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Adaptive Optics for Industry and Medicine: Proceedings of the 4th International Workshop Münster, Germany, Oct. 19-24, 2003

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No detectada 2005 SpringerLink

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Micromachined Membrane Deformable Mirrors

G. Vdovin

Development of adaptive optics, initiated 50 years ago with the article of Babcock [1], resulted in impressive technical and scientific results in military and astronomical applications. These results were obtained on a high price using custom-developed complex adaptive optical systems. Adaptive optics has a great potential to be applied in a range of optical systems, including imaging, ophthalmic, laser, optical communications and information processing. These systems are marketed widely and use relatively inexpensive parts with high performance. This article presents a fragmentary analysis of the current state and possible future development of inexpensive deformable mirrors for the industry and medicine. The analysis is mainly based on the results obtained by the author and his colleagues at the TU Delft and OKO Technologies during 1993-2003.

Part I - Wavefront Correctors and Mirror Control | Pp. 3-8

The Development and Optimisation of High Bandwidth Bimorph Deformable Mirrors

D. Rowe; L. Laycock; M. Griffith; N. Archer

Our first mirror designs were based on a standard bimorph construction and exhibited a resonant frequency of 1 kHz with a maximum stroke of ±5 μm. These devices were limited by the requirement to have a “dead space” between the inner active area and the mirror boundary. This was necessary to ensure that the requirements for both the stroke and the static boundary conditions at the edge of the mirror could be met simultaneously, but there was a significant penalty to pay in terms of bandwidth, which is inversely proportional to the square of the full mirror diameter. In a series of design iteration steps, we have created mounting arrangements that seek not only to reduce dead space, but also to improve ruggedness and temperature stability through the use of a repeatable and reliable assembly procedure. As a result, the most recently modeled mirrors display a resonance in excess of 5 kHz, combined with a maximum stroke in excess of ±10 μm. This has been achieved by virtually eliminating the “dead space” around the mirror. By careful thermal matching of the mirror and piezoelectric substrates, operation over a wide temperature range is possible. This paper will discuss the outcomes from the design study and present our initial experimental results for the most recently assembled mirror.

Part I - Wavefront Correctors and Mirror Control | Pp. 9-15

Deformable Mirrors with Thermal Actuators

G. Vdovin; M. Loktev

Adaptive optics is applied in lasers, scientific instrumentation, ultrafast sciences, ophthalmology and material processing. For successful use in these applications, the deformable mirrors must be simple, inexpensive, reliable and efficient. Most of the currently used technologies based on piezoelectric, electrostrictive, electromagnetic and electrostatic actuation are rather expensive. We report on a novel type of ultra-low-cost deformable mirror with actuators based on thermal expansion. The 19-channel one inch deformable mirror has response time of ≈ 15 s, actuator stroke of about 6 μm, temporal stability of about ⋋/10 rms in the visible range. The mirror has shown good correction ability for low-order Zernike polynomials, therefore it can be used for correction of rather large aberrations with slow changing amplitudes in both temporal and spatial domains.

Part I - Wavefront Correctors and Mirror Control | Pp. 17-24

Technology and Operation of a Liquid Crystal Modal Wavefront Corrector

M. Loktev; G. Vdovin

Possibilities for implementation of a reflective-type liquid crystal modal wavefront corrector (LC-MWC) based on the silicon technology are discussed. Two possible corrector’s configurations are considered; the first one is based on a continuous thin-film resistive layer, and the second one uses a network of discrete IC resistors. Technological issues of the manufacturing of silicon-based LC-MWC are discussed. Results of analysis of correction efficiency and various operation modes are presented.

Part I - Wavefront Correctors and Mirror Control | Pp. 25-33

Aberration Compensation Using Nematic Liquid Crystals

S. Somalingam; M. Hain; T. Tschudi; J. Knittel; H. Richter

We have developed a novel transmissive nematic liquid crystal device which is capable of compensating spherical wavefront aberration that occurs during the operation of optical pickup systems. In order to increase the storage capacity, next generation optical data storage systems beyond CD and DVD will use according to the Blu-Ray specification (BD) blue laser light and an objective lens with high numerical aperture (N.A.) of 0.85. However, such high N.A. systems have an inherent higher sensitivity on aberrations. For example spherical aberration is inversely proportional to the wavelength and grows with the fourth power of N.A. of the objective lens. In an optical pickup system there are two sources for spherical aberration: The first one is the variation of the substrate thickness due to manufacturing tolerances under mass production conditions. The second one concerns disks with multiple data-layers, which cause spherical aberration when layers are switched, as the objective lens can only be optimized for a single layer thickness. We report a method for effective compensation of spherical aberration by utilizing a novel liquid crystal device, which generates a parabolic wavefront profile. This particular shape makes the device highly tolerant against lateral movement. A sophisticated electrode design allows us to reduce the number of driving electrodes down to two by using the method of conductive ladder mashing. Further evaluation in a blue-DVD test drive has been carried out with good results. By placing the device into an optical pick-up we were able to readout a dual-layer ROM disk with a total capacity of 50 gigabytes (GB). A data-to-clock jitter of 6.9% for the 80 μm and of 8.0% for the 100 μm cover layer could be realized.

