FIRST (Fibered Imager foR a Single Telescope) is an original instrument dedicated to imaging at high contrast and high angular resolution in the visible tight binary systems, potentially with low-mass companions like brown dwarfs. Its principle is based on the pupil remapping technique, which turns a monolithic telescope into an interferometer. Thanks to monomode optical fibers the wavefront is filtered from speckle noise due to atmospheric turbulence which improves the contrat performance of the instrument. In its second version (FIRSTv2), passive integrated optics are used to perform the recombination of the sub-pupils sparsely chosen on the telescope pupil. Two types of photonics were tested in laboratory to show their capability. This method allows the detection of companions at angular separations shorter than the diffraction limit of the telescope. In addition, the interferometric recombination of FIRST is coupled with a spectrometer, allowing the spectral characterization of the detected companions, a unique capability at such spatial scales. I will first present the lab characterization of the integrated optics tested on the bench, then the preleminary interferometric capabilities of the instrument and finally show the future work planned at the 8m Subaru telescope after its integration.

The path towards an imaging interferometric array including a large number of telescopes and kilometric baselines such as the Planet Formation Imager represents an exciting but formidable challenge. In this perspective, heterodyne interferometry is considered as one potential technology, in particular in the mid-infrared N- and Q-band, despite its well documented sensitivity penalty but its lower requirements on hard infrastructure and its stronger scalability to a large number of telescopes. In this presentation we present the V8 concept, whose goal is to enable the combination of the 8 VLTI telescopes (Unit and Auxiliary) as a coherent array using infrared heterodyne interferometry. Exploiting the potential of state of the art technology in the field of high bandwidth detectors, laser frequency combs, fiber links synchronization and innovative photonics correlator, we propose a rapid overview of the developments needed towards an 8 beam imaging instrument. The presentation will focus in particular on the concept of photonics correlator, which enable the correlation of very high bandwidths heterodyne signals through photonics components. Experimental demonstrations will be presented as well as ideas to scale the concept to a large number of telescopes and a large number of telescopes. An early version of a photonic correlator is soon to be implemented in a complete heterodyne detection chain at 10 microns, capable of combining 2 telescopes with a Quantum Cascade Laser (QCL) local oscillator, currently under development at IPAG, of which we will present the current status and the perspectives.

The characterization and study of exoplanets by interferometric nulling has experienced a resurgence in recent years owing to new innovative concepts and advances in technology. Current active endeavors in nulling interferometry for extrasolar planets are the Hi-5 project (Defrère et al, 2018) and LIFE (Quanz et al, 2018). Martinache & Ireland (2018) introduced the kernel-nulling architecture designed to exploit the use of a four-telescope interferometer and provide a reliable high-contrast imaging solution robust to small instrumental perturbations. The main aim of our current study is to evaluate the astrophysical potential of the kernel-nuller as the prime high-contrast imaging mode of the VLTI. We also show how we plan to test and characterize an H-band prototype of the kernel-nuller which has been made with photonic circuitry and includes active on-chip phase control.

I will provide a brief outline of the topics and talks that will be part of the session

The highly miniaturized photonic chips offer great flexibility in terms of coherent manipulation of photons. Such photonic spectrographs are well-suited to disperse the light from directly imaged planets (post-coronagraph stage, collected using a single-mode fiber) to characterize exoplanet atmospheres.

Given the high costs of photonic fabrication, multi-project wafer (MPW) silicon nitride (SiN) offerings provide a convenient and affordable avenue to develop this technology. In this work, we study the potential of two commonly used SiN waveguide geometries by MPW foundries, i.e., square and rectangular profiles, to determine how they affect the performance of mid/high-resolution arrayed waveguide grating (AWG) spectrometers around 1.5μm. Specifically, we present results from detailed simulations on the impact of phase errors on the throughput and cross talk as well as experimental results of coupling and propagation losses.

Further, we present the results from a proof-of-concept high-resolution AWG spectrograph. This chip uses the low-loss SiN platform (SiN core, SiO2 cladding) with square waveguides (800 nm x 800 nm). The AWG has a measured resolving power (lambda/delta_lambda) of ~ 12000 and a free spectral range (FSR) of 3 nm. While the FSR is small, the chip operates over a broad band (1200 - 1700 nm).  The peak on-chip throughput (excluding the coupling efficiency) is about 40% (4 dB) in the TE mode. Thanks to the high-confinement waveguide geometry, the chip is highly miniaturized with a size of only 7.4 mm x 2 mm.

The discovery of the "Hanbury Brown and Twiss effect" in the 1950s has been a key in the birth of modern quantum optics. But Hanbury Brown and Twiss were astronomers! What they actually did is to develop a new technique, called intensity interferometry, which consists in inferring the diameter of a star by measuring the spatial intensity correlation function with two separated telescopes.

