EURASIP Journal on Applied Signal Processing 
Volume 2005 (2005), Issue 15, Pages 2559-2572
doi:10.1155/ASP.2005.2559

Adaptive DFT-Based Interferometer Fringe Tracking

Edward Wilson,1 Ettore Pedretti,2,3 Jesse Bregman,4 Robert W. Mah,4 and Wesley A. Traub2

 1Intellization, 454 Barkentine Lane, Redwood Shores, 94065, CA, USA
2Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge 02138, MA, USA
3Astronomy Department, University of Michigan, 914 Dennison Building, Ann Arbor 48109, MI, USA
4NASA Ames Research Center, Mail Stop 269-1, Moffett Field 94035, CA, USA
Received 1 June 2004; Revised 29 October 2004

Abstract

An automatic interferometer fringe tracking system has been developed, implemented, and tested at the Infrared Optical Telescope Array (IOTA) Observatory at Mount Hopkins, Arizona. The system can minimize the optical path differences (OPDs) for all three baselines of the Michelson stellar interferometer at IOTA. Based on sliding window discrete Fourier-transform (DFT) calculations that were optimized for computational efficiency and robustness to atmospheric disturbances, the algorithm has also been tested extensively on offline data. Implemented in ANSI C on the 266 MHz PowerPC processor running the VxWorks real-time operating system, the algorithm runs in approximately 2.0 milliseconds per scan (including all three interferograms), using the science camera and piezo scanners to measure and correct the OPDs. The adaptive DFT-based tracking algorithm should be applicable to other systems where there is a need to detect or track a signal with an approximately constant-frequency carrier pulse. One example of such an application might be to the field of thin-film measurement by ellipsometry, using a broadband light source and a Fourier-transform spectrometer to detect the resulting fringe patterns.