Diode-pumped mid-infrared fiber laser with 50% slope efficiency

Until now, the field of mid-infrared fiber laser research has been constrained by the limitation imposed by the Stokes efficiency limit. The conversion of high-power diode light emission operating at near-infrared wavelengths into midinfrared light invariably results in the deposition of significant amounts of heat in the fiber. This issue is compounded by the fact that mid-infrared transmitting glasses are thermomechanically weak, which means scaling the output power has been a longstanding challenge. In this report, we show that by cascading the adjacent transitions of the erbium ion at 2.8 and 1.6 µm in combination with a low-loss fluoride fiber, the slope efficiency for emission at 2.8 µmcan reach 50%, thus exceeding the Stokes limit by 15%.We also show that by highly resonating the 1.6 µm transition, a highly non-resonant excited-state absorption process efficiently recycles the excitation back to the upper laser level of the mid-infrared transition. This demonstration represents a significant advancement for the field that paves the way for future demonstrations that will exceed the 100 W power level.