Mikhail Loktev

Correction of low order aberrations using continuous deformable mirrors.

By analyzing the Poisson equation describing the static behavior of membrane and bimorph deformable mirrors and biharmonic equation describing the continuous facesheet mirror with push-pull actuators, we found that to achieve a high quality correction of low-order aberrations these mirrors should have sufficient number of actuators positioned outside the correction aperture. In particular, any deformable mirror described by the Poisson equation requires at least two actuators to be placed outside the working aperture per period of the azimuthal aberration of the highest expected order. Any deformable mirror described by the biharmonic equation, such as a continuous facesheet mirror with push-pull actuators, requires at least four actuators to be placed outside the working aperture per period of the azimuthal aberration of the highest expected order, and these actuators should not be positioned on a single circle.

Planar double-grating microspectrometer

We report on a miniature spectrometer with a volume of 0.135 cm(3) and dimensions of 3x3x11 mm, mounted directly on the surface of a CCD sensor. The spectrometer is formed by two flat diffraction gratings that are designed to perform both the dispersion and imaging functions, eliminating the need for any spherical optics. Two separate parts of the device were fabricated with the single-mask 1 mum lithography on a single glass wafer. The wafer was diced and the device was assembled and directly mounted onto a CCD sensor. The resolution of 3 nm, spectral range of 450 to 750 nm and the optical throughput of ~9% were measured to be in a complete agreement with the model used for the development of the device.

On the possibility of intraocular adaptive optics.

We consider the technical possibility of an adaptive contact lens and an adaptive eye lens implant based on the modal liquid crystal wavefront corrector, aimed to correct the accommodation loss and higher-order aberrations of the human eye. Our first demonstrator with 5 mm optical aperture is capable of changing the focusing power in the range of 0 to +3 diopters and can be controlled via a wireless capacitive link. These properties make the corrector potentially suitable for implantation into the human eye or for use as an adaptive contact lens. We also discuss possible feedback strategies, aimed to improve visual acuity and to achieve supernormal vision with implantable adaptive optics.

Deformable mirror with thermal actuators.

Low-cost adaptive optics is applied in lasers, scientific instrumentation, ultrafast sciences, and ophthalmology. These applications demand that the deformable mirrors used be simple, inexpensive, reliable, and efficient. We report a novel type of ultralow-cost deformable mirror with thermal actuators. The device has a response time of ~5 s , an actuator stroke of ~6mum , and temporal stability of ~lambda/10 rms in the visible range and can be used for correction of rather large aberrations with slow-changing amplitude.

Comparison study of the performance of piston, thin plate and membrane mirrors for correction of turbulence-induced phase distortions

Abstract In the framework of the Kolmogorov turbulence model and analytical and numerical models of deformable mirrors, we investigated and intercompared the correction performances of piston, membrane and flexible faceplate (with either free edge, supported edge or clamped edge) adaptive mirrors featuring orthogonal, hexagonal and segmented ring actuator structures. It was shown that all modal correctors provide similar correction performance, which is close to the performance of the ideal corrector with Karhunen–Loeve-type influence functions.

Liquid-crystal intraocular adaptive lens with wireless control

We present a prototype of an adaptive intraocular lens based on a modal liquid-crystal spatial phase modulator with wireless control. The modal corrector consists of a nematic liquid-crystal layer sandwiched between two glass substrates with transparent low- and high-ohmic electrodes, respectively. Adaptive correction of ocular aberrations is achieved by changing the amplitude and the frequency of the applied control voltage. The convex-shaped glass substrates provide the required initial focusing power of the lens. A loop antenna mounded on the rim of the lens delivers an amplitude-modulated radio-frequency control signal to the integrated rectifier circuit that drives the liquid-crystal modal corrector. In vitro measurements of a 5-mm clear aperture prototype with an initial focusing power of +12.5 diopter, remotely driven by a radio-frequency control unit at ~6 MHz, were carried out using a Shack-Hartmann wavefront sensor. The lens based on a 40-mum thick liquid-crystal layer allows for an adjustable defocus of 4 waves, i. e. an accommodation of ~2.51 dioptres at a wavelength of 534 nm, and correction of spherical aberration coefficient ranging from -0.8 to 0.67 waves. Frequency-switching technique was employed to increase the response speed and eliminate transient overshoots in aberration coefficients. The full-scale settling time of the adaptive modal corrector was measured to be ~4 s.

Speckle imaging through turbulent atmosphere based on adaptable pupil segmentation

We report on the first results to our knowledge obtained with adaptable multiaperture imaging through turbulence on a horizontal atmospheric path. We show that the resolution can be improved by adaptively matching the size of the subaperture to the characteristic size of the turbulence. Further improvement is achieved by the deconvolution of a number of subimages registered simultaneously through multiple subapertures. Different implementations of multiaperture geometry, including pupil multiplication, pupil image sampling, and a plenoptic telescope, are considered. Resolution improvement has been demonstrated on a ∼550 m horizontal turbulent path, using a combination of aperture sampling, speckle image processing, and, optionally, frame selection.

Customized CMOS wavefront sensor

We report on an integrated Hartmann wavefront sensor (WFS) using passive-pixel architecture and pixels clustered as position-sensitive detectors for dynamic wavefront analysis. This approach substitutes a conventional imager, such as a CCD or CMOS imager, by a customized detector, thus improving the overall speed performance. CMOS (complementary-metal- oxide-semiconductor) technology enables on-chip integration of several analog and digital circuitry. The sensor performance depends on the feature size of the technology, noise levels, photosensitive elements employed, architecture chosen and reconstruction algorithm.

Cylindrical adaptive lenses

The correction of low-order aberrations is important in many adaptive optics applications. Modal cylindrical adaptive lenses can be used to correct several low order aberrations. Furthermore, the same technology can be used for creating arrays of controllable lenses. The most significant feature of these cylindrical lenses is a modal control system based on nematic liquid crystals. Modal control allows the precise control of the spatial phase distribution in order to achieve an aberration-free lens. This has been investigated both by computer simulation and experiment. We found that the introduction of a 180-degree phase shift between the second or higher order harmonics and no phase shift between the first harmonic components of the control voltages improves the optical performance of the device. These extra harmonics eliminate the strong dependence of the liquid crystal orientation on the impedance of the device. This is especially important for devices with small apertures. It also was found that modal cylindrical lens controlled by two-harmonic voltages can produce a slit-like beam whose transverse structure has the shape of a pulse which remains unchanged over a long range in the direction of propagation. We investigated a device with two-crossed 1D control electrodes and produced a lens with controllable focus and astigmatism.

Liquid crystal wavefront corrector with modal response based on spreading of the electric field in a dielectric material

A novel liquid crystal (LC) wavefront corrector with smooth modal influence functions is proposed and realized. The device consists of a thin layer of planar aligned nematic LC sandwiched between a glass plate with a conductive electrode and a plate made of ceramic material with a very high dielectric constant. Control electrodes are positioned on the back side of the ceramic plate, opposite to the LC. The modal character of the response is determined by spreading of the electric field in the ceramic plate. The device implemented is operating in a reflective (mirror) mode; however, similar principles can be used to build a transmissive device. Low cost and simplicity of control make it a good alternative to continuous face-sheet deformable mirrors.