The maximum resolution of a multiple-input multiple-output (MIMO) imaging system is determined by the size of the synthetic aperture. The synthetic aperture is determined by a coordinate shift using the relative positions of the illuminators and receive apertures. Previous methods have shown non-iterative phasing for multiple illuminators with a single receive aperture for intra-aperture synthesis. This work shows non-iterative phasing with both multiple illuminators and multiple receive apertures for inter-aperture synthesis. Simulated results show that piston, tip, and tilt can be calculated using inter-aperture phasing after intra-aperture phasing has been performed. Use of a fourth illuminator for increased resolution is shown. The modulation transfer function (MTF) is used to quantitatively judge increased resolution.

This excerpt gives a succinct explanation of polarization birefringent grating beam steering.

We describe our Innovative Multi Aperture Gimbaless Electro-Optical (IMAGE) testbed which uses coherent detection of the complex field reflected off a diffuse target with seven hexagonally arranged apertures. The seven measured optical fields are then phased with a digital optimization algorithm to synthesize a composite image whose angular resolution exceeds that of a single aperture. This same post-detection phasing algorithm also corrects aberrations induced by imperfect optics and a turbulent atmospheric path. We present the coherent imaging sub-aperture design used in the IMAGE array as well as the design of a compact range used to perform scaled tests of the IMAGE array. We present some experimental results of imaging diffuse targets in the compact range with two phase screens which simulates a similar to 7[Km] propagation path through distributed atmospheric turbulence.

The day is coming when engineers will be able to replace mechanical complex gimbal and steering mirror assemblies in electro-optical systems with thin, conformal devices that either have no moving parts or only make use of micro-motion.

"Active" sensors (systems that employ lasers as an illumination source) have several advantages. In particular, multiple phenomenologies can be measured. This paper describes the types of measurements possible and the required source and receiver components.

By using resets of multiple wavelengths to limit dispersion, we show conceptually that we will be able to steer passive, broadband, electro-optical sensors over wide angles using optical phased array technology. The dispersion associated with optical phased array beam steering can be limited to values of 20 to 80 times the diffraction limit for very large angle beam steering. Much of the remaining factor of 20 - 80 may be correctable using digital techniques. A significant obstacle associated with widespread implementation of optical phased array beam steering is wavelength dispersion associated with resets. We discuss optical phased array beam steering using resets of greater than one wavelength. By using larger resets the unfolded phase front for wavelengths other than the design wavelength can be maintained closer to a prism, thus limiting dispersion. This technique can be implemented with the variable period beam steering approach or the variable blaze beam steering approach. One way to implement the variable blaze beam steering approach is using moveable lenslets. This method of implementation is amenable to large value resets, so it is an attractive method of implementation for this multiple wavelength reset, limited dispersion, beam steering technique.

Here we investigate a novel approach to steering broadband imagery with a Liquid Crystal Optical Phased Array (LCOPA). Our approach overcomes the deleterious blurring and echoing effects inherent in the use of such a device. We develop a model for the LCOPA and formulate a method in which a steered, graybody scene may be restored through the application of a Wiener filter. We also show this approach may be extended to scenes that are not strictly composed of graybodies but instead are only spectrally smooth over an appropriate bandwidth. Experimental results are presented that demonstrate the effectiveness of this approach.

A statistical spectral band selection procedure and classifiers for an active multispectral laser radar (LADAR) sensor are described. The sensor will operate in the 1 to 5 mu m wavelength region. The algorithms proposed are tested using library reflectance spectra for some representative background materials. The material classes considered include both natural (vegetation and soil) and man-made (camouflage cloth and tar-asphalt). The analysis includes noise statistics due to Gaussian receiver noise and target induced speckle variations in the LADAR return signal intensity. The results of this analysis are then directly applied to an artificially generated spatial template of a scene consisting of these four material classes. The performance of four different classifier algorithms, which include a minimum distance classifier, a log-domain minimum distance classifier, a Bayes speckle-only classifier, and a Bayes speckle-Gaussian classifier, are evaluated. We show that the Bayesian classifier designed for speckle and Gaussian noise statistics outperforms the other classifiers. Our results also indicate that even when exact knowledge of the observation model is available, the classifier performance for speckled images can be poor unless the number of integrated speckle cells is large. (C) 1998 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(98)00403-6].

