Showing papers on "Wavefront sensor published in 1995"

Wavefront sensor for a staring imager

Abstract: A wavefront sensor for use in a staring imaging system for generating focused and defocused images of an object scene that is being imaged by the imaging system from a single input optical beam impinged thereon. The wavefront sensor includes a beamsplitter for splitting an optical input beam into identical first and second half beam portions and for focusing the first and second half beam portions onto respective first and second focal planes. A phase plate is interposed between the beamsplitter and the second focal plane for inducing a known focus error in the second half beam portion. A photodetector is also provided and is effective to form a best focused image and a slightly out of focus image at the first and second focal planes, respectively. The focused and defocused images are combinable using phase diversity techniques to estimate wavefront aberrations present in the imaging system.

Adaptive wavefront shaping with liquid crystals

Abstract: The authors are interested in high resolution imaging systems, and in particular new techniques for wavefront sensing and correction.

First tests of the Cassegrain adaptive optics system of the Mount Wilson 100-in telescope

Abstract: In January 1994, we began construction of a modern adaptive optics system for the newly refurbished 100-inch telescope. The design philosophy of the adaptive optics system is to achieve a working system in the visible in a short time at relatively low cost. This means wavefront sensing with natural guide stars and implementation at the bent Cassegrain focus of the telescope. The system has an integrated wavefront sensor and finder camera, and is automated for one-person operation. It uses off-the-shelf components where possible. The deformable mirror, which has 241 actuators, is on loan from the U.S. Air Force. The use of an existing mirror imposes constraints that have driven some of the design considerations. The system is operating at the telescope, with early results described below.

Adaptive optics wave measurement and correction system

Abstract: An adaptive optics system which simultaneously measures the phase function and corrects the phase distortion of a wavefront using a liquid crystal device placed in an interferometer. The liquid crystal device inserts the conjugate phase function in the path and obtains a null in the interference pattern.

Modeling an interferometer for non-null testing of aspheres

Abstract: Aspheric surface testing would be greatly facilitated if the requirement for a null condition were removed. Testing an optic in a non-null configuration introduces aberrations into the wavefront. The wavefront measured at the sensor is different from the wavefront initially produced by the test surface, and the interferometer must be calibrated if useful measurements of aspheres are to be made. One potential calibration technique is reverse optimization, where a lens design program is used to retrieve the prescription of the interferometer. Various problems in modeling an interferometer, and potential solutions, are discussed. A defocused sphere was used to generate a non-null wavefront with 100(lambda) of departure at the surface. The reverse optimization results matched the experimental data to better than (lambda) /4 PV.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Sensing three-dimensional index-of-refraction variations by means of optical wavefront sensor measurements and tomographic reconstruction

Abstract: Light waves propagating through a region of inhomogeneous index of refraction are aberrated Turbulent air flows cause such inhomogeneities to arise from density fluctuations within the flow, which affect the index of refraction of air Here we describe a technique for measuring the 3-D structure of the index-of-refraction distribution in a turbulent air flow using optical wavefront sensor measurements and tomographic signal reconstruction Use of wavefront sensor measurements offers higherspeed measurement than previously used holographic methods, while providing high-fidelity reconstructions We discuss Hartmann sensor measurements and wavefront reconstruction Sampling requirements are also addressed, including the effects of a limited number of projections and wavefront subaperture size Theory, simulation, and experimental results are presented

WIYN telescope active optics system

Abstract: In this paper we describe the active optics system of the WIYN 3.5 meter telescope put into operation in the spring of 1995 on Kitt Peak, Arizona. The active optics system provides real time collimation of the telescope and optical figure control of the primary mirror. The individual subsystems are first described. These include the wavefront curvature sensing technique, the support and articulation of the secondary mirror, the control of the primary mirror figure and rigid body motion, and the mechanics and electronics used for controlling and monitoring the optics. Algorithms for the complete loop are then discussed. This involves mapping coma terms used to actively correct the collimation, while residual phase errors are corrected by active control of the forces supporting the primary mirror. In the next section we compare two operational modes: open loop using mapped collimation and optical figure corrections, and closed loop using feedback from the wavefront sensor directly. Finally, preliminary stellar images obtained with the actively controlled telescope are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Micro-precision interferometer pointing control system

Abstract: The paper describes the development of the wavefront tilt (pointing) control system for the JPL micro-precision interferometer (MPI). This control system employs piezoelectric actuators and a digital imaging sensor with feedback compensation to reject errors in instrument pointing. Stringent performance goals require large feedback, however, several characteristics of the plant tend to restrict the available bandwidth. A robust 7th-order wavefront tilt control system was successfully implemented on the MPI instrument, providing sufficient disturbance rejection performance to satisfy the established interference fringe visibility requirement.