Part I - Wavefront Correctors and Mirror Control | Pp. 35-43

Wireless Control of an LC Adaptive Lens

G. Vdovin; M. Loktev; X. Zhang

We consider using liquid crystal adaptive lenses to correct the accommodation loss and higher-order aberrations of the human eye. In this configuration, the adaptive lens is embedded into the eye lens implant and can be controlled through a wireless inductive link. In this work we experimentally demonstrate a wireless control of a liquid crystal adaptive lens in a wide range of its focusing power by using two coupled coils with the primary coil driven from a low-voltage source through a switching control circuit and the secondary coil used to drive the lens.

Part I - Wavefront Correctors and Mirror Control | Pp. 45-51

Summary of Adaptive Optics at Stanford

P. Lu; Y.-A. Peter; E. Carr; U. Krishnamoorty; I.-W. Jung; O. Solgaard; R. Byer

The status of adaptive optics at Stanford is summarized. Particular focus is given to the fabrication and testing of segmented, micro-mirror SLMs developed under the CCIT (Coherent Communications, Imaging, and Targeting) project [1]. Square and hexagonal “5 ß 5” and “32 ß 32” arrays have been fabricated using MEMS technology, and “5 ß 5” arrays have been characterized.

Part I - Wavefront Correctors and Mirror Control | Pp. 53-59

Control of a Thermal Deformable Mirror: Correction of a Static Disturbance with Limited Sensor Information

M. de Boer; K. Hinnen; M. Verhaegen; R. Fraanje; G. Vdovin; N. Doelman

This research considers the control of a Thermal Deformable Mirror (TDM), which is used to correct a static aberration in a light beam. The TDM is a cheap, but relatively slow, deformable mirror suited for the correction of static and thermal aberrations. Correction of quasi-static disturbances can, for example, be used to enhance microscope images, or to improve the quality of femto-second light wave packages. The control objective is to maximize the light power that drops through a pinhole after focusing of the beam in a minimal time span. Only the light power behind the pinhole is available for feedback. The slow dynamics of the mirror and the lack of measurement information complicate the control task. The optimization algorithm (OA) used to maximize the light power is based on consecutive line searches in the direction of several Zernike modes. The OA operates on the surface shape of the mirror, rather than the mirror input voltages. To speed up and to improve the performance of the algorithm, model predictive control (MPC) is used to adjust the mirror shape to the shape provided by the OA. The effectiveness of using MPC has been experimentally validated. The disturbance in the experiments consists of a focus and tilt misalignment, and the zero-input shape of the TDM. Without MPC, the light power reaches 40% of the maximum light power within 3000 iterations. When applying the MPC controller, 40% of the maximum intensity is obtained within 700 iterations, and 50% within 2200 iterations.

Part I - Wavefront Correctors and Mirror Control | Pp. 61-71

A Novel Microprocessor-Controlled High-Voltage Driver for Deformable Mirrors

H.-M. Heuck; I. Buske; U. Buschmann; H. Krause; U. Wittrock

Genetic algorithms have found widespread use in adaptive optics. One important advantage compared to the matrix inversion method is that it is not necessary to measure the wavefront of the optical beam on which the deformable mirror acts. Instead, any signal, that is monotonic increasing with the quality of the desired wavefront, is sufficient as a feedback signal. Often, this signal derives from a power-in-the-bucket measurement in the far-field of the beam. In coherent control of chemical reactions with temporally shaped femtosecond laser pulses the signal derives from the rate at which the desired chemical is produced. In our adaptive optics experiments we use micro-machined silicon membrane mirrors with up to 59 electrostatic actuators. We have developed a microprocessor-controlled highvoltage driver for up to 60 channels because we could not find a suitable driver on the market. The driver is a fully self-contained unit that only needs input power and communicates with a personal computer via a USB interface. It is especially suited for controlling adaptive mirrors with a genetic algorithm. The driver can store up to 100 voltage patterns, apply them sequentially to the mirror, store up to 4 feedback signals for each voltage pattern, and relay these feedback signals back to the host computer. We will discuss performance issues and tradeoffs like speed, bit resolution, and number of electrodes in our presentation.

