The crucial issue of space-based interferometers is the laser interferometric metrology systems to monitor with very high accuracy optical path differences. Although classical high-resolution laser interferometers using a single wavelength are well developed, this type of incremental interferometer has a severe drawback: any interruption of the interferometer signal results in the loss of the zero reference, which requires a new calibration, starting at zero optical path difference. We propose in this paper an absolute metrology system based on multiplewavelength interferometry.
This paper presents a non-contact optical metrology measuring the pistons and tip/tilt angles of the 61 hexagonal
segments of a compact-sized segmented mirror. The instrument has been developed within the scope of a design
study for a European Extremely Large Telescope (E-ELT). It is used as reference sensor for cophasing of the
mirror segments in closed loop control. The mirror shape is also measured by different types of stellar light-based
phasing cameras whose performances will be evaluated with regard to a future E-ELT. Following a description
of the system architecture, the second part of the paper presents experimental results demonstrating the level of
precision: 0.48nm RMS in piston and 0.074 μrad RMS in tip and tilt.
This paper presents the fibre-optics low coherence interferometric sensor LISE and its applications in the optics
manufacturing industry. The sensor works as a comparator of optical group delays. The group delay along the
optical axis of the probe interferometer arm containing an object, for example a lens assembly, is compared with
the group delay of the reference arm containing a movable delay line. The light source is a super luminescent diode
(SLD) emitting at 1.31 &mgr;m with a coherence length of typically 25 &mgr;m. Thanks to the limited temporal coherence
of the source, multiple surfaces of the object can be detected during a single scan of the delay line. Measurement
ranges are between a few mm up to 600 mm (optical thickness). The measurement range can be placed at a
working distance of up to several meters away from the instrument's exit. Two classes of accuracy exist. While
the standard system has an absolute accuracy of ±1 &mgr;m for position and distance measurements, the second
generation, high-accuracy system reaches an accuracy of better than ±200 nm for distance measurements while
maintaining the ±1 &mgr;m accuracy for position measurements. The paper starts by explaining the measurement
principle in Section 1. The following Section 2 describes the system design and the individual system components.
The definition and validation of the absolute accuracy are discussed in Section 3, followed by a description of
the complete detection procedure for the high accuracy system in Section 4. In its final Section 5 the paper
gives examples of applications in the optics manufacturing industry. This description ranges from the centre
thickness measurement of lenses to the "global" on-axis metrology of completely mounted optical systems such
as objectives where all lens thicknesses and airgaps are measured without touching or disassembling the optical
system. Practical considerations concerning alignment, focusing of the measurement beam, model-based signal
identification, dispersion and longitudinal resolution are discussed.
This technical summary presents the fiber-optics interferometric sensor LISE and its applications in the optics
industry. The summary explains the measurement principle (Section 1), describes the hardware system components
(Section 2) and gives results of an experimental accuracy validation (Section 3). Section 4 illustrates the
application of the sensor as a metrology tool for optics manufacturing.
This paper presents the second generation LISE-LI of the fibre-optics Low coherence Interferometric Sensor (LISE), recently developed by FOGALE nanotech. Based on the proven concept of low coherence interferometry, the LISE system works as a comparator of optical group delays. The group delay along the optical axis in the probe interferometer arm containing the object to be measured is compared with the group delay along the optical axis of the reference interferometer arm containing a delay line. The latter consists of a mirror that can be linearly displaced on a translation stage. The light source is a super luminescent diode emitting at a near infrared wavelength (typically 1.31 μm) with a spectral bandwidth of a few tens of nm. Thanks to the limited temporal coherence of the source, multiple surfaces of the object can be detected during a single scan of the delay line. Measurement ranges are between a few mm up to 600 mm (optical thickness). The measurement zone can be placed at a working distance of up to several meters away from the instrument's exit. Applications in industry and in research laboratories include thickness measurements of individual optical elements (e.g. lenses), technical multi-layer glasses, display cover glasses, semiconductor wafers, and position measurements of multiple elements of an optical system (e.g. a photographic lens). Compared to the first generation of the system, the absolute accuracy of the second generation system is about ten times better, reaching a level of ±100 nm for thickness measurements over the full measurement range. Following an introductory description of the measurement principle, the first part of the paper focuses on the key elements in the system design, both in hardware and detection algorithm, that ensure the high accuracy level. The second part of the paper presents an experimental validation of the achieved accuracy level. We present results of thickness measurements on distance pieces made of Zerodur. The measured results demonstrate the absolute accuracy over the measurement range as well as the excellent long-term stability of the system.
