Holographic Synthetic Aperture Research Articles

Phase Calibration in Holographic Synthetic Aperture Radar: An Innovative Method for Vertical Shift Correction

Phase Calibration in Holographic Synthetic Aperture Radar: An Innovative Method for Vertical Shift Correction

Aberration free synthetic aperture second harmonic generation holography.

Second harmonic generation (SHG) microscopy is a valuable tool for optical microscopy. SHG microscopy is normally performed as a point scanning imaging method, which lacks phase information and is limited in spatial resolution by the spatial frequency support of the illumination optics. In addition, aberrations in the illumination are difficult to remove. We propose and demonstrate SHG holographic synthetic aperture holographic imaging in both the forward (transmission) and backward (epi) imaging geometries. By taking a set of holograms with varying incident angle plane wave illumination, the spatial frequency support is increased and the input and output pupil phase aberrations are estimated and corrected - producing diffraction limited SHG imaging that combines the spatial frequency support of the input and output optics. The phase correction algorithm is computationally efficient and robust and can be applied to any set of measured field imaging data.

Single-shot wide-field topography measurement using spectrally multiplexed reflection intensity holography via space-domain Kramers-Kronig relations.

Surface topology measurements of micro- or nanostructures are essential for both scientific and industrial applications. However, high-throughput measurements remain challenging in surface metrology. We present single-shot full-field surface topography measurement using Kramers-Kronig holographic imaging and spectral multiplexing. Three different intensity images at different incident angles were simultaneously measured with three different colors, from which a quantitative phase image was retrieved using spatial Kramers-Kronig relations. A high-resolution topographic image of the sample was then reconstructed using synthetic aperture holography. Various patterned structures at the nanometer scale were measured and cross-validated using atomic force microscopy.

Holographic SAR Tomography 3-D Reconstruction Based on Iterative Adaptive Approach and Generalized Likelihood Ratio Test

Holographic synthetic aperture radar (HoloSAR) tomography is an attractive imaging mode that can retrieve the 3-D scattering information of the observed scene over 360° azimuth angle variation. To improve the resolution and reduce the sidelobes in elevation, the HoloSAR imaging mode requires many passes in elevation, thus decreasing its feasibility. In this article, an imaging method based on iterative adaptive approach (IAA) and generalized likelihood ratio test (GLRT) is proposed for the HoloSAR with limited elevation passes to achieve super-resolution reconstruction in elevation. For the elevation reconstruction in each range-azimuth cell, the proposed method first adopts the nonparametric IAA to retrieve the elevation profile with improved resolution and suppressed sidelobes. Then, to obtain sparse elevation estimates, the GLRT is used as a model order selection tool to automatically recognize the most likely number of scatterers and obtain the reflectivities of the detected scatterers inside one range-azimuth cell. The proposed method is a super-resolving method. It does not require averaging in range and azimuth, thus it can maintain the range-azimuth resolution. In addition, the proposed method is a user parameter-free method, so it does not need the fine-tuning of any hyperparameters. The super-resolution power and the estimation accuracy of the proposed method are evaluated using the simulated data, and the validity and feasibility of the proposed method are verified by the HoloSAR real data processing results.

A Phase Calibration Method Based on Phase Gradient Autofocus for Airborne Holographic SAR Imaging

Holographic synthetic aperture radar tomography can realize full 3-D reconstructions of objects over 360° with very high resolution, and thus becomes an interesting 3-D imaging technique. However, due to the uncompensated platform motion errors, phase errors among different tracks usually exist in this imaging mode, which will hinder the 3-D image focusing. In this letter, a phase calibration method based on phase gradient autofocus is proposed to mitigate the influence of these phase errors. It exploits the space-invariant characteristic of the phase errors in a limited scene and uses an efficient and robust multibaseline autofocus to calibrate the phase errors among the multiple circular tracks. Experimental results with the GOTCHA real data are carried out to demonstrate the validity and feasibility of the proposed method.