Part I - Wavefront Correctors and Mirror Control | Pp. 73-81

Preliminary Investigation of an Electrostatically Actuated Liquid-Based Deformable Mirror

E.M. Vuelban; N. Bhattacharya; J.M. Braat

The increased emphasis on corporate social responsibility has provided much-needed efforts in helping young people in a wide variety of ways. Societal, economic, and human resources reasons have encouraged more organizations to help youth. In addition, many forward-thinking organizations have done much to volunteer their employees’ time, develop programs, and donate resources. There are countless examples of companies that support youth development; however, the results of their programs are not widely disseminated, leaving companies that want to start their own efforts often either duplicating what might already be available or developing a program that will be less than effective. Many of the best-practice companies realize the steps necessary to lead to successful programs and have partnered with existing agencies to serve those who need it most. Their accomplishments are commendable and should be shared with others.

Part I - Wavefront Correctors and Mirror Control | Pp. 83-89

Interferometer-Based Adaptive Optical System

O. Soloviev; G. Vdovin

Interferometer-based adaptive optics has an advantage of direct measurement of the wavefront profile. Nevertheless the majority of adaptive optical systems, realized so far, use other types of wavefront sensors, such as Hartmann sensors. Interferometric sensors have two problems: (1) a source of a coherent reference wave should be present and (2) in many cases it is impossible to reconstruct the wavefront unambiguously from a single interferogram, due to the illposed nature of the phase unwrapping problem. In the case of an adaptive optical system with a limited number of degrees of freedom, one can expect that the ambiguity of the solution will be partly or even completely eliminated by looking for the wavefront reconstruction only in the existing basis. We report on the expected performance of interferometric adaptive optical system with a 37- channel membrane deformable mirror, based on a computer model of a complete system.

Part I - Wavefront Correctors and Mirror Control | Pp. 91-99

Extended Hartmann-Shack Wavefront Sensor

B. Schäfer; K. Mann; M. Dyba

A combination of a Hartmann-Shack sensor and a standard farfield measurement on one single detector is proposed. The technique is fast, manages without moveable parts, thus permitting a very compact design. It is not only suited for characterisation of the wavefront distribution, but may also be considered for determination of the important parameters beam width, beam divergence and beam propagation ratio M of partially coherent laser beams. First results indicate that a fairly thorough beam characterisation including spatial coherence, propagation characteristics and beam quality can be achieved with this method.

Part II - Wavefront Sensors | Pp. 103-110

High Resolution Wavefront Sensing

J.E. Oti; V.F. Canales; M.P. Cagigal

High resolution wavefront sensors are devices with a great practical interest since they are becoming a key part in an increasing number of applications like extreme Adaptive Optics. We describe theoretically a novel wavefront sensor, which basically consists of a telescopic system with a linearly increasing amplitude mask placed at the intermediate common focal plane. This sensor offers high resolution and an easy adjustment of the sampling and of the dynamic range. The parameters and performance of the new sensor are discussed, and a comparison with commonly used Hartmann-Shack sensors is carried out. Our sensor presents several advantages. Resolution is higher, and consequently a larger number of modes can be estimated for reconstructing the wavefront, and the dynamic range and sampling can be easily adjusted. Furthermore, we show that a proper election of the mask parameters allows an acceptable performance even in adverse photon noise conditions.

Part II - Wavefront Sensors | Pp. 111-118

Distorted Grating Wavefront Sensing in the Midwave Infrared

D.M. Cuevas; L.J. Otten; P. Harrison; P. Fournier

Kestrel Corporation has extensive experience using distorted grating wavefront sensors (DGWFS) in a number of applications. The DGWFS has previously been demonstrated in the visible range (400–700 nm) by Kestrel and others. An experimental system was built in a laboratory environment to show that the DGWFS could recover wavefront characteristics of a midwave infrared (MWIR) laser. This paper describes the theory of the DGWFS and the experimental procedures implemented to run the system with the MWIR laser. The sensitivity to the type of gratings employed will be addressed. The results of sensitivity, dynamic range and thermal noise measurements will be discussed.

