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

Full-Field Optical Coherence Microscopy

International audiencePresentation of the recent developments in Full-Field Optical Coherence Microscop

A study of the application of adaptive optics (AO) in optical coherence tomography (OCT) and confocal microscopy for the purpose of high resolution imaging

A problem is presented when imaging the eye in that optical aberrations are introduced by tissues of the anterior eye such as the cornea and lens. Adaptive optics (AO) and scanning laser ophthalmoscopy (SLO) have been combined to detect and compensate for these aberrations through the use of one or more correcting devices. Di erent corrector options exist, such as a liquid crystal lens or a deformable mirror (DM), such as that used in this thesis. This study seeks to use the ability of the DM to add focus/defocus aberrations to the closed loop AO system. This procedure could allow for dynamic focus control during generation of B-scan images using spectral domain optical coherence tomography (SD-OCT), where typically this is only possible using slower time domain techniques. The confocal gate scanning is controlled using the focus altering aberrations created by changing the shape of the deformable mirror. Using the novel master-slave interferometry method, multiple live en-face images can be acquired simultaneously. In this thesis, application of this method to an AO system is presented whereby en-face images may be acquired at multiple depths simultaneously. As an extension to this research, an OCT despeckle method is demonstrated. Further to this work is the investigation of the role in AO for optimisation of optical systems without the requirement for direct aberration measurement. Towards this end, genetic algorithms (GA) may be employed to control the DM in an iterative process to improve the coupling of light into fibre

Effect of Stress Magnitude and Stress Rate on Elastic Properties of the Reservoir Rocks

We studied the effect of stress magnitude and stress rate on elastic properties of the reservoir rocks. We designed and developed experiments to study: (i) the dynamic and static elastic moduli of reservoir rocks, (ii) quantifying the effects of wave’s amplitude on the longitudinal and transverse velocities in porous media, and (iii) anisotropy of sandstone subjected to stress in dry and saturated statuses

Intra-cavity optical and mechanical dissipation of a coating membrane for gravitational wave detectors

openIl lavoro di ricerca di questa tesi è motivato dal fatto che la sensibilità della prossima generazione di rilevatori di onde gravitazionali sarà limitata dal rumore termico dei rivestimenti altamente riflettenti degli specchi. Quest'ultimo può essere correlato alle dissipazioni meccaniche del materiale del rivestimento attraverso il teorema di fluttuazione-dissipazione quando il sistema è in equilibrio termico. Questo rumore fondamentale limita la sensibilità dell'interferometro nella banda di rilevazione tra 20 e 200 Hz, quindi è estremamente importante ridurre significativamente il rumore termico del rivestimento per consentire la rilevazione di una gamma più ampia di fonti. Inoltre, i rivestimenti devono possedere un'estrema trasparenza alla lunghezza d'onda di funzionamento, al fine di non interagire in modi indesiderati con il laser ad alta potenza che circola nell'interferometro. I rivestimenti utilizzati negli strumenti per le onde gravitazionali sono multistrati di ossidi amorfi come Tantala (Ta2O5) e Silice (SiO2). Attualmente, c'è uno sforzo considerevole nella comunità internazionale per lo sviluppo di nuovi materiali con un livello estremamente basso di perdite meccaniche e ottiche. Tuttavia, entrambe queste dissipazioni (meccaniche e ottiche) sono così piccole che è difficile misurarle sui rivestimenti ottici, specialmente a basse temperature, a causa di interazioni non controllate con il substrato su cui vengono depositati. Con questo progetto di tesi miriamo a sviluppare un setup sperimentale innovativo per misurare sia le perdite ottiche che le dissipazioni meccaniche di una membrana free-standing, sia a temperatura ambiente che in condizioni criogeniche. L'apparato sperimentale utilizzato include un criostato a bassa vibrazione, all'interno del quale sarà installata una cavità. La membrana da testare sarà posizionata all'interno della cavità ottica e fatta interagire con il campo risonante nella cavità. Analizzando come i parametri della cavità cambiano in base all'accoppiamento tra la membrana e il campo risonante, è possibile estrarre sia l'assorbimento ottico che le dissipazioni meccaniche del materiale.The research work of this thesis is motivated by the fact that the next generation of gravitational wave detectors is expected to be limited by the thermal noise of the highly reflective mirror coatings. The latter can be related to the mechanical dissipations of the coating material through the fluctuation dissipation theorem when the system is at thermal equilibrium. This fundamental noise limits the interferometer sensitivity in the detection band between 20 and 200 Hz, therefore it is extremely important to significantly reduce the coating thermal noise to enable the detection of a wider range of sources. Furthermore the coatings have to possess extreme transparency at the operation wavelength, in order not to interact in unwanted ways with the high-power laser circulating into the interferometer. The coating used in gravitational wave instruments are multilayers of amorphous oxides such as Tantala (Ta2O5) and Silica (SiO2). A strong interdisciplinary effort is nowadays being devoted among the international community in developing new materials with an extremely low level of mechanical and optical losses. However, both of these dissipations (mechanical and optical) are so small that they are difficult to be measured on optical coatings, especially at low temperatures, due to uncontrolled interaction with the substrate they are deposited upon. With this thesis project we aim to develop an innovative experimental setup to measure both the optical losses and the mechanical dissipations of a freestanding coating membrane, both at room temperature and in cryogenic conditions. The experimental apparatus that I am using includes a low-vibration cryostat, inside of which a cavity will be installed. The membrane to be tested will be positioned within the optical cavity and made to interact with the intra-cavity field by means of precise nano-positioning systems. Analysing how the cavity parameters change according to the coupling between the membrane and the intracavity field, it is possible to extract both the optical absorption and the mechanical dissipations of the material

