David J. Rabb

Distributed aperture synthesis

Distributed aperture synthesis is an exciting technique for recovering high-resolution images from an array of small telescopes. Such a system requires optical field values measured at individual apertures to be phased together so that a single, high-resolution image can be synthesized. This paper describes the application of sharpness metrics to the process of phasing multiple coherent imaging systems into a single high-resolution system. Furthermore, this paper will discuss hardware and present the results of simulations and experiments which will illustrate how aperture synthesis is performed.

Binary optical true-time delay based on the white cell: design and demonstration

An optical true-time delay device that uses a binary counting system in a modified White cell is demonstrated. The switching engine uses four spherical mirrors and a three-state digital micromirror array. The delay part, as designed, provides 6 bits of delay ranging from 78 ps to 5 ns, using a combination of dielectric blocks for short delays and lens trains for longer ones. Long lens trains are folded for compactness. The authors describe the design and demonstrate two of the 6 bits of delays experimentally. Delays were accurate to within the measurement resolution of 1.25 ps. The insertion loss varied from 3.1-5.2 dB, depending on delay. It was found that the micromirrors do not contribute significantly to the loss.

Demonstrated resolution enhancement capability of a stripmap holographic aperture ladar system

Holographic aperture ladar (HAL) is a variant of synthetic aperture ladar (SAL). The two processes are related in that they both seek to increase cross-range (i.e., the direction of the receiver translation) image resolution through the synthesis of a large effective aperture. This is in turn achieved via the translation of a receiver aperture and the subsequent coherent phasing and correlation of multiple received signals. However, while SAL imaging incorporates a translating point detector, HAL takes advantage of a two-dimensional translating sensor array. For the research presented in this article, a side-looking stripmap HAL geometry was used to sequentially image a set of Ronchi ruling targets. Prior to this, theoretical calculations were performed to determine the baseline, single subaperture resolution of our experimental, laboratory-based system. Theoretical calculations were also performed to determine the ideal modulation transfer function (MTF) and expected cross-range HAL image sharpening ratio corresponding to the geometry of our apparatus. To verify our expectations, we first sequentially captured an oversampled collection of pupil plane field segments for each Ronchi ruling. A HAL processing algorithm incorporating a high-precision speckle field registration process was then employed to phase-correct and reposition the field segments. Relative interframe piston phase errors were also removed prior to final synthetic image formation. By then taking the Fourier transform of the synthetic image intensity and examining the fundamental spatial frequency content, we were able to produce experimental modulation transfer function curves, which we then compared with our theoretical expectations. Our results show that we are able to achieve nearly diffraction-limited results for image sharpening ratios as high as 6.43.

Multi-transmitter aperture synthesis with Zernike based aberration correction

Multi-transmitter aperture synthesis increases the effective aperture in coherent imaging by shifting the backscattered speckle field across a physical aperture or set of apertures. Through proper arrangement of the transmitter locations, it is possible to obtain speckle fields with overlapping regions, which allows fast computation of optical aberrations from wavefront differences. In this paper, we present a method where Zernike polynomials are used to model the aberrations and high-order aberrations are estimated without the need to do phase unwrapping of the difference fronts.

Spherical Fourier Cell and Application for Optical True Time Delay

A new optical configuration for switching light beams called a spherical Fourier cell is explained. Its use for optical true time delay is outlined. An experimental apparatus was constructed for a 6-bit delay system, with 2 bits demonstrated. Delays of 0, 2.1, 4.1, and 6.2 ns were measured. Loss and crosstalk measurements are also given.

Real-time all-optical quality of service monitoring by use of correlation and a network protocol to exploit it

We propose to use optical correlation to measure the quality of an optical link in real time, staying completely within the optical domain. We transmit a test signal of 010 and correlate the received (degraded) signal with 010. The strength and shape of the output measure dispersion and attenuation in just 3 bit periods (75 ps at 40 Gb/s) compared with minutes by traditional methods. Correlation becomes feasible owing to the recent development of tapped delay lines with very large numbers of taps. We present simulations showing that this technique can detect attenuation, dispersion, noise, and jitter. With this instantaneous quality-of-service information available to all nodes in a network, new protocols will enable the network to select paths based on quality, allowing service providers to take into account the systems physical impairments when selecting new light paths or when restoring existing ones and to guarantee varying levels of service. We present one such protocol.

Phase gradient algorithm method for three-dimensional holographic ladar imaging

Three-dimensional (3D) holographic ladar uses digital holography with frequency diversity to add the ability to resolve targets in range. A key challenge is that since individual frequency samples are not recorded simultaneously, differential phase aberrations may exist between them, making it difficult to achieve range compression. We describe steps specific to this modality so that phase gradient algorithms (PGA) can be applied to 3D holographic ladar data for phase corrections across multiple temporal frequency samples. Substantial improvement of range compression is demonstrated with a laboratory experiment where our modified PGA technique is applied. Additionally, the PGA estimator is demonstrated to be efficient for this application, and the maximum entropy saturation behavior of the estimator is analytically described.

Holographic Aperture Ladar with Range Compression

Simultaneous range compression and aperture synthesis is experimentally demonstrated with a stepped linear frequency modulated waveform and holographic aperture ladar. The resultant three-dimensional (3D) data has high resolution in the aperture synthesis dimension and is recorded using a conventional low bandwidth focal plane array. Individual cross-range field segments are coherently combined using data driven registration and phase correction methods allowing range compression to be performed without the benefit of a coherent waveform. Furthermore, we demonstrate a synergistically enhanced ability to discriminate image objects due to the coaction of range compression and aperture synthesis. We show that two objects can be precisely located in 3D space, despite being unresolved in two directions, due to resolution gains in both the range and azimuth cross-range dimensions.

Sharpness-based correction methods in holographic aperture ladar (Conference Presentation)

Turbulence mitigation has long been a challenge in long-distance imaging systems. Holographic aperture ladar (HAL), a multiwavelength spatio-temporal heterodyne 3D imaging modality, is also vulnerable to the deleterious effects of volumetric turbulence along the path of propagation. In this work, we develop a sharpness-maximization turbulence mitigation algorithm using a single phase plane approximation to volumetric turbulence. We show preliminary results that simulated HAL fields aberrated by a single turbulent phase screen, coincident with the pupil, are well corrected using this sharpness-maximization algorithm for turbulence profiles with D/r0 values of up to 50.

Special Section Guest Editorial: Computational Approaches to Imaging LADAR

This is the guest editorial for the Optical Engineering Special Section on Computational Approaches to Imaging LADAR.