2021, Qi, Xue, Schaarschmidt, Kay, Li, Guangrui, Junaid, Saher, Scheibinger, Ramona, Lühder, Tilman, Schmidt, Markus A.

Ultrafast supercontinuum generation crucially depends on the dispersive properties of the underlying waveguide. This strong dependency allows for tailoring nonlinear frequency conversion and is particularly relevant in the context of waveguides that include geometry-induced resonances. Here, we experimentally uncovered the impact of the relative spectral distance between the pump and the bandgap edge on the supercontinuum generation and in particular on the dispersive wave formation on the example of a liquid strand-based photonic bandgap fiber. In contrast to its air-hole-based counterpart, a bandgap fiber shows a dispersion landscape that varies greatly with wavelength. Particularly due to the strong dispersion variation close to the bandgap edges, nanometer adjustments of the pump wavelength result in a dramatic change of the dispersive wave generation (wavelength and threshold). Phase-matching considerations confirm these observations, additionally revealing the relevance of third order dispersion for interband energy transfer. The present study provides additional insights into the nonlinear frequency conversion of resonance-enhanced waveguide systems which will be relevant for both understanding nonlinear processes as well as for tailoring the spectral output of nonlinear fiber sources.

2021, Junaid, Saher, Bierlich, Joerg, Hartung, Alexander, Meyer, Tobias, Chemnitz, Mario, Schmidt, Markus A.

We demonstrate supercontinuum generation in a liquid-core microstructured optical fiber using carbon disulfide as the core material. The fiber provides a specific dispersion landscape with a zero-dispersion wavelength approaching the telecommunication domain where the corresponding capillary-type counterpart shows unsuitable dispersion properties for soliton fission. The experiments were conducted using two pump lasers with different pulse duration (30 fs and 90 fs) giving rise to different non-instantaneous contributions of carbon disulfide in each case. The presented results demonstrate an extraordinary high conversion efficiency from pump to soliton and to dispersive wave, overall defining a platform that enables studying the impact of non-instantaneous responses on ultrafast soliton dynamics and coherence using straightforward pump lasers and diagnostics.

2022, Qi, Xue, Scheibinger, Ramona, Nold, Johannes, Junaid, Saher, Chemnitz, Mario, Schmidt, Markus A.

Soliton-based supercontinuum generation is a powerful approach for generating light with the desired properties, although limited dispersion tuning capabilities remain a key challenge. Here, we introduce liquid-core fibers (LCFs) with longitudinally controlled dispersion of a higher-order mode, achieved by axial modulation of the liquid core diameter. This approach provides a versatile photonic platform with unique dispersion control capabilities that are particularly relevant to ultrafast, non-linear frequency conversion. Our tuning concept uses LCFs with anomalous dispersion at telecommunication wavelengths (TE01-mode) and relies on the strong dependence of dispersion on the core diameter. Non-monotonic, complex dispersion profiles feature multiple dispersive waves formation when launching ultrashort pulses. For example, this effect has been used to fill spectral gaps in fibers with linearly decreasing core diameter in order to spectrally smooth the output spectra. Our results highlight the potential of LCFs for controlling dispersion, particularly along the fiber axis, thus yielding novel dispersion landscapes that can reveal unexplored nonlinear dynamics and generate tailored broadband spectra.