Real-time Experimental Demonstrations of a Photonic Lantern Wave-front Sensor

• Published in: Astrophysical journal. Letters 2023-12, Vol.959 (2), p.L34
• Main Authors: Lin, Jonathan W., Fitzgerald, Michael P., Xin, Yinzi, Kim, Yoo Jung, Guyon, Olivier, Norris, Barnaby, Betters, Christopher, Leon-Saval, Sergio, Ahn, Kyohoon, Deo, Vincent, Lozi, Julien, Vievard, Sébastien, Levinstein, Daniel, Sallum, Steph, Jovanovic, Nemanja
• Format: Article
• Language: English
• Subjects: Aberration; Adaptive optics; Adaptive systems; Astronomical instrumentation; Astronomical instruments; Control theory; Error correction; Extrasolar planets; Fabrication; Focal plane; Optics; Photonics; Plane waves; Planet detection; Pupils; Radial velocity; Real time; Sensors; Spectrographs; Telescopes; Wave front control; Wave front sensors; Waveguides
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad12a4

The direct imaging of an Earth-like exoplanet will require sub-nanometric wave-front control across large light-collecting apertures to reject host starlight and detect the faint planetary signal. Current adaptive optics systems, which use wave-front sensors that reimage the telescope pupil, face two challenges that prevent this level of control: non-common-path aberrations, caused by differences between the sensing and science arms of the instrument; and petaling modes: discontinuous phase aberrations caused by pupil fragmentation, especially relevant for the upcoming 30 m class telescopes. Such aberrations drastically impact the capabilities of high-contrast instruments. To address these issues, we can add a second-stage wave-front sensor to the science focal plane. One promising architecture uses the photonic lantern (PL): a waveguide that efficiently couples aberrated light into single-mode fibers (SMFs). In turn, SMF-confined light can be stably injected into high-resolution spectrographs, enabling direct exoplanet characterization and precision radial velocity measurements; simultaneously, the PL can be used for focal-plane wave-front sensing. We present a real-time experimental demonstration of the PL wave-front sensor on the Subaru/SCExAO testbed. Our system is stable out to around ±400 nm of low-order Zernike wave-front error and can correct petaling modes. When injecting ∼30 nm rms of low-order time-varying error, we achieve ∼10× rejection at 1 s timescales; further refinements to the control law and lantern fabrication process should make sub-nanometric wave-front control possible. In the future, novel sensors like the PL wave-front sensor may prove to be critical in resolving the wave-front control challenges posed by exoplanet direct imaging.