KEYWORDS: Sensors, Mirrors, Sensing systems, Humidity, Temperature metrology, Control systems, Simulation of CCA and DLA aggregates, Actuators, Environmental sensing, Large telescopes
The 10-m class Southern African Large Telescope (SALT) at Sutherland, South Africa, was inaugurated in November 2005, following completion of all its major sub-systems. It is the largest single optical telescope in the southern hemisphere. The SAMS (Segment Alignment Measurement System) is a unique capacitive edge sensing solution for the active alignment of the SALT primary mirror. Twelve thin film edge sensors are bonded directly onto the edges of each of the 91 segments, with heat-generating control electronics housed remotely in temperature-controlled enclosures. The SAMS is capable of measuring the tip/tilt and piston of each segment, as well as the change in global radius of curvature, a mode normally undetected by such a system. The primary objective was to build a system that offered an excellent cost-to-performance ratio without sacrificing measurement accuracy, a very necessary requirement because of the scale and number of sensors required for large segmented mirrors. This paper describes the results obtained during the commissioning and calibration of the completed system.
This paper presents a non-contact, optical metrology system that allows to measure the pistons and tip/tilt angles of a compact-sized, flat segmented mirror, with nanometer resolution, from a long working distance of a few meters. The system is developed in the scope of the APE project (Active Phasing Experiment), and will be installed on the Very Large Telescope (VLT). It will serve for two purposes: (1) as the sensor within the closed loop control of the mirror segments, and (2) as internal reference metrology to qualify the accuracy of different types of phasing cameras that will be developed and tested within the scope of the APE project. The sensor is based on the concept on instantaneous phase shifting, low-coherence, dual wavelength interferometry. By simultaneously acquiring four interferograms at phase shifts of 90° the system is inherently very insensitive to vibrations. Working at two wavelengths allows absolute distance measurements within the range of half the synthetic wavelength, i.e. in our case within a range of about 25 μm. The concept of low coherence interferometry avoids phase contributions of parasitic reflections and speckle effects. The low coherent light sources are two superluminescent diodes emitting at visible (red) wavelengths with coherence lengths in the range of 25 μm that can be increased by spectral filtering. Dual wavelengths measurements are performed at a rate of 20 Hz. The targeted accuracy is better than 5 nm rms (wavefront). The paper starts with a description of the system architecture. The different subsystems (light source, interferometer core with phase modulator and signal decoder, computer) are discussed. After having presented the optical design, the paper provides a closer look to the measurement and calibration algorithms. Finally, the paper presents the experimental setup and the "first fringes" obtained in the laboratory.
This paper presents the second generation LISE-LI of the fibre-optics Low coherence Interferometric Sensor (LISE), recently developed by FOGALE nanotech. Based on the proven concept of partial coherence interferometry, the LISE system works as a comparator of optical group delays. The group delay along the optical axis in the probe interferometer arm containing the object to be measured is compared with the group delay along the optical axis of the reference interferometer arm containing a delay line. The latter consists of a mirror that can be linearly displaced on a translation stage. The light source is a super luminescent diode emitting at near infrared wavelength (typically 1.31 μm) with a spectral bandwidth of a few tens of nm. Thanks to the limited temporal coherence of the source, multiple surfaces of the object can be detected during a single scan of the delay line. Measurement ranges are between a few mm up to 600 mm (optical thickness). The measurement zone can be placed at a working distance of up to several meters away from the instrument's exit. Applications in industry and in research laboratories include thickness measurements of individual optical elements (e.g. lenses), technical multi-layer glasses, glue and varnish layers deposited on various substrates, Si or GaAs wafers, and position measurements of multiple elements of an optical system (e.g. a photographic lens). Compared to the first generation of the system, the absolute accuracy of the second generation system is about ten times better, reaching a level of ±100 nm for thickness measurements over the full measurement range. Following an introductory description of the measurement principle, the first part of the paper focuses on the key elements in the system design, both in hardware and detection algorithm, that ensure the high accuracy level. The second part of the paper presents an experimental validation of the achieved accuracy level. We present results of thickness measurements on distance pieces made of Zerodur®. The measured results demonstrate the absolute accuracy over the measurement range as well as the excellent long-term stability of the system.
We present a novel concept of quasi-distributed optical fibre extensometers for embedment into concrete, with optimized, continuous transfer of the strain field to the fibre, and fully corrected from thermal variations. These sensors have been developed for structural health monitoring applications. They are composed of a combination of optical cavities and Bragg gratings cascaded along a single fibre. The cavities, which are the parts sensitive to the concrete strain, are formed by partially reflecting elements inserted into the fibre. Their length, which is also the measurement basis, can range from 10 cm to several metres. Several cavities can be cascaded along a single fibre, allowing quasi-distributed strains measurements. Bragg Gratings are inserted along the same fibre, close to the cavities, and are used to measure the temperature locally. Both types of sensors are read by a fibre optic low coherence interferometer featuring a temporal delay line. The interferometer is used in an original way to measure simultaneously the length variations of the cavities and the wavelength shifts of the Bragg gratings. In this paper we present the design of the sensor, and in particular the study of the composite packaging whose shape and mechanical properties have been optimized by finite elements modelling to minimize the intrusion effect and ensure a continuous transfer of the strain field when embedded into concrete. We develop the optical reading method, presenting the theory of the sensor interrogation, or how to get the strain and temperature information. We describe also the instrumentation. Finally we present some laboratory experiments that show very good agreement between standard sensors and OFS, and an example of implementation into a bridge near Angouleme, France.
