Space interferometers consisting of several free flying telescopes, such as the planned Darwin mission, require a complex metrology system to make all the components operate as a single instrument. This metrology system consists of various sub-systems to monitor distances, angles and speeds. Our research focuses on one of these sub-systems that measures the absolute distance between two satellites with high accuracy. For Darwin the required accuracy would be in the order of 10 μm over 250 meter.
To measure this absolute distance, we are currently building a frequency sweeping interferometer. It is operated by first measuring a phase in the interferometer, sweeping a tunable laser over a known frequency interval and finally measuring a second phase. By also counting the number of fringes during the sweep it is possible to determine the absolute path length difference without ambiguities. We plan on actively stabilizing the wavelength at the endpoints of the sweep on a Fabry-Perot cavity using the Pound-Drever-Hall technique. In this way the unknown distance is directly referenced to the length of the Fabry-Perot cavity.
KEYWORDS: Distance measurement, Phase measurement, Interferometers, Interferometry, Calibration, Digital signal processing, Metrology, Space telescopes, Control systems, Tunable lasers
One of ESA’s future missions is the Darwin Space Interferometer, which aims to detect planets around nearby stars using optical aperture synthesis with free-flying telescopes. Since this involves interfering white (infra-red) light over large distances, the mission is not possible without a complex metrology system that monitors various speeds, distances and angles between the satellites. One of its sub-systems should measure absolute distances with an accuracy of around 70 micrometer over distances up to 250 meter. To enable such measurements, we are investigating a technique called frequency sweeping interferometry, in which a single laser is swept over a large known frequency range. Central to our approach is the use of a very stable, high finesse Fabry-P´erot cavity, to which the laser is stabilized at the endpoints of the frequency sweep. We will discuss the optical set-up, the control system that controls the fast sweeping, the calibration and the data analysis. We tested the system using long fibers and achieved a repeatability of 50 micrometers at a distance of 55 meters. We conclude with some recommendations for further improvements and the adaption for use in space.
Future space missions, among which the Darwin Space Interferometer, will consist of several free flying satellites. A complex metrology system is required to have all the components fly accurately in formation and have it operate as a single instrument. Our work focuses on a possible implementation of the sub-system that measures the absolute distance between two satellites with high accuracy. For Darwin the required accuracy is on the order of 70 micrometer over a distance of 250 meter. We are exploring a technique called frequency sweeping interferometry, which involves interferometrically measuring a phase difference while sweeping the wavelength of a tunable laser. This phase difference is directly proportional to the absolute distance. A very high finesse Fabry-Perot cavity is used as a reference standard, to which the laser is locked at the end-points of the sweep. We will discuss our measurement scheme, our set-up and some first measurements.
KEYWORDS: Interferometers, Signal processing, Digital signal processing, Interferometry, Distance measurement, Metrology, Satellites, Phase measurement, Control systems, Semiconductor lasers
Future space missions, among which the Darwin Space Interferometer, will consist of several free flying satellites. A complex metrology system is required to have all the components fly accurately in formation and have it operate as a single instrument. Our work focuses on a possible implementation of the sub-system that measures the absolute distance between two satellites with high accuracy. For Darwin the required accuracy is on the order of 70 micrometer over a distance of 250 meter.
We are exploring a technique called frequency sweeping interferometry, which involves interferometrically measuring a phase difference while sweeping the wavelength of a tunable laser. This phase difference is directly proportional to the absolute distance. A very high finesse Fabry-Perot cavity is used as a reference standard, to which the laser is locked end-points of the sweep. We will discuss the control system that drives the setup and show some first experimental results.
Space interferometers consisting of several free flying telescopes, such as the planned Darwin mission, require a complex metrology system to make all the components operate as a single instrument. Our research focuses on one of its sub-systems that measures the absolute distance between two satellites with high accuracy. For Darwin the required accuracy would be in the order of 10 μm over 250 meter. To measure this absolute distance, we are currently exploring the frequency sweeping interferometry technique. Its measurement principle is to first measure a phase in the interferometer, sweep a tunable laser over a known frequency interval and finally measure a second phase. By also counting the number of fringes during the sweep it is possible to determine the absolute path length difference without ambiguities. The wavelength at the endpoints of the sweep is stabilized on a Fabry-Perot cavity. In this way the unknown distance is directly referenced to the length of the Fabry-Perot cavity.
In preparation for the planet-finding missions DARWIN (ESA) and the Terrestrial Planet Finder (NASA) a range of precursor missions are being defined, aimed at testing and validating the technology needed to make the planet-finder missions feasible from a technology point of view. In Europe the SMART-2 mission is meant to test high critical technologies for the DARWIN and the gravitation wave mission LISA (ESA/NASA). The mission SMART-2 consists of two spacecraft. These two spacecraft will demonstrate the feasibility of formation flying related to the DARWIN mission. Furthermore SMART-2 will simulate a stellar interferometer by combining white light from the two spacecraft in an interferometric focus. Two fringe-tracking modes of operations will be tested. In the standard fringe-tracking mode an onboard optical delay line is commanded to keep the optical path difference within the coherence length of the combined light. In the second mode the optical path difference is equalised by commanding the FEEPS (Field Emission Electric Propulsion) thrusters. In both modes a range of metrology systems are needed to measure deviations from the nominal configuration of the two spacecraft. Here we report on the work related to metrology systems for the SMART-2 mission needed to measure the longitudinal distance with nanometer accuracy and the lateral position of one spacecraft with respect to the second spacecraft with 5 mm accuracy. We discuss the present concepts for the metrology systems for SMART-2 and we will elaborate on the possibility to integrate the different optical metrology systems into a single system reducing complexity, risks and mass.
An external cavity diode laser is used as source for a frequency modulated continuous wave (FMCW) interferometer intended to determine absolute optical path differences (OPD) of up to 3mm with a target accuracy of 0.3 microns. This interferometer will eventually be paired with a heterodyne interferometer to extend the accuracy of the system to 0.1nm. Rather than using injection current modulation for the FMCW subsystem, the frequency sweep is provided by moving a mirror of the external cavity with a piezo element. A noise source analysis for this particular setup is provided, and estimates of the maximum attainable accuracy are made.
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