Part II - Wavefront Sensors | Pp. 119-127

Comparative Results from Shack-Hartmann and Distorted Grating Wavefront Sensors in Ophthalmic Applications

P. Harrison; G.R.G. Erry; P. Fournier; D.M. Cuevas; L.J. Otten; A. Larichev

The aim of this work was to determine the relative performance of a Shack-Hartmann (SH) wavefront sensor and a distorted grating wavefront curvature sensor (DGWFS) when used to measure the aberrations in the human eye. Previous work carried out by Kestrel and others suggests that the DGWFS is able to successfully reconstruct wavefronts in severely scintillated conditions in which SH sensors typically fail to give a good reconstruction. The poor performance of conventional SH sensors in scintillated conditions prevents their use in ophthalmic aberrometers with human subjects who have medical conditions such as cataracts. This limitation substantially restricts the percentage of the population that can take advantage of emerging technology enabled by having accurate aberration data for the anterior segment. The SH sensor utilized has a novel dithered reference source which mitigates scintillation problems. However, the DGWFS potentially offers a simpler, lower cost and more robust solution.

Part II - Wavefront Sensors | Pp. 129-139

Shack-Hartmann Sensors for Industrial Quality Assurance

J. Pfund; M. Beyerlein; R. Dorn

The measurement of the deviations of a test sample from its ideal shape is the key to quality assurance in fabrication processes of the optical industry. Here, interferometers are often used as standard tools. However, in recent years Shack-Hartmann sensors have been introduced in various applications with competitive performance. We show two typical application examples, the null test of a sphere where the highest accuracy is obtained and a non null test of an aspherical surface. In the latter case the Shack-Hartmann sensor is superior to an interferometer due to its higher dynamic range.

Part II - Wavefront Sensors | Pp. 141-150

Single-Chip Neural Network Modal Wavefront Reconstruction for Hartmann-Shack Wavefront Sensors

T. Nirmaier; G. Pudasaini; C. Alvarez Diez; J. Bille; D.W. de Lima Monteiro

We describe existing concepts for CMOS-based Hartmann-Shack wavefront sensors and propose the next step for a further miniaturization of adaptive optics by integrating the modal wavefront reconstruction on chip. In conventional Hartmann-Shack wavefront sensing a CCD camera is placed on the focal plane of a microlens array. The spot pattern in the focal plane is captured, analyzed by image processing and the wavefront is often decomposed into orthogonal modes, usually expressed in coe.cients of Zernike polynomials. The single-chip modal wavefront reconstructor would include all these processing steps in a single chip. We propose to use already existing CMOS-based wavefront sensor concepts and add a hardware arti.cial neural network for modal wavefront reconstruction.

Part II - Wavefront Sensors | Pp. 151-161

CMOS Technology in Hartmann-Shack Wavefront Sensing

D.W. de Lima Monteiro; T. Nirmaier

We assess, in this paper, the use of CMOS technology (Complementary-Metal-Oxide-Semiconductor) for the fabrication of fast wavefront sensors based on the Hartmann-Shack technique. We briefly recapitulate the core of this technology and we point out its pros and cons with respect to the sensor performance. Focusing on fast operation, we compare conventional image sensors with custom layout approaches that make use of position-sensitive detectors (PSDs). We also present the results obtained with three different CMOS sensor concepts implemented so far:

Heidelberg1: 0.6 μm — AMS — 16 × 16-PSDs alternate — winner-take-all digital readout

Heidelberg2: 0.35 μm — AMS — 8 × 8-PSDs chessboard-like — winner-take-all / resistive ring [2]

Delft1: 1.6 μm — DIMES — 8 × 8-PSDs quad cell — passive pixels [3, 4] The aim is to identify the practical capabilities of standard CMOS technology in wavefront sensing.

Part II - Wavefront Sensors | Pp. 163-175

Generalised Phase Diversity Wavefront Sensor

A.H. Greenaway; H.I. Campbell; S. Restaino

Phase-Diversity is an algorithm for reconstruction of wavefront phase from data corresponding to images of the input wavefront intensity on two planes normal to the direction of propagation and located at different positions along the axis of propagation. These planes are generally described as symmetrically placed about the image plane, but can equally well be symmetrically placed about the system input pupil. In this case the phase diversity algorithm becomes essentially the same as the wavefront curvature algorithm. For reconstruction of the wavefront phase the inverse problem is presented in terms of the di.erential Intensity Transport Equation and solved either iteratively or through use of Green’s functions. Here we will explore what other aberrations, other than defocus, can be used in a generalised phase diversity wavefront reconstruction. The possible advantages of this approach will be considered.

Part II - Wavefront Sensors | Pp. 177-186

Generalised Phase Diversity: Initial Tests

S. Zhang; H.I. Campbell; A.H. Greenaway

Some early results demonstrating the performance of the Generalised Phase Diversity Wavefront Sensor were presented. In these computer simulations we would seek to validate the theoretical analysis that we have previously published and to explore the optimisation of the sensor for various forms of wavefront error. Consideration would be given to the extent to which optimisation that exploits information about the wavefront decreases the chance to detect other wavefront characteristics.

