![]() To overcome this hurdle, a post-draw tapering procedure was developed, which allows for more accurate control over the flow of the molten core, resulting in crystalline silicon optical fibres with the smallest core and with losses comparable to their planar waveguide counterparts 22. This is because, at the temperatures required to soften the silica cladding, the silicon core is molten, and high drawing speeds can lead to break-up of the core due to Rayleigh instabilities 21. Until recently, one of the main limitations to producing SCFs using a fibre-drawing tower was obtaining the small, few-micrometre core sizes needed to achieve efficient nonlinear processing. As these fibres are clad in silica, they are robust, stable, and fully compatible with standard fibre fabrication procedures, thus increasing the device yield and reducing costs 20. Silicon core fibres (SCFs) represent an emerging platform that combines the benefits of fibre geometry with the advantages of semiconductor material systems. With a view towards extending the long wavelength edge, more complicated structures (i.e., suspended waveguides 14) and material systems (e.g., silicon-on-sapphire 15, 16, 17, silicon–germanium (SiGe)-on-silicon 18, and suspended III–V semiconductors-on-silicon waveguides 19) have been considered, though these come with increased fabrication costs and integration complexity. As a result, the best demonstrations of SC generation in silicon-on-insulator (SOI) waveguides, which are the most common semiconductor waveguide platforms, have so far been limited to wavelengths <3.3 μm 12, 13. In this case, the trade-off is that these small core waveguides suffer from low power conversion efficiency due to both the high on-chip coupling losses (typically 5–10 dB per facet) and propagation losses associated with increased core/cladding interactions 11. Alternatively, planar-based SC systems employing highly nonlinear group IV materials (e.g., silicon) and compound III–V semiconductors (e.g., GaAs and AlGaAs) can avoid these issues and offer advantages in terms of compactness and on-chip integration 9, 10, which are important considerations for the development of portable systems. However, there are challenges when working with these materials, as they are not as stable or robust as their silica counterparts and, in the case of the chalcogenides, they often contain toxic compounds. To date, the broadest and brightest spectra have been demonstrated in fibre systems made from non-silica soft glasses (e.g., chalcogenides 6, fluorides 7, or tellurites 8), primarily due to their capability to handle high power levels. Mid-IR SC spectra have been demonstrated in a range of material systems, in both fibre and planar platforms. ![]() To be effective, these sources must exhibit several key features, including coherence, high brightness, robustness, stability, and for healthcare applications, safe handling 5. For applications that require broad spectral bandwidths, such as spectroscopic sensing 3 and high-resolution imaging 4, supercontinuum (SC) sources based on extreme nonlinear phenomena have emerged as the most popular option. The mid-infrared (mid-IR) region is an important spectral region in which strong molecular absorption bands and atmospheric transmission windows can be exploited for practical use in medicine, food production, imaging, environmental monitoring, and security 1, 2. These waveguides exhibit many of the benefits of fibre platforms, such as a high coupling efficiency and power handling capability, allowing for the generation of mid-infrared supercontinuum spectra with high brightness and coherence spanning almost two octaves (1.6–5.3 µm). The waveguides are fabricated from a silicon core fibre that is tapered to engineer mode properties to ensure efficient nonlinear propagation in the core with minimal interaction of the mid-infrared light with the cladding. Here, we design and demonstrate a compact silicon core, silica-clad waveguide platform that has low losses across the entire silicon transparency window. However, most research in this area has made use of small core waveguides fabricated from silicon-on-insulator platforms, which suffer from high absorption losses of the use of silica cladding, limiting their ability to generate light beyond 3 µm. ![]() In recent years, silicon has attracted great interest as a platform for nonlinear optical wavelength conversion in this region, owing to its low losses (linear and nonlinear) and high stability. Broadband mid-infrared light sources are highly desired for wide-ranging applications that span free-space communications to spectroscopy.
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