Abstract
We report mid-infrared (MIR) nonlinear absorption in As2S3 glasses which results from two-photon excitation of valence electron to the Urbach extension followed by strong linear absorption of excited states. The measured MIR nonlinear absorption can be 3 to 4 orders of magnitude stronger than the two-photon absorption in the near-infrared for similar laser intensities and does not result from contaminants, but it is intrinsic to As2S3 glasses. As2S3 fibers are widely used to generate supercontinuum by pumping them with high peak power laser pulses. For a 100 kilowatt peak power MIR soliton propagating in single mode As2S3 fiber, the nonlinear absorption can be of similar magnitude than the fiber background loss. Finally, for laser peak power around 1 MW, the MIR nonlinear absorption can be ~2 orders of magnitude larger than the fiber background loss in single mode As2S3 fiber.
Highlights
Supercontinuum (SC) generation in chalcogenide fiber is widely tested in order to generate broadband spectrum spanning from visible up to the far-infrared [1,2,3,4,5,6]
In order to minimize the nonlinear absorption (NLA) in chalcogenide fiber when pumped with NIR lasers, it is commonly expected that As2S3 fibers are preferable as compared to the As2Se3 because of its wider bandgap [9]
Depending of the input laser intensity, the MIR NLA observed in As2S3 can be many orders of magnitude higher than its linear absorption
Summary
Supercontinuum (SC) generation in chalcogenide fiber is widely tested in order to generate broadband spectrum spanning from visible up to the far-infrared [1,2,3,4,5,6]. The fabrication of tapered As2S3 fiber [2,4,11,12] with core diameter reduced to 2-3 ȝm was used to increase the laser intensity in the fiber core over a short distance in order to enhance the spectral broadening through nonlinear intensity dependent effects as self-phase modulation and self-frequency shift Raman soliton. With all these techniques, the broadest reported 20 dB spectral bandwidth SC spans only over ~3000 nm, and none of them was shown to extend beyond 4.2-4.5 ȝm. Intensities of pump laser pulses injected in these chalcogenide fibers or glass samples were high, from 10 GW/cm to 1 TW/cm2 [13,14,15,16,17], and SC generation was covering the complete transmission of chalcogenide materials
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