Specialized wavefront sensors for adaptive optics

Abstract: The performance of an adaptive optical system is strongly dependent upon correctly measuring the wavefront of the arriving light. The most common wavefront measurement techniques used to date are the shearing interferometer and the Shack-Hartmann sensor. Shack-Hartmann sensors rely on the use of lenslet arrays to sample the aperture appropriately. These have traditionally been constructed using ULM or step and repeat technology, and more recently with binary optics technology. Diffractive optics fabrication methodology can be used to remove some of the limitations of the previous technologies and can allow for low-cost production of sophisticated elements. We have investigated several different specialized wavefront sensor configurations using both Shack-Hartmann and shearing interferometer principles. We have taken advantage of the arbitrary nature of these elements to match pupil shapes of detector and telescope aperture and to introduce magnification between the lenslet array and the detector. We have fabricated elements that facilitate matching the sampling to the current atmospheric conditions. The sensors were designed using a far-field diffraction model and a photolithography layout program. They were fabricated using photolithography and RIE etching. Several different designs will be presented with some experimental results from a small-scale adaptive optics brass-board.

Performance measurements of generation III wavefront sensors at the Starfire Optical Range

Abstract: The adaptive optics system for the 1.5-m telescope at the Starfire Optical Range, Kirtland AFB, New Mexico has recently been upgraded. Two of the key components in the new system are improved Generation III Shack-Hartmann Wavefront Sensors (WFSs) built by Adaptive Optics Associates (AOA). The performance of the new WFSs has been measured. Measurements indicate a factor of two improvement in noise performance and less inter- subaperture pixel crosstalk resulting in improved closed loop stability. System design and performance measurements are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Wave-front sensing by pseudo-phase-conjugate interferometry.

Abstract: A wave-front sensor based on pseudo-phase-conjugate interferometry is presented. We show that a pseudo-phase-conjugate interferometer is suitable for the measurement of phase distribution on a propagating wave. This new method may be employed for optical workshop applications and wave-front sensing for adaptive optics. The theoretical sensitivity of the interferometer is twice that of the Hartmann-Shack wave-front sensor. Preliminary laboratory experiments demonstrate excellent performance and consistency with computer simulations.

A Michelson interferometer and reference mirrors integrated on glass

Abstract: A Michelson interferometer for displacement measurements is presented using integrated optics. Loops on single-mode waveguides are proposed and demonstrated as reference mirrors. The waveguides are made by potassium ion exchange on glass, and with a simple design this makes the sensor well adapted to low-cost mass production. The elements of the sensor are described and interference fringes from vibration measurements are obtained. The ratio between signal and detector noise is more than 30 dB. This makes the precision of the sensor independent of detector noise, and it is thus limited by the wavelength stability of the diode laser. Because of the strong signal amplitude, the resolution is likely to be better than 1 nm.

One-dimensional wavefront sensor development for tomographic flow measurements

Abstract: Optical diagnostics are extremely useful in fluid mechanics because they generally have high inherent bandwidth, and are nonintrusive However, since optical probe measurements inherently integrate all information along the optical path, it is often difficult to isolate out-of-plane components in 3D flow events It is also hard to make independent measurements of internal flow structure Using an arrangement of 1D wavefront sensor, we have developed a system that uses tomographic reconstruction to make 2D measurements in arbitrary flow These measurements provide complete information in a plane normal to the flow We have applied this system to the subsonic free jet because of the wide range of flow scales available These measurements rely on the development of a series of 1D wavefront sensors that are used to measure line-integral density variations in the flow of interest These sensors have been constructed using linear CCD cameras and binary optics lenslet arrays In designing these arrays, we have considered the coherent coupling between adjacent lenses and have made comparisons between theory and experimental noise measurements This paper will present examples of the wavefront sensor development, line-integral measurements as a function of various experimental parameters, and sample tomographic reconstructions

Shot noise performance of Hartmann and shearing interferometer wavefront sensors

Abstract: The performance of a ground-based optical imaging system is severely degraded from the diffraction limit by the random effects of the atmosphere. Adaptive optics techniques have been used to compensate for atmospheric turbulence effects. A critical component in the adaptive optics system is the wavefront sensor. Presently, two types of sensors are common: the Hartmann-Shack wavefront sensor and the shearing interferometer wavefront sensor. In this paper we make a direct performance comparison of these two sensors. The performance calculations are restricted to common configurations of these two sensors and the fundamental limits imposed by shot noise and atmospheric effects. These two effects encompass the effects of extended reference beacons and sensor subaperture spacings larger than the Fried parameter, ro. Our results indicate comparable performance for good seeing conditions, and small beacons. However, for poor seeing conditions and extended beacons, the Hartmann sensor has lower error levels than the Shearing interferometer.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Laser beacon system for the UnISIS adaptive optics system at the Mount Wilson 2.5-m telescope

