The application of Moiré effect for testing of a lithographic projection lens is reported. The arrangement presented allows measuring magnification, distortion, field curvature and telecentricity of the lens and can be used for its fine tuning. The method is based on two matched two-dimensional gratings, positioned in mutually conjugated planes; one of them can be translated. Visual interpretation of Moiré fringe pattern allows quick diagnostics of position errors exceeding critical dimension, whereas lateral scanning is applied for measuring of smaller magnitude errors. Field curvature and telecentricity are measured by 3D scanning. Presented results are in a good agreement with those obtained elsewhere.
We describe an approach that oers an almost real time image enhancement through turbulent and wavy me-
dia. The approach consists in a combination of optimization-based adaptive optics with digital multi-frame
post-processing. Applications in astronomical and terrestrial imaging { where the image features are initially
unresolved due to loss of contrast, blur, vibrations and image wander { have been illustrated by experimental
results. A new software from Flexible Optical BV is presented
Based on the analysis of factors that influence atmospheric imaging over long turbulent horizontal paths, we
consider a number of practical configurations of opto-electronic surveillance systems with optimized performance.
Our approach is based on simultaneous quasi real-time processing of a number of images obtained through
uncorrelated atmospheric paths, using either temporal or spatial multiplexing. Practical results obtained on a
4.25 km imaging path using newly developed imaging system, based on temporal multiplexing, combined with
image restoration based on projection on convex sets, are reported. Potential applications include optical and
IR long-range security and military surveillance, unmanned aircraft imaging systems and naval optical imaging
and warning systems.
In this paper we present some experimental results of speckle imaging for near-diffraction-limited observation
of ground-based scenery and astronomical objects through atmospheric turbulence. The method of alternating
projections onto convex sets is used for iterative reconstruction of the point-spread function (PSF), combined
with Wiener filtering for deconvolution and several pre-processing techniques. A modification of the optical
system with aperture segmentation is considered. The results of imaging on a horizontal path and astronomical
imaging are reported and compared with time averaged and best frame images. Apparent image improvement
is demonstrated in a field much wider than the isoplanatic patch size.
By analysis of the mechanical equations, describing deformable mirrors, we show that the print-through is
the natural property of a deformable mirror, which is completely defined by the geometry and mechanics of
deformation. The print-through causes increased scattering in imaging applications, and can result in hot/cold
spots in laser applications. Further development of adaptive optics for extreme UV applications would also
require to address the print-through problem. We describe different ways to reduce, or completely eliminate the
print-through in continuous faceplate and membrane deformable mirrors. In combination with simple hysteresis
compensation, our approach allows for high-precision feedforward control of these deformable mirrors, directly
in terms of Zernike modes.
We describe a compact integrated module implementing a low-cost adaptive optics system. It is targeted as a
correction system for small telescopes with primary mirror diameter up to 1 m, operating on a natural guide star
with magnitude at least 4 (for a 25 cm telescope). It is supposed to provide stable diffraction-limited imaging
of stars, double stars, planets and artificial bodies in various seeing conditions. Besides, it allows correcting for
static aberrations of the telescope, observer's ocular aberrations and fine adjustment of focus. The first on-sky
tests have demonstrated closed-loop operation with correction for aberrations.
The development of adaptive optics for the human eye to correct aberrations, to restore accommodation after lens extraction due to cataract and to correct age-related presbyopia have interest of academia and industry. We report on optics for a new accommodative intraocular lens which uses a two-element varifocal Alvarez lens. This lens has two refractive elements with cubic surfaces which, in combination, form a varifocal lens when the elements are shifted relatively to each other perpendicular to the optical path. The accommodative function of the lens will be driven by the ocular ciliary muscle. The refractive elements of the dual-optic intraocular lens are designed to provide a near emmetropic on-axis vision with a >4 dioptre accommodation range. The anterior element has a spherical lens to correct for the overall refraction of the eye, aspheric terms to correct the corneal asphericity and a cubic term as accommodative component; the posterior element has a cubic shaped surface only. The modular transfer function shows that the image on the retina reaches a diffraction limited performance for the on-axis vision in combination with the aspheric correction for aberrations of the cornea. We conclude that the varifocal lens is uniquely suitable for application as an intraocular accommodative lens because of its optical quality and ample accommodative power.
