Abstract
We report transmission measurements of germanium on silicon waveguides in the 7.5-8.5 μm wavelength range, with a minimum propagation loss of 2.5 dB/cm at 7.575 μm. However, we find an unexpected strongly increasing loss at higher wavelengths, potential causes of which we discuss in detail. We also demonstrate the first germanium on silicon multimode interferometers operating in this range, as well as grating couplers optimized for measurement using a long wavelength infrared camera. Finally, we use an implementation of the "cut-back" method for loss measurements that allows simultaneous transmission measurement through multiple waveguides of different lengths, and we use dicing in the ductile regime for fast and reproducible high quality optical waveguide end-facet preparation.
Highlights
The mid-infrared (MIR) wavelength range is fundamentally interesting for a wide variety of sensing applications, because many gases and chemicals exhibit strong absorption lines in this region, and in the so-called "fingerprint range" (8-14 μm)
Several waveguide platforms which use a Si waveguide core and different lower cladding materials have been demonstrated: silicon-on-insulator (SOI) has been demonstrated up to 3.8 μm wavelengths [1], beyond which SiO2 absorption becomes prohibitive; silicon-on-sapphire (SOS) has been demonstrated up to 5.5 μm [2]; in silicon on silicon nitride low loss waveguiding has been shown at 3.39 μm in [4] and quantum cascade laser integration has been demonstrated at 4.8 μm [5]; and suspended silicon waveguides, which are expected to be able to exploit the full Si MIR transparency range have been demonstrated up to 3.8 μm wavelength [6]
SiGe graded index and Germanium on silicon (Ge-on-Si) material platforms have been proposed for use at longer wavelengths at which Si absorption is too high for its use as a waveguide core material [7]
Summary
The mid-infrared (MIR) wavelength range is fundamentally interesting for a wide variety of sensing applications, because many gases and chemicals exhibit strong absorption lines in this region, and in the so-called "fingerprint range" (8-14 μm). SiGe graded index and Germanium on silicon (Ge-on-Si) material platforms have been proposed for use at longer wavelengths at which Si absorption is too high for its use as a waveguide core material [7]. SiGe waveguides have been demonstrated with a loss of only 2.0 dB/cm at 7.4. Because SiGe waveguides contain a mixture of Si and Ge, the Ge-on-Si platform with a fully Ge core would be expected to be transparent over an even wider range. To date Ge-on-Si waveguides with a loss of only 0.6 dB/cm have been demonstrated at 3.8 μm [9], and have been used at 5.8 μm with a loss of < 2.5 dB/cm [10], and at 7.5 μm with a loss of 3.0±0.8 dB/cm [11]
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