We demonstrate a technique for restoring imagery using a computational imaging camera with a phase mask that produces a blurred, space-variant point spread function (PSF). To recover arbitrary images, we first calibrate the computational imaging process utilizing Karheunen-Loeve Decomposition, where the PSFs are sampled across the field of view of the camera system. These PSFs can be transformed into a series of spatially invariant "eigen-PSFs", each with an associated coefficient matrix. Thus the act of performing a spatially varying image deconvolution can be changed into a weighted sum of spatially invariant deconvolutions. After demonstrating this process on simulated data, we also show real-world results from a camera system modified with a diffractive waveplate, and provide a brief discussion on processing time and tradeoffs inherent to the technique.
In this talk, Abbie will present an overview of imaging through degraded environments such as turbid water and dense fog using range-gated digital holography. Digital holography provides the unique ability to separate out ballistic photons in a scattering and turbid environment through range-gating, coherence gating and polarization gating. The complex fields of the detected holograms and then post-processed using coherent averaging, incoherent averaging and singular value decomposition techniques to improve the SNR and reconstruct imagery in extremely challenging imaging conditions.
Communication in maritime environments presents unique challenges often requiring the secure transfer of information over long distances in a complex dynamic environment. Here a system for generating orbital angular momentum (OAM) beams, multiplexing, transmitting, and demultiplexing using a convolutional neural network (CNN) is presented. A single input from a 1550 nm seed laser is amplified, split into four separate beams that are directed and modulated by four switches, and the resulting beams directed into phase plates to create beams carrying OAM. These four beams constitute the individual channels. The beams are passed through several optical elements, coherently combined, and transmitted to a receiver at a range of 12 m. The resulting OAM beam spatial patterns are captured using a high speed short-wave infrared detector, concurrently transmitted to a workstation for storage, and processed in real-time using a trained CNN. Results from short range and quiescent environmental state show a pattern detection accuracy of <99%.
We have developed a multi-spectral SWIR lidar system capable of measuring simultaneous spatial-spectral information for imaging and spectral discrimination through partial obscurations. Our system utilizes a supercontinuum laser source and eight narrowband spectral channels in the 1000 nm to 1600 nm region. The system employs a steering mirror, which enables us to scan the region of interest and collect spectral and spatial data as a point-by-point scan. The system is designed to detect weak signal returns in the few-photon regime. The technique promises more capable classification and target detection of spectrally diverse targets in obscured environments with potential applications for mapping of ground type through forest canopy, pollution monitoring of water ways, and intelligence, surveillance, reconnaissance and target detection (ISRT). Custom targets designed to provide distinct spectral response are employed to ascertain the system’s response. The lidar system is calibrated by measuring the return signal from a highly reflective flat Spectralon target; this enables us to determine the reflectivity of the objects of interest. Spectral response of the targets are analyzed and their estimated reflectivity is reported. The same targets are studied in the presence of two partial obscurants. The objects are easily identified even though the return signal is attenuated by a factor of seven. The general spectral shape of the targets are preserved in the presence of the obscurants. More challenging objects and environments and various methods to recover the spectral response of the objects are currently being pursued.
We experimentally design, construct and test a compact optical system featuring a vortex diffractive waveplate placed in the aperture stop of the lens system. The wavefront coding camera diffracts polarized coherent illumination in the system's focal plane and reduces the peak intensity of the beam by two orders of magnitude while simultaneously recovering an unpolarized incoherent background image.
A Point Spread Function (PSF) engineered imaging system provides functionality at the expense of image distortion. Deconvolution and other post-processing techniques may partially restore the image if the PSF is known. We compare how various phase mask functions (e.g., vortex, axicon, cubic, and circular harmonic) may functionally protect a sensor from a coherent beam, and we discuss the subsequent trade-off between peak irradiance and the integrated modulation transfer function (Strehl ratio). Both experimental and numerical examples demonstrate that the peak irradiance may be suppressed by orders of magnitude without intolerable loss of image fidelity. The design of an optimal phase mask that accomplishes this task is made difficult by the nonlinear relationship between peak irradiance and Strehl. Results from experimental and numerical optimization schemes like simulated annealing, differential evolution, and Nelder-Mead will be compared.
