The characterization of exoplanets is critical to understanding planet formation, and the search for life. To directly characterize Earth-like exoplanets, a contrast ratio of 10−10 between the star and planet must be overcome. The GLINT instrument is an integrated photonic nulling interferometer fabricated using ultrafast laser inscription, where all routing, beam splitting, phase shifting and recombination is done within a single photonic chip. The instrument uses pupil sub-aperture’s to create different baselines. An achromatic π phase shift is then applied to baseline pairs and recombined in a tricoupler. This suppresses the on-axis starlight while off-axis planet light is observed in the “null” output. The Achromatic phase shifters are designed using differential waveguide dispersion where lengths of three different widths combinations are used to create a differential achromatic phase shift. Adiabatic tapers are then used to transition between regions of different widths to reduce losses. Using this method, a 3.6 mm long 180° achromatic phase shifter was fabricated, and measured to have a phase shift of 171.5 ± 4° from 1440-1640 nm. This phase shift is comparable to GRAVITYs 90° phase shifters (85 ± 3°).
KEYWORDS: Calibration, Data modeling, Signal to noise ratio, Education and training, Nulling interferometry, Equipment, Point spread functions, Wavefronts, Sensors, Optical path differences
Photonic technologies have enabled a generation of nulling interferometers, such as the guided light interferometric nulling technology instrument, potentially capable of imaging exoplanets and circumstellar structure at extreme contrast ratios by suppressing contaminating starlight, and paving the way to the characterization of habitable planet atmospheres. But even with cutting-edge photonic nulling instruments, the achievable starlight suppression (null-depth) is only as good as the instrument’s wavefront control and its accuracy is only as good as the instrument’s calibration. Here, we present an approach wherein outputs from non-science channels of a photonic nulling chip are used as a precise null-depth calibration method and can also be used in real time for fringe tracking. This is achieved using a deep neural network to learn the true in-situ complex transfer function of the instrument and then predict the instrumental leakage contribution (at millisecond timescales) for the science (nulled) outputs, enabling accurate calibration. In this method, this pseudo-real-time approach is used instead of the statistical methods used in other techniques (such as null self calibration, or NSC) and also resolves the severe effect of read-noise seen when NSC is used with some detector types.
Nulling interferometry is one of the most promising technologies for imaging exoplanets within stellar habitable zones. The use of photonics for carrying out nulling interferometry enables the contrast and separation required for exoplanet detection. So far, two key issues limiting current-generation photonic nullers have been identified: phase variations and chromaticity within the beam combiner. The use of tricouplers addresses both limitations, delivering a broadband and achromatic null together with phase measurements for fringe tracking. We present a derivation of the transfer matrix of the tricoupler, including its chromatic behavior, and our 3D design of a fully symmetric tricoupler, built upon a previous design proposed for the guided-light interferometric nulling technology instrument. It enables a broadband null with symmetric, baseline-phase-dependent splitting into a pair of bright channels when inputs are in antiphase. Within some design trade space, either the science signal or the fringe tracking ability can be prioritized. We also present a tapered-waveguide 180-deg-phase shifter with a phase variation of 0.6 deg in the 14-to-1.7-μm band, producing a near-achromatic differential phase between beams for optimal operation of the tricoupler nulling stage. Both devices can be integrated to deliver a deep, broadband null together with a real-time fringe phase metrology signal.
KEYWORDS: Data modeling, Calibration, Wavefronts, Signal to noise ratio, Point spread functions, Nulling interferometry, Adaptive optics, Telescopes, Sensors, Machine learning
Photonic technologies have enabled a new generation of nulling interferometers such as the GLINT instrument, potentially capable of imaging exoplanets and circumstellar structure at extreme contrast ratios by suppressing contaminating starlight, and paving the way to the characterisation of habitable planet atmospheres. But even with cutting edge photonic nulling instruments, the achievable starlight suppression (null-depth) is only as good as the instrument’s wavefront control, and its accuracy is only as good as the instrument’s calibration. Here we present a new approach wherein outputs from non-science channels of a photonic nulling chip are used as a precise null-depth calibration method, and can also be used in realtime for fringe tracking. This is achieved by using a deep neural network to learn the true in-situ complex transfer function of the instrument, and then predict the instrumental leakage contribution (at millisecond timescales) for the science (nulled) outputs, enabling accurate calibration. In this method, this pseudo-realtime approach is used instead of the statistical methods used in other techniques (such as numerical self calibration, or NSC), and also resolves the severe effect of read-noise seen when NSC is used with some detector types.
Nulling interferometry is one of the most promising techniques for imaging exoplanets at solar system scales as it simultaneously meets the stringent requirements for contrast and angular resolution. The GLINT instrument, operating at Subaru telescope behind the SCExAO extreme adaptive optics system, has delivered significant advances in performance, paving the way for a science-ready instrument. Results from previous commissioning runs have confirmed that integrated optics and self-calibration methods yield a robust pathway to detecting exoplanets below the diffraction limit of the telescope. Furthermore, limitations to the current performance have been identified. The most critical are the compensation of remaining phase fluctuations and delivering deep achromatic nulls. Specifically, one technology that offers enormous promise to resolve these issues is the use of tricouplers to perform simultaneous nulling and fringe tracking, or wavefront characterization and correction free of non-common path aberrations. By adding a broadband π radian phase-shifter, fringe tracking and path length stabilization is performed around a deep achromatic null, providing excellent high contrast performance at small angular scales. In this paper, we present the commissioning of GLINT, its current challenges and describe the modelling of devices to overcome them. We discuss the expected performance of a nuller based on these principles of phase control and null depth as implemented within the GLINT instrument. We further present laboratory characterization of 3D-written tricouplers made using ultrafast laser inscription.
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