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Author: Roth, Martin × DDC: 530 ×

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    High-Resolution Arrayed-Waveguide-Gratings in Astronomy: Design and Fabrication Challenges
    (Basel : MDPI, 2017) Stoll, Andreas; Zhang, Ziyang; Haynes, Roger; Roth, Martin
    A comprehensive design of a folded-architecture arrayed-waveguide-grating (AWG)-device, targeted at applications as integrated photonic spectrographs (IPS) in near-infrared astronomy, is presented. The AWG structure is designed for the astronomical H-band (1500 nm-1800 nm) with a theoretical maximum resolving power R = 60,000 at 1630 nm. The geometry of the device is optimized for a compact structure with a footprint of 5.5 cm × 3.93 cm on SiO2 platform. To evaluate the fabrication challenges of such high-resolution AWGs, effects of random perturbations of the effective refractive index (RI) distribution in the free propagation region (FPR), as well as small variations of the array waveguide optical lengths are numerically investigated. The results of the investigation show a dramatic degradation of the point spread function (PSF) for a random effective RI distribution with variance values above ~10-4 for both the FPR and the waveguide array. Based on the results, requirements on the fabrication technology for high-resolution AWG-based spectrographs are given in the end.
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    Seeking celestial positronium with an OH-suppressed diffraction-limited spectrograph
    (Washington, DC : The Optical Society, 2021) Robertson, Gordon; Ellis, Simon; Yu, Qingshan; Bland-Hawthorn, Joss; Betters, Christopher; Roth, Martin; Leon-Saval, Sergio
    Celestially, positronium (Ps) has been observed only through gamma-ray emission produced by its annihilation. However, in its triplet state, a Ps atom has a mean lifetime long enough for electronic transitions to occur between quantum states. This produces a recombination spectrum observable in principle at near IR wavelengths, where angular resolution greatly exceeding that of the gamma-ray observations is possible. However, the background in the near IR is dominated by extremely bright atmospheric hydroxyl (OH) emission lines. In this paper, we present the design of a diffraction-limited spectroscopic system using novel photonic components—a photonic lantern, OH fiber Bragg grating filters, and a photonic TIGER 2D pseudo-slit—to observe the Ps Balmer alpha line at 1.3122 µm for the first time, to our knowledge.
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    Performance limits of astronomical arrayed waveguide gratings on a silica platform
    (Washington, DC : Soc., 2020) Stoll, Andreas; Madhav, Kalaga; Roth, Martin
    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.
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    Reflectivity of Venus’s Dayside Disk During the 2020 Observation Campaign: Outcomes and Future Perspectives
    ([Bristol] : IOP Publishing, 2022) Lee, Yeon Joo; García Muñoz, Antonio; Yamazaki, Atsushi; Quémerais, Eric; Mottola, Stefano; Hellmich, Stephan; Granzer, Thomas; Bergond, Gilles; Roth, Martin; Gallego-Cano, Eulalia; Chaufray, Jean-Yves; Robidel, Rozenn; Murakami, Go; Masunaga, Kei; Kaplan, Murat; Erece, Orhan; Hueso, Ricardo; Kabáth, Petr; Špoková, Magdaléna; Sánchez-Lavega, Agustín; Kim, Myung-Jin; Mangano, Valeria; Jessup, Kandis-Lea; Widemann, Thomas; Sugiyama, Ko-ichiro; Watanabe, Shigeto; Yamada, Manabu; Satoh, Takehiko; Nakamura, Masato; Imai, Masataka; Cabrera, Juan
    We performed a unique Venus observation campaign to measure the disk brightness of Venus over a broad range of wavelengths in 2020 August and September. The primary goal of the campaign was to investigate the absorption properties of the unknown absorber in the clouds. The secondary goal was to extract a disk mean SO2 gas abundance, whose absorption spectral feature is entangled with that of the unknown absorber at ultraviolet wavelengths. A total of three spacecraft and six ground-based telescopes participated in this campaign, covering the 52–1700 nm wavelength range. After careful evaluation of the observational data, we focused on the data sets acquired by four facilities. We accomplished our primary goal by analyzing the reflectivity spectrum of the Venus disk over the 283–800 nm wavelengths. Considerable absorption is present in the 350–450 nm range, for which we retrieved the corresponding optical depth of the unknown absorber. The result shows the consistent wavelength dependence of the relative optical depth with that at low latitudes, during the Venus flyby by MESSENGER in 2007, which was expected because the overall disk reflectivity is dominated by low latitudes. Last, we summarize the experience that we obtained during this first campaign, which should enable us to accomplish our second goal in future campaigns.
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