Piezoelectric and Magnetoelastic Strain in the Transduction and Frequency Control of Nanomechanical Resonators

Stress and strain play a central role in semiconductors, and are strongly manifested at the nanometer-scale regime. Piezoelectricity and magnetostriction produce internal strains that are anisotropic and addressable via a remote electric or magnetic field. These properties could greatly benefit the nascent field of nanoelectromechanical systems (NEMS), which promises to impact a variety of sensor and actuator applications. The piezoelectric semiconductor GaAs is used as a platform for probing novel implementations of resonant nanomechanical actuation and frequency control. GaAs/AlGaAs heterostructures can be grown epitaxially, are easily amenable to suspended nanostructure fabrication, have a modest piezoelectric coefficient roughly twice that of quartz, and if appropriately doped with manganese, can form dilute magnetic compounds. In ordinary piezoelectric transducers there is a clear distinction between the metal electrodes and piezoelectric insulator. But this distinction is blurred in semiconductors. An integrated piezoelectric actuation mechanism is demonstrated in a series of suspended anisotype GaAs junctions, notably pin diodes. A dc bias was found to alter the resonance amplitude and frequency in such devices. The results are in good agreement with a model of strain based actuation encompassing the diode’s voltage-dependent carrier depletion width and impedance. A bandstructure engineering approach is employed to control the actuation efficiency by appropriately designing the doping level and thickness of the GaAs structure. Actuation and frequency are also sensitively dependent on the device’s crystallographic orientation. This combined tuning behavior represents a novel type of depletion-mediated electromechanical coupling in piezoelectric semiconductor nanostructures. All devices are actuated piezoelectrically, whereas three techniques are demonstrated for sensing: optical interferometry, piezoresistance and piezoelectricity. Finally, a nanoelectromechanical GaMnAs resonator is used to obtain the first measurement of magnetostriction in a dilute magnetic semiconductor. Resonance frequency shifts induced by field-dependent magnetoelastic stress are used to simultaneously map the magnetostriction and magnetic anisotropy constants over a wide range of temperatures. Owing to the central role of carriers in controlling ferromagnetic interactions in this material, the results appear to provide insight into a unique form of magnetoelastic behavior mediated by holes