title: Open-loop phase shifting for fast acquisition of interferograms in low light levels.
abstract: Phase shifting interferometry relies on sets of interferograms taken at multiple known phase offsets to deduce the instantaneous phase of a quasi-static fringe pattern. The traditional method for introducing these phase shifts has been either to step a mirror, and measure the fringe pattern at each step, or to scan a mirror, integrating the fringe pattern for discrete time intervals while the fringes "move" on the detector. A stepping mirror eliminates this fringe smear but has typically required a closed-loop controller to ensure that the optical path introduced is accurately known. Furthermore, implementing rapid stepping of a moderately sized optic can prove difficult if the fringe phase needs to be measured on a short time scale. We report results demonstrating very fast (>100 Hz) and precise phase shifting using a piezomodulated mirror operated in open-loop without any position feedback. Our method exploits the use of a synthetic driving waveform that is optimized to match the complex frequency response of the modulator and its supported optic. For phase measurements in the near-infrared at 2.15 μm, and with a time between steps as small as 0.2 ms, we report errors below λ/100 in the desired position of our optic, i.e., an effective optical path difference error of ~λ/55. For applications in near-infrared stellar interferometry, this implies an enhancement in the fringe-tracking sensitivity of roughly 20% (in the photon-limited regime) over that which is conventionally realized using a swept mirror.
authors.affiliation: Magdalena Ridge Observatory Interferometer, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. tmccracken@mro.nmt.edu
authors.forename: Tyler M
authors.initials: TM
authors.lastname: McCracken
authors.forename: Colby A
authors.initials: CA
authors.lastname: Jurgenson
authors.forename: Chris A
authors.initials: CA
authors.lastname: Haniff
authors.forename: David F
authors.initials: DF
authors.lastname: Buscher
authors.forename: John S
authors.initials: JS
authors.lastname: Young
authors.forename: Michelle
authors.initials: M
authors.lastname: Creech-Eakman