Up to now Nd:YAG-lasers in the power range up to 3 kW are in industrial use. Higher output powers, up to 6 kW, were realised in laser laboratories. In these cases the beams of several lasers beams are combined anyhow side by side on the workpiece. This yields always a deterioration of the beam quality. Another possibility to increase the laser power on the workpiece under conservation of the beam parameter product is to use amplifiers behind the resonator. The maximum output power scales with the numbers of the rods. This number is limited by practical and adjustment reasons up to 8 rods in a line. For higher numbers these systems will be very complicated to adjust.
A new idea is to combine laser and amplifier with an optical fiber. This set-up reduces the adjustment problems of the system considerable. For such a system the rods in the oscillator and also the rods in the amplifier can be adjusted independently.
A four rod laser delivers 2 kW output power through a step index fiber with 600 μm core diameter into four amplifier rods. The amplified beam with about 4 kW power was coupled into another 600 μm fiber. This beam can be used for material processing or can be coupled once more in an amplifier to increase the power.