High Efficiency FiberCoupled Diode Lasers for Resonant Pumping of Fiber Lasers

Highpower (kilowattclass) lasers are of great interest to the Department of Energy (DOE) and other commercial users, for many areas of accelerator applications: for implementation of future colliders, in generation and manipulation of electron beams, in electromagnetic radiation and particle beam generation. For many of these and other applications, it is required that these lasers operate at high repetition rates and short, subpicosecond pulse widths, with greater than 20% electrical to optical efficiencies. The laser technology of choice to meet these requirements is solidstate lasers, which use gain media based on crystals or glasses doped with rare earth or transition metal ions. Of particular interest are Erbium and Thulium fiber lasers emitting in the 1.5–1.6 and 1.7–2.1 μm wavelength ranges, respectively. Systems based on the current state of the art are unable to meet the efficiency requirements sought by DOE. Traditionally, the wallplug efficiency of fiber lasers has been limited by three factors – the opticaltooptical conversion efficiency of the fiber gain medium, the wallplug efficiency of the semiconductor pump source, and the optical losses associated with coupling the semiconductor pump into the dualclad fiber. The work proposed herein will enable a dramatic improvement in the system wallplug efficiency of fiber lasers for accelerator technology by simultaneously addressing all three of these issues. Resonant pumping of Erbium and Thulium fiber lasers offers significantly reduced quantum defect (and hence high opticaltooptical conversion efficiency) compared to pumping at shorter wavelengths. By directly pumping the upper level of the laser with a long wavelength pump, the energy loss associated with nonradiative decay to the upper laser level can be avoided, improving overall efficiency. Unfortunately the wallplug efficiency of long wavelength fibercoupled InPbase diode pumps is low (35% at 1.5 μm and 15% at 1.9 μm). This prevents the resonant pumping approach from yielding significant improvements in total system efficiency. To address this problem, this program will develop high power fibercoupled laser diode modules operating at 1.5 and 1.9 μm. The proposed approach is expected to make a leap forward in efficiency, ultimately delivering >50% and >20% rated power conversion efficiency at 1.5 and 1.9 μm, respectively; these projected values are inclusive of the optical losses associated with fiber coupling. The envisioned product at the end of this program will be a fibercoupled module that can be incorporated into a fieldspecific system. Key Words: diode laser, laser pump, high power laser, high efficiency laser, long wavelength laser, accelerators, fiber lasers. This program will develop high efficiency and high power diode laser pumps operating at wavelengths around 1550 nm and 1900 nm. These high performance laser pumps will enable the application of Erbium and Thulium doped fiber lasers in DOE accelerators, as well as in other high impact commercial areas.