A Photonic Lantern (PL) is a novel device that efficiently converts a multi-mode fiber into several single-mode fibers. When coupled with an extreme adaptive optics (ExAO) system and a spectrograph, PLs enable high throughput spectroscopy at high angular resolution. The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system of the Subaru Telescope recently acquired a PL that converts its multi-mode input into 19 single-mode outputs. The single mode outputs feed a R~4,000 spectrograph optimized for the 600 to 760 nm wavelength range. We present here the integration of the PL on SCExAO, and study the device performance in terms of throughput, field of view, and spectral reconstruction. We also present the first on-sky demonstration of a Visible PL coupled with an ExAO system, showing a significant improvement of x12 in throughput compared to the use of a sole single-mode fiber. This work paves the way towards future high throughput photonics instrumentation at small angular resolution.
HISPEC (High-resolution Infrared Spectrograph for Exoplanet Characterization) is an infrared (0.98 to 2.46 microns) cross-dispersed, R=100,000 single-mode fiber-fed diffraction-limited echellette spectrograph for the Keck II telescope’s adaptive optics (AO) system. MODHIS (Multi-Objective Diffraction-limited High-resolution Infrared Spectrograph) shares similar specifications as HISPEC while being optimized for TMT’s first-light AO system NFIRAOS. Keck-HISPEC, currently in full-scale development and slated for first light in 2026, and TMTMODHIS, currently in conceptual design phase, will provide increasingly compelling science capabilities from exoplanet atmosphere characterization through both transit and direct high-contrast spectroscopy, to detection and mass measurements through infrared precision radial velocity (RV). The science cases include the precise RV measurements of stars orbiting the Galactic Center, Solar System studies, and the chemodynamical history of nearby dwarf galaxies and the galactic halo.
The High-Resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC) is a new instrument for the W. M. Keck Observatory that enables R∼100,000 spectroscopy simultaneously across the y, J, H, and K astronomical bands (0.98-2.5 μm). The fiber delivery subsystem of HISPEC is responsible for routing science and calibration light throughout the observatory efficiently. It consists of high-performance single mode fibers, a photonic lantern, mechanical and MEMS-based fiber switchers that allow for the reconfiguration of light paths. To efficiently cover this large wavelength range, a silica fiber is used for the y&J bands and the 1×3 photonic lantern while a ZBLAN fiber is used for the H&K bands. The HK fiber is a custom design by Le Verre Fluore. The fibers route the science light from the focal point of the adaptive optics system to spectrographs in the basement ∼65 m away, hence, the fibers must be very efficient. To calibrate the instrument, several mechanical fiber switchers can be used to direct calibration light to the spectrograph or the front of the optical train. Some switchers must make over 800 cycles annually, while maintaining sub-3% coupling losses between fibers with core sizes of 4.4 μm. To achieve this, extensive testing was conducted, in which throughput and dust accumulation were monitored to determine how these parameters are impacted by switch preparation procedures and ambient environmental conditions. We developed systems to automatically and remotely clean and image fiber end faces in situ. We have created a protocol that allows us to achieve thousands of switch connections reliably. Additionally, through the 25,000+ switch cycles ran during testing, we identified shortcomings in the design of these mechanical fiber switchers which will be remedied for the final instrument.
Photonic lantern nulling (PLN) is a method for enabling the detection and characterization of close-in exoplanets by exploiting the symmetries of the ports of a mode-selective photonic lantern (MSPL) to cancel out starlight. A six-port MSPL provides four ports where on-axis starlight is suppressed, while off-axis planet light is coupled with efficiencies that vary as a function of the planet’s spatial position. We characterize the properties of a six-port MSPL in the laboratory and perform the first testbed demonstration of the PLN in monochromatic light (1569 nm) and in broadband light (1450 to 1625 nm), each using two orthogonal polarizations. We compare the measured spatial throughput maps with those predicted by simulations using the lantern’s modes. We find that the morphologies of the measured throughput maps are reproduced by the simulations, though the real lantern is lossy and has lower throughputs overall. The measured ratios of on-axis stellar leakage to peak off-axis throughput are around 10−2, likely limited by testbed wavefront errors. These null-depths are already sufficient for observing young gas giants at the diffraction limit using ground-based observatories. Future work includes using wavefront control to further improve the nulls, as well as testing and validating the PLN on-sky.
