Fassil Ghebremichael

Holography-based wavefront sensing

We describe a modal wavefront sensing technique of using multiplexed holographic optical elements (HOEs). The phase pattern of a set of aberrations is angle multiplexed in a HOE, and the correlated information is obtained with a position sensing detector. The recorded aberration pattern is based on an orthogonal basis set, the Zernike polynomials, and a spherical reference wave. We show that only two recorded holographic patterns for any particular aberration type are sufficient to allow interpolated readout of aberrations to lambda/50. In this paper, we demonstrate the capability of detecting errors between +/-2lambda PV for each orthogonal set at rates limited only by the speeds of the detection electronics, which could be up to 1 MHz. We show how we take advantage of the unavoidable intermodal and intramodal cross talks in determining the type, amplitude, and orientation of the wavefront aberrations.

Dynamic correction of a distorted image using a photorefractive polymeric composite

We demonstrate, for the first time, the dynamic correction of aberrated images in real-time using a polymeric composite with fast response times. The current novel experimental design is capable of restoring a phase aberrated, image carrying laser beam, to nearly its original quality. The ability to reconstruct images in real-time is demonstrated through the changing of the aberrating medium at various speeds. In addition, this technique allows for the correction of images in motion, demonstrated through the oscillatory movement of the resolution target. We also have demonstrated that important parameters of the materials in the study such as response times, diffraction efficiencies and optical gains all retain high figures of merit values under the current experimental conditions.

Fast, compact, autonomous holographic adaptive optics

We present a closed-loop adaptive optics system based on a holographic sensing method. The system uses a multiplexed holographic recording of the response functions of each actuator in a deformable mirror. By comparing the output intensity measured in a pair of photodiodes, the absolute phase can be measured over each actuator location. From this a feedback correction signal is applied to the input beam without need for a computer. The sensing and correction is applied to each actuator in parallel, so the bandwidth is independent of the number of actuator. We demonstrate a breadboard system using a 32-actuator MEMS deformable mirror capable of operating at over 10 kHz without a computer in the loop.

Modeling, synthesis, and characterization of third-order nonlinear optical salts

A number of dithioacetate and dithiolate mono- and dianions have been synthesized and characterized through Z-scan measurements, with some showing significant third order nonlinear optical (NLO) behavior. Tetralkylphosphonium cations were utilized in tandem with the nonlinear anions so as to minimize electrostatic interactions within the salt, consequently resulting in the materials being room temperature ionic liquids (RTILs), which have numerous advantages over typical organic-based materials. Anions composed of metal-ligand systems were also tested for NLO behavior as components of novel ionic liquid materials. These RTILs introduce a new class of materials with potential applications in optical limiting and other all-optical devices.

Beam Cleanup and Image Restoration with a Photorefractive Polymeric Composite

We demonstrate, for the first time to our knowledge, the use of a photorefractive polymeric composite to clean a phase-distorted laser beam and reconstruct a badly distorted image. Advantageous qualities including relatively high figures of merit, ease of processability, and low cost make this class of materials attractive when compared with their inorganic crystalline counterparts. In addition, we used four-wave-mixing and holographic techniques to obtain an internal diffraction efficiency of approximately 31% at 54.5 V/micron and a two-beam-coupling gain coefficient of gamma = 17 cm-1 at 54.5 V/micron under our experimental conditions.

Improved interferometer studies of linear electro-optic effects of dye-doped polymers

The basic Mach-Zehnder interferometer was modified for use in in situ temperature-dependent linear electro-optic (LEO) measurements of thin films of 4-dimethylamino-4?-nitrostilbene (DANS) doped into poly(methyl methacrylate) (PMMA). Optimum interferometer phase stability was possible because of an incorporated electronic feedback system. Film thickness variation was compensated for to obtain more accurate LEO coefficient measurements and thus the second-order susceptibility. Moreover, both the alpha relaxation associated with the glass transition, T(g), and beta relaxation associated with the secondary transition occurring below T(g) of PMMA + 2 wt.% DANS was obtained.

HALOS: fast, autonomous, holographic adaptive optics

We present progress on our holographic adaptive laser optics system (HALOS): a compact, closed-loop aberration correction system that uses a multiplexed hologram to deconvolve the phase aberrations in an input beam. The wavefront characterization is based on simple, parallel measurements of the intensity of fixed focal spots and does not require any complex calculations. As such, the system does not require a computer and is thus much cheaper, less complex than conventional approaches. We present details of a fully functional, closed-loop prototype incorporating a 32-element MEMS mirror, operating at a bandwidth of over 10kHz. Additionally, since the all-optical sensing is made in parallel, the speed is independent of actuator number - running at the same bandwidth for one actuator as for a million.

Holographic wavefront sensor: fast sensing, no computing

We present results of a fast holographic wavefront sensor. The modal device consists of a multiplexed hologram designed to diffract a single input beam into multiple output beams depending on the amplitude of particular Zernike terms. The aberration and amplitude are determined by the spatial location and intensity of the reconstructed focused spots. The sensing does not require any calculations, so the device is simple, compact and fast. In fact, using several position sensing detectors (PSD), a full description of the wave aberration can be obtained at rates in excess of 100 kHz. The holographic wavefront sensor can be reconfigured for any type of basis set, and is easily adaptable to laser mode profiling. In this talk we will present results of the both the theory and operation of our holographic wavefront sensor.

HALOS: fast compact, autonomous adaptive optics for UAVs

We present an adaptive optics system which uses a multiplexed hologram to deconvolve the phase aberrations in an input beam. The wavefront characterization is extremely fast as it is based on simple measurements of the intensity of focal spots and does not require any complex calculations. Furthermore, the system does not require a computer in the loop and is thus much cheaper, more compact and more robust as well. A fully functional, closed-loop prototype incorporating a 32-element MEMS mirror has been constructed. The unit has a footprint no larger than a laptop but runs at bandwidths over an order of magnitude faster than comparable, conventional systems occupying a significantly larger volume. Additionally, since the sensing is based on parallel, all-optical processing, the speed is independent of actuator number – running at the same bandwidth for one actuator as for a million.

Holographic adaptive laser optics system (HALOS)

We present results from a fast holographic adaptive laser optics system (HALOS) incorporating a MEMS-based deformable mirror and an off-the-shelf, photon counting avalanche photodiode array. A simple digital circuit has been constructed to provide autonomous control and the entire system is no larger than a shoebox. Our results demonstrate that this device is largely insensitive to obscuration and in principle can run as fast with one actuator as with one million. We further show how HALOS can be used in image correction, laser beam projection as well as phased-array beam combination.