Photonic components play an important role in the development of astronomical instruments, both to increase the capabilities of instruments at existing telescope facilities and for the next generation of extremely large (30-m class) telescopes. These chip- and fiber-based astrophotonic components [1] can reduce the footprint of instruments as well as their sensitivity to undesired environmental changes. Stellar interferometry has particularly benefited from photonic developments. Here, information about the astronomical source can be extracted from interferometric signals obtained by overlapping light from different telescopes. Photonic beam combiners can overlap multiple beams within a em-scale device, drastically reducing the required space compared to using bulk optics. In addition, the single-mode waveguides provide spatial filtering, which increases measurement precision. Photolithography-based photonic beam combiners have been successfully used in instruments (e.g. GRAVITY at VLTI), but developments in ultrafast laser inscription (ULI)-based beam combiners are still ongoing. These are currently at technology readiness level (TRL) 3. Due to low photon flux, these photonic components are required to have high throughput, uniform wavelength response, polarisation insensitivity, with operation over a broad wavelength range (several 100 nm). Performance has to be tested in the laboratory, followed by validation on the telescope (TRL 5).

While conventional methods like diamond turning can achieve the necessary precision for manufacturing image slicers, they are often expensive in cost and time, and restrictive in terms of the materials to which they can be readily applied. Ultrafast laser-assisted etching (ULAE) is an emerging manufacturing technology that could potentially be used to manufacture free form reflective optics using fused silica, as it enables mu m-level precision shaping of fused silica over several millimetres scales. Here we demonstrate the potential of ULAE for manufacturing fused silica image slicers by fabricating precision 8 x 1mm flat fused silica surfaces using ULAE. The waviness meets the required level for this application, staying below 1 mu m. Specifically, we measured S-10z = 0.164 mu m. The roughness varies with surface inclination; for a flat surface at 0 degrees inclination, we measured S-q = 109 nm, while at a 5 degrees inclination, it increased to S-q = 204 nm. If combined with a suitable polishing technique to remove the high spatial frequency roughness, we believe this work demonstrates that ULAE provides a new route to manufacture freeform reflective monolithic fused silica optics such as image slicers for ground- and space-based applications.

A fiber-connectorized K-band integrated-optics two-telescope beam combiner was developed for long-baseline interferometry at the CHARA telescope array utilizing the ultrafast laser inscription (ULI) technique. Single-mode waveguide insertion losses were measured to be ∼1.1dB over the 2–2.3 µm window. The development of asymmetric directional couplers enabled the construction of a beam combiner that includes a 50:50 coupler for interferometric combination and two ∼75:25 couplers for photometric calibration. The visibility of the bare beam combiner was measured at 87% and then at 82% after fiber-connectorization by optimizing the input polarization. These results indicate that ULI technique can fabricate efficient fiber-connectorized K-band beam combiners for astronomical purposes.

The goal of the CHara ARray Integrated Optics Testbench (CHARIOT) is to establish a fully characterized (nulling) interferometry setup for on-sky tests of novel astrophotonic 2D or 3D beam combiners for the interferometry community worldwide. CHARIOT is planned for four telescope beams covering the J-, H-, and K-bands with plug-and-play fiber interfaces. Verifying novel astrophotonics on-sky with CHARIOT will enable the development of components and advances in instruments in many fields, including nulling and spectro-interferometry.

Multimode fibers (MMFs) can facilitate acquisition of higher resolution images than multicore fibers (MCFs) [1]. MMF imaging is hindered by the transmission matrix (TM) of the MMF changing if its path is altered, and access to the distal-end is required for recalibration, although work is ongoing to address this [2], [3]. In [4] we showed that an MCF terminated with a photonic lantern (PL) [5] may provide some of the sought-after advantages of MMF imaging. We also suggested they might enable interrogation of the TM without access to the multimode (MM) distal end for imaging modalities requiring coherent beam shaping. Here, we demonstrate that following calibration, polarization maintaining (PM) MCF PLs facilitate controlled coherent beam shaping at the distal end after the path of the PL is altered, using only information gained from the proximal end.