Part II - Wavefront Sensors | Pp. 187-196

Prime Microlens Arrays for Hartmann-Shack Sensors: An Economical Fabrication Technology

D.W. de Lima Monteiro; O. Akhzar-Mehr; G. Vdovin

A Hartmann-Shack wavefront sensor consists basically of two elements: a microlens array and a photosensitive detector. This paper presents a technique to fabricate close-packed microlens arrays compliant to the sensor requirements. The method is based on bulk-silicon anisotropic etching and requires a single etch mask. We first etch a micromirror array in a KOH solution and use it later as a mold for the replication of microlens arrays. The elements in the fabricated microlens arrays feature excellent fit to a parabolic mirror surface, 100% optical fill factor, excellent parallelism of the optical axes and very high precision of the array pitch. The uniformity of the focal length of the microlenses is high (in the order of 5%) and the surface roughness — expressed in terms of wavefront — is of the order of 8–13 nm. This technology also enables simple single-mask fabrication of arbitrary aspherical optical surfaces.

Part II - Wavefront Sensors | Pp. 197-205

A Proposal for Wavefront Retrieval from Hartmann Test Data

V.M. Duran-Ramirez; D. Malacara-Doblado; D. Malacara-Hernandez; D.P. Salas-Peimbert; G. Trujillo-Shiaffino

In the classical Hartmann test analysis, the transverse aberrations are represented by small straight segments joining two consecutive measured points in the pupil. In the analysis presented in this paper we propose the wavefront to be synthesized by many nonflat functions whose domains are the squares defined by the square unit cell defined by an array of holes in the Hartmann screen placed at the exit pupil of the system. Two advantages of this method are that a higher precision in the wavefront retrieval is obtained and second that the local curvatures and astigmatism with their corresponding axes are obtained.

Part II - Wavefront Sensors | Pp. 207-214

Use of Intracavity Adaptive Optics in Solid-State Lasers Operation at 1 µm

W. Lubeigt; P. van Grol; G. Valentine; D. Burns

An intracavity 37-element deformable membrane mirror (DMM) has been used in order to control the transverse mode profile of a diode-pumped solidstate laser. Automatic spatial mode and output power optimisation of Nd:YVO end-pumped and Nd:YAlO side-pumped lasers are demonstrated using a closed-loop genetic algorithm. Transverse mode and power optimisation of a diode-pumped, grazing incidence Nd:GdVO laser has been performed successfully. The optimisation procedure featured a genetic algorithm ensuring the global maximum is attained. Using a Michelson interferometer with the DMM operating intracavity, the DMM was found to present negligible deformation when used with a power density of 115W/cm but noticeable deformation appeared with a power density of 1.25 kW/cm.

Part III - Laser resonators and laser amplifiers | Pp. 217-227

Intracavity Use of Membrane Mirrors in a Nd:YVO Laser

P. Welp; I. Buske; U. Wittrock

10mm and 15mm diameter adaptive membrane mirrors have been implemented into a Nd:YVO laser. The laser performance has been investigated and compared to a similar laser without an adaptive mirror. While some membrane mirrors did not change the laser behaviour, several other membrane mirrors lead to a switching between transverse modes. With less than 3 kW/cm, the maximum intensity on the mirror membrane was small compared to a damage threshold of more than 144 kW/cm.

Part III - Laser resonators and laser amplifiers | Pp. 229-236

Adaptive Optics for High-Power Laser Beam Control

A. Kudryashov; V. Samarkin; A. Alexandrov; A. Rukosuev; V. Zavalova

This paper presents an adaptive optical closed loop system with a bimorph mirror as a wavefront corrector and a Shack-Hartmann wavefront sensor to compensate for the aberrations of high power lasers. An adaptive system can correct for the low-order aberrations in the real-time — the frequency of corrected aberrations is less then 25 (30) Hz. The amplitude of such aberrations — about 7 microns. These parameters are mostly determined by the utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations — up to 30th Zernike polynomial (excluding tip-tilt). We are presenting the results of the use of our adaptive system in several TW laser systems such as ATLAS, LULI and Beijing Institute of Physics.

Part III - Laser resonators and laser amplifiers | Pp. 237-248

Aberrations of a Master-Oscillator-Power-Amplifier Laser with Adaptive Optics Correction

I. Buske; H.-M. Heuck; P. Welp; U. Wittrock

Beam quality and efficiency of high-power solid state lasers are limited by aberrations of the active medium. The aberrations are due to temperature gradients in the laser crystals that in turn are due to the inevitable heat generation in the crystal. The aberrations lead to high diffraction losses of the laser resonator and reduced output power.

