Development of optical ground verification method for mum to sub-mm reflectors

in 5th International Conference on Space Optics (2004, June 01)

Large reflectors and antennas for the IR to mm wavelength range are being planned for many Earth observation and astronomical space missions and for commercial communication satellites as well. Scientific ... [more ▼]

Large reflectors and antennas for the IR to mm wavelength range are being planned for many Earth observation and astronomical space missions and for commercial communication satellites as well. Scientific observatories require large telescopes with precisely shaped reflectors for collecting the electro-magnetic radiation from faint sources. The challenging tasks of on-ground testing are to achieve the required accuracy in the measurement of the reflector shapes and antenna structures and to verify their performance under simulated space conditions (vacuum, low temperatures). Due to the specific surface characteristics of reflectors operating in these spectral regions, standard optical metrology methods employed in the visible spectrum do not provide useful measurement results. The current state-of-the-art commercial metrology systems are not able to measure these types of reflectors because they have to face the measurement of shape and waviness over relatively large areas with a large deformation dynamic range and encompassing a wide range of spatial frequencies. 3-D metrology (tactile coordinate measurement) machines are generally used during the manufacturing process. Unfortunately, these instruments cannot be used in the operational environmental conditions of the reflector. The application of standard visible wavelength interferometric methods is very limited or impossible due to the large relative surface roughnesses involved. A small number of infrared interferometers have been commercially developed over the last 10 years but their applications have also been limited due to poor dynamic range and the restricted spatial resolution of their detectors. These restrictions affect also the surface error slopes that can be captured and makes their application to surfaces manufactured using CRFP honeycomb technologies rather difficult or impossible. It has therefore been considered essential, from the viewpoint of supporting future ESA exploration missions, to develop and realise suitable verification tools based on infrared interferometry and other optical techniques for testing large reflector structures, telescope configurations and their performances under simulated space conditions. The first one is an IR-phase shifting interferometer with high spatial resolution. This interferometer shall be used specifically for the verification of high precision IR, FIR and sub-mm reflector surfaces and telescopes under both ambient and thermal vacuum conditions. The second one presented hereafter is a holographic method for relative shape measurement. The holographic solution proposed makes use of a home built vacuum compatible holographic camera that allows displacement measurements from typically 20 nanometres to 25 microns in one shot. An iterative process allows the measurement of a total of up to several mm of deformation. Uniquely the system is designed to measure both specular and diffuse surfaces. [less ▲]

Etat de l'art de l'holographie dynamique : systèmes appliqués à la métrologie et au contrôle non destructif

Conference (2004, January)

Dans ce papier, nous présentons le résultat de plus de dix années de recherche et développement au Centre Spatial de Liège (CSL) concernant une caméra holographique (utilisant la technique de l ... [more ▼]

Dans ce papier, nous présentons le résultat de plus de dix années de recherche et développement au Centre Spatial de Liège (CSL) concernant une caméra holographique (utilisant la technique de l'interférométrie holographique) capable d'effectuer des mesures, sans contact et sur un grand champ, de déplacements dans la gamme des microns et submicrons. Les principales applications montrées sont la recherche de défauts, l'identification de modes de vibration et la métrologie de déplacements. [less ▲]

Holographic interferometry with a 90 degrees photorefractive crystal geometry

in Delaye, Ph; Denz, C.; Mager, L. (Eds.) Photorefractive Effects, Materials, and Devices (2003, June)

Holographic interferometers are generally designed in the common geometry where both interfering waves (reference and object) enter the crystal by the same window (co-propagating configuration). The ... [more ▼]

Holographic interferometers are generally designed in the common geometry where both interfering waves (reference and object) enter the crystal by the same window (co-propagating configuration). The alternative presented here consists in using a reference beam entering the crystal by a lateral face. The crystal has to be cut adequately in order to obtain electro-optic properties similar to that of the co-propagating configuration but with a smaller optical scattering noise. Therefore higher signal-to-noise ratios are expected in the interferograms. We will present the main features and constraints of this configuration together with the experimental study of its noise characteristics. This work has been performed for sillenite and semiconductor crystals [less ▲]

Study and comparison of multibeam photorefractive recordings and photorefractive crystals stack to holographic interferometry

in Delaye, Ph; Denz, C.; Mager, L. (Eds.) Photorefractive Effects, Materials, and Devices (2003)

Two techniques with the common objective to integrate multiple view holographic interferometry in one optical head are presented: multi recordings in a photorefractive BSO crystal and the use of a series ... [more ▼]

Two techniques with the common objective to integrate multiple view holographic interferometry in one optical head are presented: multi recordings in a photorefractive BSO crystal and the use of a series of BSO crystals as the recording medium. After a description of the principles we will show their application to holographic interferometry. The results will be presented and a comparison of the two methods, based on the quality of the interferograms, will be performed, showing the respective advantages and limitations of each technique compared to the other and to classical experiment of holographic interferometry. Up to four holograms have been recorded in one crystal, or two holograms in two crystals, corresponding to the four positions of illumination of the object [less ▲]

Holographic interferometers based on photorefractive crystals for vibration measurements

Conference (2002, June)

Photorefractive crystals offer many advantages over other classical holographic recording media. They allow fast in-situ processable holographic recording as well as indefinite reusability. Moreover high ... [more ▼]

Photorefractive crystals offer many advantages over other classical holographic recording media. They allow fast in-situ processable holographic recording as well as indefinite reusability. Moreover high signal-to-noise ratio can be reached as well as high resolutions. We present several types of holographic instruments using such materials. Here we will emphasize their applications in vibration measurements. A first one is based on the classical stroboscopic technique, a second one uses a pulsed laser. [less ▲]

Multiple exposure holographic interferometry with sillenite crystals applied to vibration measurement

in Nolte, D.; Salamo, G.; Siahmakoun, A. (Eds.) et al Photorefractive Effects, Materials, and Devices (2001, July)

A holographic camera is presented, which uses a sillenite crystal as dynamic recording medium. The camera operates in a pulse regime and is used to provide vibration analysis of an object. A 4 pulse real ... [more ▼]

A holographic camera is presented, which uses a sillenite crystal as dynamic recording medium. The camera operates in a pulse regime and is used to provide vibration analysis of an object. A 4 pulse real-time holographic technique was implemented to measure the amplitude and phase of the vibration. The implementation of this technique required bringing under control a Q-switched YAG laser. We present the experimental results for a metallic plate driven by a vibrating loud speaker. [less ▲]

Mesure de vibrations par holographie dynamique multi-expositions

in Actes du Premier colloque francophone Méthodes et Techniques Optiques pour l'Industrie (2000, November)