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Adaptive Optics for Industry and Medicine: Proceedings of the 4th International Workshop Münster, Germany, Oct. 19-24, 2003
Ulrich Wittrock (eds.)
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Palabras clave – provistas por la editorial
Optics; Optoelectronics; Plasmonics and Optical Devices; Optics and Electrodynamics; Engineering; general; Medicine/Public Health; general; Physics; general
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Micromachined Membrane Deformable Mirrors
Development of adaptive optics, initiated 50 years ago with the article of Babcock , 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 . 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