Improving the output performance from mid-infrared fibre lasers

The strong current interest in fibre laser research arises from the high efficiency and high power particular examples such as the Yb³⁺-doped silica fibre laser [1] and the Yb³⁺, Er³⁺-doped silica fibre laser [2] have recently demonstrated. Extending the wavelength range of the output emitted from fibre lasers into the mid-infrared region is an active research field within fibre laser research. Over recent years there have been a large number of demonstrations of mid infrared fibre lasers and of them, the ³F₄ → ³H₆ ∼2 μm transition of Tm³⁺, the ⁵I₇ → ⁵I₈ ∼2 μm transition of Ho³⁺, the ⁵I₆ → ⁵I₇ ∼3 μm transition of Ho³⁺, and the ⁴In_13/2 ∼ ⁴I_11/2 ∼ 3 μm transition of Er³⁺, have displayed the highest output power and are considered to be the most important transitions for mid-infrared fibre lasers that have been studied so far. Recently a milestone was reached when 100 W level output at 2.05 μm was demonstrated from a directly diode-pumped Tm³⁺-doped silica fibre laser [3]; a milestone which has cemented the Tm³⁺-doped silica fibre laser as one of the most convenient generators of mid-infrared laser radiation. In this talk, I will briefly review the recent progress in the field of mid-infrared fibre lasers with a special emphasis on the various methods that have been directed towards the augmenting of both the efficiency and output power that is produced from fibre lasers that emit in the 2 - 3 μm region of the spectrum. I will show that diode lasers, Yb³⁺-doped silica fibre lasers, Tm³⁺ -doped silica fibre lasers and Raman fibre lasers are all useful pump sources for mid-infrared fibre lasers and I will review the various dopants in both single and co-doped combinations that are useful for the generation of mid-infrared output. I will also discuss possible areas of future research that are aimed at extending the wavelength range of high power mid-infrared fibre lasers.