Paul S. Lebow

Phase conjugation through Brillouin-enhanced four-wave mixing over an extended atmospheric path

We report what is to our knowledge the first demonstration of Brillouin-enhanced four-wave mixing over a long atmospheric path. Weak laser light reflected from a remote retroreflector was input to a high-gain conjugate mirror. The resulting amplified output returns to the retroreflector owing to its phase-conjugate nature. Despite atmospheric effects the phase-conjugate return energy measured at the retroreflector increased by a factor of 10(4) over the initial weak beam.

Weak-signal iterative holography.

An iterative holographic table-top experiment is presented, where a recorded hologram is used to re-illuminate the initial target. With this beam shaping setup, more light is directed to the target for each iteration until a convergence limit is met. We experimentally examine convergence properties of this iterative hologram reconstruction approach for weak object signals and compare with theory.

Limits of bootstrapping in a weak-signal holographic conjugator

We explore the effect of noise on the energy convergence for extremely weak signals in the object field of a holographic experiment. The impact of noise for the energy-on-target in the iterative, bootstrapping process of a holographic phase conjugator system is theoretically derived to obtain a recursive analytical solution. Theoretical results are compared with numerical simulations for a weak-signal holographic conjugator.

Dynamic holography for extended object beam shaping

We describe a laboratory experiment to improve the energy-on-target for an extended object. We utilize an iterative approach combining digital holography for detection and SLM beam shaping for object re-illumination. We developed a technique to modify the SLM phase to prevent oversharpening of glints and other high intensity return signal points that cause the beam to collapse to a single point with further iterations. Instead, the gain is increased as more light uniformly hits the intended target with each iteration. We present laboratory results to verify this approach and demonstrate the increased gain resulting from this dynamic beam-shaping.

Optimization of the field of view of a Brillouin-enhanced four-wave-mixing phase conjugator

A Brillouin-enhanced four-wave mixing phase conjugator featuring a field of view that varies by approximately an order of magnitude, i.e., from 2 to 20 mrad, is described. We control this variable field of view by utilizing a slightly noncollinear pumping geometry and by adjusting the pump-pump frequency difference to optimize the phase mismatch for the overall process. A model that describes the interplay between frequency and geometry to the Brillouin-enhanced four-wave-mixing phase mismatch is found to predict the expandable field of view.

Designing manufacturable filters for a 16-band plenoptic camera using differential evolution

A 16-band plenoptic camera allows for the rapid exchange of filter sets via a 4x4 filter array on the lenss front aperture. This ability to change out filters allows for an operator to quickly adapt to different locales or threat intelligence. Typically, such a system incorporates a default set of 16 equally spaced at-topped filters. Knowing the operating theater or the likely targets of interest it becomes advantageous to tune the filters. We propose using a modified beta distribution to parameterize the different possible filters and differential evolution (DE) to search over the space of possible filter designs. The modified beta distribution allows us to jointly optimize the width, taper and wavelength center of each single- or multi-pass filter in the set over a number of evolutionary steps. Further, by constraining the function parameters we can develop solutions which are not just theoretical but manufacturable. We examine two independent tasks: general spectral sensing and target detection. In the general spectral sensing task we utilize the theory of compressive sensing (CS) and find filters that generate codings which minimize the CS reconstruction error based on a fixed spectral dictionary of endmembers. For the target detection task and a set of known targets, we train the filters to optimize the separation of the background and target signature. We compare our results to the default 16 at-topped non-overlapping filter set which comes with the plenoptic camera and full hyperspectral resolution data which was previously acquired.

Gated holographic imaging for structured illumination through obscurations

We describe a pulsed phase conjugator system that projects a laser beam possessing a scene-dependent energy distribution. We expand upon the inherent properties of optical phase conjugation (OPC) to include not only corrections for phase aberrations but to provide targeted laser beam illumination of partially obscured objects. For instance, if incorporated into a foliage-penetrating lidar system, OPC could significantly enhance performance. We demonstrate this OPC beam projection concept experimentally.

Pulsed holographic system for imaging through spatially extended scattering media

Imaging through scattering media is a highly sought capability for military, industrial, and medical applications. Unfortunately, nearly all recent progress was achieved in microscopic light propagation and/or light propagation through thin or weak scatterers which is mostly pertinent to medical research field. Sensing at long ranges through extended scattering media, for example turbid water or dense fog, still represents significant challenge and the best results are demonstrated using conventional approaches of time- or range-gating. The imaging range of such systems is constrained by their ability to distinguish a few ballistic photons that reach the detector from the background, scattered, and ambient photons, as well as from detector noise. Holography can potentially enhance time-gating by taking advantage of extra signal filtering based on coherence properties of the ballistic photons as well as by employing coherent addition of multiple frames. In a holographic imaging scheme ballistic photons of the imaging pulse are reflected from a target and interfered with the reference pulse at the detector creating a hologram. Related approaches were demonstrated previously in one-way imaging through thin biological samples and other microscopic scale scatterers. In this work, we investigate performance of holographic imaging systems under conditions of extreme scattering (less than one signal photon per pixel signal), demonstrate advantages of coherent addition of images recovered from holograms, and discuss image quality dependence on the ratio of the signal and reference beam power.

Real-time CW Holographic Phase Conjugator

We demonstrate real-time correction of tip-tilt errors introduced by a dynamic phase aberrator in a directed beam through the use of a holographic phase conjugator system.

Inverse Masking for Scene-specific Beam Projection

Inverse masking allows for scene-specific beam shaping based on discriminators within the initial image. Experimental work to detect an image, encode phase on a spatial modulator, and illuminate specific regions in a scene are performed.