Supercontinuum generation and nonlinearity in soft glass fibres

IntroductionThere are many applications for broad bandwidth infrared laser sources, including optical frequency metrology (Udem et al., 2002), precision spectroscopy (Holzwarth et al., 2000) and optical tomography (Hartl et al., 2001), and moving into the mid-infrared (mid-IR), uses for wavelengths beyond 2 μm include LIDAR, molecular spectroscopy and active hyperspectral imaging. Fibre-based supercontinuum sources are attractive for these applications due to their combination of high brightness and broad bandwidth in comparison to alternative thermal or laser sources. Current high brightness mid-IR sources are typically based on optical parametric oscillators (OPO) or quantum cascade lasers (QCL). While OPOs achieve excellent performance they require large pump lasers and can be rather complex and costly to maintain, and QCLs are hard to scale up in power and cannot at present be used to access the important 2–3 μm regime. New fibre-based technology could create an important additional source of robust and lower cost broad bandwidth mid-IR light for the future.Beyond a wavelength of 2 μm, due to the onset of losses in silica, it is necessary to consider the use of non-silica glasses. The fundamental material properties of these glasses can enhance supercontinuum generation across the mid-IR since these glasses can have intrinsic nonlinearities ∼ 10 × to 100 × that of silica. However, the zero-dispersion wavelengths of these materials are generally longer than for silica, implying the need for longer wavelength pump lasers.