Non-contact measurement machine for freeform optics

The performance of high-precision optical systems using spherical optics is limited by aberrations. By applying aspherical and freeform optics, the geometrical aberrations can be reduced or eliminated while at the same time also reducing the required number of components, the size and the weight of the system. New manufacturing techniques enable creation of high-precision freeform surfaces. Suitable metrology (high accuracy, universal, non-contact, large measurement volume and short measurement time) is key in the manufacturing and application of these surfaces, but not yet available. In this thesis, the design, realization and testing of a new metrology instrument is described. This measurement machine is capable of universal, noncontact and fast measurement of freeform optics up to Ø500 mm, with an uncertainty of 30 nm (2s). A cylindrical scanning setup with an optical distance probe has been designed. This concept is non-contact, universal and fast. With a probe with 5 mm range, circular tracks on freeform surfaces can be measured rapidly with minimal dynamics. By applying a metrology frame relative to which the position of the probe and the product are measured, most stage errors are eliminated from the metrology loop. Because the probe is oriented perpendicular to the aspherical best-fit of the surface, the sensitivity to tangential errors is reduced. This allows for the metrology system to be 2D. The machine design can be split into three parts: the motion system, the metrology system and: the non-contact probe. The motion system positions the probe relative to the product in 4 degrees of freedom. The product is mounted on an air bearing spindle (??), and the probe is positioned over it in radial (r), vertical (z) and inclination (¿) direction by the R-stage, Z-stage and ¿- axis, respectively. The motion system provides a sub-micrometer repeatable plane of motion to the probe. The Z-stage is hereto aligned to a vertical plane of the granite base using three air bearings, to obtain a parallel bearing stage configuration. To minimize distortions and hysteresis, the stages have separate position and preload frames. Direct drive motors and high resolution optical scales and encoders are used for positioning. Mechanical brakes are applied while measuring a track, to minimize power dissipation and to exclude encoder, amplifier and EMC noise. The motors, brakes and weight compensation are aligned to the centres of gravity of the R and Zstage. Stabilizing controllers have been designed based on frequency response measurements. The metrology system measures the position of the probe relative to the product in the six critical directions in the plane of motion of the probe (the measurement plane). By focussing a vertical and horizontal interferometer onto the ¿-axis rotor, the displacement of the probe is measured relative to the reference mirrors on the upper metrology frame. Due to the reduced sensitivity in tangential direction at the probe tip, the Abbe criterion is still satisfied. Silicon Carbide is the material of choice for the upper metrology frame, due to its excellent thermal and mechanical properties. Mechanical and thermal analysis of this frame shows nanometer-level stabilities under the expected thermal loads. Simulations of the multi-probe method show capabilities of in process separation of the spindle reference edge profile and the spindle error motion with sub-nanometer uncertainty. The non-contact probe measures the distance between the ¿-axis rotor and the surface under test. A dual stage design is applied, which has 5 mm range, nanometer resolution and 5° unidirectional acceptance angle. This enables the R and Z-stage and ¿-axis to be stationary during the measurement of a circular track on a freeform surface. The design consists of a compact integration of the differential confocal method with an interferometer. The focussing objective is positioned by a flexure guidance with a voice coil actuator. A motion controller finds the surface and keeps the objective focused onto it with some tens of nanometers servo error. The electronics and software are designed to safely operate the 5 axes of the machine and to acquire the signals of all measurement channels. The electronics cabinet contains a real-time processor with many in and outputs, control units for all 5 axes, a safety control unit, a probe laser unit and an interferometry interface. The software consists of three main elements: the trajectory planning, the machine control and the data processing. Emphasis has been on the machine control, in order to safely validate the machine performance and perform basic data-processing. The performance of the machine assembly has been tested by stability, single track and full surface measurements. The measurements focus on repeatability, since this is a key condition before achieving low measurement uncertainty by calibration. The measurements are performed on a Ø100 mm optical flat, which was calibrated by NMi VSL to be flat within 7 nm rms. At standstill, the noise level of the metrology loop is 0.9 nm rms over 0.1 s. When measuring a single track at 1 rev/s, 10 revolutions overlap within 10 nm PV. The repeatability of three measurements of the flat, tilted by 13 µm, is 2 nm rms. The flatness measured by the uncalibrated machine matches the NMi data well. Ten measurements of the flat tilted by 1.6 mm repeat to 3.4 nm rms. A new non-contact measurement machine prototype for freeform optics has been developed. The characteristics desired for a high-end, single piece, freeform optics production environment (high accuracy, universal, non-contact, large measurement volume and short measurement time) have been incorporated into one instrument. The validation measurement results exceed the expectations, especially since they are basically raw data. Future calibrations and development of control and dataprocessing software will certainly further improve these results

