Conversion of ultrashort laser pulses to wavelengths above 3 mm in tapered germanate fibres

Tapered germanate fibres are proposed for effective adiabatic conversion of Raman soliton pulses to the mid-IR region. A theoretical analysis demonstrates that, in fibres with anomalous group velocity dispersion decreasing along their length, wavelengths of up to 3.5 mm can be reached, which are unattainable in fibres with a constant core diameter at the same parameters of a 2-mm input signal. The analysis relies on a one-way wave equation that takes into account the combined effect of dispersion, Kerr and Raman nonlinearities, nonlinear dispersion and optical losses and the frequency dependence of the effective fundamental transverse mode size.