Holographic SAR Tomography Image Reconstruction by Combination of Adaptive Imaging and Sparse Bayesian Inference

In this letter, we propose an imaging algorithm for the holographic synthetic aperture radar tomography in the circumstance of sparse and nonuniform elevation circular passes. Considering the anisotropic behavior of scatterers and the off-grid effect of sparse signal recovery, the algorithm combines the 2-D adaptive imaging method for circular SAR and the sparse Bayesian inference-based method for elevation reconstruction. For each circular pass, the azimuth-range 2-D image can be formed by the adaptive imaging method, which depends on the preretrieved maximum azimuth response angle and the azimuth persistence width. To deal with the off-grid effect in elevation reconstruction, which is caused by the deviation between the true scatterers and the discretized imaging grids, the off-grid sparse Bayesian inference method jointly estimates the scatterers and elevation off-grid error by applying their hierarchical priors. Compared with the conventional compressive sensing method that does not concern the off-grid effect, the proposed algorithm can provide more accurate 3-D reconstruction for pointlike targets, which is verified by the real-data experiments.

Image-based registration for synthetic aperture holography

High pixel count apertures for digital holography may be synthesized by scanning smaller aperture detector arrays. Characterization and compensation for registration errors in the detector array position and pitch and for phase instability between the reference and object field is a major challenge in scanned systems. We use a secondary sensor to monitor phase and image-based registration parameter estimators to demonstrate near diffraction-limited resolution from a 63.4 mm aperture synthesized by scanning a 5.28 mm subaperture over 144 transverse positions. We demonstrate 60 μm resolution at 2 m range.

High resolution digital holographic synthetic aperture applied to deformation measurement and extended depth of field method

This paper discusses the potential of the synthetic-aperture method in digital holography to increase the resolution, to perform high accuracy deformation measurement, and to obtain a three-dimensional topology map. The synthetic aperture method is realized by moving the camera with a motorized x-y stage. In this way a greater sensor area can be obtained resulting in a larger numerical aperture (NA). A larger NA enables a more detailed reconstruction combined with a smaller depth of field. The depth of field can be increased by applying the extended depth of field method, which yields an in-focus reconstruction of all longitudinal object regions. Moreover, a topology map of the object can be obtained.

Synthetic aperture holography: a novel approach to three-dimensional displays

We describe an electro-optic apparatus capable of displaying a computer-generated hologram in real time. The computer-generated hologram is calculated by a supercomputer, read from a fast frame buffer, and transmitted to a wide-bandwidth acousto-optic modulator. Coherent light is modulated by the acousto-optic modulator and optically processed to produce a three-dimensional image with horizontal parallax. We evaluate different display geometries and their effect on the optical parameters of the system. We then show how the display resolution can be increased by simultaneously writing three acoustic columns on a single crystal and optically multiplexing the resulting holograms. We finally describe some improvements that follow from the analysis.

Limitations in microwave holographic synthetic aperture imaging over a lossy half-space

Experimental results are presented which illustrate the capabilitity of a single-frequency microwave holographic scheme to image both metallic and plastic pipes at a depth of 0.33 m in wet soil with a covering layer of concrete paving stones. The data acquisition system utilises a crossed dipole antenna scanning above the interface recording two orthogonal polarisations at each point on a two-dimensional grid, from which improved surface rejection is obtained. Maximum imaging depth is limited primarily by the ratio of soil attenuation to surface and environmental effects on the antenna response, thereby preventing any significant increase in the conditions quoted. Theoretical investigation into the influence of the lossy dielectric half-space on the synthetic aperture in a one-dimensional model has illustrated that scan height is an important consideration for resolution and antenna system design. It is shown that scanning in close proximity (≤0.1 λ) to the interface increase both lateral and depth resolution. To take advantage of this effect, it is concluded that a noncontacting antenna operating in close surface proximity must be insensitive to height fluctuations.