Charles Higgs

Beam control of a 2D polarization maintaining fiber optic phased array with high-fiber count

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.

Active tracking using multibeam illumination

We have conducted active-tracking experiments in support of the Air Forces 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.

Adaptive optics compensation using active illumination

We have conducted atmospheric compensation experiments using active illumination for both adaptive-optics and tracking. Tests were performed in support of the Air Forces 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.

Dynamic target board for ABL-ACT performance characterization

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.

Active tracking of a ballistic missile in the boost phase

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.

Coherent beam combining of a large number of PM fibers in a 2D fiberarray

We report coherent combining of a record thirty-two PM fibers. Strehl degrades by <1 dB and phase error is <lambda/50. The use of a 2D fiberarray as the beamformer makes this approach scalable to >1000 fibers.

Multibeam laser illuminator approach

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.

Adaptive phase compensation in a Raman look-through configuration.

Raman look through is a technique for extending the use of low-power adaptive optics to the control of high-power laser beams. This is done by incorporating a Raman amplifier into an adaptive-optics system. We report the results of a Raman look-through experiment, which is to our knowledge the first demonstration of this technique.

Initial results from the Advanced-Concepts Laboratory for adaptive optics and tracking

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.

Infrared Hartmann wavefront sensor

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.