High-energy lasers see performance gains

The first talk reviewed progress at the Lawrence Livermore National Laboratory (LLNL) in developing a mobile laser unit with a target average power level of 100 kW. As well as high power, the laser must also deliver high efficiency, since overheating is a crucial problem that limits the system's run time, as well as reasonable beam quality.

The current system, based on a diode-pumped ceramic Nd:YAG laser emitting at 1064 nm, offers an average output power of 25 kW. Scaling up to higher powers is made difficult by the cost and reliability of 808 nm diode pumps, but also because the wavefront errors in the systems scale linearly as the power increases.

LLNL has implemented four specific measures over the past six months that together have delivered a 40-fold hike in performance. Perhaps most obvious has been to minimize the absorption losses within the laser cavity, which on its own has improved performance by a factor of four, while other measures include reducing the depolarized power, adding an intracavity telescope into the design, and introducing holographic diffusers to increase the run time (but with a significant power penalty).

Reducing surface contamination is now a key priority to achieve further performance improvements, while edge-pumped lasers could in the future be used to reduce wavefront errors at higher powers.

Walter Fink of the Air Force Research Lab followed by discussing the potential of high-power fiber lasers for military applications. The big attraction here is that fiber lasers have no need for free-space optics, which makes them ideal for harsh military environments. Other key advantages are the high system reliability and efficiency.

Fink said that while Yb-doped fiber lasers currently deliver power levels of up to about 3 kW, the benchmark for military applications is 100 kW and above. The High Energy Laser Joint Technology Office (HEL-JTO), an agency funding the US Department of Defense, hopes to increase output powers by using single-aperture scaling to boost the power in individual fibers, as well as both coherent and spectral beam combining.

Fink also said that “robust development would be needed in diode bars for enhanced efficiency coupling”. He also foresees rapid maturation of the technology, perhaps within the next 2-5 years, via an accelerated development program funded by the DoD. That could just be what companies specializing in fibre lasers and high-power diode lasers were hoping to hear — providing they are in the US, of course.