Fiber lasers already have proved capable of remarkable continuous-wave performance, generating more than 1 kW with excellent beam quality. Scientists at the University of Southampton and at Southampton Photonics in the UK now have shown that fiber technology also can scale picosecond sources to average powers of more than 100 W. They have obtained 321 W from such a laser and believe that it could readily be boosted to 500 W. Previous picosecond fiber lasers have been limited by nonlinear effects to average outputs of ~100 W.Figure 1. The master oscillator power amplifier (MOPA) comprises a gain-switched diode laser as the master oscillator, three fiber preamplifiers and a final fiber power amplifier. The master oscillator is seeded with a narrowband fiber laser to force single-longitudinal-mode oscillation. High-average-power picosecond lasers operating at gigahertz rates have important applications in micromachining and laser projection. They also could become the central component of future high-speed, free-space systems for intersatellite or even interplanetary communication.The investigators’ master oscillator power amplifier (MOPA) system comprises a gain-switched, Fabry-Perot diode laser at 1060 nm serving as the master oscillator, a chain of three ytterbium-doped fiber preamplifiers and a final ytterbium-doped power amplifier (Figure 1). The researchers seed the master oscillator with a continuous-wave, narrow-bandwidth fiber laser to force its oscillation in a single longitudinal mode. They drive the gain-switched master oscillator with a 1-GHz signal to generate a train of strongly chirped, 56-ps pulses, which are compressed to 20 ps in a chirped fiber Bragg grating. (They used a 3-dB coupler into the compressor because it was available, but a circulator would be more efficient.)Figure 2. The MOPA produced 321 W of output with no sign of rollover at the upper end, indicating that further power scaling is likely. The gigahertz train of 20-ps pulses enters the chain of preamplifiers with 0.5 W of average power and emerges with 3 W. A bulk (free-space) isolator protects the preamplifier chain from backreflections from the power amplifier, an 8-m-long, double-clad, ytterbium-doped fiber with a 43-μm core that is pumped by up to 430 W of counterpropagating radiation from a 975-nm diode laser stack. The scientists believe that the large core is the key to avoiding nonlinear effects in the laser. Although it is slightly multimode, that drawback is outweighed by the accompanying reduction in nonlinear effects, higher damage threshold and better pump absorption. Another enhancement in pump absorption results from the fiber’s D-shaped cladding, which prevents cladding modes that are not coupled to the core.Figure 3. The output bandwidth increased from the 0.19-nm width of the seed pulse to 0.49 nm at the maximum power of 321 W. The broadening was due to self-phase modulation; no stimulated Raman or stimulated Brillouin scattering was observed. Operating the preamplifiers at a constant maximum power and varying the pump power to the final amplifier, the researchers observed up to 321 W of output with an M2 beam quality of 2.4 in recent experiments (Figure 2). The slope efficiency was 78 percent with respect to launched pump power. As indicated in the inset of Figure 2, the 1060-nm peak was 20 dB above background noise, and they believe this figure could be increased by spectral filtering prior to the power amplifier.Stimulated Raman scattering was not present even at the highest power level. The 20-ps pulses exhibited no stretching or distortion from the lowest powers up to 221 W, although autocorrelation observations above 221 W were obscured by thermally induced birefringence in the fiber. The MOPA’s spectral width increased somewhat at the higher power levels, an effect the investigators attribute to the onset of self-phase modulation in the power amplifier.IEEE Photonics Technology Letters, May 1, 2006, pp. 1013-1015.