Enhancing Direct Exoplanet Spectroscopy with Apodizing and Beam Shaping Optics

Abstract

Direct exoplanet spectroscopy aims to measure the spectrum of an exoplanet while simultaneously minimizing the light collected from its host star. Isolating the planet light from the starlight improves the signal-to-noise ratio (S/ N) per spectral channel when noise due to the star dominates, which may enable new studies of the exoplanet atmosphere with unprecedented detail at high spectral resolution (>30,000). However, the optimal instrument design depends on the flux level from the planet and star compared to the noise due to other sources, such as detector noise and thermal background. Here we present the design, fabrication, and laboratory demonstration of specially-designed optics to improve the S/N in two potential regimes in direct exoplanet spectroscopy with adaptive optics instruments. The first is a pair of beam-shaping lenses that increase the planet signal by improving the coupling efficiency into a single-mode fiber at the known position of the planet. The second is a grayscale apodizer that reduces the diffracted starlight for planets at small angular separations from their host star. The former especially increases S/N when dominated by detector noise or thermal background, while the latter helps reduce stellar noise. We show good agreement between the theoretical and experimental point spread functions in each case and predict the exposure time reduction (∼33%) that each set of optics provides in simulated observations of 51 Eridani b using the Keck Planet Imager and Characterizer instrument at W. M. Keck Observatory.

Design considerations of photonic lanterns for diffraction-limited spectrometry

“…5. As indicated in panel a, we see that beam shaping optics offer a significant ∼30% boost to SMF coupling, consistent with results from [8,9]. We additionally see that these beam-shaping optics also offer a significant ∼25% boost to coupling into a 3-port lantern, but offer negligible ∼5% gain for a 6-port lantern.…”

“…Such "beam-shaping" optics have been shown to work well (e.g. [8,9], on SCExAO and KPIC respectively). However, PIAA optics only correct for errors in field amplitude, not phase.…”

“…3e and 3f show apodized PSFs with and without turbulence-induced aberrations. Without beam-shaping optics, coupling efficiencies into SMFs for optical systems with Keck-like pupils (secondary obstruction ratio of ∼30%) would be limited to ∼65%; with such optics, coupling efficiencies can reach upwards of 80% [8,9]. In this work we consider how these optics may also be applied to lantern-based fiber injectors.…”

Learning the Lantern: Neural network applications to broadband photonic lantern modelling

“…When attempting to inject light from an AO-corrected telescope into a fiber or PL, the usual approach is to maximise the Strehl ratio of the beam (as is usual in AO operations) in order to maximise coupling. 19,20 Sometimes apodiziation, such as phase-induced amplitude apodization, 20,40,41 is used to further improve coupling. However, other potential applications exist wherein a specific distribution of intensities in the output waveguides from a PL is desired.…”

Fundamental limit of single-mode integral-field spectroscopy

“…PIAA optics are a set of freeform optics placed close to or in the pupil that reshape the incoming wavefront from the telescope to match the fundamental mode of the fiber without any loss, except for the Fresnel losses. A similar approach is envisioned for the SMF injection unit of KPIC [21]. For integral-field spectroscopy the situation is not as simple.…”