Abstract: A 50 watt excimer laser (lambda equals 351 nm) has been installed at the Mt. Wilson 2.5- meter telescope in California as part of the UnISIS adaptive optics system. This laser is used to produce Rayleigh guide stars 18 km above Mt. Wilson. In its initial configuration the projection optics are used to create a single laser guide star. The optical system is designed to allow an easy switch to accommodate three laser guide stars if (1) the laser return signal is sufficiently bright and (2) the laser guide star wavefront sensor has a read noise low enough to detect the split signal. The three guide stars are projected simultaneously in a triangular configuration above the telescope pupil. This three laser guide star system design is the first to confront directly the problem of focal anisoplanatism with an array of laser guide stars. The three guide star array provides a test for theoretical analyses of arrays of laser guide stars which will be an inevitable part of the adaptive optics systems of 8-meter and 10-meter class telescopes.

Performance of laser guide star adaptive optics at Lick Observatory

Abstract: A sodium-layer laser guide star adaptive optics system has been developed at Lawrence Livermore National Laboratory (LLNL) for use on the 3-meter Shane telescope at Lick Observatory. The system is based on a 127-actuator continuous-surface deformable mirror, a Hartmann wavefront sensor equipped with a fast-framing low-noise CCD camera, and a pulsed solid-state-pumped dye laser tuned to the atomic sodium resonance line at 589 nm. The adaptive optics system has been tested on the Shane telescope using natural reference stars yielding up to a factor of 12 increase in image peak intensity and a factor of 6.5 reduction in image full width at half maximum (FWHM). The results are consistent with theoretical expectations. The laser guide star system has been installed and operated on the Shane telescope yielding a beam with 22 W average power at 589 nm. Based on experimental data, this laser should generate an 8th magnitude guide star at this site, and the integrated laser guide star adaptive optics system should produce images with Strehl ratios of 0.4 at 2.2 {mu}m in median seeing and 0.7 at 2.2 {mu}m in good seeing.

At-wavelength testing of optics for EUV

Abstract: Optical systems for extreme ultraviolet (EUV) lithography require optical elements with wavefront aberrations limited to a fraction of an EUV wavelength to achieve diffraction-limited performance. Achieving wavefront and surface figure metrology at this level of accuracy is one of the key challenges in the development of EUV lithography. We have successfully built and operated a prototype EUV point diffraction interferometer which is capable of performing wavefront measurement of EUV optical elements at their operational wavelength. Initial experiments to characterize the interferometer, and to measure the optical wavefront diffracted from a Fresnel zone plate lens are discussed.

Dynamic wavefront sensor system using spatial light modulators

Abstract: Liquid crystal televisions (LCTVs) have become very popular spatial light modulators Their polarization and phase modulation capabilities allow them to be used as inexpensive spatial light modulators in a wide variety of applications The design of a dynamic Hermann wavefront sensor system is described A LCTV is used as an aberration generator in an optical system A LCTV is also used as a Harmann wavefront sensor to measure the aberrations Experimental results characterizing the LCTVs performance as an aberration generator and Harmann wavefront sensor are presented

Optimum matching of wavefront sensor and wavefront corrector

Abstract: In an adaptive optical system, the effectiveness of wavefront correction is influenced by relative configuration (matching) of the subapertures of the wavefront sensor and actuators of the wavefront corrector. In this paper, the computer simulations for the systems using Hartmann-Shack wavefront sensor, deformable mirror with continuous face plate, and discrete actuators are reported. In the simulation we use the algorithm of direct wavefront slope control. For different configurations of wavefront sensor and wavefront corrector, the wavefront reconstruction matrixes are established based on the measured influence function of deformable mirrors developed in the Institute of Optics and Electronics. The process of wavefront sensing, reconstruction, and correction for different Zernike terms and a series of wavefront induced by Kolmogorov turbulence are simulated. The criteria for evaluating the configurations are residual error of correction and stability of reconstruction matrix (condition number). The results of this investigation show that the configurations in which each subaperture is controlled mainly by three actuators have smaller residual error and better stability. The arrangements of square subaperture with 4 actuators at the corners are unstable and result in a checkerboard pattern of residual wavefront.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Comparison of Shack-Hartmann and curvature sensing for large telescopes

Abstract: This paper reports the results of a simulation comparing the performance of the following wavefront sensors when used in a closed loop astronomical adaptive optics (AO) system: a 48- element Shack-Hartmann, a 20-element Shack-Hartmann, and a 20-element curvature sensor. The method chosen for wavefront reconstruction in each case is based on a modal interaction matrix technique with zernike polynomials chosen as the basis modes. No attempt is made to include a real mirror model in the simulation, thus the evaluation of the sensing technique is decoupled from specific mirror technologies. Two different seeing conditions are simulated with various guide star magnitudes. The wavefront distortions are sensed in the visible and the point spread function of the corrected wavefront is recorded in the near infra-red. The effects of photon statistics and various levels of sensor pixel readout noise are also included in the simulations.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Method for optimizing closed-loop adaptive optics wavefront reconstruction algorithms on the basis of experimentally measured performance data