We use a birefringence-compensation scheme consisting of a relay-imaging telescope and a 90° polarization rotator to eliminate the stress-induced birefringence. In order to further improve the beam quality, the remaining aberrations of the thermal lens have to be eliminated. It is important to know the type and strength of the aberrations to determine the requirements of the adaptive mirror.

We present the investigation of aberrations in a MOPA cw-Nd:YVO/Nd:YAG laser system in which we employ an adaptive membrane mirror in order to compensate for the aberrations of the power amplifier. A genetic algorithm is used to control the adaptive mirror. A suitable power-in-the-bucket measurement behind a diffraction-limited aperture generates the beam quality signal for the feedback loop. The beam quality (M) is improved by a factor of 2.8.

Part III - Laser resonators and laser amplifiers | Pp. 250-259

Dynamic Aberrations Correction in an ICF Laser System

Y. Zhang; Z. Yang; C. Guan; H. Wang; P. Jiang; B. Xu; W. Jiang

The wavefront aberrations in inertial confinement fusion (ICF) laser system consists of static aberration such as material inhomogeneous, optical figure error and assemble error and so on, and dynamics aberration such as pumping induced thermal distortion, nonlinear effect induced index fluctuation and so on. Two Years ago, an adaptive optical system with 45 correction elements was established for the wavefront control of ICF laser system. And the static aberration correction had been realized. Recently this adaptive optical system has been used to correct the lamp pumping induced thermal distortion. For the nanosecond scale pulse laser output, the directly close-loop operation of adaptive optical system is impossible. So the pre-correction method of the wavefront control has been adopted. The dynamic thermal distortion pre-correction has been realized. The system also successfully corrected the compound aberration both of static and thermal aberration. This work would be helpful for shortening the operation period of the ICF laser system.

Part III - Laser resonators and laser amplifiers | Pp. 261-271

Adaptive Shaping of High-Power Broadband Femtosecond Laser Pulses

T. Witting; G. Tsilimis; J. Kutzner; H. Zacharias; M. Köller; H. Maurer

We demonstrate the implementation of a feedback controlled pulse shaping device in a femtosecond high-power Ti:sapphire laser system. The laser system consists of a mirror dispersion controlled oscillator and a multipass amplifier with a pairing double prism compressor. The system provides pulses with a duration of 30 fs and an energy of up to 1.2 mJ per pulse at 1 kHz repetition rate. The phase distorted output pulses are phase modulated with a high resolution spatial light modulator (SLM). The pulse shaper consists of an all-reflective zero-dispersion compressor equipped with a liquid crystal array. For adaptive compression of the amplified pulses a feedback loop is implemented. A two-photon process is used to monitor the temporal pulse characteristics. To achieve the shortest possible pulse an evolutionary algorithm controls the pulse shaper utilizing the two-photon signal as feedback. With this set-up transform limited pulses are achieved. Detailed investigations of algorithm parameters and their effect on convergence behaviour have been performed and are compared with the experimental findings.

Part III - Laser resonators and laser amplifiers | Pp. 273-282

Wavefront Measurement and Adaptive Optics at the PHELIX Laser

H.-M. Heuck; U. Wittrock; C. Häfner; S. Borneis; E. Gaul; T. Kühl; P. Wiewior

The PHELIX (Petawatt High Energy Laser for Heavy Ion Research) laser project has been initiated to build a high energy, ultra high power laser for research purposes in connection with the heavy ion accelerator of the GSI. The PHELIX laser should provide ns-pulses with an energy up to 5 kJ and, alternatively, fs-pulses reaching 1 petawatt with an energy of 500 kJ. Aberrations due to beam transport and due to the amplification process limit the focus ability and the intensity on the target. For the amplification of the fs-pulse, the CPA (chirped pulse amplification) technique is used. Distortions in the phase also entail longer pulses during the compression in the CPA process.

Part III - Laser resonators and laser amplifiers | Pp. 283-290

ISTC Projects from RFNC-VNIIEF Devoted to Improving Laser Beam Quality

F. Starikov; G. Kochemasov

Information is given about the Projects # 1929 and # 2631 supported by ISTC and concerned with improving laser beam quality and interesting for adaptive optics community. One of them, Project # 1929 has been recently finished. It has been devoted to development of an SBS phase conjugation mirror of superhigh conjugation quality employing the kinoform optics for high-power lasers with nanosecond scale pulse duration. With the purpose of reaching ideal PC fidelity, the SBS mirror includes the raster of small lenses that has been traditionally used as the in Shack-Hartmann wavefront sensor in adaptive optics. The second of them, Project # 2631, is concerned with the development of an adaptive optical system for phase correction of laser beams with wavefront vortex. The principles of operation of modern adaptive systems are based on the assumption that the phase is a smooth continuous function in space. Therefore the solution of the Project tasks will assume a new step in adaptive optics.

