A -- PHASED ARRAY OF PHASED ARRAYS

Notice Date

January 16, 2001

Contracting Office

Department of the Air Force, Air Force Materiel Command, AFRL -- PL "Directed Energy Directorate", 2251 Maxwell Ave, Kirtland AFB, NM, 87117

ZIP Code

87117

E-Mail Address

Barbara Steinbock (steinbob@plk.af.mil)

Description

PAPA -- PHASED ARRAY OF PHASED ARRAYS The Air Force Research Laboratory is conducting a three-month, DARPA-funded study of the technical challenges and possible solutions in developing optical phased arrays as electronically-steered transmitters for fiber lasers, as well as receivers for focal plane array detectors. The end goal is to evaluate the feasibility of concepts for complete laser systems based on this technology and, with promising results, recommend a program plan to develop and demonstrate the specific implementation approaches that will enable concept reality. The Phased Array of Phased Arrays (PAPA) investigation, being led by Paul McManamon in the Sensors Directorate and Bill Thompson and Craig Denman in the Directed Energy Directorate, was begun in mid December with the kickoff of several contracts to investigate potential implementation approaches and architectures. A study progress review, to be held at Wright-Patterson AFB on 7 & 8 Feb 2001, will include open sessions for additional presentations and discussion. The study findings will be presented to DARPA in early March 2001. Why Phased Arrays? Traditional methods of optical remote sensing and projecting laser beams rely on mechanical gimbals and deformable mirrors to correct for wavefront aberrations. These systems are often limited in performance, are complex and costly, and can have significant impacts on the platform that will carry the system. Rapid steering of large aperture laser beam directors, with the sub-micro radian accuracies required for many applications, will require massive systems and large power expenditures. A promising alternative approach for both sensing and transmitting is to use Optical Phased Arrays (OPAs). Similar to a microwave phased array, an OPA can be composed of multiple liquid crystal or micro-mirror elements that modify the optical phase of an incident optical signal in transmission or reflection. These phase modifications can be used in beam steering and/or aberration correction. The liquid crystal elements, called writeable gratings, are solid state devices that are manufactured using semiconductor fabrication techniques. The dynamically-variable grating modifies the phase of an incident optical signal via a voltage-controlled index of refraction. Creation of a linear gradient in the optical path delay tilts the phase front and steers the optical beam. The realized linear gradient is actually folded, similar to a blazed grating. Current devices are just over 4 cm square. An OPA can be flood illuminated by a single high power laser source or, alternatively, each element of the array can be fed by an individual laser. The latter approach is the focus of the Phased Array of Phased Arrays (PAPA) research program. In the PAPA concept, each element of the OPA is flood illuminated by a fiber laser. All of the fiber laser amplifiers are driven by light derived from a single master oscillator. This Master Oscillator Power Amplifier (MOPA) approach ensures that the phases of the fiber lasers are consistent. With proper OPA phasing, a high power laser beam (or possibly multiple beams) can be generated from these individual fiber lasers, corrected for atmospheric and other distortions, and rapidly steered. Although the PAPA approach would be valuable for many low power applications, it is potentially scalable to very high power uses by coupling large numbers of fiber lasers and increasing the power per fiber. The complex optical systems currently required for such high power systems would be greatly simplified by eliminating expensive and somewhat fragile gimbals and by reducing the number of optical elements. In addition, an OPA would provide the additional benefit of multiple, independently-steered and compensated laser beams, if needed, without requiring additional gimbals. Phased array, or subaperture, imaging, in which an incoming signal is modified by the writeable grating and optically switched to an FPA, would also be a highly desirable feature for many military platforms. If this sensing and imaging capability is feasible, then it may be possible to incorporate the receiving functions inherent in most laser system applications (such as target acquisition and tracking and atmospheric compensation) into the same OPA components that transmit the laser energy. Airborne OPAs would be extremely attractive for situational awareness, self defense, and force application. In space, OPAs could be scaled to the very large apertures and high powers required for many Space Based Laser applications while keeping the power-area ratio low and without requiring complex deployable optics configurations. For both airborne and space applications, an OPA would also be a distributed architecture that would provide graceful degradation. If you would like to participate in the PAPA project status review at Wright-Patterson AFB, please contact Dr. Marc Hallada, Schafer Corporation (Albuquerque), at 505-338-2853, mhallada@schafercorp.com. Although this will be a status review for on-going contracted efforts for the PAPA project, there is still great interest in innovative implementation approaches that will enable the vision of an integrated laser system based on OPA technology. If you would like to present at this review, please email an abstract of the proposed presentation to Dr. Hallada. At this review, we will begin the process of discussing a technical development and demonstration program plan for presentation to DARPA in the March time period.

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Record

Loren Data Corp. 20010118/SPMSC001.HTM (D-016 SN50A9P7)