Mikhail A. Vorontsov

Automated video enhancement from a stream of atmospherically-distorted images: the lucky-region fusion approach

An automated video enhancement technique capable of image fusion from a stream of randomly-distorted images of a still scene is presented in this paper. The technique is based on the lucky-region fusion (LRF) approach and aims to improve locally the image quality according to the following steps: (1) for each image of the video stream an image quality map (IQM) which characterizes locally the image quality is computed, (2) each IQM is compared to that of the current fused image leading to the selection of best quality regions (the lucky-regions), and (3) the selected regions are merged into the fused video stream. While the LRF approach succeeds in producing images with significantly improved image quality compared to the source images, its performance depends on the imaging conditions and requires adjustment of its fusion parameter - the fusion kernel size - in order to adapt to an evolving environment (e.g. a turbulent atmosphere). Parameter selection was so far performed manually using a trial-and-error approach which causes the technique to be impractical for a real world implementation. The automated LRF technique presented is relaxed from this requirement and selects automatically the fusion parameter based on the analysis of the source images making it more suitable for practical systems. The improved LRF technique is applied to imaging through atmospheric turbulence for various imaging conditions and scenes of interest. In each case automatically-fused video streams demonstrate increases in image quality comparable to that obtained with manual selection of the fusion parameter.

Adaptive phase-locked fiber array with wavefront phase tip-tilt compensation using piezoelectric fiber positioners

In this paper, we present the recent development of a conformal optical system with three adaptive phase-locked fiber elements. The coherent beam combining based on stochastic parallel gradient descent (SPGD) algorithm is investigated. We implement both phase-locking control and wavefront phase tip-tilt control in our conformal optical system. The phase-locking control is performed with fiber-coupled lithium niobate phase shifters which are modulated by an AVR micro-processor based SPGD controller. The perturbation rate of this SPGD controller is ~95,000 iterations per second. Phase-locking compensation bandwidth for phase distortion amplitude of 2π-radian phase shift is >100Hz. The tip-tilt control is realized with piezoelectric fiber positioners which are modulated by a computer-based software SPGD controller. The perturbation rate of the tip-tilt SPGD controller is up to ~950 iterations per second. The tip-tilt compensation bandwidth using fiber positioners is ~10Hz at 60-μrad. jitter swing angle.

Atmospheric compensation with a speckle beacon in strong scintillation conditions: directed energy and laser communication applications.

Wavefront control experiments in strong scintillation conditions (scintillation index, approximately equal to 1) over a 2.33 km, near-horizontal, atmospheric propagation path are presented. The adaptive-optics system used comprises a tracking and a fast-beam-steering mirror as well as a 132-actuator, microelectromechanical-system, piston-type deformable mirror with a VLSI controller that implements stochastic parallel gradient descent control optimization of a system performance metric. The experiments demonstrate mitigation of atmospheric distortions with a speckle beacon typical for directed energy and free-space laser communication applications.

Phase-locking of tiled fiber array using SPGD feedback controller

We present the laboratory experiments of phase locking of a multi-channel tiled fiber array using a stochastic parallel gradient descent (SPGD) feedback controller demonstrating the compensation effect of the simulating phase-induced distortions based on the model-free optimization of the received signal strength. An all-polarization-maintaining (PM)-fiber optical configuration is used to simplify the free-space transceiver system. The atmospheric aberrations are simulated by a multi-channel integrated optical phase modulator which obtains input control voltages from an array of multi-channel independent sinusoidal signal generators. A similar multi-channel phase modulator which obtains input control voltages from a computer-based SPGD controller is used to compensate the simulating phase distortions. The experimental results show that the constructive interference state is reached through phase locking of the multi-channel tiled fiber array for phase distortions up to 180 hertz for each channel. The update rate of the computer-based SPGD controller is ~16,000 iterations per second. The average compensation bandwidth is about 310 Hz

Laser beam projection with adaptive array of fiber collimators. II. Analysis of atmospheric compensation efficiency

We analyze the potential efficiency of laser beam projection onto a remote object in atmosphere with incoherent and coherent phase-locked conformal-beam director systems composed of an adaptive array of fiber collimators. Adaptive optics compensation of turbulence-induced phase aberrations in these systems is performed at each fiber collimator. Our analysis is based on a derived expression for the atmospheric-averaged value of the mean square residual phase error as well as direct numerical simulations. Operation of both conformal-beam projection systems is compared for various adaptive system configurations characterized by the number of fiber collimators, the adaptive compensation resolution, and atmospheric turbulence conditions.

Free-space laser communications with adaptive optics: Atmospheric compensation experiments

Refractive index inhomogeneities of the turbulent air cause wave-front distortions of optical waves propagating through the atmosphere, leading to such effects as beam spreading, beam wander, and intensity fluctuations (scintillations). These distortions are responsible for severe signal fading in free-space optical communications systems and therefore compromise link reliability. Wave-front distortions can be mitigated, in principle, with adaptive optics, i.e., real-time wave-front control, reducing the likeliness of signal fading. However, adaptive optics technology, currently primarily used in astronomical imaging, needs to be adapted to the requirements of free-space optical communication systems and their specific challenges.

Adaptive phase distortion correction in strong speckle-modulation conditions

We introduce beam-quality metrics for adaptive wave-front control that permit estimation of the degree of laser beam energy concentration on a remotely located extended object based upon the backscattered wave intensity distribution at the receiver. A 37-control-channel adaptive optics system with phase correction of the output wave capable of operating in the presence of speckle-field-induced strong intensity modulation is presented. System operation is based on optimization of the speckle-field-based metric by the stochastic parallel gradient descent technique. Results demonstrate that adaptive wave-front correction using speckle-field-based beam-quality metrics can significantly improve laser beam concentration on extended objects.

Scintillation resistant wavefront sensing based on multi-aperture phase reconstruction technique

A scintillation resistant sensor that allows retrieval of an input optical wave phase using a multi-aperture phase reconstruction (MAPR) technique is introduced and analyzed. The MAPR sensor is based on a low-resolution lenslet array in the classical Shack-Hartmann arrangement and two high-resolution photo-arrays for simultaneous measurements of pupil- and focal-plane intensity distributions, which are used for retrieval of the wavefront phase in a two stage process: (a) phase reconstruction inside the sensor pupil subregions corresponding to lenslet subapertures and (b) recovery of subaperture averaged phase components (piston phases). Numerical simulations demonstrate the efficiency of the MAPR technique in conditions of strong intensity scintillations and the presence of wavefront branch points.

Coherent combining of multiple beams with multi-dithering technique: 100KHz closed-loop compensation demonstration

We demonstrate the coherent combining of three beams with a phase-locking controller using VLSI multi-dithering technique. Three fiber-coupled phase shifters are used to compensate phase distortions in the beam propagation path. The highest dither frequency in our system is ~70MHz. The achieved closed-loop compensation bandwidth of three beamlets is up to 100KHz.

Atmospheric compensation over a 2.3 km propagation path with a multi-conjugate (piston-MEMS/modal DM) adaptive system

We discuss the expansion of wavefront distortion compensation based on stochastic parallel gradient descent (SPGD) optimization to the control of several wavefront correctors. We describe then a SPGD adaptive optics system that uses a low-order deformable mirror with modal control and a high-resolution (either 132 or 320 control channels) piston-type MEMS mirror. The system was installed at a 2.3km near-horizontal propagation and used for atmospheric compensation experiments. Results obtained for different system configurations are presented.