Current SC sources are typically based on silica nonlinear fibers with zero-dispersion wavelength and low attenuation in the visible/near-IR range allowing to generate spectra covering the 400 to 1600 nm spectral region. Extending high brightness SC sources to the mid-infrared is expected to lead to substantial improvements in optical diagnostics, rendering possible e.g. to synthesize mid-IR frequency combs and enabling spectroscopy at a completely new level in terms of precision and sensitivity in the molecular fingerprint region. Whilst silica-based fibers are optimum for the visible and near-IR wavelengths range, the high material losses in the IR generally prevents reaching wavelengths beyond 2 microns. In order to circumvent this limitation, several fibers composed of highly nonlinear soft-glasses such as tellurite, chalcogenide or fluoride with losses orders of magnitude smaller than that of silica in the mid-IR have been recently developed. Among those alternative materials, fluoride or ZBLAN fibers exhibit the lowest nonlinearity but possess significantly lower absorption in the mid-IR range up to 4 micron. The refractive index of ZBLAN is also close to that of silica so that ZBLAN fibers can be readily spliced to standard silica fibers. And indeed the potential of ZBLAN fibers for SC generation in the mid-IR has been recently demonstrated with SC sources extending from 1 to 4 microns.
It is clear that key advances in understanding SC properties have been achieved by accurate modeling of the underlying ultrafast dynamics using the generalized nonlinear Schrödinger equation. It is thus evident that the detailed knowledge of the pump and fiber parameters is paramount for precise prediction and optimization of the SC characteristics. Whilst silica is a well-known material, the properties of soft glasses on the other hand are much less known and for optimization of mid-IR SC generation in soft-glass fibers it is crucial to have a precise knowledge of the material parameters, including losses, waveguide properties and Raman gain characteristics.
In this paper, Agger et. al. perform a detailed experimental study of a commercially available ZBLAN fiber used for SC generation in the mid-IR. By accurately characterizing the material losses, the waveguide parameters of the fiber, and the frequency-dependence of the Raman gain they are able, for the first time, to conduct an extensive comparison between numerical simulations and experiments of SC generation in this type of fiber. There results open up the route for exploring with confidence various regimes of SC generation using in ZBLAN fibers and optimizing the output characteristics for specific applications in the mid-IR spectral range.
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