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[EN] The sixth generation (6G) mobile systems will create new markets, services, and industries making possible a plethora of new opportunities and solutions. Commercially successful rollouts will involve scaling enabling technologies, such as cloud radio access networks, virtualization, and artificial intelligence. This paper addresses the principal technologies in the transition towards next generation mobile networks. The convergence of 6G key-performance indicators along with evaluation methodologies and use cases are also addressed. Free-space optics, Terahertz systems, photonic integrated circuits, softwarization, massive multiple-input multiple-output signaling, and multi-core fibers, are among the technologies identified and discussed. Finally, some of these technologies are showcased in an experimental demonstration of a mobile fronthaul system based on millimeter 5G NR OFDM signaling compliant with 3GPP Rel. 15. The signals are generated by a bespoke 5G baseband unit and transmitted through both a 10 km prototype multi-core fiber and 4 m wireless V-band link using a pair of directional 60 GHz antennas with 10 degrees beamwidth. Results shown that the 5G and beyond fronthaul system can successfully transmit signals with both wide bandwidth (up to 800 MHz) and fully centralized signal processing. As a result, this system can support large capacity and accommodate several simultaneous users as a key candidate for next generation mobile networks. Thus, these technologies will be needed for fully integrated, heterogeneous solutions to benefit from hardware commoditization and softwarization. They will ensure the ultimate user experience, while also anticipating the quality-of-service demands that future applications and services will put on 6G networks., This work was partially funded by the blueSPACE and 5G-PHOS 5G-PPP phase 2 projects, which have received funding from the European Union's Horizon 2020 programme under Grant Agreements Number 762055 and 761989. D. PerezGalacho acknowledges the funding of the Spanish Science Ministry through the Juan de la Cierva programme.

This article presents an experimental demonstration of a high-capacity millimeter-wave 5G NR signal transmission with analog radio-over-fiber (ARoF) fronthaul over multi-core fiber and full real-time processing. The demonstration validates the core of the blueSPACE fronthaul architecture which combines ARoF fronthaul with space division multiplexing in the optical distribution network to alleviate the fronthaul capacity bottleneck and maintain a centralized radio access network with fully centralized signal processing. The introduction of optical beamforming in the blueSPACE architecture brings true multi-beam transmission and enables full spatial control over the RF signal. The proposed ARoF architecture features a transmitter that generates the ARoF signal and an optical signal carrying a reference local oscillator employed for downconversion at the remote unit from a single RF reference at the central office. A space division multiplexing based radio access network with multi-core fibre allows parallel transport of the uplink ARoF signal and reference local oscillator at the same wavelength over separate cores. A complete description of the real-time signal processing and experimental setup is provided and system performance is evaluated. Transmission of an 800 MHz wide extended 5G NR fronthaul signal over a 7-core fibre is shown with full real-time signal processing, achieving 1.4 Gbit/s with a bit error rate $$ < 3.8 × 10 - 3 and thus below the limit for hard-decision forward error correction with 7% overhead.

This paper presents an experimental demonstration of analog radio-over-fiber (ARoF) fronthaul for high-bandwidth, high-capacity millimeter wave (mm-wave) extended fifth generation mobile network (5G) new radio (NR) signals over an optical distribution network with optical space division multiplexing (SDM). ARoF is shown to alleviate fronthaul capacity bottlenecks, transporting an 800 MHz wide extended 5G NR signal and allowing to maintain full centralization in a centralized radio access network (C-RAN). The proposed ARoF fronthaul architecture features a transmitter that generates the ARoF signal and an optical signal carrying a reference local oscillator (LO) employed for downconversion at the remote unit (RU) from a single radio frequency (RF) reference at the central office (CO). An SDM based RAN with 7-core multi-core fiber (MCF) allows parallel transport of the uplink ARoF signal and reference LO at the same wavelength over separate cores. Transmission of an 800 MHz wide extended 5G NR fronthaul signal over 7-core MCF is shown with full real-time processing, achieving 1.4 Gbit/s with BER