This paper presents a fibre-based low coherence interferometric sensor developed by FOGALE nanotech. Based on the proven principle of partial coherence interferometry the sensor works as a comparator of optical path lengths. The optical path lengths along the optical axis in the measurement interferometer arm containing a target object are compared with the optical path lengths in an internal delay line. Multiple, partially reflecting surfaces of the target can be detected during one scan of the delay line.
Standard measurement ranges are between a few mm up to 400 mm (optical thickness). Longer measurement ranges are available on request. The measurement zone can be placed at a distance of up to several meters away from the instrument's exit. The sensor reaches an absolute accuracy on position measurements down to ±100 nm over the full measurement range. The system has been successfully applied in industry (glass and optical) as well as in research environments, e.g. for the dimensional metrology of large astronomical telescopes. This paper focuses on an innovative application of the sensor in the optics manufacturing industry.
The paper starts with a description of the measurement technique, the system hardware concept, the detection and signal processing scheme. We present a modeling-based approach for the dimensional metrology of optical components (e.g. single lenses, windows, prisms) or complete, mounted systems where the positions of all individual elements can be detected. A comprehensive propagation model including dispersion and phase effects is used to extract the distances from the optical path differences. Prior to a measurement, the optimum tailoring of the measurement beam is obtained by a simulation of the beam propagation through the target object. This ensures that each surface to be measured delivers a sufficiently strong signal. To illustrate our approach we present its application in the fabrication process of complex optical systems where "global" metrology of complete systems can be performed with a very high accuracy in a short time.
The future European Extremely Large Telescope will be composed of one or two giant segmented mirrors (up to 100 m of
diameter) and of several large monolithic mirrors (up to 8 m in diameter). To limit the aberrations due to misalignments and defective surface quality it is necessary to have a proper active optics system. This active optics system must include a phasing system to limit the degradation of the PSF due to misphasing of the segmented mirrors. We will present the lastest design and development of the Active Phasing Experiment that will be tested in laboratory and on-sky connected to a VLT at Paranal in Chile. It includes an active segmented mirror, a static piston plate to simulate a secondary segmented mirror and of four phasing wavefront sensors to measure the piston, tip and tilt of the segments and the aberrations of the VLT. The four phasing sensors are the Diffraction Image Phase Sensing Instrument developed by Instituto de Astrofisica de Canarias, the Pyramid Phasing Sensor developed by Arcetri Astrophysical Observatory, the Shack-Hartmann Phasing Sensor developed by the European Southern Observatory and the Zernike Unit for Segment phasing developed by Laboratoire d'Astrophysique de Marseille. A reference measurement of the segmented mirror is made by an internal metrology developed by Fogale Nanotech. The control system of Active Phasing Experiment will perform the phasing of the segments, the guiding of the VLT and the active optics of the VLT. These activities are included in the Framework Programme 6 of the European Union.
This paper presents a fibre-based low coherence interferometric
sensor developed by FOGALE nanotech. Based on the well-established
principle of low coherence interferometry the sensor works as a
comparator of optical path lengths. The optical path length in the
measurement interferometer arm containing a target object is
compared with the optical path length in an internal delay line.
Multiple, partially reflecting surfaces of the target can be
detected during one scan of the delay line.
Measurement ranges are between a few mm up to 400 mm (optical
thickness). The measurement zone can be placed at a distance of up
to several meters away from the instrument's exit. The sensor
reaches an absolute accuracy down to 100 nm over the full
measurement range. The system has been applied both for the glass
and for the optical industry. This paper focuses on innovative
applications of the sensor in the optics manufacturing industry.
First, the measurement technique and the system concept including
the detection scheme and signal processing are explained. We
present a modeling-based approach for the dimensional metrology of
optical components (e.g. single lenses, windows, prisms) or
complete lenses where the positions of all individual elements can
be detected. A comprehensive propagation model including
dispersion and phase effects is used to extract the distances from
the optical path differences. Prior to a measurement, the optimum
tailoring of the measurement beam is obtained by a simulation of
the beam propagation through the target object. This ensures that
each surface to be measured delivers a sufficiently strong signal.
