Demonstration of an electric field conjugation algorithm for improved starlight rejection through a single mode optical fiber

Abstract

Linking a coronagraph instrument to a spectrograph via a single-mode optical fiber is a pathway toward detailed characterization of exoplanet atmospheres with current and future ground- and space-based telescopes. However, given the extreme brightness ratio and small angular separation between planets and their host stars, the planet signal-to-noise ratio will likely be limited by the unwanted coupling of starlight into the fiber. To address this issue, we utilize a wavefront control loop and a deformable mirror to systematically reject starlight from the fiber by measuring what is transmitted through the fiber. The wavefront control algorithm is based on the formalism of electric field conjugation (EFC), which in our case accounts for the spatial mode selectivity of the fiber. This is achieved by using a control output that is the overlap integral of the electric field with the fundamental mode of a single-mode fiber. This quantity can be estimated by pairwise image plane probes injected using a deformable mirror. We present simulation and laboratory results that demonstrate our approach offers a significant improvement in starlight suppression through the fiber relative to a conventional EFC controller. With our experimental setup, which provides an initial normalized intensity of 3  ×  ^{10  −  4} in the fiber at an angular separation of 4λ  /  <i>D</i>, we obtain a final normalized intensity of 3  ×  ^{10  −  6} in monochromatic light at λ  =  635  nm through the fiber (100  ×   suppression factor) and 2  ×  ^{10  −  5} in Δλ  /  λ  =  8  %   broadband light about λ  =  625  nm (10  ×   suppression factor). The fiber-based approach improves the sensitivity of spectral measurements at high contrast and may serve as an integral part of future space-based exoplanet imaging missions as well as ground-based instruments.