Planar integrated photonics spectrograph on silicon-nitride-on-insulator: densely integrated systems for astrophotonics and spectroscopy

A conventional Arrayed Waveguide Grating (AWG) has been modified, without output receiver waveguides, for nonconventional applications such as Astrophotonics and spectroscopy sensing where the input signal can have information over the entire band and a continuum of light/spectrum. The material system chosen for the AWG design is siliconnitride/ SiO2/Si (Si3N4-SiO2-Si) for its relatively high refractive index, which for a given channel spacing allowing a more compact device than Silicon-on-Silica. Further, CMOS compatibility and the presence of high non-liner optical coefficient would be an added advantage to design and fabricate densely integrated photonic sub-systems, such as calibration source and AWG, for astrophotonics and spectroscopy. The proposed AWG utilizes a flat image plane optimized for minimal aberration. An analytical calculation, based on Gaussian beam approximation, was used to determine the optimal flat plane position where the non-uniformity in 1/e electric field widths is minimal. This plane can be used as the dicing plane to re-image the entire output of the AWG onto a detector array to sample the entire spectrum. Tailored AWG, with flat image-plane, designed to resolve 48 spectral channels with 0.4nm (50GHz) resolution and adjacent channel cross-talk level within a 0.2nm window (ITU-grid) ~ -28dB. Calculated insertion loss non-uniformity is close to 3dB. The foot-print of high index contrast (Δn=23%) IPS is ~ 12x8.5 mm2. The modelled mean spectral resolving power, R, at the flat image-plane is ~ 7,600. The design principle could be utilised for devices using other material systems with different parameters.

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