To illustrate our approach we present its successful application
in the fabrication process of high-performance imaging optics.
This paper reports on the design and realization of a new optical fibre sensor body meant for embedding distributed optical fibre sensors into concrete. We have carried out a finite element analysis which shows that the proposed wave-like sensor enables continuous bonding between optical fibre and concrete, symmetrical response in traction and compression whatever the contact conditions may be. The stiffness of the optical fibre can so be adapted to the concrete, which reduces strain concentrations and need for a theoretical calibration factor. Experimental validations are carried out, showing that despite its wave-shape, the proposed sensor body does not introduce any losses or strain that would result from fibre macrobendings. The process of realization is very cheap, thus dedicated to distributed sensors. We have realized optical fibre extensometers based on this technique and on low-coherence interferometry method for the measurements, including an original approach of thermal effect compensation. Preliminary results are very encouraging.
A common problem in the large telescopes is the accurate positioning of the optical elements. In the Salt Telescope, there is no secondary mirror, but a light collection system mounted on an optical payload, which is dynamically positioned above the primary mirror. Fogale Nanotech has developed an optical ranging system, the LISE LS40-LD, which is derived from a commercial product, to measure the distance from the payload to the primary mirror, and adjust their relative position. The LISE system is a fibre optic low-coherence interferometre illuminated by an infrared broadband source. It consists in a control unit placed under the telescope and a small collimator mounted on the payload, which focuses the measurement beam on the mirror and collects the reflected light. The collimator is linked to the control unit by a 50 m long optic fibre. It measures a true absolute distance. The distance to measure is 13.505 m, the measuring range is +/-20 mm and the required accuracy is +/- 10 μm. The system has been installed on the Salt Telescope on July, 2003. The results shows that the specifications are met.
There are currently several projects for giant telescopes with segmented mirrors under way. These future telescopes will have their primary mirror made of several thousand segments. The main advantage of segmentation is that it enables the active control of the whole mirror, so as to suppress the deformations of the support structure due to the wind, gravity, thermal inhomogeneities etc. ..., thus getting the best possible stigmatism. However, providing active control of segmented mirrors requires numerous accurate edges sensors. It is acknowledged that capacitance-based technology nowadays offers the best metrological performances-to-cost ratio. As the leader in capacitive technology, FOGALE nanotech offers an original concept which reduces the cost of instrumentation, sensors and electronics, while keeping a very high level of performances with a manufacturing process completely industrialised. We present here the sensors developed for the Segment Alignment Measurement System (SAMS) of the Southern African Large Telescope (SALT). This patented solution represents an important improvement in terms of cost, to market the Position Sensors for Segmented Mirrors of ELTs, whilst maintaining a very high performance level. We present here the concept, the laboratory qualification, and the first trials on the 7 central segments of SALT. The laboratory results are good, and we are now working on the on-site implementation to improve the immunity of the sensors to environment.
The point spread function of a segmented aperture is seriously affected by the misalignment of the segments. Stringent requirements apply to position sensors and their calibration. The Active Phasing Experiment (APE) will be a technical instrument aimed at testing possible phasing techniques for a European Giant Optical Telescope (EGOT) in a representative environment. It will also integrate simultaneous control of segmented and monolithic, active surfaces. A mirror composed of 61 hexagonal segments is conjugated to the primary mirror of the VLT. Each segment can be moved in piston, tip and tilt and can be controlled in open or closed loop. Three new types of Phasing Wave Front Sensors dedicated to the measurement of segmentation errors will be tested, evaluated and compared: a modified Mach-Zehnder sensor developed by the LAM and ESO, a Pyramid Sensor developed by Arcetri, and a Curvature Sensor developed by IAC. A reference metrology developed by FOGALE will be added to measure directly the deformation of the segmented mirror and check the efficiency of the tested wavefront sensors. This metrology will be based on a synthetic wavelength instantaneous phase stepping method. This experiment will first run in the laboratory with point-like polychromatic sources and a turbulence generator. In a second step, it will be mounted at a Nasmyth focus of a VLT unit telescope. These activities are included in a proposal to the European Commission for funding within Framework Program 6.
The Phase Reference Imaging and Micro-arcsecond Astrometry facility (PRIMA) requires an accurate metrology system to measure the internal optical path difference (OPD) with an accuracy to 5 nm over the baseline length of the telescopes (up to 130 m). The actual concept of PRIMA plans to use an incremental metrology for measuring the internal OPD, combined with a zero-OPD calibration. As a future improvement of the PRIMA Laser Metrology System, we propose in this paper an absolute metrology system based on multiple-wavelength interferometry. It could reduce the complexity of the internal optical path calibration scheme and lead to an accurate determination of the baseline length, thus increasing the overall performance of PRIMA.
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