Numerical Design of $4\,\mu {\rm m}$-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
⁶
H
_11/2
→
⁶
H
_13/2
laser transition of Dy
³⁺
using an indium fluoride (InF
₃
) 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 InF
₃
fiber could deliver 15 μJ 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 Tb
³⁺
co-dopants are explored as ways to overcome this limitation.