Towards a multi-input astrophotonic AWG spectrograph

Abstract

Astrophotonics is the new frontier technology to make suitable diffraction-limited spectrographs for the next generation of large telescopes. Astrophotonic spectrographs are miniaturized, robust and cost-effective. For various astronomical studies, such as probing the early universe, observing in near infrared (NIR) is crucial. Therefore, our research group is developing moderate resolution (R ∼ 1500) on-chip photonic spectrographs in the NIR bands (J Band: 1.1−1.4 µm; H band: 1.45−1.7 µm). To achieve this, we use the concept of arrayed waveguide gratings (AWGs). We fabricate the device using a silica-on-silicon substrate. The waveguides on this AWG are 2 µm wide and 0.1 µm high Si 3 N 4 core buried inside a 15 µm thick SiO 2 cladding.To make the maximal use of astrophotonic integration such as coupling the AWGs with multiple single-mode fibers coming from photonic lanterns or fiber Bragg gratings (FBGs), we require a multi-input AWG design. In a multi-input AWG, the output spectrum due to each individual input channel overlaps to produce a combined spectrum from all inputs. This on-chip combination of light effectively improves the signal-to-noise ratio as compared to spreading the photons to several AWGs with single inputs. In this paper, we present the design and simulation results of an AWG in the H band with three input waveguides (channels). The resolving power of individual input channels is ∼1500, while the overall resolving power with three inputs together is ∼500, 600, 750 in three different configurations simulated here. The device footprint is only 16 mm × 7 mm. The free spectral range of the device is ∼9.5 nm around a central wavelength of 1600 nm. For the standard multi-input AWG, the relative shift between the output spectra due to adjacent input channels is about 1.6 nm, which roughly equals one spectral channel spacing. In this paper, we discuss ways to increase the resolving power and the number of inputs without compromising the free spectral range or throughput.