High accuracy absolute long distance metrology using Femto-Second lasers: optical heterodyning, electrical heterodyning and applying FSL-systems

Femto-Second based Laser (FSL) systems enable a large range of concepts for distance measurement that can easily be used for formation flying. ESA has acknowledged this by initiating two parallel studies (HAALDM) to investigate the full range of possibilities. TNO leads one of the two study teams, as manufacturer of space optical systems. To complete the teams' expertise on the field of laser technology, optical distance measurement and space missions the Laser Center of the Vrije Universiteit in Amsterdam, INETI and EADS Astrium GmbH participate in the studies. Since the involved new and exciting technology of Frequency Comb Laser based metrology is completely new to many people in the field of space systems the work is presented in two papers that are strongly linked. In the first paper the fundamentals of FS-lasers and frequency combs are explained as well as the major study results of the first team. In the second paper (this paper) the results of the second study team are presented and a more elaborate section on the application of FSL-systems in space is included. Length metrology systems based of FS-Lasers offer an unrivalled dynamic range and yields absolute distance measurements. At this stage its dynamic range is limited by the external references (like atomic clocks or Ultra Stable Oscillators) used to stabilize the systems. Publications prove that an accuracy of the absolute distance measurement of few micrometers, is feasible over a working range as large as 240m. [1] Substantially larger working ranges are feasible as well as accuracies of tens of nanometers. [2] The TNO study team distinguishes two groups of measurements systems, the Electrical Heterodyning (EH) and the Optical Heterodyning (OH). Both types are analyzed in depth in this paper and the studied FSL-systems are linked to potential missions. This paper includes systems with accuracy in the micrometers as well as tens of nanometers. Special attention is given to the implementation, since a large gain can be obtain by combining with other metrology functions and by careful source and element selection. Proposed systems are benchmarked against normal ranging systems, such as laser rangers, absolute and relative interferometers in terms of accuracy, range, power consumption, mass and volume, non-FSL systems problems and perspectives. Since the FSL-systems use materials not yet used in space, the qualification of these materials is treated. In a similar way the potential perturbations are analyzed, to ascertain their influence on the systems performance. The last part of the paper deals with the outlooks of the proposed metrology and the further development. This document describes the activities performed and the results found, so far, in the HAALDM project. (High Accuracy Absolute Long Distance Measurement with (sub-) micrometer Accuracy for Formation Flight Applications, ESA Contract number 20183/NL/HE).