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- ItemUltrafast laser inscription of asymmetric integrated waveguide 3 dB couplers for astronomical K-band interferometry at the CHARA array(Washington, DC : Soc., 2021) Benoît, Aurélien; Pike, Fraser A.; Sharma, Tarun K.; MacLachlan, David G.; Dinkelaker, Aline N.; Nayak, Abani S.; Madhav, Kalaga; Roth, Martin M.; Labadie, Lucas; Pedretti, Ettore; Brummelaar, Theo A. ten; Scott, Nic; Coudé du Foresto, Vincent; Thomson, Robert R.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±0.5dB 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.
- ItemPhotonic lanterns: a practical guide to filament tapering(Washington, DC : OSA, 2021) Davenport, John J.; Diab, Momen; Deka, Pranab J.; Tripathi, Aashana; Madhav, Kalaga; Roth, Martin M.We present a detailed method of tapering and drawing photonic lanterns using a filament glass processing system. Single-mode fibers (SMFs) were stacked inside a low refractive index, fluorine-doped capillary, which was then heated and tapered to produce a transition from single-mode to multi-mode. Fabrication parameters were considered in four categories: method of preparation and stacking of SMFs into a capillary, heat and filament dimensions of the glass processor, capillary ID, and the use of vacuum during tapering. 19- and 37- fiber lanterns were drawn, demonstrating good fusion between SMF claddings, a clear differentiation between core and cladding in the multimode (MM) section, and well-ordered arrangements between SMFs, which is controlled during the tapering process. The transmission efficiency of a 19-fiber photonic lantern, compared to an MMF with the same core diameter and NA, has a relative transmission efficiency of 1.19 dB or 67.1%. The steps and parameters provided in this paper form a framework for fabricating quality photonic lanterns.
- ItemPerformance limits of astronomical arrayed waveguide gratings on a silica platform(Washington, DC : Soc., 2020) Stoll, Andreas; Madhav, Kalaga; Roth, MartinWe 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.