Extending the supercontinuum so far into the MIR is made possible by using optical fibers made of chalcogenide glasses offering high MIR transmission, in this case As2Se3 in the core and AsSe2 in the cladding. By optimizing the core diameter Cheng and colleagues could shape the dispersion profile so that the dispersion remains low and flat within the wavelength region of interest. In fact, since working in the MIR has the advantage of being far away from material resonances in the UV, it is possible to get extremely flat dispersion profiles: in the present case the dispersion varies between only -12 to +10 ps/nm/km within 4 to 20 μm. For comparison, a microstructured silica fiber can have a dispersion typically in the range -100 to +100 ps/nm/km in the range ~0.6 to ~2 μm.
One disadvantage is that the approach taken requires a complex laser system for pumping at 9.8 μm, and that the average power is only on the order of 1 mW; for the huge spectral width this means that the spectral power density (mW/nm) becomes very small. Also, the generated supercontinuum could perhaps have broadened even longer into the MIR than 15 μm, but seems to be limited by absorption from Se-H contamination in the glass. Maybe future improvements in the glass fabrication will lead to new records being set.
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