Part III - Laser resonators and laser amplifiers | Pp. 291-301

Adaptive Optical System for Retina Imaging Approaches Clinic Applications

N. Ling; Y. Zhang; X. Rao; C. Wang; Y. Hu; W. Jiang; C. Jiang

We presented “A small adaptive optical system on table for human retinal imaging” at the 3rd Workshop on Adaptive Optics for Industry and Medicine. In this system, a 19 element small deformable mirror was used as wavefront correction element. High resolution images of photo receptors and capillaries of human retina were obtained. In recent two years, at the base of this system a new adaptive optical system for human retina imaging has been developed. The wavefront correction element is a newly developed 37 element deformable mirror. Some modifications have been adopted for easy operation. Experiments for different imaging wavelengths and axial positions were conducted. Mosaic pictures of photoreceptors and capillaries were obtained. 100 normal and abnormal eyes of different ages have been inspected.The first report in the world concerning the most detailed capillary distribution images cover ±3° by ± 3° field around the fovea has been demonstrated. Some preliminary very early diagnosis experiment has been tried in laboratory. This system is being planned to move to the hospital for clinic experiments.

Part IV - Medical Applications | Pp. 305-315

Adaptive Optics to Simulate Vision with a Liquid Crystal Spatial Light Modulator

S. Manzanera; P.M. Prieto; J. Salort; E.J. Fernández; P. Artal

Adaptive optics for the human eye has two main applications: to obtain high-resolution images of the retina and to produce aberration-controlled retinal images to simulate vision. We built an adaptive optics prototype specially designed for visual testing based on the use of a liquid crystal spatial light modulator (Hamamatsu X8267). The system consists of a measurement channel with switchable red (633 nm) or infrared (780 nm) illumination, a real time Hartmann-Shack sensor (25 Hz), and an additional channel allowing subjects to perform visual tasks in green light through the adaptive optics system. We tested the modulator, both as aberration generator and as corrector, first in an artificial eye and then routinely in different living eyes. This device has advantages in terms of effective stroke and mode independence allowing production and compensation of a larger range of aberrations than with other correcting devices. This opens new possibilities for visual applications of adaptive optics. However, a low temporal response and diffraction effects may be important drawbacks of the modulator for some particular applications. Examples of the performance of the system and a discussion of its limitations and potential for performing visual optics experiments will be presented.

Part IV - Medical Applications | Pp. 317-324

Confocal Scanning Retinal Imaging with Adaptive Optics

I. Iglesias; B. Vohnsen; P. Artal

We have developed a prototype of a confocal scanning laser ophthalmoscope that incorporates adaptive optics to correct for the wavefront aberrations of the eye and those induced by the optical system. Two corrector devices were tried out in the experiments: a membrane deformable mirror and a liquid crystal spatial light modulator. We obtained high-resolution images of different parts of the retina with and without the wavefront correction. We also explored alternative adaptive optics configurations to improve on the imaging performance of the system.

Part IV - Medical Applications | Pp. 325-332

A High-Resolution Adaptive Optics Fundus Imager

G.R.G. Erry; L.J. Otten; A. Larichev; N. Irochnikov

The spatial resolution of retinal images is limited by the presence of time-varying aberrations present within the eye. A low-order adaptive optics system using a Shack-Hartmann wavefront sensor and a bimorph deformable mirror is described. Using a standard Hitachi video camera and a PC running Windows 2000, control of low-order aberrations within the eye can be achieved, enabling high resolution images to be obtained. The wavefront sensor employs a novel dithered reference technique to reduce speckle recorded in the hartmannogram. As a measurement of the residual errors within the image is known, it is possible to use post-processing techniques to further improve the resolution of the image. Example images of retinas taken with the system and then post-processed are shown, demonstrating the ability to calculate near diffraction-limited images. Data on the performance of the system are presented.

