Jed Khoury

Resolution limits in imaging ladar systems

We introduce a new design concept of laser radar systems that combines both phase comparison and time-of-flight methods. We show from signal-to-noise ratio considerations that there is a fundamental limit to the overall resolution in three-dimensional imaging range laser radar (ladar). We introduce a new metric, volume of resolution, and we show from quantum noise considerations that there is a maximum resolution volume that can be achieved for a given set of system parameters. Consequently, there is a direct trade-offbetween range resolution and spatial resolution. Thus, in a ladar system, range resolution may be maximized at the expense of spatial image resolution and vice versa. We introduce resolution efficiency eta(r) as a new figure of merit for ladar that describes system resolution under the constraints of a specific design, compared with its optimal resolution performance derived from quantum noise considerations. We analyze how the resolution efficiency could be utilized to improve the resolution performance of a ladar system. Our analysis could be extended to all ladars, regardless of whether they are

Photoconductive optically driven deformable membrane for spatial light modulator applications utilizing GaAs substrates.

The fabrication and characterization of an optically addressable deformable mirror for a spatial light modulator is described. Device operation utilizes an electrostatically driven pixellated aluminized polymeric membrane mirror supported above an optically controlled photoconductive GaAs substrate. A 5 microm thick grid of patterned photoresist supports the 2 microm thick aluminized Mylar membrane. A conductive ZnO layer is placed on the back side of the GaAs wafer. A standard Michelson interferometer is used to measure mirror deformation data as a function of illumination, applied voltage, and frequency. A simplified analysis of device operation is also presented.

Electrically Tunable Surface Plasmon Source for THz Applications

In this paper, we propose a design for a widely tunable solid-state optically and electrically pumped terahertz source based on the Smith-Purcell free-electron laser. Our design consists of a thin dielectric layer sandwiched between an upper corrugated structure and a lower layer of thin metal, semiconductor, or high-electron-mobility material. The lower layer is for current streaming, which replaces the electron beam in the Smith-Purcell free-electron laser design. The upper layer consists of two microgratings for optical pumping, and a nanograting to couple with electrical pumping in the lower layer. The optically generated surface plasmon waves from the upper layer and the electrically induced surface plasmon waves from the lower layer are then coupled. Emission enhancement occurs when the plasmonic waves in both layers are resonantly coupled.

Patterned Multipixel Membrane Mirror MEMS Optically Addressed Spatial Light Modulator With Megahertz Response

In this letter, we report the fabrication, modeling and characterization of an all-optically addressed spring patterned silicon-nitride deformable mirror microelectromechanical-systems device. Combinations of high-frequency ac and dc bias voltages are applied across the device. The experimental verified theoretical modeling for this device shows the mirror deflection saturation as a function of light intensity appropriate for the dynamic range compression deconvolution. Frequency response up to 10MHz, practical for correcting very fast turbulences, was measured

Nonlinear dynamic range compression deconvolution.

We introduce a dynamic range image compression technique for nonlinear deconvolution; the impulse response of the distortion function and the noisy distorted image are jointly transformed to pump a clean reference beam in a two-beam coupling arrangement. The Fourier transform of the pumped reference beam contains the deconvolved image and its conjugate. In contrast to standard deconvolution approaches, for which noise can be a limiting factor in the performance, this approach allows the retrieval of distorted signals embedded in a very high-noise environment.

An all optically driven integrated deformable mirror device

We demonstrate a technique for actuating micromirrors vertically cascaded on wafer fused GaAs-GaP photodiodes. Unlike traditional actuation schemes, the electrostatic drive of the individual capacitive actuators is addressed optically in this device. Vertical mirror displacements of up to 500 nm were observed using interferometry while addressing the photodetectors with a 5 mW optical signal. Microlenses were used to address a 900 pixel device with patterned conductive pillars and thin film load resistors for each actuator-detector element. This approach can enable realization of faster and denser adaptive optics wave front corrector arrays.

Mapping approach for image correction and processing for bidirectional resonant scanners

We have developed a mapping algorithm for correcting sinusoidally scanned images from their distortions. Our algorithm is based on the close relationship between linear and sinusoidal scanning. Straightforward implementation of this algorithm showed that the mapped image has either missing lines or redundant lines. The missing lines were filled by fusing the mapped image with its median-filtered or interpolated version. The implementation of this algorithm shows that it is possible to retrieve up to 98% of the original image (depending on the algorithm used for data fusion) as measured by the recovered energy. Excellent correction was obtained for both simulated scanned images and actual images from a scanning laser radar system.

Binary phase-only filtering for turbulence compensation in fiber-coupled free-space laser communication systems

Binary wavefront control in the focal plane (i.e., binary phase-only filtering) for partial compensation of atmospheric turbulence in fiber-coupled free-space laser communication systems is investigated. Numerical results from wave-optics simulations show that in an air-to-air scenario, the combination of binary phase-only filtering and centroid tracking provides mean fiber coupling efficiency close to that resulting from ideal least-squares adaptive optics, but without the requirement for direct wavefront sensing. This result suggests a simpler and less computationally demanding turbulence mitigation system that is more readily applied to tactical applications.

Resolution limits for time-of-flight imaging laser radar

In previous work we introduced a new metric for the image resolution volume that couples the spatial resolution with the range resolution. We showed from a quantum noise consideration that there is a constant volume resolution and that one can trade-off spatial resolution at the expense of range resolution, and vise-a-versa. This theory was developed for a heterodyne LADAR system. In this paper we extend our previous heterodyne LADAR system theory to develop a image resolution volume metric for time-of-flight LADAR system, where device parameters such as optical amplifier noise are included.

Adaptive defect and pattern detection in amplitude and phase structures via photorefractive four-wave mixing.

This work comprises the theoretical and numerical validations of experimental work on pattern and defect detection of periodic amplitude and phase structures using four-wave mixing in photorefractive materials. The four-wave mixing optical processor uses intensity filtering in the Fourier domain. Specifically, the nonlinear transfer function describing four-wave mixing is modeled, and the theory for detection of amplitude and phase defects and dislocations are developed. Furthermore, numerical simulations are performed for these cases. The results show that this technique successfully detects the slightest defects clearly even with no prior enhancement. This technique should prove to be useful in quality control systems, production-line defect inspection, and e-beam lithography.