After impressive success in the 1960s-1970s, this technique was abandoned because of its poor sensitivity, in particular compared to direct (“amplitude”) interferometry. However, intensity interferometry presents the important advantage of being much easier to implement. This fact, as well as the progress achieved in photon detectors and associated digital electronics, have sparked a renewed interest in intensity interferometry involving several research groups worldwide.

I will present the current status of our work on the revival of intensity interferometry. In short, we have already demonstrated intensity correlations using stellar light for the first time since the 1970s, using moderate-size telescopes at Calern Observatory, and modern photonic technologies (fibers, single-photon detectors). We also have applied the technique on a star with a strong emission line (P Cygni), which allowed us to (slighltly) refine the evaluation of the star's distance. In addition, I will briefly discuss some potential significant improvements that can be foreseen by the use of other modern photonic tools such as superconducting nanowire single photon detectors or waveguide gratings.

Exoplanet discovery is vital for our understanding of star and planet formation, and evaluating the likelihood of life existing on planets other than Earth. Interferometric nulling beam  combination in space is one of the key methods for detecting planets and searching for signs of life. Photonic chips are a light weight, cheap, alternative to current bulk optic beam combination systems. With a complete beam combining circuit able to fit in the palm of one’s hand, these chips are ideal for space based telescopes and, due to being robust to environmental noise, and are also an upgrade to any Earth based telescope system. The multimode interference coupler (MMI) is a photonic device that uses the self-interference of light to either combine or split incoming information. There are many applications for an MMI in photonic systems such as quantum systems, photonic gyroscopes, or chemical sensing systems, that take advantage of the combination and splitting effects simultaneously. For exoplanet detection the beam combination of an MMI is used to remove starlight from an exoplanet signal. The null depth and bandwidth of the MMI are the key features for demining the usefulness of an MMI for exoplanet detection. In this talk we will show simulations of an MMI as a beam combiner with a potential null depth of 10⁴ at a bandwidth of 400 nm, at a cost of some transmission, along with experimental verification of the MMI as a splitter. This will then be compared to alternative devices that do not sacrifice transmission for large bandwidths but do suffer from fabrication issues that the MMI does not. Fabrication tolerances can be unachievable for alternative designs, preventing the exoplanet requirement from being met. The MMI being robust to common fabrication issues make it a front runner for future exoplanet detection systems and, due to the devices matching simulations, can be relied on to work with each chip iteration.

Intensity Interferometry offers an interesting path forward for astronomical interferometry at very long baselines and at short wavelengths, all leading to extremely high angular resolution. Single photon counting detectors (and associated correlator technology) have evolved to allow a sensitivity gains of two orders of magnitude since the seminal experiments of Hanbury-Brown and Twiss in the 1960s, but sensitivity remains the challenge to make this technique widely useful: extremely high angular resolution requires high surface brightness by definition.

The simplest way to increase the sensitivity of an intensity interferometer is to obtain multiple correlation measurements at different wavelengths since these are uncorrelated and improve the SNR on the visibility as √(N), with N spectral channels. The first issue to address is the break-even point since the throughput of spectrographs can be low; for example a throughput of 50% requires at least 4 spectral channel to break even, while a throughput of 10% requires 100 channels. The next issue we wish to address is one of reliability and ease of use as we intend to deploy these spectrographs at multiple locations simultaneously. In this presentation, we propose and compare three concepts, each with its advantages and drawbacks: a classical multimode fiber-fed spectrograph, a photonics lantern fed focal plane based concept and an integrated optics solution using photonics lanterns and an Arrayed Waveguide Grating Spectrograph.

We hope to build a simple demonstrator of whichever concept we end up choosing for testing in the context of the I2C project.

By limiting the precision of the fringe visibility measurement, photon noise is a major obstacle to the capability of interferometers to detect exoplanets at the smallest angular separations. To circumvent this limitation, the SCIFY project aims to design, build and commission Hi-5: the first nulling instrument for the VLTI, operating in the L' band, a sweet spot for the detection of young giant exoplanets. Based on an integrated optics chip combining all four VLTI beams into a double Bracewell configuration, it will enable medium spectral resolution (R=2000) in the L' band of giant exoplanets between 5 and 50 mas from their star.
The end-to-end simulator SCIFYsim is being developed to predict the performance of this new instrument in the presence of a wide variety of instrumental errors, like optical path difference residuals from fringe tracking, wavefront error at the injection, longitudinal dispersion, chromaticity of the combiner chip, and more. As it was designed for modularity, this simulator could also be used to study the performance of other types of integrated beam combiners operating in realistic conditions, like ABCD pairwise combiners, or the more elaborate kernel-nulling combiner envisioned for the VIKiNG project.
I will present both the current state of the design of the high-contrast combiner Hi-5, and some of the features and early results of SCIFYsim.