A real-time holographic system is demonstrated for directing energy at a retroreflective target in the presence of atmospheric beam distortions and target motion. The system searches and tracks over a five-degree field of regard. The method relies upon optical phase conjugation using an off-axis holographic configuration to generate a precompensated beam using a reflective phase-only spatial light modulator. The system operates at a 147-Hz update rate and compensates for time-dependent effects in the beam path. The performance was demonstrated over a 22-m indoor range using a rotating glass wedge to simulate beam wander.
Laser propagation through deep turbulence requires adaptive optic systems capable of correcting wavefront distortions with large phase shifts. The large phase shifts, can be overcome by using dual deformable mirrors in a woofer-tweeter configuration. In this configuration the woofer corrects low spatial-frequency aberrations and the tweeter corrects higher order distortions. In this work, we perform a simulated side-by-side comparison of various control methods for woofer-tweeter adaptive optics systems. Attention is focused on the deep turbulence regime, where the wavefront distortions contain discontinuities. Our simulated wavefronts originate from a cooperative point-like beacon placed at the target plane. The light from the beacon propagates horizontally through multiple kilometers of turbulent atmosphere to the receiving aperture. At the receiver, the wavefronts are interfered with a local reference wave to produce a hologram. Through digital holographic processing, we recover the complex wave field at the receiver, in which the phase contains both branch cuts and branch points. These discontinuities pose a challenging problem for continuous surface mirrors required for high-energy applications. Our study explores the ability of previously published control methods to correct wavefronts with branch points and cuts. Various control methods for woofer-tweeter systems, including zonal and modal methods, are compared using the Strehl ratio and stroke efficiency as performance metrics. The investigation of these control methods will enable future applications to maximize the the stroke of dual deformable mirror adaptive optics systems leading to better energy on target in deep turbulence conditions.
KEYWORDS: Point spread functions, Imaging systems, Modulation transfer functions, Spatial light modulators, Optical transfer functions, Computational imaging, Diffraction, Deconvolution, Optical filters, Signal to noise ratio
A phase-only filter is placed in the pupil plane of an imaging system to engineer a new point spread function with a low peak intensity. Blurred detected images are then reconstructed in post-processing through Wiener Deconvolution. A Differential Evolution algorithm is implemented to optimize these filters for high SNR across the MTF. These filters are tested experimentally using a reflective Spatial Light Modulator (SLM) in the pupil of a system and successfully show the peak intensity reduced 100 times the diffraction limit. Results are compared to expected performance.
Large angle, nonmechanical beam steering is demonstrated at 4.62 μm using the digital light processing technology. A 42-deg steering range is demonstrated, limited by the field-of-view of the recollimating lens. The measured diffraction efficiency is 8.1% on-axis and falls-off with a sin2 dependence with the steering angle. However, within the 42-deg steering range, the power varied less than 25%. The profile of the steered laser beam is Gaussian with a divergence of 5.2 mrad. Multibeam, randomly addressable beam steering, is also demonstrated.
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.
Vortex and axicon phase masks are introduced to the pupil plane of an imaging system, altering both the point spread function and optical transfer function for monochromatic and broadband coherent and incoherent light. Each phase mask results in the reduction of the maximum irradiance of a localized coherent laser source, while simultaneously allowing for the recovery of the incoherent background scene. We describe the optical system, image processing, and resulting recovered images obtained through this wavefront encoding approach for laser suppression.
Multibeam, multicolor, large-angle beam-steering is demonstrated in the visible spectral region by imprinting Fresnel zone plates (FZP) on a liquid crystal spatial light modulator. Spectral dispersion, both diffractive and refractive, is observed but does not prevent the use of this technology for beam steering applications. The experimental results show that while diffractive dispersion dominates over refractive dispersion, wavelength-specific FZPs can be rendered to direct those beams on target, either simultaneously or consecutively. Only a slight correction in the FZP positon is necessary to compensate for refractive dispersion. The position, intensity, and wavelength of each beam can be controlled independently.
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.
Building on the work of Goodman and Lawrence , we have extended digital holographic imaging to gigapixel scales
with 2-D aperture synthesis. Sub-pixel registration algorithms were required to mosaic together thousands of arrays of
data, and phase-error correction algorithms were required to correct for system instabilities.
Phase errors introduced in the object beam of a digital hologram degrade the image quality of the
object. We present computer simulations showing the effect of multiple planes of phase errors in the
propagation path. By using a nonlinear optimization technique to maximize sharpness metrics, we show
results that account for aberrations in multiple planes and correct anisoplanatic blur. This paper demonstrates
this technique for two and three phase screens.