Short-range synthetic aperture imaging at 633 nm by digital holography

Abstract

We present an active optical synthetic aperture-imaging system. A phase-step digital holographic setup is used as a wavefront sensor in the far field. The overlap of the holograms enables the estimation and compensation of their relative positions and phase with a speckle cross-correlation algorithm. Experimental results on a short-range synthetic aperture setup at 633 nm are presented that are based on 128 x 128 holograms. The synthesis is executed in one direction by means of rotation of the object. Test images show a significant gain of resolution in the synthesis direction. Processing errors are estimated through experiment. Random processing errors of a synthetic pupil composed of 33 merged holograms are negligible, but biases induced by unknown optical aberrations ofthe reference wave induce defocusing and astigmatism.

Synthetic aperture single-exposure on-axis digital holography

“…This expression indicates the limit for the finest resolution that can be attained, provided that no errors are present in the synthesis process. An ameliorated resolution has been shown in a scheme involving heterodyne holography [14] and in a phase-shifting configuration [15][16]. A synthetic aperture has been reported as well in off-axis digital holography [17][18].…”

“…A drawback for many of these proposals is their intricate set-up and cumbersome procedure. Alternatively, an active synthetic aperture, achieved with a relative movement of the sample and the CCD camera, can also enhance resolution [14][15][16][17][18].…”

Image Quality Improvement of Digital Holography by Multiple Wavelengths or Multiple Holograms

“…To reduce the speckles, it is effective to use a hologram with a large extent (Binet et al, 2002). For example, if we record M holograms to be synthesized into a large extent hologram for each hologram, a good quality image will be reconstructed.…”

Speckle reduction of reconstructions of digital holograms using three dimensional filtering

“…Using a wavelengthtunable laser, they added together reconstructions from each of the captured DHs on an intensity basis, thus reducing the speckle content. The imple-mentation of a synthetic aperture for digital holography has been realised recently [20,21,22,23,24,25]. It is achieved by moving either the CCD or the sample, capturing multiple DHs between successive moves, and then stitching together the resulting DHs to form a larger DH.…”