will be performed as a function of total signal. The simulation of these three profiles will be based on using a constant base image with added noise. The variation in the tilt estimate is due to the added noise since each frame would be identical otherwise. The variation as a function of total signal is performed by scaling the base image. Experimental data is analyzed to determine the SNR by dividing the mean by the rms. This data was taken at several different intensity levels so that the total counts would change. Finally, the results from the simulations and the experimental data are compared. The dominant noise not simulated is due to scintillation. Currently, it seems that this last noise source dominates both of the sources included in the simulation.
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.
An infrared Hartmann-type wavefront sensor was assembled from a 32 X 32 lenslet array, fabricated by a binary-optic process on a germanium substrate, and a 128 X 128 pixel InSb detector, manufactured by Amber Engineering, Inc. The sensor was used to measure the wavefront of a hydrogen-fluoride laser beam from the TRW Alpha Verification Module.
The SABLE experiment investigated the ability of phase-conjugate adaptive-optics to compensate strong thermal blooming and turbulence encountered during atmospheric propagation of a high-power laser beam. The experiments utilized a 10 kW hydrogen fluoride laser, selected because its spectrum is strongly absorbed by the atmosphere. Characteristics of the beam at both the transmitter and receiver, of the atmosphere along the propagation path, and of the adaptive optics were measured and recorded during the tests and used to test the accuracy of a time-dependent computer propagation code. Wind variations along the propagation path were shown to significantly improve system performance and to suppress the phase conjugate instability.
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