Numerical design of 4μm-class dysprosium fluoride fiber lasers

In this report we show using numerical simulations that, with judicious choice of pump and fiber parameters that it is possible to achieve lasing at wavelengths longer than 4 µm from the ⁶H_11/2 ⁶H_13/2 laser transition of Dy³⁺ using an indium fluoride (InF₃) glass host. The key issue for this transition is the short lifetime of the upper laser level of 1.25 µs relative to the long lifetime of the lower laser level which is 640 µs. Whilst continuous wave (CW) cascade lasing at 4.4 µm and 3.3 µm could be achieved, the large pump power requirement at threshold of >30 W is impractical. Pulsed pumping combined with gain switching however offers a viable alternative. We calculate that 200 ns, 160 µJ pump pulses launched into InF₃ fiber could deliver 15µ J output at 4.4 µm. Scaling of average output power is constrained by population buildup in the lower laser level. Boosting the output power by quenching this lower level lifetime using OH^-1 or Tb³⁺ co-dopants are explored as ways overcome this limitation.