BROADBAND NULLING USING A PRISM PHASE SHIFTER

摘要

Achromatic phase shifters are key components of broadband nulling interferometers. Such interferometers are currently developed for the detection of earth-like planets at nearby stars. The work reported here was carried out in the context of ES A's space-interferometry mission Darwin, scheduled for launch in 2015. To detect a weak object such as an earth-like planet close to a much brighter star requires first of all a very high spatial resolution and in addition a considerable suppression of the starlight. The high spatial resolution is obtained by an array of 6 free-flying telescopes, constituting a synthetic optical aperture. Suppression of the starlight is achieved by creating at the location of the star a small area where destructive interference (nulling) occurs. The remaining, virtually unobstructed planet light, is subjected to low-resolution spectrometry. The wavelength region for these observations is optimized, not only for detecting spectral features indicative of habitable planets, but also for minimizing the brightness gap. For Darwin this resulted in the selection of the wavelength band between 6 and 18 μm. The starlight suppression or "null depth" to be achieved in this case still amounts to a factor 10~6. Deep nulling at the location of the star, can be obtained with destructive interferometry provided that intensity, phase, polarisation, optical path and wavefront error differences are all extremely small. Since the nulling should be performed over a wide spectral range, a phase difference of π (or another value, depending on the array geometry) needs to be achieved within a few mrad. This requires an achromatic phase shifter providing a λ/2 optical pathlength difference for each wavelength λ within the spectral range. Achromatic phase shifting can be accomplished in many ways. At TNO TPD most efforts have been directed to a dispersive method developed in cooperation with the Delft University of Technology. This paper describes a breadboard operating in the visible region based on a dispersive phase shifter realized with prisms.