Ground-based exoplanet science relies on the correction of aberrations induced by both atmosphere and instrument. However, current pupil-plane adaptive optics faces two major challenges: non-common-path aberrations and petaling modes. One solution is to add a wavefront sensor which operates in the focal plane, such as a photonic lantern (PL), a waveguide that efficiently couples aberrated light into single-mode fibers. We present a first experimental verification of real-time closed-loop control with the photonic lantern wavefront sensor (PLWFS), using a linear phase-retrieval algorithm, and on-sky demonstrations. We also discuss non-linear reconstruction using a neural network, and consider potentials for spectrally dispersed sensing.
FIRST is a post Extreme Adaptive-Optics (ExAO) spectro-interferometer operating in the Visible (600-800 nm, R∼400). Its exquisite angular resolution (a sensitivity analysis of on-sky data shows that bright companions can be detected down to 0.25λ/D) combined with its sensitivity to pupil phase discontinuities (from a few nm up to dozens of microns) makes FIRST an ideal self-calibrated solution for enabling exoplanet detection and characterization in the future. We present the latest on-sky results along with recent upgrades, including the integration and on-sky test of a new spectrograph (R∼3,600) optimized for the detection of Hα emission from young exoplanets accreting matter.
The Photonic Lantern (PL) is a novel optical technology consisting of a multi-mode fiber adiabatically merged to several single-mode fibers. PLs efficiently split light into its individual modes, revealing both phase and amplitude information. This makes them attractive for use in focal plane wavefront sensing and spectroscopy. Spectro-astrometry, a technique that involves searching for wavelength-dependent centroid shifts in spectrally-dispersed datasets, can be conducted with PLs to resolve circumstellar structures with extremely small angular separations that are not accessible with traditional imaging techniques. Here, we investigate the application of PLs for spectro-astrometry of young stars hosting protoplanetary disks with embedded accreting planets. Although spectro-astrometry of point-source accreting companions with PLs has been numerically explored in the past, those simulations did not include the effects of scattered light by the protoplanetary disk. We carry out numerical simulations of accretion signatures inside protoplanetary disks to understand the feasibility of using PLs to detect accreting planets under realistic conditions. We simulate the response of a 6 port PL to young stars with a circumstellar disk containing an accretion hotspot centered on the Paschen beta hydrogen line. We discuss the lower limit of the hotspot-to-star contrast detectable by a PL in the context of contamination by disk signals after introducing both random and systematic noise sources. The simulations also demonstrate the effects of scattered light by the circumstellar disk on the PL response to an embedded accreting protoplanet with a fixed planet-to-star contrast.
Photonic Lanterns (PLs) are tapered waveguides that can efficiently couple multi-mode telescope light into a multi-mode fiber entrance at the focal plane and coherently convert it into multiple single-mode beams. Each SMF samples its unique mode (lantern mode) of the telescope light in the pupil, analogous to subapertures in aperture mask interferometry. In this study, we show the concept and potential of coherent imaging with PLs. It can be enabled by interfering SMF outputs and applying path length modulation, which can be achieved using a photonic chip beam combiner at the backend (e.g., the ABCD beam combiner). Using numerically simulated lantern modes of a six-port PL, we calculate interferometric observables for various input scenes. Our simulated observations suggest that PLs may offer significant benefits in the photon-noise limited regime and for resolving small-scale (⪅ λ/2D) asymmetries.