Part IV - Medical Applications | Pp. 333-341

Perceived Image Quality Improvements from the Application of Image Deconvolution to Retinal Images from an Adaptive Optics Fundus Imager

P. Soliz; S.C. Nemeth; G.R.G. Erry; L.J. Otten; S.Y. Yang

: The objective of this project was to apply an image restoration methodology based on wavefront measurements obtained with a Shack-Hartmann sensor and evaluating the restored image quality based on medical criteria.: Implementing an adaptive optics (AO) technique, a fundus imager was used to achieve low-order correction to images of the retina. The high-order correction was provided by deconvolution. A Shack-Hartmann wavefront sensor measures aberrations. The wavefront measurement is the basis for activating a deformable mirror. Image restoration to remove remaining aberrations is achieved by direct deconvolution using the point spread function (PSF) or a blind deconvolution. The PSF is estimated using measured wavefront aberrations. Direct application of classical deconvolution methods such as inverse filtering, Wiener filtering or iterative blind deconvolution (IBD) to the AO retinal images obtained from the adaptive optical imaging system is not satisfactory because of the very large image size, dificulty in modeling the system noise, and inaccuracy in PSF estimation. Our approach combines direct and blind deconvolution to exploit available system information, avoid non-convergence, and time-consuming iterative processes. : The deconvolution was applied to human subject data and resulting restored images compared by a trained ophthalmic researcher. Qualitative analysis showed significant improvements. Neovascularization can be visualized with the adaptive optics device that cannot be resolved with the standard fundus camera. The individual nerve fiber bundles are easily resolved as are melanin structures in the choroid. : This project demonstrated that computer-enhanced, adaptive optic images have greater detail of anatomical and pathological structures.

Part IV - Medical Applications | Pp. 343-352

Adaptive Aberrometer for Acuity Measurements and Testing

A. Larichev; N. Irochnikov; S. Gorbunov

We present the novel instrument combining the Shack-Hartman wavefront sensor, compensator of low order aberrations and bimorph adaptive mirror for high order aberration correction. We have tested the developed instrument in the clinical environment on human subjects. The measurement rate was 77 frames per second. The typical residual error of correction was between 0.1-0.15 microns was achieved. The wavefront can be reconstructed in form of up to 36 Zernike polynomials. Automatic low order aberration compensator allows introduction of spherical and astigmatic terms with amplitudes up to ±15D and ±6D correspondingly and accuracy of 0.05 D. An 18 electrode bimorph deformable mirror corrector makes it possible to model Zernike aberrations up to the 3-th order + Spherical aberration with RMS amplitudes up to 1 micron. The results of measurements were compared with clinical refraction and good correspondence was found.

Part IV - Medical Applications | Pp. 353-362

Adaptive optics with strong scintillation and optical vortices for optical communication

C. Paterson; C.R. Walker

In the classical Hartmann test analysis, the transverse aberrations are represented by small straight segments joining two consecutive measured points in the pupil. In the analysis presented in this paper we propose the wavefront to be synthesized by many nonflat functions whose domains are the squares defined by the square unit cell defined by an array of holes in the Hartmann screen placed at the exit pupil of the system. Two advantages of this method are that a higher precision in the wavefront retrieval is obtained and second that the local curvatures and astigmatism with their corresponding axes are obtained.

Part V - Atmospheric Propagation | Pp. 365-375

Wavefront Measurement over an Extended Horizontal Path Using a Wavefront Curvature Sensor

J. Burnett; S. Woods; A. Turner; A. Scott

This paper reports on the results of wavefront curvature sensor measurements over horizontal paths of 66m and 4 km. The wavefront curvature sensor used has been developed at QinetiQ and is based on the use of a quadratically distorted diffraction grating to enable the simultaneous recording of two symmetrically separated planes about the entrance pupil of a telescope. The measurements allow us to characterize the spatio-temporal nature of the wavefront errors and therefore enable us to estimate the wavefront sensor (WFS) and deformable mirror (DM) requirements for the development of an adaptive optic system (AOS). For the 66m path the dynamic range and frame-rate of the WFS camera was found to be adequate to drive the AOS, although the software based control resulted in intermittent performance. The data for the 4 km path suggested that the frame-rate of the WFS camera was at least a factor of 3 slower than would be necessary to either drive the AOS or make any detailed conclusions about the spatial analysis.

Part V - Atmospheric Propagation | Pp. 377-387

The Detection of Atmospheric Tip-Tilt and its Program Construction in Lunar Laser Ranging

G. Rui; X. Yaoheng

Based on the fact that the returned photon numbers are too low in Lunar Laser Ranging (LLR) and the new method that effects of atmospheric tiptilt are considered, the two algorithms for computation of atmospheric tip-tilt, absolute difference and cross correlation, are investigated. Programs are applied to the extended target of the lunar surface. The atmospheric tip-tilt is computed in different conditions, the best algorithm is selected. A new method is put forward to reduce the time in atmospheric tip-tilt computing, and the possibility on real-time compensation in Lunar Laser Ranging is positive.

Part V - Atmospheric Propagation | Pp. 389-395

Información

Tipo: libros

ISBN impreso

978-3-540-23978-9

ISBN electrónico

978-3-540-28867-1

Editor responsable

Springer Nature

País de edición

Reino Unido

Fecha de publicación