Due to the increasing dimension, complexity and cost of the future astronomical surveys, new technologies enabling more compact and simpler systems are required. The development of curved detectors allows us to enhance the performances of the optical system used (telescope or astronomical instrument), while keeping the system more compact. One of the emerging astronomical areas that would greatly benefit from this technology is the observation of the low surface brightness Universe (LSB). I will present the compact and almost fully reflective Calar Alto Schmidt-Lemaitre Explorer (CASTLE) aimed at testing the curved detector technologies and observing the LSB Universe. This telescope delivers a Point Spread Function (PSF) with extremely-compact wings, key factor for the detection of LSB features in the sky. I present here the design and the first results obtained through full-system photon Monte Carlo simulations along with the current status of the project. Thanks to its wide field of view (2.36ºx1.56º ) and future robotic capabilities, CASTLE can be used also for transients search and detection (e.g. gravitational wave electro-magnetic counterparts). 

High angular resolution astronomical observations are performed by using interferometry thanks to telescope arrays to observe astronomical targets such as proto-planetary discs or active galactic nuclei that cannot be imaged with classical monolithic telescopes.

In this framework, the mid and far-infrared spectral domains are very challenging. To manage the thermal radiation from the environment that downgrade the interferometric observables, current instruments like MATISSE at the VLTI, use expensive and complex cryogenic systems.

In this context, we propose an alternative solution. The ALOHA (Astronomical Light Optical Hybrid Analysis) project consists in the joined use of guided optics and nonlinear optics to design a new kind of interferometer. The collected mid-infrared light is converted in a nonlinear crystal towards the near-infrared spectral domain, allowing the use of optical fibre links between each telescope and the mixing station.

In 2015, we successfully obtained fringes on the sky at 1.55 µm with the CHARA instrument at Mount Wilson observatory (USA). On each arm of the interferometer, the near-infrared radiation at 1.55μm was converted to the visible domain at 630 nm thanks to a Periodically Poled Lithium Niobate nonlinear crystal and a 1064 nm pump laser. We reached a limited magnitude of H_mag = 3.

In parallel, in 2019, we started the development of an in-laboratory up-conversion interferometer at 3.5 μm. The MIR source is shifted to 820 nm thanks to a 1064 nm pump laser. The full in-lab demonstration was successful and it has validated its ability to analyse the spatial coherence of a black body source.

We are now ready to demonstrate the operation of our instrument on the sky in real conditions at CHARA…

FIRST/SUBARU is an instrument that enables high contrast imaging and spectroscopy thanks to a unique combination of sparse aperture masking, spatial filtering unsign single mode waveguides and cross-dispersion in the visible. In the context of the new generation instruments, we are exploring different ways to introduce on-chip active phase modulation prior to beam recombination. In this paper we will present our results on a 5T and 9T hybrid photonic chips, were the inputs' splitting + electro-optic phase modulation is obtained in a Lithium Niobate crystal, coupled to a passive Glass chip, devoted to pairwise beam combination. Performances in terms of global transmission, polarisation (birefringence) issues and electro-optic figure of merit will be presented.

We will also show some interesting alternatives such as thermo-optic modulation in borosilicate ion-diffused waveguides and preliminary results on 3D laser-written waveguides.

Characterisation of exoplanets is key to understanding their formation, composition and potential for life. However, detecting their glint drown in the overwhelming star glare is challenging.
Nulling interferometry, combined with extreme adaptive optics, is among the most promising techniques to address it and to advance this goal.
We present an integrated-optic nuller whose design is directly scalable to future science-ready interferometric nullers: the Guided-Light Interferometric Nulling Technology, deployed at the Subaru Telescope.
It combines four beams and delivers spatial and spectral information.
We demonstrate the capability of the instrument, achieving a null depth better than 0.001 with a precision of 0.0001 for all baselines, in laboratory conditions with simulated seeing applied.
On sky, the instrument delivered angular diameter measurements of stars that were 2.5 times smaller than the diffraction limit of the telescope.
These successes pave the way for future design enhancements: scaling to more baselines, improved photonic component and handling low-order atmospheric aberration within the instrument, all of which will contribute to enhanced sensitivity and precision.

I will present the main features of the core of the new fringe tracker developed for the CHARA in the framework of the CHARA/SPICA developments. It is based on a new generation 6-beam ABCD beam combiner working in the H band. I will detail the different levels of characterization we have made and the performance reached during the first commissioning runs.