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Single-mode and low-loss operation of optical waveguides is typically limited to a 200-500 nm wide wavelength range. The lower limit is the boundary between single and multi-mode operation, and the upper limit comes from the decreasing confinement of the fundamental mode inside the core, which eventually leads to too large bending radii, waveguide cross-talk and poor integration density. Many interferometric waveguide components, such as grating couplers and multi-mode interference (MMI) couplers, have even narrower wavelength range. This paper demonstrates photonic integrated circuits (PICs) with ultra-broadband operation from 1.2 to 2.4 μm wavelength based on 3 μm thick silicon-on-insulator (SOI) waveguides. Such thick waveguides maintain ultra-high mode confinement for over 1 μm bandwidth, which supports dense integration with low-loss crossings, Euler bends and total internal reflection (TIR) mirrors. While some parts of the PICs are based on multi-moded strip waveguides, mode filters with rib-waveguides allow to keep the PICs effectively single-moded. The focus of the paper is on passive PICs, although the platform also enables active components. Ultra-broadband test results are provided for long waveguide spirals and waveguide-fiber coupling, as well as for echelle gratings, arrayed waveguide gratings (AWGs) and different types of 2x2 couplers. Low-loss operation is demonstrated with continuous transmission spectra measured from 1.25 μm up to 2.4 μm wavelength, i.e. up to 1.15 μm bandwidth. The measured bandwidths are limited by the available measurement setup, rather than the PIC components themselves. Remaining challenges for ultra-broadband operation, such as anti-reflection coatings, are discussed. Applications for broadband operation in communication, imaging and sensing are also presented.
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