Chirped grating IR-filter on a waveguide for sensing applications

Very recently, we proposed a novel concept for a mid-infrared bandpass filter based on a single linearly chirped Bragg grating. [1] By its design, the filter structure features a large rejection bandwidth and can be used to create narrowband emitters in the mid-infrared region. Here we present results for a specific filter design for silicon waveguides, which features transmission wavelength and bandwidth well suited for carbon dioxide sensing.

Simulations were performed using Comsol Multiphysics® and the design was optimized for a central wavelength of 4.26 µm. Furthermore, we included real-world effects like the discrete resolution of the design grid as well as process specific fabrication tolerances. The devised structures were based on a photonic waveguide concept, which was developed recently for evanescent-field based sensing applications. [2] In short, a thick SiO₂ cladding layer and an intermediate thin Si₃N₄ layer with a thickness of 140 nm are deposited on a Si substrate. The waveguide itself consists of poly-silicon and it is deposited on top of the S₃N₄ layer with a height of 660 nm. Slab waveguides with gratings on top (with a modulation depth of 100 nm) as well as strip waveguides with sidewall gratings were investigated.

The concept and design are discussed in detail and very first results from the first fabrication run of test structures are presented.

[1] C. Ranacher, B. Jakoby, C. Consani, A. Tortschanoff, “Design of a Mid-Infrared Bandpass Filter With Large Rejection Bandwidth of Silicon Photonics,” J. of Lightwave Technology, vol. 37, no 15, pp. 3770-3776, 2019

[2] C. Ranacher, C. Consani, N. Vollert, A. Tortschanoff, M. Bergmeister, T. Grille, and B. Jakoby, “Characterization of evanescent field gas sensor structures based on silicon photonics,” IEEE Photonics Journal , vol. 10, no. 5, pp. 1–14, 2018