Abstract: An experimental method is presented for optimizing the wavefront reconstruction algorithm of a closed-loop adaptive optics system employed with an astronomical telescope. This technique uses wavefront sensor measurements from an independent scoring sensor to calculate adjustments to the reconstruction algorithm which will minimize the residual mean-square phase distortion. The method applies to closed-loop adaptive optics systems incorporating one or more guidestars, a wavefront reconstruction algorithm that is equivalent to a matrix multiply, and one or more deformable mirrors. Simulation results are reported for the case of a hybrid adaptive optics system incorporating one natural guidestar, one laser guidestar, and one deformable mirror. Differences in the spatial resolution of the wavefront sensors for the two guidestars are considered, particularly reduced resolution natural guidestar sensors for use with dim stars.

Pupil-imaging wavefront gradient sensor

Abstract: A wavefront sensor design is presented The wavefront sensor will be applied typically to adaptive correction of a ground based astronomical telescope It comprises a pair of crossed biprisms to act as star image splitters in the telescope image plane, splitting the incident wave into four beams The biprisms are followed by a lens which forms four pupil images These exhibit intensity map differences related to the phase gradient of the incident wave, providing that the point spread function overlaps the biprism crossing This allows standard wavefront reconstruction algorithms to be used to reconstruct the phase map over the aperture The results from a computer diffraction simulation of this system are presented to show the useful range of the sensor In addition, a design for an experimental prototype system is presented

Photon-counting image-acquisition technique and its applications

Abstract: An experimental setup of photon counting real-time image acquisition system is introduced, wherein a photon image head (an image intensifier with high radiant emittance gain) is coupled with a high frame rate CCD camera by a super powerful relay lens. The restrictions on luminous emittance of object are analyzed for multiphoton and single photon imaging modes. The methods of determining readout noise are introduced. The application examples of system in adaptive optics wavefront sensor operated with a faint object, and in experimental study on the optical wave-particle duality and the uncertainty relation are presented.

Measurements of profile of energy density and wavefront of pulsed laser beam propagating through near-resonant medium

Abstract: We measured the energy density profile and wavefront of the laser beam through the near resonator sodium heat pipe using a linear sensor camera and a Shack-Hartmann type wavefront sensor. Development of the wavefront was obtained as a function of laser wavelength, and self-focusing and -defocusing were observed for a small laser beam diameter. The energy density profile of the laser beam shows good agreement with the result of our simulation code based on the coupled Maxwell-Density Matrix equations including transverse and time- dependent variations. For the laser beam with large diameter, change of wavefront from the convex to the concave in the shorter wavelength region was observed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Aspherical wavefront testing with several defocusing steps

Abstract: A deep aspheric wavefront produces many interference fringes even when the optimum defocusing term is selected. The large number of fringes imposes a limit to the degree of asphericity that may be measured. A procedure using several interferograms with several defocusing values is described.

PAMELA: phase I testing and verification on a 0.5-m diameter telescope with a 36-segment adaptive primary mirror

Abstract: The Phased Array Mirror, Extendible Large Aperture (PAMELA) prototype telescope phase I testing and verification has been completed. The prototype telescope is the first to have a fully adaptive primary mirror which consists of 36 hexagonal injection-molded Pyrex segments that are seven centimeters flat-to-flat. The segments are mounted on three long-throw voice-coil actuators for tip, tilt, and piston motion. The segments' tilts are measured with a Hartmann- Shack wavefront sensor, and the piston errors between adjacent segments are measured via inductive edge-sensors. The 0.5 meter telescope was successfully operated with simultaneously closed tilt and piston control loops for the entire array. Phase I test and verification results are shown for the closed loop operations.

Dual-wavelength interferometer for testing projection lenses

Abstract: A custom-built phase shifting interferometer is described for measuring the wavefront quality of Ultratech Stepper 1X projection lenses over a field of 25 mm X 50 mm using wavelengths of 363.8 nm (for i-line systems) and 442.0 nm (for gh-systems). Over an N.A. of 0.42 the wavefront is determined at 240 X 240 points with a calibrated accuracy of (lambda) /20 peak-to-valley (at (lambda) equals 363.8 nm). The interferometer is an unequal path Twyman-Green interferometer with an Argon-laser and a HeCd-laser as light sources. The interferometer test system is integrated on a 10 ft X 4 ft vibration isolation table together with an xy-stage to position the projection lens at different field points. The safe handling of the projection lens is supported by a load/unload mechanism on the table. The measurement wavefront maps at various field points of the tested projection lens are used to optimize the state of alignment of the projection lens.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.