Wavefront control for minimization of speckle coupling into a fiber injection unit based on the electric field conjugation algorithm

Abstract

A fiber injection unit situated in the focal plane behind a coronagraph feeding a high resolution spectrograph can be used to couple light from an exoplanet to obtain high resolution spectra with improved sensitivity. However, the signal-to-noise ratio of the planet signal is limited by the coupling of starlight into the single mode fiber. To minimize this coupling, we need to apply a control loop on the stellar wavefront at the input of the fiber. We present here a wavefront control algorithm based on the formalism of the Electric Field Conjugation (EFC) controller that accounts for the effect of the fiber. The control output is the overlap integral of the electric field with the fundamental mode of a single mode fiber. This overlap integral is estimated by sending probes to a deformable mirror. We present results from simulations, and laboratory results obtained at the Caltech Exoplanet Technology Lab’s transmissive testbed. We show that our approach offers a significant improvement in starlight suppression through the fiber relative to a conventional EFC controller. This new approach improves the contrast of a high contrast instrument and could be used in future missions.