Spectral Peaking in the Mode-Locked Mid-Infrared Fibre Laser

Ultrafast mid-infrared (mid-IR) fibre lasers operating in the 3–5 μm range are of great interest in the molecular spectroscopy, medicine, frequency metrology and many other emerge fields [1]. As the fundamental vibrational bands of many air molecules reside in this “fingerprint” region, the mid-IR light propagating through free space undergoes strong absorptions, particularly water <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\sim 2.8\mu \mathrm{m})$</tex> and CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf><tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\sim 4.3\mu \mathrm{m})$</tex>. This molecule-induced modulations coupled with nonlinear effects, bring significant pulse spectra-temporal distortions, thereby limiting the performance of the laser systems [2], [3]. Here, we report the generation of intense water-induced spectral peaks in an Er-doped mode-locked fibre laser operating at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2.8 \mu \mathrm{m}$</tex> (Figs.1(a-c)).