James Leger, University of Minnesota; Thang Hai Nam Au, University of Minnesota; Harsha Torke, University of Minnesota
Keywords: Lasar radar, Optical phased array, Imaging, Beam forming
Abstract:
PROBLEM: Active imaging of satellites at GEO, x-GEO, and cislunar distances is challenging on many fronts. Conventional RF radar technology does not offer sufficient spatial resolution due to its large wavelength. Even at optical wavelengths, the required resolution and scanning/tracking performance stretches current technology to its limits, and many imaging scenarios are currently not practical. The required laser conditioning and scanning is conventionally achieved by mounting a collimated source on a steerable gimbal so that the center of the beam is pointed in the desired direction. However, this results in a cumbersome and often expensive mechanical system with a large-angle scan speed that is limited by the mass of the optics. For scenarios that require large apertures such as GEO, x-GEO, and cislunar imaging, this approach rapidly becomes impractical.
A potential alternative configuration uses an array of coherent and phase-steerable optical sub-apertures to steer the beam. This phased-array technology replaces the large single aperture by many smaller ones and achieves scanning with no mechanical motion by electronic phasing. This technology, including the required active phase control to compensate for atmospheric aberrations has been pioneered by the directed energy community. Unfortunately, while offering an effective solution for low-resolution RF systems, there are two inherent problems with using optical phased arrays for high resolution imaging. The first is that high resolution imaging requires a large array diameter, whereas large scanning angles require a small distance between apertures. Thus, systems that require high resolution and large scanning angles require an impractical number of optical apertures. For example, to image an object in a GEO orbit at 35,000 km with a Rayleigh resolution of 1 meter using light with a wavelength of one micrometer requires an array that is 35 meters in diameter. Conventional optical phased arrays, on the other hand, require aperture spacing that is smaller than approximately 60 micrometers to electronically scan a beam over one degree in angle, with larger scanning angles requiring even smaller spacing or complex mechanical scanning. Clearly, optical arrays do not offer a solution to a high-resolution system that is scannable over a large angular range with minimum mechanical motion. An additional problem with phased arrays is that the total number of resolvable points in an electronically scanned image can be shown to be approximately equal to the total number of apertures in the array. Thus, for a high resolution, large area scan space, an impractically large number of apertures is again required.
SOLUTION: We propose a radically new optical design that enables large angle laser beam steering, focusing, and beam forming over large optical apertures with a minimum of mechanical motion. Compared with conventional optical phased arrays, our design is capable of vastly larger steering angles, can address orders-of-magnitude larger numbers of object points, and is fully focusable, all accomplished by small displacements of light-weight fibers. Importantly, this imaging modality is capable of acquiring and tracking objects over substantial angles with no macro-motion. Second-order compensation of atmospheric turbulence can also be achieved with no additional hardware, resulting in improved beam quality on target.
Our architecture starts with a conventional coherent beam combining layout consisting of a single master oscillator laser driving an array of fiber amplifiers. Each fiber amplifier is collimated by a modest-sized collimating optic to form a sub-aperture of the array. Active phase control establishes coherence across the array of sub-apertures, resulting in a large diameter, low divergence beam at the transmitter and a high-resolution illumination beam at the target. A key innovation of our system consists of the micro-motion of the fiber amplifier delivery tip at each subaperture. Motion in the x- and y-directions produces an angular tip to the beam in each module. Motion in the z-direction provides a spherical curvature. A coordinated motion of these three directions at each module can generate a large, steerable and focusable wavefront that extends across the entire array. The diameter of each sub-aperture can be modest in size (e.g. 30 cm) making a large array of many coherently combined modules practical while still allowing beam steering over large angles and significant beam focusing.
A second innovation is to use the principle of diffractive optics to wrap the phase of the wavefront at the edge of each sub-aperture. Integer multiples of a wavelength are subtracted from the phase of each sub-aperture in such a manner that the phase at adjacent edges of two sub-apertures is identical (modulo 2pi). If the linewidth of the laser is sufficiently narrow, this wrapped beam will perform identically to its continuous counterpart. Note that each sub-aperture assembly is highly modular, making the construction of large arrays considerably more practical.
OUTCOMES: Because the wavefront contained in each transmitting sub-aperture has a completely adjustable piston, tilt and misfocus component, any piecewise second-order wavefront can be established across the phased array. This flexibility in wavefront generation can be used to tailor the illumination beam to an arbitrary shape at the target. We use an iterative Fourier transform algorithm to solve for the phase distribution that produces the desirable beam shape at the target. Thus, along with a conventional raster scanning and imaging mode, our system is capable of identifying targets by comparing their reflected power to a dictionary of manufactured illumination shapes. This mode holds the promise of high-speed target detection combined with macro-motionless target tracking. The goal of this research program is to explore engineering designs, calculate performance characteristics, and demonstrate key components of this new active imaging architecture.
ACKNOWLEDGMENTS: The research reported here is supported by a subaward of AFOSR grant no. FA9550-23-1-0536.
Date of Conference: September 17-20, 2024
Track: SDA Systems & Instrumentation