Dragonfly: On-chip pupil remapping for optical stellar interferometry

Abstract

Summary form only given. Aperture masking has gained widespread use within the optical stellar interferometry community as a way of obtaining high fidelity imaging data on various classes of stellar targets. Aperture masking involves apodizing the starlight at the pupil plane of a telescope, typically using a plate with many small sub-apertures, and then recombining the beams in a Fizeau interferometric scheme at the detector/camera. By appropriate analysis of the spatial interference pattern, it is possible to reconstruct an image with high fidelity. This has proven to be an extremely successful technique when imaging from ground-based telescopes as structures within the immediate vicinity of the diffraction-limited core are extremely difficult to recover with competing methods. Recent reformulations of this basic idea propose replacing the mask with a number of single-mode optical waveguides which sample the pupil plane. Waveguides offer several advantages. Firstly, the light within a single-mode guide propagates with planar wavefronts. As a result, the phase is flat across each sub-aperture, which is important for precise calibration of the interference pattern. Secondly, waveguides can be routed from a 2D input plane to ID output plane (known as pupil remapping) so that interferometry can be performed on an integrated chip (via couplers). An ideal technology for achieving the reformatting in 3D is the laser direct write technique. Finally, waveguides allow a redundant 2D array of sub-apertures to be made non-redundant at the output. This means that all of the light in the pupil plane can be utilised (by contrast, an aperture mask may discard more than 95% of the starlight), hence higher throughputs can be achieved. We discuss the design, fabrication and performance of a pupil remapping system consisting of a 2D to ID (femtosecond laser written) chip with 8 single mode waveguides designed to operate at 1550 nm (C-band or H band) over a bandwidth of ~300 nm, a micro-lens array and computer controlled segmented mirror (MEMS). The combination of a micro-lens array and MEMS were used to maximise or minimise the coupling into the pupil remapping chip so as to be able to switch between particular baselines and hence simulate a non-redundant output distribution. We demonstrate relatively high throughputs, 55-65% across all guides and minimal cross coupling between any two guides (<;0.2%). High fringe visibilities were achieved with both monochromatic and polychromatic light across all possible baselines from the 8 guides available. These results demonstrate the feasibility of the technology for stellar interferometry, with the first on sky tests scheduled for the 21st/22nd of May, 2011 at the Australian Astronomical Telescope (AAT).