Synthetic-aperture imaging laser radar: laboratory demonstration and signal processing

Abstract

The spatial resolution of a conventional imaging laser radar system is constrained by the diffraction limit of the telescope's aperture. We investigate a technique known as synthetic-aperture imaging laser radar (SAIL), which employs aperture synthesis with coherent laser radar to overcome the diffraction limit and achieve fine-resolution, long-range, two-dimensional imaging with modest aperture diameters. We detail our laboratory-scale SAIL testbed, digital signal-processing techniques, and image results. In particular, we report what we believe to be the first optical synthetic-aperture image of a fixed, diffusely scattering target with a moving aperture. A number of fine-resolution, well-focused SAIL images are shown, including both retroreflecting and diffuse scattering targets, with a comparison of resolution between real-aperture imaging and synthetic-aperture imaging. A general digital signal-processing solution to the laser waveform instability problem is described and demonstrated, involving both new algorithms and hardware elements. These algorithms are primarily data driven, without a priori knowledge of waveform and sensor position, representing a crucial step in developing a robust imaging system.

Incoherent digital holography with phase-only spatial light modulators

“…Techniques to overcome the diffraction limit have been suggested and studied by many researchers. [54][55][56][57][58][59][60][61] Synthetic aperture (SA) imaging is a well-researched approach to improve the resolving capabilities of an imaging system. In SA imaging, for a set distance between the observed target and the imager, the effective aperture of a system is synthetically extended, leading to an increased NA.…”

“…This pattern can be considered as being acquired by a virtual imager of an effective aperture that is wider than the actual physical aperture, and thus of a higher NA. [56][57][58][59][60][61] In this sense, SA is considered as a super-resolution method, since it increases the resolving power beyond the resolution limit of a single-finite aperture of the physical system. It is not surprising that techniques of holographic imaging are frequently used for implementing SA.…”

Review of three-dimensional holographic imaging by Fresnel incoherent correlation holograms

“…The use of optical holography for synthetic aperture is usually restricted to coherent imaging [21][22][23] . Therefore, the use of this technique is limited only to those applications in which the observed targets can be illuminated by a laser.…”

Enhanced-resolution using modified configuration of Fresnel incoherent holographic recorder with synthetic aperture

“…The ability to exceed the resolving power beyond the diffraction limits is a widely studied topic among many research groups [2][3][4][5][6][7][8][9]. A common approach to achieving this goal is by forming a synthetic aperture (SA).…”

“…According to this method, several interference patterns are captured by an aperture-limited system (i.e., a system limited by its physical aperture) from various points of view. These interferograms are tiled into a new larger pattern, which could be considered to be formed by a virtual equivalent imaging system with a much wider aperture [2,3,[5][6][7][8][9]. This approach enlarges the effective aperture of the system, which is now equivalent to an imaging system with a higher numerical aperture (NA) value.…”