New applications of adaptive optics, especially in the potentially mass markets such as laser optics, imaging and medicine, require development of new components with high quality and low price. These requirements are equally applicable to wavefront sensors, wavefront reconstructors and wavefront correctors. The whole concept of adaptive optics as a science-intensive technology needs to be altered, to facilitate low-cost and service-free deployment
and user-unaware exploitation. As an example of a technology, that has a good low-cost potential, we describe the technology of piezoelectric deformable mirrors with actuators based on the transversal piezoelectric effect, as an inexpensive alternative to the deformable mirrors with stacked actuators.
A reflective-type liquid crystal (LC) wavefront corrector with modal addressing is described. The corrector's backplane has an array of pixel electrodes interconnected by a network of discrete resistors. The resistive network serves to form the local voltage profile that controls the phase distribution generated in the liquid crystal layer. This design is realized in a bipolar silicon technology. Preliminary numerical analysis is presented; technology and experimental results are discussed.
We designed and built an experimental setup for subjective (manually controlled) correction of up to 12 orthogonal terms (excluding tilts and defocus) of the aberration of the human eye. In our experiments, the subject was looking through an adaptive optical system at the USAF resolution chart. By using the arrow keys of the computer keyboard, the subject was able to control the amplitudes of up to 12 orthogonal aberration terms, introduced by a deformable mirror, optically conjugated to the pupil of the eye. Preliminary statistical analysis on a group of 12 participants with 6 correction attempts per participant, demonstrated satisfactory correlation of aberration coefficients obtained by the same person in different correction attempts. The majority of the participants were able to improve the visual acuity by subjective optimization of the figure of the deformable mirror.
Liquid crystal (LC) wavefront correctors with modal addressing are described. Three different approaches are considered. The first one is based on a continuous thin-film resistive layer. This layer is used for forming of the local voltage profile that controls the phase distribution across the corrector’s aperture. The second approach is a modification of the first one, where the continuous resistive coating is replaced by a network of discrete resistors. It is based on silicon technology. The third approach makes use of distributed electric field in thick dielectric layers for forming of the modal response of an actuator. Technologies, methods of control and experimental results are discussed for each case.
We present an overview of the results of our recent research in the field of adaptive optical components based on silicon microtechnologies, including membrane deformable mirrors, spatial light modulators, liquid-crystal correctors, wavefront sensors, and both spherical and aspherical micro-optical components. We aim at the realization of adaptive optical systems using standard-technology solutions.
37-channel modal liquid crystal wavefront correctors with a 30 and 80 mm diameter aperture are developed. Optical response, voltage-phase and dynamic properties of the devices have been studied. The possibility of synthesis of low order aberrations was experimentally demonstrated.
Low-cost adaptive optics is applied in lasers, scientific instrumentation, ultrafast sciences and ophthalmology. These applications demand the deformable mirrors to be simple, inexpensive, reliable and efficient. We report on a novel type of ultra-low-cost deformable mirror with thermal actuators. The device has response time of ~5 s, actuator stroke of about 6 μm, temporal stability of about λ/10 rms in the visible range and can be used for correction of rather large aberrations with slow changing amplitude.
We have extended the technology of fabrication of optical spherical
mirrors by using single-mask bulk micromachining to fabricate
highly-uniform spherical arrays of micro-mirrors and to mold
polymer-on-glass microlenses. The arrays fabricated feature 100%
optical fill factor and very high field uniformity of optical
characteristics of individual micro-mirrors (lenses). The technology
is specially suitable for the fabrication of uniform arrays of
spherical mirrors with small numerical apertures for use in
Hartmann-Shack wavefront sensors. Optical tests with the hexagonal
array of molded microlenses with pitch of 300μm and focal length
of ~30mm demonstrated that the contribution of microlens
imperfections into the wavefront reconstruction error does not exceed
λ/50 rms.
