Final design and on-sky testing of the iLocater SX acquisition camera: broad-band single-mode fibre coupling

ABSTRACT Enabling efficient injection of light into single-mode fibres (SMFs) is a key requirement in realizing diffraction-limited astronomical spectroscopy on ground-based telescopes. SMF-fed spectrographs, facilitated by the use of adaptive optics (AO), offer distinct advantages over comparable seeing-limited designs, including higher spectral resolution within a compact and stable instrument volume, and a telescope independent spectrograph design. iLocater is an extremely precise radial velocity (EPRV) spectrograph being built for the Large Binocular Telescope (LBT). We have designed and built the front-end fibre injection system, or acquisition camera, for the SX (left) primary mirror of the LBT. The instrument was installed in 2019 and underwent on-sky commissioning and performance assessment. In this paper, we present the instrument requirements, acquisition camera design, as well as results from first-light measurements. Broad-band SMF coupling in excess of 35 per cent (absolute) in the near-infrared (0.97–1.31 ${\mu {\rm m}}$) was achieved across a range of target magnitudes, spectral types, and observing conditions. Successful demonstration of on-sky performance represents both a major milestone in the development of iLocater and in making efficient ground-based SMF-fed astronomical instruments a reality.

Download Full-text

Nulling at short wavelengths: theoretical performance constraints and a demonstration of faint companion detection inside the diffraction limit with a rotating-baseline interferometer

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2163 ◽

2019 ◽

Vol 489 (1) ◽

pp. 1291-1303 ◽

Cited By ~ 3

Author(s):

E Serabyn ◽

B Mennesson ◽

S Martin ◽

K Liewer ◽

J Kühn

Keyword(s):

Adaptive Optics ◽

Near Infrared ◽

Single Mode ◽

Fluctuation Analysis ◽

Beam Diameter ◽

Primary Star ◽

Performance Constraints ◽

Beam Combination ◽

Final Design ◽

And Performance

ABSTRACT The Palomar Fiber Nuller (PFN) is a rotating-baseline nulling interferometer that enables high-accuracy near-infrared (NIR) nulling observations with full azimuth coverage. To achieve NIR null-depth accuracies of several x 10−4, the PFN uses a common-mode optical system to provide a high degree of symmetry, single-mode-fibre beam combination to reduce sensitivity to pointing and wavefront errors, extreme adaptive optics to stabilize the fibre coupling and the cross-aperture fringe phase, rapid signal calibration and camera readout to minimize temporal effects, and a statistical null-depth fluctuation analysis to relax the phase stabilization requirement. Here, we describe the PFN’s final design and performance and provide a demonstration of faint-companion detection by means of nulling-baseline rotation, as originally envisioned for space-based nulling interferometry. Specifically, the Ks-band null-depth rotation curve measured on the spectroscopic binary η Peg reflects both a secondary star 1.08 ± 0.06 × 10−2 as bright as the primary, and a null-depth contribution of 4.8 ± 1.6 × 10−4 due to the size of the primary star. With a 30 mas separation at the time, η Peg B was well inside both the telescope’s diffraction-limited beam diameter (88 mas) and typical coronagraphic inner working angles. Finally, we discuss potential improvements that can enable a number of small-angle nulling observations on larger telescopes.

Download Full-text

Starlight coupling through atmospheric turbulence into few-mode fibres and photonic lanterns in the presence of partial adaptive optics correction

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3752 ◽

2020 ◽

Vol 501 (2) ◽

pp. 1557-1567

Author(s):

Momen Diab ◽

Aline N Dinkelaker ◽

John Davenport ◽

Kalaga Madhav ◽

Martin M Roth

Keyword(s):

Atmospheric Turbulence ◽

Adaptive Optics ◽

Degrees Of Freedom ◽

Signal To Noise Ratio ◽

Spatial Coherence ◽

Single Mode ◽

Cost Effective ◽

Single Mode Fibre ◽

Astronomical Instruments ◽

Efficient Coupling

ABSTRACT Starlight corrupted by atmospheric turbulence cannot couple efficiently into astronomical instruments based on integrated optics as they require light of high spatial coherence to couple into their single-mode waveguides. Low-order adaptive optics in combination with photonic lanterns offer a practical approach to achieve efficient coupling into multiplexed astrophotonic devices. We investigate, aided by simulations and an experimental testbed, the trade-off between the degrees of freedom of the adaptive optics system and those of the input waveguide of an integrated optic component leading to a cost-effective hybrid system that achieves a signal-to-noise ratio higher than a standalone device fed by a single-mode fibre.

Download Full-text

Modal noise mitigation for high-precision spectroscopy using a photonic reformatter

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1950 ◽

2020 ◽

Vol 497 (3) ◽

pp. 3713-3725

Author(s):

F A Pike ◽

A Benoît ◽

D G MacLachlan ◽

R J Harris ◽

I Gris-Sánchez ◽

...

