Power scaling of high-power fiber lasers and amplifiers

Rare-earth-doped fibers have established themselves as an attractive and power-scalable solid-state laser concept. Using step-index large-mode-area fibers in continuous-wave operation, output powers approaching the kW regime with diffraction-limited beam quality have been demonstrated. In the pulsed regime, average powers on the order of 100 W even for femtosecond fiber laser systems have been reported. Thermo-optical and nonlinear effects within the fiber itself limit the linear power scaling of these systems. Microstructuring the fiber adds several useful features to the fiber for overcoming these restrictions. We review the advantages of rare-earth-doped photonic-crystal fibers for the power scaling of fiber lasers to the multi-kW range with excellent beam quality. The application of the techniques of coherent and incoherent beam combining of fiber lasers and amplifiers for further power scaling are discussed.