Astrophysical research into exoplanets has delivered thousands of confirmed planets orbiting distant stars. These planets span a wide range of size and composition, with diversity also being the hallmark of system configurations, the great majority of which do not resemble our own solar system. Unfortunately, only a handful of the known planets have been characterized spectroscopically thus far, leaving a gaping void in our understanding of planetary formation processes and planetary types. To make progress, astronomers studying exoplanets will need new and innovative technical solutions. Astrophotonics – an emerging field focused on the application of photonic technologies to observational astronomy – provides one promising avenue forward. In this paper we discuss various astrophotonic technologies that could aid in the detection and subsequent characterization of planets and in particular themes leading towards the detection of extraterrestrial life.
Coronagraphs allow for faint off-axis exoplanets to be observed, but are limited to angular separations greater than a few beam widths. Accessing closer-in separations would greatly increase the expected number of detectable planets, which scales inversely with the inner working angle. The Photonic Lantern Nuller (PLN) is an instrument concept designed to characterize exoplanets within a single beam-width, using a device called the Mode-Selective Photonic Lantern (MSPL), a photonic mode-converter that maps linearly polarized modes into individual single-mode outputs. The PLN leverages the spatial symmetry of an MSPL to create nulled ports, which cancel out on-axis starlight but allow off-axis exoplanet light to couple. However, the quality of the nulls is dependent on the symmetry of the lantern modes, which affects how well the starlight can be suppressed. We present results from our laboratory characterization of an MSPL, including measurements of lantern port throughputs (60-90%), images of the mode intensities, and reconstructions of the mode electric fields using off-axis holography. We discuss the implications on the level of starlight suppression that this MSPL can achieve.
Inner working angle is a key parameter for enabling scientific discovery in direct exoplanet imaging and characterization. Approaches to improving the inner working angle to reach the diffraction limit center on the sensing and control of wavefront errors, starlight suppression via coronagraphy, and differential techniques applied in post-processing. These approaches are ultimately limited by the shot noise of the residual starlight, placing a premium on the ability of the adaptive optics system to sense and control wavefront errors so that the coronagraph can effectively suppress starlight reaching the science focal plane. Photonic lanterns are attractive for use in the science focal plane because of their ability to spatially filter light using a finite basis of accepted modes and effectively couple the results to diffraction-limited spectrometers, providing a compact and cost-effective means to implement post-processing based on spectral diversity. We aim to characterize the ability of photonic lanterns to serve as focal-plane wavefront sensors, allowing the adaptive optics system to control aberrations affecting the science focal plane and reject additional stellar photon noise. By serving as focal-plane wavefront sensors, photonic lanterns can improve sensitivity to exoplanets through both direct and coronagraphic observations. We have studied the sensing capabilities of photonic lanterns in the linear and quadratic regimes with analytical and numerical treatments for different lantern geometries (including non-mode-selective, mode-selective, and hybrid geometries) as a function of port number. In this presentation we report on the sensitivity of such lanterns and comment on the relative suitability and sensitivity impacts of different lantern geometries for focal-plane wavefront sensing.
A focal plane wavefront sensor offers major advantages to adaptive optics, including removal of non-commonpath error and providing sensitivity to blind modes (such as petalling). But simply using the observed point spread function (PSF) is not sufficient for wavefront correction, as only the intensity, not phase, is measured. Here we demonstrate the use of a multimode fiber mode converter (photonic lantern) to directly measure the wavefront phase and amplitude at the focal plane. Starlight is injected into a multimode fiber at the image plane, with the combination of modes excited within the fiber a function of the phase and amplitude of the incident wavefront. The fiber undergoes an adiabatic transition into a set of multiple, single-mode outputs, such that the distribution of intensities between them encodes the incident wavefront. The mapping (which may be strongly non-linear) between spatial modes in the PSF and the outputs is stable but must be learned. This is done by a deep neural network, trained by applying random combinations of spatial modes to the deformable mirror. Once trained, the neural network can instantaneously predict the incident wavefront for any set of output intensities. We demonstrate the successful reconstruction of wavefronts produced in the laboratory with low-wind-effect, and an on-sky demonstration of reconstruction of low-order modes consistent with those measured by the existing pyramid wavefront sensor, using SCExAO observations at the Subaru Telescope.
