Integrated photonic building blocks for next-generation astronomical instrumentation II: the multimode to single mode transition

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 figure

Multiband processing of multimode light: combining 3D photonic lanterns with waveguide Bragg gratings

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.Comment: 5 pages, 4 figures, accepted to Laser & Photonics Review

Integrated photonic building blocks for next-generation astronomical instrumentation I: the multimode waveguide

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.Comment: 15 pages, 11 figures, accepted to Optics Expres

Transition technologies towards 6G networks

[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.Raddo, TR.; Rommel, S.; Cimoli, B.; Vagionas, C.; Pérez-Galacho, D.; Pikasis, E.; Grivas, E.... (2021). Transition technologies towards 6G networks. EURASIP Journal on Wireless Communications and Networking. 2021(1):1-22. https://doi.org/10.1186/s13638-021-01973-91222021

Overcoming the effects of the Earth's atmosphere on astronomical observations with 3D integrated photonic technologies

Theoretical thesis.Bibliography: pages 157-174.1. Introduction -- 2. Femtosecond direct write technique background -- 3. Design and modelling of photonic lanterns -- 4. Experimental methods -- 5. Photonic lantern fabrication and characterisation -- 6. Photonic lanterns with waveguide Bragg gratings -- 7. High-resolution spectra with slit reformatting devices -- 8. Conclusions and future work -- Appendices.The earth's atmosphere prevents telescopes, astronomical instruments and astrophotonic devices from working at their full capabilities. Atmospheric turbulence introduces wavefront distortions, while hydroxyl molecules in the ozone layer produce strong emission lines in the infrared part of the spectrum. Most techniques to address these problems are inefficient and expensive, forcing the instrumentation to be of large size and cost. By using a femtosecond laser direct-write technique, 3D optical circuitry called photonic lanterns were fabricated. The devices enable the creation of compact optical instruments and integrated photonic devices on ground-based telescopes. The objective of this thesis was to optimise the design of photonic lanterns to increase their efficiency, and create an on-sky prototype instrument to demonstrate the feasibility of the approach. A detailed optimisation of the three main building blocks of photonic lanterns and slit reformatting devices was performed. After optimising design for multimode waveguides, the mode evolution along a transition section between the multimode and isolated single-mode waveguides sections was analysed, and three types and lengths of transition to find the adiabatic regime were studied. An optimal design was identified for which back-to-back photonic lanterns and slit reformatting devices are >90% efficient. In the second part of the thesis, fully integrated photonic lanterns with multiple waveguide Bragg gratings were created. Multiple gratings were placed in the waveguide using the point-by-point femtosecond laser inscription technique. Devices which filter out one, two, three or four wavelengths were fabricated. The best performance was demonstrated by the device with three gratings for wavelengths of 1545 nm, 1552nm and 1559 nm, featuring grating strengths of 5.12 dB, 5.60 dB, and 2.87 dB, respectively. In the third part of the thesis, a slit reformatting device and a single-mode high resolution spectrograph were used to create a prototype on-sky demonstrator instrument and conduct a proof-of-concept test. The astronomical data were acquired on the 0.4m telescope at the Macquarie University Observatory, Sydney. The spectrum of Antares revealed multiple CO band heads in the astronomical H band, as well as sharp atmospheric water absorption lines demonstrating the feasibility of the technique -- abstract.1 online resource (xxvii, 174 pages

Exploration of integrated photonic lanterns fabricated by femtosecond laser inscription

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.3 page(s

Enabling photonic technologies for seeing-limited telescopes: Fabrication of integrated photonic lanterns on a chip

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

Exploration of integrated photonic lanterns fabricated by femtosecond laser inscription

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

Enabling photonic technologies for seeing-limited telescopes : fabrication of integrated photonic lanterns on a chip

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.8 page(s