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An On-chip Photonic Spectrograph Using Arrayed Waveguide Gratings

Published onJun 01, 2020
An On-chip Photonic Spectrograph Using Arrayed Waveguide Gratings

As we enter the era extremely large telescopes in the coming decade, the volume, mass, and cost of the instruments on these telescopes is poised to grow in a superlinear fashion. This is particularly true for spectrographs. Astrophotonics is an innovative approach which applies the versatile photonic technologies to manipulate guided light on a chip or fiber to achieve various scientific objectives in astronomy in an efficient and cost-effective way. We present an astrophotonic spectrograph in the near-IR H-band (1.45 -1.65 micron) and a spectral resolution of 1500. The key dispersing element in this spectrograph is a photonic chip based on Arrayed-Waveguide-Grating technology. This chip-based spectrograph is fed by single-mode fibers and the dispersion is achieved on the chip by splitting the light into multiple on-chip waveguides and introducing increasing path delays (= spectral order x central wavelength) in consecutive waveguides. The light from these waveguides is coherently combined within the chip to create a 1D spectrum. The 1D spectrum produced on the focal plane of the AWG contains overlapping spectral orders, each spanning a 10 nm band in wavelength. These spectral orders are cross-dispersed in the perpendicular direction using a cross-dispersion setup which consists of collimating lenses and a secondary grating and the 2D spectrum is thus imaged onto a near-IR detector. The photonic action on a chip (such as dispersion) can only be achieved using a single-mode input. However, the astronomical light received by the ground-based telescopes is multi-moded in nature, and can only be captured efficiently by a large-diameter multi-mode fiber (MMF). A photonic lantern is an adiabatic taper to convert a multi-mode fiber into several single-mode fibers. In the present design, we use a few-mode photonic lantern to capture the light and feed the emanating single-mode outputs to the AWG chip for dispersion. The total size of the setup is 50 cm x 30 cm x 20 cm, nearly the size of a shoebox. This spectrograph will be used for an on-sky test on the University of Maryland College Park Observatory telescope. This demonstration will pave the way for future miniaturized integrated photonic spectrographs on large telescopes, particularly for the purpose of multi-object spectroscopy.

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