KEYWORDS: Single mode fibers, Point spread functions, Telescopes, Power meters, Physics, Device simulation, Spectrographs, Near infrared, Turbulence, Sensors
Efficiently coupling light from large telescopes to photonic devices is challenging. However, overcoming this challenge would enable diffraction-limited instruments, which offer significant miniaturization and advantages in thermo-mechanical stability. By coupling photonic lanterns with high performance adaptive optics systems, we recently demonstrated through simulation that high throughput diffraction-limited instruments are possible (Lin et al., Applied Optics, 2021). Here we build on that work and present initial results from validation experiments in the near-infrared to corroborate those simulations in the laboratory. Our experiments are conducted using a 19-port photonic lantern coupled to the state-of-the-art SCExAO instrument at the Subaru Telescope. The SCExAO instrument allows us to vary the alignment and focal ratio of the lantern injection, as well as the Strehl ratio and amount of tip/tilt jitter in the beam. In this work, we present experimental characterizations against the aforementioned parameters, in order to compare with previous simulations and elucidate optimal architectures for lantern-fed spectrographs.
New frontiers of astronomical science push the imaging capabilities of modern AO-equipped telescopes. However, precision measurement at the diffraction limit is made challenging by time-varying residual aberrations in AO-corrected wavefronts. Photonic lanterns (PLs) are a novel technology whose spatial filtering and coherence properties may be exploited to enable new capabilities in precision measurement at the diffraction limit. We aim to determine the potential of AO-fed PL fiber spectrometers for spectroastrometry. We define spectroastrometric signals for a 6-port PL and perform numerical simulations to calculate expected signals for a binary point source model, as a function of contrast, separation, and position angle. In addition, we simulate the effects of AO residual wavefront error on spectroastrometric signals. We also present simulated spectroastrometric signals for accreting planets, which are expected to show strong hydrogen emission lines.
KEYWORDS: Single mode fibers, Signal to noise ratio, Point spread functions, Signal attenuation, Waveguides, Sensors, Light wave propagation, Turbulence, Atmospheric propagation, Cladding
The coupling of large telescopes to astronomical instruments has historically been challenging due to the tension between instrument throughput and stability. Light from the telescope can either be injected wholesale into the instrument, maintaining high throughput at the cost of point-spread function (PSF) stability, or the time-varying components of the light can be filtered out with single-mode fibers (SMFs), maintaining instrument stability at the cost of light loss. Today, the field of astrophotonics provides a potential resolution to the throughput- stability tension in the form of the photonic lantern (PL): a tapered waveguide which can couple a time-varying and aberrated PSF into multiple diffraction-limited beams at an efficiency that greatly surpasses direct SMF injection. As a result, lantern-fed instruments retain the stability of SMF-fed instruments while increasing their throughput. To this end, we present a series of numerical simulations characterizing PL performance as a function of lantern geometry, wavelength, and wavefront error (WFE), aimed at guiding the design of future diffraction-limited spectrometers. These characterizations include a first look at the interaction between PLs and phase-induced amplitude apodization (PIAA) optics. We find that Gaussian-mapping beam-shaping optics can enhance coupling into 3-port lanterns but offer diminishing gains with larger lanterns. In the y- and J - band (0.97–1.35 µm) region, with moderately high WFE (∼ 10% Strehl ratio), a 3-port lantern in conjunction with beam-shaping optics strikes a good balance between pixel count and throughput gains. If pixels are not a constraint, and the flux in each port will be dominated by photon noise, then larger port count lanterns will provide further coupling gains due to a greater resilience to tip-tilt errors. Finally, we show that lanterns can maintain high operating efficiencies over large wavelength bands where the number of guided modes at the lantern entrance drops, if care is taken to minimize the attenuation of weakly radiative input modes.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.