Optical phased array technology has the potential to dramatically reduce the cost of pointing, tracking, beam stabilization, focusing, and beam fanouts. Optical phased array technology will also be able to provide an adaptive technology that allows internal optical system design 'reprogramming' as well as the traditional beam clean up functions. Flexible optical systems that can be reprogrammed will allow easier upgrade to include new technology or new requirements, a critical need for long life aircraft in a rapidly progressing and changing world. The vision for a final aperture is a simple flat surface similar to a flat panel display that provides random access rapid beam steering and other beam deflection functions. The vision for intermediate optical elements is one of reprogrammability to adapt to future requirements and technology developments. Three major approaches are considered to provide the physical implementation of optical phased array capability. They are liquid crystal writable grating technology, deformable micro- mirrors, and lenslet arrays.

In an effort to increase achievable postdetection signal-to-noise ratios (SNRs) of continuous-wave, 1-μm all-solid-state ladar systems, a prototype rare-earth-doped optical-fiber amplifier has been included in the optical return signal path of both a heterodyne and a direct-detection ladar system. We provide numerical predictions for SNR increases according to our previously developed theory. We also detail our experimental efforts and provide the results of SNR measurements for four distinct cases: direct ladar detection with and without a fiber amplifier, and heterodyne ladar detection with and without a fiber amplifier. Experimentally measured increases in SNRs for ladar systems incorporating an optical-fiber amplifier are then compared with our earlier predictions. Specifically, we have found that for direct detection with a fiber amplifier in place, the predicted SNR increase is 42.0 dB, and we have measured an increase of 36.5 dB. Similarly, for heterodyne ladar detection with a fiber amplifier, the predicted SNR increase is 3.8 dB, and we have measured an increase of 8.0 dB.

In this paper we consider some of the implications of steering broad spectral band radiation using dynamic gratings. The similarities and differences in dispersion between dynamic gratings formed with electro-optic phase arrays and with microlens arrays are highlighted. Optical design considerations are presented that limit the dispersion in the image. System designs that use refractive, diffractive, and combinations of refractive and diffractive components to partially correct for the dispersion are analyzed.

A method for microscanning in imaging sensors is developed that allows liquid-crystal beam steerers to be used as nonmechanical microscan devices. This submicroscanning method involves using liquid-crystal beam steerers to shift images on a focal plane array by a fraction of the amount used in typical microscan methods. Interpolation techniques based on interlaced sampling are used to produce images free of aliasing out to twice the Nyquist frequency determined by the focal plane array. Since a continuous phase ramp is produced by the liquid-crystal beam steerer, dispersion effects due to the grating-like nature of the devices are avoided. Simulations for both 1D and 2D cases are presented, as well as experimental results using a 3 to 5 micrometers imaging sensor and a liquid-crystal beam steerer designed for operation at 1.064 micrometers .

Liquid crystal writable grating technology is being developed for beam steering in laser radar systems. To date, steering of 10.6 micrometers , 1.06 micrometers , and 0.53 micrometers wavelengths has been demonstrated. Preliminary results are described. In this paper we also consider the ability of writable gratings to steer broad spectral band radiation for use in passive sensors. We find that there is potential for these devices in microscan systems because there is little or no dispersion for the small scan angles required in microscanning. The dispersion that is present is less than the resolution of the sensor considered here. For large angle steering we find that dispersion correction or a narrowing of the spectral bandwidth is required. Approaches for dispersion correction and post-detection compensation are discussed.

In an effort to increase the SNR of a continuous wave, 1-mu m all solid state ladar system, a rare-earth-doped optical fiber amplifier is investigated as a preamplifier for radar return signals. The experimental system is detailed and a theoretical analysis of the fiber amplifier's effect on both heterodyne and direct detection schemes is provided. Beginning with the optical powers incident on the detector, the signal and noises are analyzed, through the detector electronics, to predict the SNR. The SNR is then plotted as a function of the return signal power, and a SNR threshold is defined to determine a minimum detectable signal power. The return signals required to attain the SNR threshold are then compared for four cases: direct detection with and without the fiber amplifier and heterodyne detection with and without the fiber amplifier. For the direct detection scheme considered, our results predict a sensitivity increase of 20.6 dB with the addition of the fiber amplifier, yet for heterodyne detection the predicted sensitivity increase is only 3.1 dB.