The introduction of millimeter wave (mm-wave) frequency bands for cellular communications with significantly larger bandwidths compared to their sub-6 GHz counterparts, the resulting densification of network deployments and the introduction of antenna arrays with beamforming result in major increases in fronthaul capacity required for 5G networks. As a result, a radical re-design of the radio access network is required since traditional fronthaul technologies are not scaleable. In this article the use of analog radio-over-fiber (ARoF) is proposed and demonstrated as a viable alternative which, combined with space division multiplexing in the optical distribution network as well as photonic integration of the required transceivers, shows a path to a scaleable fronthaul solution for 5G. The trade-off between digitized and analog fronthaul is discussed and the ARoF architecture proposed by blueSPACE is introduced. Two options for the generation of ARoF two-tone signals for mm-wave generation via optical heterodyning are discussed in detail, including designs for the implementation in photonic integrated circuits as well as measurements of their phase noise performance. The proposed photonic integrated circuit designs include the use of both InP and SiN platforms for ARoF signal generation and optical beamforming respectively, proposing a joint design that allows for true multi-beam transmission from a single antenna array. Phase noise measurements based on laboratory implementations of ARoF generation based on a Mach–Zehnder modulator with suppressed carrier and with an optical phase-locked loop are presented and the suitability of these transmitters is evaluated though phase noise simulations. Finally, the viability of the proposed ARoF fronthaul architecture for the transport of high-bandwidth mm-wave 5G signals is proven with the successful implementation of a real-time transmission link based on an ARoF baseband unit with full real-time processing of extended 5G new radio signals with 800 MHz bandwidth, achieving transmission over 10 km of 7-core single-mode multi-core fiber and 9 m mm-wave wireless at 25.5 GHz with bit error rates below the limit for a 7% overhead hard decision forward error correction.

The spectral resolution of a dispersive astronomical spectrograph is limited by the trade-off between throughput and the width of the entrance slit. Photonic guided-wave transitions have been proposed as a route to bypass this trade-off, by enabling the efficient reformatting of incoherent seeing-limited light collected by the telescope into a linear array of single modes: a pseudo-slit which is highly multimode in one axis but diffraction-limited in the dispersion axis of the spectrograph. It is anticipated that the size of a single-object spectrograph fed with light in this manner would be essentially independent of the telescope aperture size. A further anticipated benefit is that such spectrographs would be free of `modal noise', a phenomenon that occurs in high-resolution multimode fibre-fed spectrographs due to the coherent nature of the telescope point-spread-function (PSF). We address these aspects by integrating a multicore fibre photonic lantern with an ultrafast laser inscribed three-dimensional waveguide interconnect to spatially reformat the modes within the PSF into a diffraction-limited pseudo-slit. Using the CANARY adaptive optics (AO) demonstrator on the William Herschel Telescope, and 1530 $\pm$ 80 nm stellar light, the device is found to exhibit a transmission of 47-53 % depending upon the mode of AO correction applied. We also show the advantage of using AO to couple light into such a device by sampling only the core of the CANARY PSF. This result underscores the possibility that a fully-optimised guided-wave device can be used with AO to provide efficient spectroscopy at high spectral resolution., Comment: 8 pages, 4 figures

The first demonstration of narrowband spectral filtering of multimode light on a 3D integrated photonic chip using photonic lanterns and waveguide Bragg gratings is reported. The photonic lanterns with multi-notch waveguide Bragg gratings were fabricated using the femtosecond direct-write technique in boro-aluminosilicate glass (Corning, Eagle 2000). Transmission dips of up to 5 dB were measured in both photonic lanterns and reference single-mode waveguides with 10.4-mm-long gratings. The result demonstrates efficient and symmetrical performance of each of the gratings in the photonic lantern. Such devices will be beneficial to space-division multiplexed communication systems as well as for units for astronomical instrumentation for suppression of the atmospheric telluric emission from OH lines.