Application of liquid crystal (LC) modulators as phase modulators for wavefront control is important for the development of inexpensive adaptive optics systems. Currently only piston-type LC correctors are available, however the implementation of a modal approach promises much higher optical performances. Besides, implementation of silicon technology allows integration of this device and part of its control electronics in a single chip. It was found that the optical performance of a modal LC corrector is comparable to that of existing deformable mirrors; besides, adjustment of frequency and phase of driving AC voltages can further improve it. We illustrate it by numerical simulation results obtained in the framework of the Kolmogorov statistical theory of atmosphere. Development of a compact integrated device imposes some additional requirements - using of low voltages (units of volts), small amount of energy consumed by LC and integrated circuitry, preferably digital design of electronics. We report the results of feedback loop operations obtained with the device manufactured using different technology. We also discuss the technology and present the design of a modal LC corrector with silicon backplane. Several control techniques are discussed in terms of optical performance, energy consumption, cost of manufacturing and possibility for integration.
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.
Static and dynamic numerical models of electro-optical characteristics of nematics are presented. A numerical model is used for the optimization of phase delay distribution in modal LC lenses. The dynamic control mode for modal LC lenses is simulated. The results are in a good agreement with experiment.
A novel method to control LC phase modulators is presented. Various modifications of modal LC correctors are described in detail, including adaptive spherical and cylindrical lenses and a 37-element wavefront corrector. Physical mechanisms are explained with the emphasis on their practical implications.
Detailed analysis of electric voltage distribution and electric currents in adaptive liquid crystal (LC) based lenses is presented. Approximation of constant LC layer impedance is shown to be useful for physical insight. More generally, a computer simulation is developed, taking into account the voltage dependencies of the LC capacitance and conductance. Computer simulated phase shift distributions are in good agreement with experimental profiles produced via Zigo interferometer measurements. Also addressed is the influence of the lens's electrical parameters on its dynamics.
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.
Model adaptive liquid crystal lenses are described with the emphasis on their performance. Process of the lens calibration is described and numerical and experimental calibrations are carried out. Imaging by the modal adaptive lens is implemented, as well as the focusing in the feedback system based on the adaptive LC lens.
A new type of phase liquid crystal modulators with distributed electrical parameters is proposed. These modulators make it feasible to use minimal number of electrodes to from a desirable phase shape. We devised methods of control and investigated the modal liquid crystal cylindrical and spherical lenses, and the multi-element wavefront corrector with controlled influence as well. The focusing by means of the adaptive lenses is presented. We demonstrate two approaches in optimal control. The first consists in computing the optimal control voltage through liquid crystal and lens parameters. The second consists in feedback use.
A novel approach to the liquid crystal modulators design is suggested under which the liquid crystal is treated as a distribution capacitor. To control the capacitor, we introduced a distribution high resistance control electrode. We devised methods of control and investigated modal liquid crystal modulators that can be used as adaptive cylindrical and spherical lenses. Analytical derivations, computer and experimental results are presented and discussed.
In reference 1 it was shown that beams containing phase singularities have enough various intensity distributions, in particular, the distribution looked like an arbitrary planar curve. In this work we present a method of synthesis of these beams by means of one-dimensional phase elements. The basis of the method is the result stated in reference 1 that Fourier transform with an additional astigmatic phase converts such beams into light fields with one-dimensional structure. Thus, the synthesis of a singular beam can be reduced to formation of one-dimensional light fields with subsequent astigmatic Fourier transform of them. One-dimensional light field synthesis is carried out by means of two one-dimensional phase elements located at some distances along the beam propagation. One-dimensional phase masks were realized experimentally on dichromated gelatin layers which were made by sensibilization of standard holographic photoplates. The masks were recorded through an exposure of layers by an argon ion laser operating at the wavelength of 0.488 mkm. The laser beam was transformed into a narrow line of 10 mkm width. The recording was made by moving a layer in its own plane step-by-step with the help of an electric motor controlled by a computer. (The step size was 5 mkm.) After the exposure these layers were developed by water vapors according to a technique described in reference 2. Experimental results of synthesis of a beam whose intensity looks like a boundary of a regular triangle are presented.
It is known that one-dimensional phase problem in optics can be reduced to a search of zero positions of the analytic continuation of a light field complex amplitude. Usually this procedure is executed by means of numerical methods based on the measurement of the field intensity on several planes. In this work it is shown that the analytic continuation can be realized by optical way. Namely, two-dimensional Fourier transform with an additional astigmatic phase converts a one- dimensional object field into a singular wavefield. The field zeros are the same as that of the analytic continuation of one-dimensional Fourier transform of the initial field. Thus, it is possible to restore the object field through one measurement. Results of computer simulations are presented.
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