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Register a new userVortex fiber nulling for exoplanet observations. I. Experimental demonstration in monochromatic light
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Vortex fiber nulling is a method for spectroscopically characterizing exoplanets at small angular separations, $lesssimlambda/D$, from their host star. The starlight is suppressed by creating an optical vortex in the system point spread function, which prevents the stellar field from coupling into the fundamental mode of a single-mode optical fiber. Light from the planet, on the other hand, couples into the fiber and is routed to a spectrograph. Using a prototype vortex fiber nuller (VFN) designed for monochromatic light, we demonstrate coupling fractions of $6times10^{-5}$ and $>0.1$ for the star and planet, respectively.
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Vortex fiber nulling (VFN) is a method that may enable the detection and characterization of exoplanets at small angular separations (0.5-2 $lambda/D$) with ground- and space-based telescopes. Since the field of view is within the inner working angle of most coronagraphs, nulling accesses non-transiting planets that are otherwise too close to their star for spectral characterization by other means, thereby significantly increasing the number of known exoplanets available for direct spectroscopy in the near-infrared. Furthermore, VFN targets planets on closer-in orbits which tend to have more favorable planet-to-star flux ratios in reflected light. Here, we present the theory and applications of VFN, show that the optical performance is approximately equivalent for a variety of implementations and aperture shapes, and discuss the trade-offs between throughput and engineering requirements using numerical simulations. We compare vector and scalar approaches and, finally, show that beam shaping optics may be used to significantly improve the throughput for planet light. Based on theoretical performance, we estimate the number of known planets and theoretical exoEarths accessible with a VFN instrument linked to a high-resolution spectrograph on the future Thirty Meter Telescope.
Vortex Fiber Nulling (VFN) is an interferometric method for suppressing starlight to detect and spectroscopically characterize exoplanets. It relies on a vortex phase mask and single-mode fiber to reject starlight while simultaneously coupling up to 20% of the planet light at separations of $lesssim1lambda/D$, thereby enabling spectroscopic characterization of a large population of RV and transit-detected planets, among others, that are inaccessible to conventional coronagraphs. VFN has been demonstrated in the lab at visible wavelengths and here we present the latest results of these experiments. This includes polychromatic nulls of $5times10^{-4}$ in 10% bandwidth light centered around 790 nm. An upgraded testbed has been designed and is being built in the lab now; we also present a status update on that work here. Finally, we present preliminary K-band (2 $mu$m) fiber nulling results with the infrared mask that will be used on-sky as part of a VFN mode for the Keck Planet Imager and Characterizer Instrument in 2021.
The Keck Planet Imager and Characterizer (KPIC) is an upgrade to the Keck II adaptive optics system that includes an active fiber injection unit (FIU) for efficiently routing light from exoplanets to NIRSPEC, a high-resolution spectrograph. Towards the end of 2019, we will add a suite of new coronagraph modes as well as a high-order deformable mirror. One of these modes, operating in $K$-band (2.2$mu m$), will be the first vortex fiber nuller to go on sky. Vortex Fiber Nulling (VFN) is a new interferometric method for suppressing starlight in order to spectroscopically characterize exoplanets at angular separations that are inaccessible with conventional coronagraph systems. A monochromatic starlight suppression of $6times10^{-5}$ in 635 nm laser light has already been demonstrated on a VFN testbed in the lab. A polychromatic experiment is now underway and coupling efficiencies of $<5times10^{-4}$ and $sim5%$ have been demonstrated for the star and planet respectively in 10% bandwidth light. Here we describe those experiments, the new KPIC VFN mode, and the expected performance of this mode using realistic parameters determined from on-sky tests done during the KPIC commissioning.
The future of exoplanet detection lies in the mid-infrared (MIR). The MIR region contains the blackbody peak of both hot and habitable zone exoplanets, making the contrast between starlight and planet light less extreme. It is also the region where prominent chemical signatures indicative of life exist, such as ozone at 9.7 microns. At a wavelength of 4 microns the difference in emission between an Earth-like planet and a star like our own is 80 dB. However a jovian planet, at the same separation exhibits 60 dB of contrast, or only 20 dB if it is hot due to its formation energy or being close to its host star. A two dimensional nulling interferometer, made with chalcogenide glass, has been measured to produce a null of 20 dB, limited by scattered light. Measures to increase the null depth to the theoretical limit of 60 dB are discussed.
One of the long-term goals of exoplanet science is the (atmospheric) characterization of a large sample (>100) of terrestrial planets to assess their potential habitability and overall diversity. Hence, it is crucial to quantitatively evaluate and compare the scientific return of various mission concepts. Here we discuss the exoplanet yield of a space-based mid-infrared (MIR) nulling interferometer. We use Monte-Carlo simulations, based on the observed planet population statistics from the Kepler mission, to quantify the number and properties of detectable exoplanets (incl. potentially habitable planets) and we compare the results to those for a large aperture optical/NIR space telescope. We investigate how changes in the underlying technical assumptions (sensitivity and spatial resolution) impact the results and discuss scientific aspects that influence the choice for the wavelength coverage and spectral resolution. Finally, we discuss the advantages of detecting exoplanets at MIR wavelengths, summarize the current status of some key technologies, and describe what is needed in terms of further technology development to pave the road for a space-based MIR nulling interferometer for exoplanet science.
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