Assembly and proving of a wave front sensing confocal Scanning Laser Ophthalmoscope

Confocal Scanning Laser Ophthalmoscopy is used to image the fundus of the living eye. In theory, this technique can be used to observe single cells of the retina. Unfortunately, vision of most eyes is decreased by higher-order aberrations, that cannot be corrected by glasses or contact lenses. This is also the reason why resolution in confocal Scanning Laser Ophthalmoscopy is not as high as expected. By the use of adaptive optics (AO) resolution can be dramatically increased. Implementing a wave front sensor into a conventional confocal Scanning Laser Ophthalmoscope (cSLO), therefore, is the first step to set up a compact adaptive-optical cSLO. In this work a Shack-Hartmann wave front sensor was implemented into a slightly modified Heidelberg Retina Tomograph (HRT) and aberrations of model eyes were measured. Results show that this system is now ready for testing on living eyes

An optical distance sensor : tilt robust differential confocal measurement with mm range and nm uncertainty

Compared with conventional high-end optical systems, application of freeform optics offers many advantages. Their widespread use, however, is held back by the lack of a suitable measurement method.The NANOMEFOS project aims at realizing a universal freeform measurement machine to fill that void.The principle of operation of this machine requires a novel sensor for surface distance measurement, the development and realization of which is the objective of the work presented in this thesis. The sensor must enable non-contact, absolute distance measurement of surfaces with reflectivities from 3.5% to 99% over 5 mm range, with 1 nm resolution and a 2s measurement uncertainty of 10 nm for surfaces perpendicular to the measurement direction and 35 nm for surfaces with tilts up to 5°. To meet these requirements, a dual-stage design is proposed: a primary measurement system tracks the surface under test by translating its object lens, while the secondary measurement system measures the displacement of this object lens. After an assessment of various measurement principles through comparison of characteristics inherent to their principle of operation and the possibilities for adaptation, the differential confocal measurement has been selected as the primary measurement method. Interferometry is used as secondary measurement method. To allow for correction of tilt dependent error through calibration, a third measurement system has been added, which measures through which part of the aperture the light returns. An analytical model of the differential confocal measurement principle has been derived to enable optimization. To gain experience with differential confocal measurement, a demonstrator has been built, which has resulted in insights and design rules for prototype development. The models show satisfactory agreement with the experimental results generated using the demonstrator, thus building confidence that the models can be applied as design and optimization tools. Various properties that characterize the performance of a differential confocal measurement system have been identified. Their dependence on the design parameters has been studied through simulations based on the models. The results of this study are applied to optimize the sensor for use in NANOMEFOS. An optical system has been designed in which the interferometer and the differential confocal systems are integrated in a compact design. The optical path of the differential confocal system has been folded using prisms and mirrors so that it can be realized within the allotted volume envelope. For the same reason, many components are adapted from commercially available parts or are custom made. An optomechanical and mechatronic design has been made around the optical system. A custom focusing unit has been designed that comprises a guidance mechanism and actuator to enable tracking of the surface. To achieve a low measurement uncertainty, it aims at accurate motion, high bandwidth and low dissipation. The lateral position of the guidance reproduces within 20 nm and from the frequency response, it is expected that a control bandwidth of at least 800 Hz can be realized. Power dissipation depends on the form of the freeform surface and is a few mW for most expected trajectories. Partly custom electronics are used for signal processing, and to drive the laser and the focusing unit. Control strategies for interferometer nulling, focus locking and surface tracking have been developed, implemented and tested. Various tests have been performed on the system to evaluate the performance. Calibrations must be carried out to achieve the required measurement uncertainty. One calibration is based on a new method to measure tilt dependency of distance sensors. The sensor realized has 5 mm measurement range, -2.5 µm to 1.5 µm tracking range, sub-nanometer resolution, and a small-signal bandwidth of 150 kHz. Using the test results, the 2s measurement uncertainty after calibration is estimated to be 4.2 nm for measurement of rotationally symmetric surfaces, 21 nm for measurement of medium freeform surfaces and 34 nm for measurement of heavily freeform surfaces. To test the performance of the machine with the sensor integrated, measurements of a tilted flat have been carried out. In these measurements, a tilted flat serves as a reference freeform with known surface form. The measurements demonstrate the reduction of tilt dependent error using the new calibration method. A tilt robust, single point distance sensor with millimeter range and nanometer uncertainty has been developed, realized and tested. It is installed in the freeform measurement machine for which it has been developed and is currently used for the measurement of optical surfaces

