July 2016
Spotlight Summary by Michael Frosz
Mid-infrared supercontinuum generation spanning 2.0 to 15.1 μm in a chalcogenide step-index fiber
There is a fierce, but unofficial, competition in the nonlinear fiber optics community to produce the widest optical spectrum with high spatial coherence, also called a supercontinuum. The record has now been broken, at least in terms of wavelength span. Cheng and colleagues demonstrate in this work the generation of a supercontinuum spanning 2.0 to 15.1 μm, an unprecedented wavelength span of more than 13 μm. In terms of frequency, the span is 130 THz, which maybe does not seem much considering that commercially available supercontinuum sources easily span more than 600 THz (0.4 to 2.4 μm). However, supercontinuum sources in the mid-infrared (MIR, 2-20 μm) can be expected to bring great benefits to researchers interested in exploiting the very strong vibrational absorption of biologically important molecules (water, CO2, O2, methane, etc.) in the MIR.
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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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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Article Information
Mid-infrared supercontinuum generation spanning 2.0 to 15.1 μm in a chalcogenide step-index fiber
Tonglei Cheng, Kenshiro Nagasaka, Tong Hoang Tuan, Xiaojie Xue, Morio Matsumoto, Hiroshige Tezuka, Takenobu Suzuki, and Yasutake Ohishi
Opt. Lett. 41(9) 2117-2120 (2016) View: Abstract | HTML | PDF