Keyword(s):

Near Infrared ◽

Broad Band ◽

Single Mode ◽

M Dwarfs ◽

Noise Mitigation ◽

Excitation Light ◽

Single Mode Fibre ◽

Spread Function ◽

Potential Applications ◽

Modal Noise

ABSTRACT Recently, we demonstrated how an astrophotonic light reformatting device, based on a multicore fibre photonic lantern and a 3D waveguide component, can be used to efficiently reformat the point spread function of a telescope to a diffraction-limited pseudo-slit. Here, we demonstrate how such a device can also efficiently mitigate modal noise – a potential source of instability in high-resolution multimode fibre-fed spectrographs. To investigate the modal noise performance of the photonic reformatter, we have used it to feed light into a bench-top near-infrared spectrograph (R ≈ 7000, λ ≈ 1550 nm). One approach to quantifying the modal noise involved the use of broad-band excitation light and a statistical analysis of how the overall measured spectrum was affected by variations in the input coupling conditions. This approach indicated that the photonic reformatter could reduce modal noise by a factor of 6 when compared to a multimode fibre with a similar number of guided modes. Another approach to quantifying the modal noise involved the use of multiple spectrally narrow lines, and an analysis of how the measured barycentres of these lines were affected by variations in the input coupling. Using this approach, the photonic reformatter was observed to suppress modal noise to the level necessary to obtain spectra with stability close to that observed when using a single mode fibre feed. These results demonstrate the potential of using photonic reformatters to enable efficient multimode spectrographs that operate at the diffraction-limit and are free of modal noise, with potential applications including radial velocity measurements of M-dwarfs.

Download Full-text

Single Mode, Extreme Precision Doppler Spectrographs

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313013264 ◽

2012 ◽

Vol 8 (S293) ◽

pp. 403-406 ◽

Cited By ~ 12

Author(s):

Christian Schwab ◽

Sergio G. Leon-Saval ◽

Christopher H. Betters ◽

Joss Bland-Hawthorn ◽

Suvrath Mahadevan

Keyword(s):

Adaptive Optics ◽

Near Infrared ◽

Single Mode ◽

Laser Frequency ◽

Single Mode Fiber ◽

Limiting Factor ◽

Multimode Fiber ◽

Line Spread Function ◽

Order Of Magnitude ◽

Novel Concept

AbstractThe ‘holy grail’ of exoplanet research today is the detection of an earth-like planet: a rocky planet in the habitable zone around a main-sequence star. Extremely precise Doppler spectroscopy is an indispensable tool to find and characterize earth-like planets; however, to find these planets around solar-type stars, we need nearly one order of magnitude better radial velocity (RV) precision than the best current spectrographs provide. Recent developments in astrophotonics (Bland-Hawthorn & Horton 2006, Bland-Hawthorn et al. 2010) and adaptive optics (AO) enable single mode fiber (SMF) fed, high resolution spectrographs, which can realize the next step in precision. SMF feeds have intrinsic advantages over multimode fiber or slit coupled spectrographs: The intensity distribution at the fiber exit is extremely stable, and as a result the line spread function of a well-designed spectrograph is fully decoupled from input coupling conditions, like guiding or seeing variations (Ihle et al. 2010). Modal noise, a limiting factor in current multimode fiber fed instruments (Baudrand & Walker 2001), can be eliminated by proper design, and the diffraction limited input to the spectrograph allows for very compact instrument designs, which provide excellent optomechanical stability. A SMF is the ideal interface for new, very precise wavelength calibrators, like laser frequency combs (Steinmetz et al. 2008, Osterman et al. 2012), or SMF based Fabry-Perot Etalons (Halverson et al. 2013). At near infrared wavelengths, these technologies are ready to be implemented in on-sky instruments, or already in use. We discuss a novel concept for such a spectrograph.

Download Full-text

Detecting exoplanets with extreme adaptive optics and a single-mode fibre fed spectrograph

2013 Conference on Lasers & Electro-Optics Europe & International Quantum Electronics Conference CLEO EUROPE/IQEC ◽

10.1109/cleoe-iqec.2013.6801229 ◽

2013 ◽

Author(s):

Nemanja Jovanovic ◽

Nick Cvetojevic ◽

Olivier Guyon ◽

Frantz Martinache ◽

Jon Lawrence

Keyword(s):

Adaptive Optics ◽

Single Mode ◽

Single Mode Fibre ◽

Mode Fibre

Download Full-text

An all-photonic focal-plane wavefront sensor

Nature Communications ◽

10.1038/s41467-020-19117-w ◽

2020 ◽

Vol 11 (1) ◽

Author(s):

Barnaby R. M. Norris ◽

Jin Wei ◽

Christopher H. Betters ◽

Alison Wong ◽

Sergio G. Leon-Saval

Keyword(s):

Adaptive Optics ◽

Focal Plane ◽

Mean Squared Error ◽

Single Mode ◽

Wavefront Sensor ◽

Wavefront Sensors ◽

Single Mode Fibre ◽

Squared Error ◽

Wavefront Error ◽

Plane Wavefront

Abstract Adaptive optics (AO) is critical in astronomy, optical communications and remote sensing to deal with the rapid blurring caused by the Earth’s turbulent atmosphere. But current AO systems are limited by their wavefront sensors, which need to be in an optical plane non-common to the science image and are insensitive to certain wavefront-error modes. Here we present a wavefront sensor based on a photonic lantern fibre-mode-converter and deep learning, which can be placed at the same focal plane as the science image, and is optimal for single-mode fibre injection. By measuring the intensities of an array of single-mode outputs, both phase and amplitude information on the incident wavefront can be reconstructed. We demonstrate the concept with simulations and an experimental realisation wherein Zernike wavefront errors are recovered from focal-plane measurements to a precision of 5.1 × 10−3 π radians root-mean-squared-error.

Download Full-text