Long baseline optical interferometry, at facilities such as the CHARA array and the VLTI, can facilitate angular resolutions that significantly exceed those provided by a single telescope. As such, long baseline optical interferometry is a key technique in many areas of astronomy [1]. One key component of an operational interferometer is the beam combiner, which interferometrically combines the light collected by the individual telescopes. Beam combiners are usually constructed using bulk optics, but this becomes increasingly challenging as the number of telescopes in the array increases. One approach to solve this is to develop beam combiners based on integrated optics, where the light is routed and combined using integrated optic (IO) waveguides. Such “astrophotonic” beam combiners have already enabled impressive results, playing a key role in instruments such as GRAVITY [2]. Given that the advantages of IO beam combiners are now acknowledged, there is significant interest in developing IO beam combiners that can provide advanced beam combination capabilities e.g., efficient operation in the mid-IR. The lack of materials suitable for developing IO circuits in mid-IR with the traditional lithographic processes, has driven the exploration of ultrafast laser inscription (ULI) as an IO beam combiner manufacturing process, since it can be used to produce optical waveguides in a wide range of glasses [4].

We report the ultrafast laser inscription (ULI) of a 2-telescope integrated optic (IO) beam combiner for K-band interferometry in commercial Infrasil glass. The ULI setup used for this work is based on a 1030 nm femtosecond laser which is paired with a spatial-light-modulator (SLM). The SLM controls the numerical aperture of the focused beam used to write waveguides in the substrate. The optimum ULI parameters were found to inscribe straight single-mode waveguides exhibiting an insertion loss of 1.1 ± 0.1 dB for a 17 mm long chip over the entire K-band. To develop optimal directional couplers, we focused our efforts on investigating the effect of varying the core-to-core separation and the effect of detuning the waveguide parameters in the coupler. By doing so, we have identified fabrication parameters that are suitable for the fabrication of a beam combiner integrating an achromatic 3 dB directional coupler and two photometric taps with a splitting ratio of 80:20. These results demonstrate the capability of the ULI fabrication technique to inscribe efficient achromatic directional couplers in the K-band range. A final fabrication step will involve simple assembly of the beam combiner with input/output fibers in preparation for on-sky testing at the CHARA array planned for July 2022.

The maximum depth that photonic structures such as volume Bragg gratings (VBGs) can be precisely fabricated inside dielectric materials using ultrafast laser inscription (ULI) is limited by the aberration imparted on the laser beam by the air-substrate interface as it is focused into the substrate. Here, we use a computer-controlled spatial light modulator (SLM) to shape the wavefront of the ULI laser before it is focused into the substrate, such that the impact of this aberration on the manufacture of VBGs is minimized. We show that this technique allows us to inscribe efficient VBGs at depths in fused silica that would otherwise result in low efficiency VBGs. We find that an optimized “reference” grating fabricated at a mean depth of 200 µm without wavefront shaping exhibited a maximum relative first-order diffraction efficiency of 48%, whereas a grating fabricated at a mean depth of 900 µm using identical parameters exhibited an efficiency of 6.2% – both measured with 633 nm light polarized perpendicularly to the grating lines. Using the SLM to control the wavefront of the ULI laser beam, we were able to pre-compensate for the effect of the substrate surface aberration and fabricate gratings at a mean depth of 900 µm that increased the first-order relative diffraction efficiency to ∼42%. A further plasma study provided significant evidence to the effectiveness of Zernike polynomials for spherical aberration correction. Combing both plasma imaging and laser writing approaches, a set of polynomials for aberration correction at a range of depths was produced with scope for arbitrary depth correction.

By using a spatial light modulator to correct for depth-dependent aberrations, we demonstrate the ultrafast laser fabrication of volume gratings at depths up to 850 μm that exhibit 45% efficiency for 633 nm light.

