Broadband vortex fiber nulling: high-dispersion exoplanet science at the diffraction limit

Abstract

As the number of confirmed exoplanets continues to grow, there is an increased push to spectrally characterize them to determine their atmospheric composition, formation paths, rotation rates, and habitability. However, there is a large population of known exoplanets that either do not transit their star or have been detected via the radial velocity (RV) method at very small angular separations such that they are inaccessible to traditional coronagraph systems. Vortex Fiber Nulling (VFN) is a new single-aperture interferometric technique that uses the entire telescope pupil to bridge the gap between traditional coronagraphy and RV or Transit methods by enabling the direct observation and spectral characterization of targets at and within the diffraction limit. By combining a vortex mask with a single mode fiber, the on-axis starlight is rejected while the off-axis planet light is coupled and efficiently routed to a radiometer or spectrograph for analysis. We have demonstrated VFN in the lab monochromatically in the past. In this paper we present a polychromatic validation of VFN with nulls of < 10 −4 across 15% bandwidth light. We also provide an update on deployment plans and predicted yield estimates for the VFN mode of the Keck Planet Imager and Characterizer (KPIC) instrument. Using PSISIM, a simulation package developed in cooperation with several groups, we assess KPIC VFN's ability to detect and characterize different types of targets including planet candidates around promising young-moving-group stars as well as known exoplanets detected via the RV method. The KPIC VFN on-sky demonstration will pave the road to deployment on future instruments such as Keck-HISPEC and TMT-MODHIS where it could provide high-resolution spectra of sub-Jupiter mass planets down to 5 milliarcseconds from their star.

Vortex fiber nulling for exoplanet observations: implementation and first light

“…45,52 VFN was demonstrated in the laboratory at visible wavelengths with monochromatic nulls 47,48 of ∼5 × 10 −5 and polychromatic nulls of <10 −4 with a 15% bandwidth. 41…”

“…In the laboratory, we have been able to measure the coupling efficiency directly by measuring the flux incident on the fiber tip with a power meter and comparing to the power at the immediate output of the fiber. 41,47 KPIC, however, does not currently have a way of sampling the power incident on the fiber or immediately after the fiber so the coupling efficiency cannot be directly determined; NIRSPEC flux and E2E throughput are the closest accessible measures. As shown in Sec.…”

“…When used in unison with conventional coronagraphs following up on Gaia-Hipparcos accelerators, VFN can improve the separation coverage and bridge the gap between direct imaging and RV and transit surveys. 41,42 VFN also has direct application to the Habitable Worlds Observatory mission recommended by the Astro2020 Decadal Review. The stringent pointing and wavefront control requirements set by the mission's coronagraph instrument, which are on the order of tens of picometers, 43,44 far exceed the requirements for VFN.…”

“…It builds on the heritage of previous fiber nulling techniques 40 that utilize the spatial and modal filtering properties of a single-mode fiber (SMF) to simplify the optical design of classical nullers. When used in unison with conventional coronagraphs following up on Gaia-Hipparcos accelerators, VFN can improve the separation coverage and bridge the gap between direct imaging and RV and transit surveys 41 , 42 . VFN also has direct application to the Habitable Worlds Observatory mission recommended by the Astro2020 Decadal Review.…”

“…Initially proposed in 2018 46 and demonstrated in the lab shortly thereafter, 41 , 47 , 48 VFN is now operating on-sky at the Keck II Telescope. This paper introduces the first VFN instrument and covers results from the on-sky commissioning phase.…”