Control of flexure in large astronomical spectrographs

This thesis describes the design, construction and testing of an experimental system for improving the imaging stability on the detectors of the Intermediate-dispersion Spectroscopic and Imaging System (ISIS), a Cassegrain spectrograph at the 4.2 metre William Hershel Telescope. This system, called ISAAC (ISIS Spectrograph Automatic Active Collimator) is based on the new concept of active compensation, where spectrum shifts, due to the spectrograph flexing under the effect of gravity, are compensated by the movement of an active optical element. ISAAC is a fine steering tip-tilt collimator mirror. The thesis provides an extensive introduction on astronomical spectrographs, active optics and actuator systems. The new concept of active compensation of flexure is also described. The problem of spectrograph flexure is analyzed, focusing in particular on the case of ISIS and on how an active compensation system can help to solve it. The development of ISAAC is explained, from the component specification and design, to the construction and laboratory testing. The performance and successful testing of the instrument at the William Herschel Telescope is then described in detail. The implications for the future of ISIS and of new spectrograph designs are then discussed, with particular stress on the new High Resolution Optical Spectrograph (HROS) for the 8-metre Gemini telescopes

Light path design for optical disk systems

No abstract

Adaptive optics for laser processing

The overall aim of the work presented in this thesis is to develop an adaptive optics (AO) technique for application to laser-based manufacturing processes. The Gaussian beam shape typically coming from a laser is not always ideal for laser machining. Wavefront modulators, such as deformable mirrors (DM) and liquid crystal spatial light modulators (SLM), enable the generation of a variety of beam shapes and furthermore offer the ability to alter the beam shape during the actual process. The benefits of modifying the Gaussian beam shape by means of a deformable mirror towards a square flat top profile for nanosecond laser marking and towards a ring shape intensity distribution for millisecond laser drilling are presented. Limitations of the beam shaping capabilities of DM are discussed. The application of a spatial light modulator to nanosecond laser micromachining is demonstrated for the first time. Heat sinking is introduced to increase the power handling capabilities. Controllable complex beam shapes can be generated with sufficient intensity for direct laser marking. Conventional SLM devices suffer from flickering and hence a process synchronisation is introduced to compensate for its impact on the laser machining result. For alternative SLM devices this novel technique can be beneficial when fast changes of the beam shape during the laser machining are required. The dynamic nature of SLMs is utilised to improve the marking quality by reducing the inherent speckle distribution of the generated beam shape. In addition, adaptive feedback on the intensity distribution can further improve the quality of the laser machining. In general, beam shaping by means of AO devices enables an increased flexibility and an improved process control, and thus has a significant potential to be used in laser materials processing

Design of the Annular Suspension and Pointing System (ASPS) (including design addendum)

The Annular Suspension and Pointing System is an experiment pointing mount designed for extremely precise 3 axis orientation of shuttle experiments. It utilizes actively controlled magnetic bearing to provide noncontacting vernier pointing and translational isolation of the experiment. The design of the system is presented and analyzed

The E-ELT adptive mirror: prototype and optical test

Ground-based optical-infrared 10m-class telescopes, such as Keck, Gemini, and Very Large Telescopes (VLT), are currently leading many of the astronomical research fields due to their unprecedented collecting areas. However, many new astrophysical quests arose from recent observations, justifying the need for even larger telescopes, with diameters > 30 m. The European scientific community, led by the European Southern Observatory (ESO), takes part in this challenging competition whit the "European-Extremely Large Telescope" (E-ELT), a revolutionary project for a 40m-class telescope that will allow us addressing many of the most pressing unsolved questions about our Universe. Building Extremely Large Telescopes asked for innovative technologies, from larger and repeatable optical manufacturing techniques to produce hundredths of aspherical off-axis hexagonal segments and large monolithic mirrors, or large thin optical shell mirrors, to better and faster controls (active and adaptive optics), together with more specialized focal plane instrumentations devoted to address specific astrophysical questions, more likely to large particle physics experiments. Moreover the challenge of building extremely large telescope pushes forward the parallel ability to measure and test optical components of large sizes. Adaptive Optics (AO) techniques allow to obtain enhanced ground-based astronomical observations, partially restoring diffraction-limited spatial resolutions, by compensating the degrading effects of the atmospheric turbulence. The delivered image quality of an AO-assisted telescope depends on the level of the correction of the wavefront error due to the atmosphere. This work focuses on the adaptive deformable mirror unit (M4AU) of the E-ELT. During my Ph.D. study, I took part in the competitive "EELT-M4 project" study, led by an Italian-French consortium (Microgate, ADS, SAGEM, INAF-O.A.Brera). Optical measurements were conceived, designed and performed on a similar, scaled-down, fully-representative prototype of the final system, to fully validate the proposed concept. Due to very stringent requirements in term of wavefront correction (10-100 nm rms), only interferometric optical setups were able to provide enough accuracy and sensitivity to carry out the work. A set of three devices has been designed for the tests: - a Ritchey-Common test setup (RCT), able to cover the full area of the M4AU DP optical surface with a single measurement, taken with a phase-shifting Fizeau interferometer; - a Stitching Interferometric Setup (SIS), where a scanning smaller optical beam is able to reconstruct the same optical surface with increased capture range and spatial resolution, useful in the first steps of the calibration procedures; - a piston sensing unit (PSU), which exploits the optical path difference between adjacent optical shells, after careful analysis of diffraction patterns. Those devices, together with purposely designed control softwares and data reduction softwares, were used during a 6-months measurement campaign, demonstrating the full compliance of the prototype system with the requirements to be obtained in the final system (flattening of each optical shell, co-phasing of the optical shells, optical stroke calibration, stability of calibration under time and changing environmental conditions). Results have been positively reviewed by the EELT project office, and were used as feedback on the design of final optical test system, to be operated on the 2.5m M4AU unit

