The generation of a supercontinuum generally involves several nonlinear processes, such as self- and cross-phase modulation, four-wave mixing, modulation instability, dispersive wave generation and Raman scattering, which act together as the wave propagates along the medium. However, in order to allow these processes to efficiently build up and give rise to a broadband supercontinuum, it is not just the nonlinearity of the medium that is important. Its chromatic dispersion characteristics play an equally important role, as does also the propagation loss across the entire spectral range of interest. Photonic crystal optical fibers have proved a rather useful technology in this respect, since their flexibly engineered waveguide properties have allowed for both precise manipulation of the waveguide dispersion and enhanced nonlinear behavior (facilitated by the large index contrast between the large core and the air-filled cladding). However, several applications would benefit from compact supercontinuum sources, and this has prompted researchers to investigate materials with extreme values of nonlinearity which could be used to facilitate nonlinear generation on a chip. Materials, such as silicon and chalcogenide, have been obvious candidates for this purpose, mainly because of their excellent nonlinear properties. Despite the impressive results achieved to date, though, widespread adoption of such highly nonlinear materials has been hampered by the typically relatively high (both linear and nonlinear, at the wavelengths of interest for most applications) optical losses.
Silica on the other hand, is a material with relatively low nonlinearity – this is one of the features that have made silica optical fibers so attractive for long-distance communications. However, when seen as a nonlinear material, the low nonlinearity of silica can be counterbalanced by its extremely low losses, which allow substantially strong nonlinear effects to be built up over long distances. This has been applied extensively in nonlinear fiber optics, where silica-based nonlinear devices are implemented in relatively long lengths of optical fiber. The work by Oh et al. has combined the advantages of silica as a low-loss material with the compactness of a waveguide chip design that has allowed 3.5 m of waveguide to be implemented in a spiral design occupying an area of just a few square centimeters. The work reports the generation of frequencies extending over more than 160 THz in the spectrum, while detailed numerical modeling of the waveguide optical properties shows that it is possible to predict accurately its nonlinear behavior. The work is significant in that it shows the potential of silica waveguides as compact and efficient nonlinear devices, whereas improvements in the engineering of the waveguide dispersion characteristics can be expected to yield devices exhibiting even more impressive performance.
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