We present the fabrication and characterization of 3 dB asymmetric directional couplers for the astronomical K-band at wavelengths between 2.0 and 2.4 µm. The couplers were fabricated in commercial Infrasil silica glass using an ultrafast laser operating at 1030 nm. After optimizing the fabrication parameters, the insertion losses of straight single-mode waveguides were measured to be ∼ 1.2 ± 0.5 d B across the full K-band. We investigate the development of asymmetric 3 dB directional couplers by varying the coupler interaction lengths and by varying the width of one of the waveguide cores to detune the propagation constants of the coupled modes. In this manner, we demonstrate that ultrafast laser inscription is capable of fabricating asymmetric 3 dB directional couplers for future applications in K-band stellar interferometry. Finally, we demonstrate that our couplers exhibit an interferometric fringe contrast of > 90 % . This technology paves the path for the development of a two-telescope K-band integrated optic beam combiner for interferometry to replace the existing beam combiner (MONA) in Jouvence of the Fiber Linked Unit for Recombination (JouFLU) at the Center for High Angular Resolution Astronomy (CHARA) telescope array.

We present a new way to mitigate focal-ratio degradation (FRD) when using optical fibers to transport multimode light. Our approach exploits a custom multicore fiber (MCF) with six dissimilar cores that are single mode at ~1550 nm wavelength and minimally coupled over 7 m. We fabricated adiabatic mode-selective photonic lanterns (PLs) at each end of the MCF to create a fiber link with multimode ports, the PLs coupling each spatial mode of the multimode ports to a specific core of the MCF and vice versa. The PL-MCF-PL link exhibits superior FRD behavior compared to a conventional multimode fiber that also supports 6 modes, because it inhibits the transfer of light from lower-order modes to higher-order modes. These results open up a potentially powerful new approach to mitigate FRD in multimode fiber links, with particular applications in astronomical instruments.

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.

We report the ultrafast laser inscription (ULI) and characterization of 3 dB directional achromatic couplers for K-band between 2 and 2.4 μm. The couplers were fabricated in commercial Infrasil glass using 1030 nm femtosecond laser pulses. Straight waveguides inscribed using optimal fabrication parameters exhibit an average propagation loss of ∼1.21 dB over full range of K-band with a single-mode behavior for a length of 17 mm. Directional couplers with different interaction lengths and waveguide widths were fabricated and characterized. We demonstrate that 3 dB achromatic directional couplers for K-band can be fabricated using ULI. These results show that ULI can fabricate highquality couplers for future applications in astronomical interferometry. Our eventual aim is to develop a two-telescope K-band integrated optical beam combiner to replace JouFLU at CHARA.

Ces travaux de these s’inscrivent dans une collaboration long-terme entre le laboratoire Xlim et l’entreprise EOLITE Systems dans le but de developper de nouvelles fibres optiques a tres grande aire modale typiquement capables de delivrer une puissance moyenne superieure a 200 W et une puissance crete de l’ordre de 1 MW avec une emission monomode transverse a 1030 nm. Pour cela, des fibres optiques a microstructuration aperiodique (FA-LPF) ont ete developpees en vue d’ameliorer les performances des fibres commerciales, en repoussant en particulier le seuil des instabilites modales transverses. Une etude experimentale sans precedents a ete menee pour mettre en evidence l’influence de la structure de la fibre, de l’architecture de la source et du diametre de champ modal sur le seuil d’apparition du phenomene. Par ailleurs, la fibre commerciale a ete remplacee par une FA-LPF dans un prototype laser industriel, et a donne lieu a une validation de principe, tant du point de vue des performances pures que du vieillissement. La faisabilite d’une fibre micro-structuree courte et efficace (de 50 cm de long), a egalement ete etudiee, en passant par une augmentation de la concentration en ions ytterbium dans le coeur de la FA-LPF ou encore par l’amelioration du ratio coeur/gaine. Finalement, un concept de fibre a coeur enterre a permis d’atteindre des dimensions de coeur superieures a 110 μm tout en maintenant une emission monomode transverse.

We report the experimental observation of nonlinear multimode beam cleaning at the multimode output of a photonic lantern by coupling fs-laser pulses into each core of a photonic lantern made from a 7-core multicore fiber.