Adaptive Optics for Stellar Interferometry

The limitations of current stellar interferometers is their low sensitivity, and the next generation will account for this by using larger apertures. The phase aberrations from seeing will need the consideration of adaptive optics (AO). Accordingly, this dissertation will first examine the problem that seeing causes in stellar interferometers. The application of Adaptive Optics in Stellar Interferometry will then consider these results to achieve the final goal: reduced losses in fringe visibility and increased sensitivity. The thesis is organised with the second chapter discussing the theory of seeing phase aberrations; their origin and effect on image resolution and fringe visibility. These are used to quantify and compare performance metrics in AO and interferometry, and the specific benefits of AO for interferometry and its method of implementation are used to highlight areas of research that are discussed in other chapters. The third chapter discusses a solution to the problem of making high sensitivity wavefront measurements is presented in this chapter. Starting with existing WFSs used in interferometer AO systems, the methods of measuring high order aberrations are considered. A new WFS method, Diffractive Phase Sensing, is presented and an implementation is described in the context of a specific WFS design: the Nine Element Sensor (NES). The fourth chapter concerns numerical simulations of the NES to evaluate its performance in an AO system. Comparisons are made with two existing WFS designs, one commonly used in astronomical AO and the other in use within current interferometer AO. The conclusions drawn specify the observation regimes for which each of the three WFS designs is most appropriate. The design and construction of a NES prototype is discussed in the fifth chapter. The prototype WFS is first tested in the laboratory, and its novel optic and CCD detector operation were analysed prior to use. The prototype was then used to make measurements of defocus phase aberrations at COAST, and results from these observations are presented and discussed to understand their implication. The final chapter considers the existing AO system at COAST—the autoguider—and its measurements of tip/tilt aberrations. The aim and method used to parameterise the atmospheric turbulence is detailed, and the results are verified with measurements from a DIMM and with fringe visibilities. Using the autoguider, the statistics of the seeing at the COAST site is presented from a year long dataset

Active magnetic bearing for ultra precision flexible electronics production system

Roll-to-roll printing on continuous plastic films could enable the production of flexible electronics at high speed and low cost, but the granularity of feature sizes is limited by the system accuracy. Technologies such as gravure printing and nanoimprint lithography demand a level of rotary motion precision that cannot be achieved with rolling element bearings. Manufacturing tolerances of the rotating parts, thermal drift and process forces in combination with structural compliance add up to additional error motions. In this master by research an active magnetic bearing (AMB) solution is designed for a new, super-sized roll-to-roll flexible electronics production machine, which was so far based on hydrostatic bearings. The magnetic bearing could actively compensate the accumulated synchronous error and maintain high accuracy under all conditions. However, the asynchronous error of a conventional AMB with the required size and power is a problem. In order to reduce the relatively high positioning uncertainty of active magnetic bearings an innovative radial position measurement based on linear, incremental encoders with optical conversion principle is proposed. A commercial encoder scanning head faces a round scale with concentric, coplanar lines on its face. By counting these lines the radial position can be measured. Because such a scale is not readily available, it is made by micro-machining. In experiments, different machining methods are compared. Then a magnetic bearing is built to demonstrate the efficacy of the proposed sensor. As a result, the best measurement noise is 3.5nm at 10kHz and a position uncertainty of approximately 0.25µm has been achieved for the magnetic bearing. These promising results are especially interesting for applications with high precision requirements at low speed of rotation