The Potsdam astroComb (POCO) Part I: Mode crossing effect in feedback resonators

We investigate theoretically and experimentally the mode interaction in an integrated Silicon Nitride (Si3N4) microring resonator with interferometric coupling realized by a feedback loop as an adjustable optical path path length connecting the ring to the bus waveguide at two coupling sections. From the transmission spectra recorded at different optical path lengths, two resonances, 1596.5~nm and 1570.5~nm, were selected for detailed investigation. Both resonances show the possibility of adjusting the resonance width and depth. However, the transmission spectra around the first resonance also show the effect of mode interaction. This is also well captured in the theoretical model, from which we can derive a coupling rate for the mode interaction of $3.4~\textrm{rad}~\textrm{ns}^{-1}$

Performance limits of astronomical arrayed waveguide gratings on silica platform

We present a numerical and experimental study of the impact of phase errors on the performance of large, high-resolution Arrayed Waveguide Gratings (AWG) for applications in astronomy. We use a scalar diffraction model to study the transmission spectrum of an AWG under random variations of the optical waveguide lengths. We simulate phase error correction by numerically trimming the lengths of the optical waveguides to the nearest integer multiple of the central wavelength. The optical length error distribution of a custom-fabricated silica AWG is measured using frequency-domain interferometry and Monte-Carlo fitting of interferogram intensities. In the end, we give an estimate for the phase-error limited size of a waveguide array manufactured using state-of-the-art technology. We show that post-processing eliminates phase errors as a performance limiting factor for astronomical spectroscopy in the H-band.Comment: 14 pages, 13 figures; to be published in Optics Expres

Design, simulation and characterization of integrated photonic spectrographs for astronomy: generation-I AWG devices based on canonical layouts

We present an experimental study on our first generation of custom-developed arrayed waveguide gratings (AWG) on a silica platform for spectroscopic applications in near-infrared astronomy. We provide a comprehensive description of the design, numerical simulation and characterization of several AWG devices aimed at spectral resolving powers of 15,000-60,000 in the astronomical H-band. We evaluate the spectral characteristics of the fabricated devices in terms of insertion loss and estimated spectral resolving power and compare the results with numerical simulations. We estimate resolving powers of up to 18,900 from the output channel 3-dB transmission bandwidth. Based on the first characterization results, we select two candidate AWGs for further processing by removal of the output waveguide array and polishing the output facet to optical quality with the goal of integration as the primary diffractive element in a cross-dispersed spectrograph. We further study the imaging properties of the processed AWGs with regards to spectral resolution in direct imaging mode, geometry-related defocus aberration, and polarization sensitivity of the spectral image. We identify phase error control, birefringence control, and aberration suppression as the three key areas of future research and development in the field of high-resolution AWG-based spectroscopy in astronomy

Characterization of a C-RED One camera for astrophotonical applications

To better understand the impact of the avalanche gain applied in the detector technology and apply this technology in our in-house astrophotonic projects, we have characterized a C-RED One camera and produced a stable and reliable method for calculating the system gain at any desired avalanche gain setting. We observed that depending on how the system gain is obtained, multiplying the system gain times the avalanche gain may not accurately produce a conversion factor from electrons to ADUs. Since the acquisition of a photon transfer curve (PTC) was possible at different avalanche gain levels, several PTCs at low avalanche gain levels were acquired. Consequently, a linear fit was produced from the acquired system gain as a function of the avalanche gain setting. Through the linear fit, the effective system gain was calculated at any desired avalanche level. The effective system gain makes possible to accurately calculate the initial system gain without the ambiguity introduced by the non-linearity of the system. Besides, the impact of the avalanche gain on the dynamic range was also analyzed and showed a stable behaviour through the measured avalanche range

Fiber Vector Bend Sensor Based on Multimode Interference and Image Tapping

A grating-less fiber vector bend sensor is demonstrated using a standard single mode fiber spliced to a multimode fiber as a multimode interference device. The ring-shaped light intensity distribution at the end of the multimode fiber is subject to a vector transition in response to the fiber bend. Instead of comprehensive imaging processing for the analysis, the image can be tapped out by a seven-core fiber spliced to the other end of the multimode fiber. The seven-core fiber is further guided to seven single mode fibers via a commercial fan-out device. By comparing the relative light intensities received at the seven outputs, both the bend radius and its direction can be determined. Experiment has shown that a slight bend displacement of 10 µm over a 1.2-cm-long multimode fiber in the X direction (bend angle of 0.382 ◦ ) causes a distinctive power imbalance of 4.6 dB between two chosen outputs (numbered C4 and C7). For the same displacement in the Y direction, the power ratio between the previous two outputs C4 and C7 remains constant, while the imbalance between another pair (C3 and C4) rises significantly to 7.0 dB. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

MARCOT Pathfinder at Calar Alto progress report

Ground-Based and Airborne Telescopes IX (2022), Montreal, Jul 17-22, 2022.--Proceedings of SPIE - The International Society for Optical Engineering vol. 12182 Article number 121820MMARCOT Pathfinder is a precursor for MARCOT (Multi Array of Combined Telescopes) at Calar Alto Observatory (CAHA) in Spain. MARCOT is intended to provide CARMENES, currently fiber-fed from the CAHA 3.5m Telescope, with a 5-15m light collecting area from a battery of several tens of small telescopes that are incoherently fed into the final joint single fiber feed of the spectrograph. The modular concept, based on commercially available telescopes, results in cost estimates that are a fraction of the ones for extremely large telescopes (ELT). As a novel approach, MARCOT will employ Multi-Mode Photonic Lanterns (MM-PL) that are being developed as a variant of classical photonic lanterns, to combine the light from the individual telescopes to a single fiber feed to the instrument. This progress report presents the overall concept of MARCOT, the pathfinder telescope and enclosure that is being commissioned at CAHA, the concept of MM-PL, and the next step of installing the Potsdam Multiplex Raman Spectrograph (MRS). MARCOT Pathfinder will be used to validate the conceptual design and predicted performance of MM-PL on sky with a 7-unit telescope prototype. © 2022 SPIE.The authors acknowledge financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrofisica de Andalucia (SEV20170709). This research has been partially funded by the Junta de Andalucia (SOMM17 5208 IAA). KM, JD, and MMR acknowledge support from BMBF grant 03Z22AN11 "Astrophotonics", DFG grant 326946494, "NAIR", and BMBF grant 03Z22AI1 "Strategic Investment", at the Zentrum fur Innovationskompetenz innoFSPEC.Peer reviewe