International audience; The advent of 30 m class Extremely Large Telescopes will require spectrographs of unprecedented spectral resolution in order to meet ambitious science goals, such as detecting Earth-like exoplanets via the radial velocity technique. The consequent increase in the size of the spectrograph makes it challenging to ensure their optimal environmental stabilization and precise spectral calibration. The multimode optical fibers used to transport light from the telescope focal plane to the separately housed environmentally stabilized spectrograph introduces modal noise. This phenomena manifests as variations in the light pattern at the output of the fiber as the input coupling and/or fiber position changes which degrades the spectrograph line profile, reducing the instrument precision. The photonic lantern is a guided wave transition that efficiently couples a multimode point spread function into an array of single modes. If arranged in a linear array at the input of the spectrograph these single modes can in principle provide a diffraction-limited mode noise free spectra in the dispersion axis. In this paper we describe the fabrication and throughput performance of the hybrid reformatter. This device combines the proven low-loss performance of a multicore fiber-based photonic lantern with an ultrafast laser inscribed three-dimensional waveguide interconnect that performs the reformatting function to a diffraction-limited pseudo-slit. The device provided an in laboratory throughput of 65 /- 2% at 1550 /- 20 nm and an on-sky throughput of 53 /- 4% at 1530 /- 80 nm using the CANARY adaptive optics system at the William Herschel Telescope.

We present the hybrid reformatter, a device combining a multicore fiber photonic lantern with an ultrafast laser inscribed waveguide interconnect to efficiently reformat a seeing-limited telescope point spread function into a diffraction-limited pseudo-slit.

This paper reports on the modal noise characterisation of a hybrid reformatter. The device consists of a multicore-fibre photonic lantern and an ultrafast laser-inscribed slit reformatter. It operates around 1550 nm and supports 92 modes. Photonic lanterns transform a multimode signal into an array of single-mode signals, and thus combine the high coupling efficiency of multimode fibres with the diffraction-limited performance of single-mode fibres. This paper presents experimental measurements of the device point spread function properties under different coupling conditions, and its throughput behaviour at high spectral resolution. The device demonstrates excellent scrambling but its point spread function is not completely stable. Mode field diameter and mode barycentre position at the device output vary as the multicore-fibre is agitated due to the fabrication imperfections., Comment: 9 pages, 9 figures, submitted to SPIE Astronomical Telescopes & Instrumentation 2016 (9912-78)

We present the opto-mechanical design and the characterization of the Replicable High-resolution Exoplanet and Asteroseismology (RHEA) spectrograph. RHEA is an ultra-compact fiber-fed echelle spectrograph designed to be used at 0.2-0.4 m class robotic telescopes where long term dedicated projects are possible. The instrument will be primarily used for radial velocity (RV) studies of low to intermediate-mass giant stars for the purpose of searching for hot Jupiters and using asteroseismology to simultaneously measure the host star parameters and de-correlate stellar pulsations. The optical design comprises a double-pass (i.e. near Littrow) configuration with prism cross-disperser and single-mode fiber (SMF) input. The spectrograph has a resolving power of R>70,000 and operates at 430–670 nm with minimum order separation of ~180 μm. This separation allows a 1x6 photonic lantern integration at a later stage which is currently under development. The current design is built with the aim of creating an inexpensive and replicable unit. The spectrograph is optimised for long-baseline RV observations through careful temperature stabilisation and simultaneous wavelength calibration. As a further improvement the echelle grating is housed in a vacuum chamber to maintain pressure stability. The performance of the current prototype is currently being tested on a 0.4 m telescope at the Macquarie University Observatory.

The challenges encountered by 3D laser written integrated photonics are both cultural and technical in nature. In this talk we will comment on the former, review the latter and highlight new 3D enabled applications.

There are numerous advantages to exploiting diffraction-limited instrumentation at astronomical observatories, which include smaller footprints, less mechanical and thermal instabilities and high levels of performance. To realize such instrumentation it is imperative to convert the atmospheric seeing-limited signal that is captured by the telescope into a diffraction-limited signal. This process can be achieved photonically by using a mode reformatting device known as a photonic lantern that performs a multimode to single-mode transition. With the aim of developing an optimized integrated photonic lantern, we undertook a systematic parameter scan of devices fabricated by the femtosecond laser direct-write technique. The devices were designed for operation around 1.55 {\mu}m. The devices showed (coupling and transition) losses of less than 5% for F/# $\geq$ 12 injection and the total device throughput (including substrate absorption) as high as 75-80%. Such devices show great promise for future use in astronomy., Comment: 12 pages, 9 figures

Since the discovery, that a tightly focused femtosecond laser beam can induce a highly localized and permanent refractive index change in a wide range of dielectrics, ultrafast laser inscription has found applications in many elds due to its unique 3D and rapid prototyping capabilities. These ultrafast laser inscribed waveguide devices are compact and lightweight as well as inherently robust since the waveguides are embedded within the bulk material. In this presentation we will review our current understanding of ultrafast laser - glass lattice interactions and its application to the fabrication of inherently stable, compact waveguide lasers and astronomical 3D integrated photonic circuits.

