[Paper Review] Capabilities of a fibered imager on an extremely large telescope

Sébastien Vievard, Nick Cvetojević+12 more|arXiv (Cornell University)|Oct 21, 2020

Adaptive optics and wavefront sensing4 references4 citations

TL;DR

This paper presents FIRST, a fibered imager that enables ultra-high angular resolution spectro-imaging beyond the diffraction limit using single-mode fibers and pupil remapping on an extreme adaptive optics (ExAO) system. On the Subaru Telescope, FIRST achieves contrast down to ~10⁻⁶ on an ELT, enabling high-contrast imaging of faint companions like exoplanets with nanometer wavefront stability and enhanced sensitivity via EMCCD detection and future upgrades to active photonic beam-combining chips.

ABSTRACT

FIRST, the Fibered Imager foR a Single Telescope instrument, is an ultra-high angular resolution spectro-imager, able to deliver calibrated images and measurements beyond the telescope diffraction limit, a regime that is out of reach for conventional AO imaging. FIRST achieves sensitivity and accuracy by coupling the full telescope to an array of single mode fibers. Interferometric fringes are spectrally dispersed and imaged on an EMCCD. An 18-Fiber FIRST setup is currently installed on the Subaru Coronographic Extreme Adaptive Optics instrument at Subaru telescope. It is being exploited for binary star system study. In the late 2020 it will be upgraded with delay lines and an active LiNb03 photonic beam-combining chip allowing phase modulation to nanometer accuracy at MHz. On-sky results at Subaru Telescope have demonstrated that, thanks to the ExAO system stabilizing the visible light wavefront, FIRST can acquire long exposure and operate on significantly fainter sources than previously possible. A similar approach on a larger telescope would therefore offer unique scientific opportunities for galactic (stellar physics, close companions) and extragalactic observations at ultra-high angular resolution. We also discuss potential design variations for nulling and high contrast imaging.

Motivation & Objective

• To develop a fibered imager (FIRST) that overcomes limitations of conventional adaptive optics and aperture masking by enabling diffraction-limited imaging with high dynamic range.
• To demonstrate the feasibility of ultra-high angular resolution spectro-imaging using single-mode fibers and pupil remapping on a ground-based 8-meter telescope.
• To estimate the performance of FIRST on an Extremely Large Telescope (ELT), particularly in terms of contrast and sensitivity for detecting faint companions.
• To explore photonics-based solutions such as on-chip kernel nulling and active photonic beam-combining chips for future high-contrast imaging on ELTs.

Proposed method

• FIRST uses pupil remapping to couple light from multiple telescope segments into single-mode fibers, spatially filtering wavefront errors and eliminating speckle noise.
• Interferometric fringes are spectrally dispersed and imaged on an EMCCD detector, enabling high-sensitivity, low-noise measurements.
• The instrument is integrated into the SCExAO system on the Subaru Telescope, using ExAO to stabilize wavefronts to within a few dozen nanometers.
• Future upgrades include active LiNbO₃ photonic beam-combining chips with phase modulation at MHz rates and nanometer accuracy.
• Sensitivity is enhanced by maximizing the number of sub-apertures per fiber and potential integration with MKIDS detectors to eliminate read noise.
• The instrument leverages closure phase stability and kernel nulling techniques to suppress atmospheric phase errors and improve contrast.

Experimental results

Research questions

• RQ1 What is the achievable contrast of FIRST on an Extremely Large Telescope, given wavefront stability and fiber coupling efficiency?
• RQ2 How does pupil remapping with single-mode fibers improve dynamic range and overcome speckle noise compared to conventional AO imaging?
• RQ3 What performance gains can be achieved by integrating active photonic beam-combining chips and MKIDS detectors with FIRST on an ELT?
• RQ4 Can kernel nulling via multimode interference couplers on-chip further enhance contrast beyond traditional coronagraphy?
• RQ5 How does the combination of ExAO correction and fiber-based spatial filtering enable long-exposure imaging of faint companions?

Key findings

• On the Subaru Telescope, FIRST has demonstrated long-exposure imaging of faint sources stabilized by ExAO, achieving contrast improvements over previous methods.
• The instrument achieves a contrast of approximately 10⁻⁶ on an ELT, based on wavefront error estimates and theoretical modeling of phase stability.
• Laboratory tests show closure phase stability at the 0.01° level, supporting high-precision measurements critical for high-contrast imaging.
• The use of single-mode fibers effectively removes speckle noise across sub-apertures, enabling imaging beyond the telescope’s diffraction limit.
• Future upgrades, including active photonic chips and MKIDS detectors, are expected to further enhance sensitivity and reduce read noise.
• The integration of kernel nulling via MMIs offers a path to robust, high-contrast imaging with immunity to second-order atmospheric phase errors.