In this paper we present theoretical and laboratory results on integrated directly-written photonic lanterns with varying taper lengths. These lanterns convert seeing-limited light into multiple diffraction limited signals, in other words, a multimode signal into multiple single-mode signals. We investigated 19-channel structures which were written within a 30-mm-long glass block and designed to operate at 1550 nm. A single structure consisted of a multimode waveguide which transitioned into an array of single-mode waveguides and then back to a multimode waveguide utilizing cosine taper transitions. Based on simulations we found that transition lengths of 6 mm were sufficient to obtain throughput at a level of ~95%. Fabricated devices showed losses (coupling and transition losses) at the level of 30% for injection F/# < 5 and taper lengths < 5 mm. We believe that such devices show great promise for future use in astronomy.

We report on the fabrication and characterization of composite multimode waveguide structures that consist of a stack of single-mode waveguides fabricated by ultrafast laser inscription. We explore 2 types of composite structures; those that consist of overlapping single-mode waveguides which offer the maximum effective index contrast and non overlapped structures which support multiple modes via strong evanescent coupling. We demonstrate that both types of waveguides have negligible propagation losses (to within experimental uncertainty) for light injected with focal ratios >8, which corresponds to the cutoff of the waveguides. We also show that right below cutoff, there is a narrow region where the injected focal ratio is preserved (to within experimental uncertainty) at the output. Finally, we outline the major application of these highly efficient waveguides; in a device that is used to reformat the light in the focal plane of a telescope to a slit, in order to feed a diffraction-limited spectrograph., 15 pages, 11 figures, accepted to Optics Express

MAMMUT (Mirror vibrAtion Metrolology systeM for the Unit Telescope) is an ESO funded feasibility project for the development of a fiber interferometer prototype designed for optical path laser-metrology along the optical train of the Unit Telescopes (UT) of the Very Large Telescope Interferometer (VLTI). Fast mechanical vibrations originating in the VLTI cause fast variations of the optical path difference between two arms of the stellar interferometer, thus reducing the contrast of measured interference fringes. MAMMUT aims at monitoring in real time the optical path variations inside the Coude train of the UT, for active control purposes. MAMMUT features a 250-meter-long optical fiber which can be used to deliver and inject a laser beam at 1353 nm into the UT. The injected beam can be dropped from the telescope in the Coude room and interfered with a phase reference, provided by the second 250-meter-long arm of the fiber interferometer. The optical path variations are measured by means of an active homodyne scheme. Coherence between the beam at the injection point and the phase reference is provided by active fiber stabilization, made possible by the implementation of an internal metrology channel in MAMMUT. Here we present the initial laboratory performance results of the MAMMUT prototype, which will be able to sense optical path variations of +/- 5 μm with sub-10 nm precision within a bandwidth of at least 100 Hz.

Transiting exoplanets (TEPs) observed just about 10 Myrs after formation of their host systems may serve as the Rosetta Stone for planet formation theories. They would give strong constraints on several aspects of planet formation, e.g. time-scales (planet formation would then be possible within 10 Myrs), the radius of the planet could indicate whether planets form by gravitational collapse (being larger when young) or accretion growth (being smaller when young). We present a survey, the main goal of which is to find and then characterise TEPs in very young open clusters., Comment: Poster contribution to Detection and Dynamics of Transiting Exoplanets (Haute Provence Observatory Colloquium, 23-27 August 2010)

We are using a femtosecond laser to inscribe a series of integrated photonic lanterns that have a range of refractive index contrasts and geometry parameters in order to determine the ideal format for optimising the single-mode to multimode transition efficiency.