Coherent beam combining has helped produce the new ultrafast fiber laser introduced by the Institute of Applied Physics (IAP) at Friedrich Schiller University Jena (University of Jena) and cooperating partners. Ph.D. student Michael Muller of the IAP presented the laser at this year’s OSA Laser Congress.
The laser delivered an average power that is approximately 10× that of current high-powered lasers, externally combining the output of 12 amplifiers. The combination enabled the laser to move beyond any issue caused by the waste heat that lasers generate when emitting light.
Fiber, slab, and thin-disk lasers feature distinct geometries that allow them to effectively dissipate waste heat; existing high-powered lasers obtain an average power of about a single kW. Using those lasers to extend beyond that range of power, however, causes the generated heat load to degrade beam quality.
The new fiber laser produced 10.4 kW average power, at 80 MHz repetition, without any distortion or degradation of beam quality. Thermographic imaging of the final beam combiner revealed a marginal heating effect, meaning the addition of even more amplifier channels (to the 12 already in place) supported power scaling up to the 100-kW level.
In operation, the laser system is turned on and optimized channel-by-channel, with each channel performing at maximum pump power. After a brief thermalization period, the optical path length in each amplifier achieves stability, and the phase lock and automated alignment control loops activated. The system maintained constructive interference throughout, optimizing the relative beam pointing and pulse overlap, respectively.
In testing, the laser performed at 96% combined efficiency, with a pulse energy of 130 μJ and pulse energy of 250 fs. The team overcame high noise in early-stage development, an effect of direct water cooling, ultimately producing a highly stable device.
“In the development stage, the laser worked marvelously at low average power for deactivated water cooling,” Muller said. “However, with the water cooling activated, and in high-power operation, we have seen an increase in noise, which made us think about the cooling. Then we changed the amplifier layout to decouple the mechanical vibrations of the cooling water from the laser amplifiers, and it did the trick.”
With a beam quality factor value of 1.2 in its current state, the laser already exhibits close to ideal beam quality, Muller said. For comparison, a perfect (or “ideal”) Gaussian beam represents a beam quality factor value of 1.
The team's research aimed to enable the construction of a laser able to overcome the heat-induced problem of beam quality degradation of individual amplifiers. Using coherent beam combining, the research team aligned multiple, high-powered beams into one. The technique compromised neither power nor beam quality while increasing brightness.
Applications such as laser-driven particle acceleration and space debris removal are extreme, yet possible, forthcoming applications for the laser, given its power/stability combination. Industrial and manufacturing-based uses include high-speed scanning and ablation cooling. Work to further refine the laser remains ongoing with the Fraunhofer Cluster of Excellence Advanced Photon Sources (CAPS).
“The most important aspect for commercialization is the assembly of the system into a sealed housing,” Muller said. At the power level the laser has achieved, contamination of the system’s optics, caused by dust, is detrimental to operational success.
Sealing the laser’s environment, Muller said, is among the capabilities of the research group’s partner, the Fraunhofer Institute for Applied Optics and Precision Engineering (Fraunhofer IOF).
Jens Limpert, head of the IAP in Jena, leads the group conducting the research and that debuted the fiber laser. The team received two European Research Council (ERC) grants (MIMAS and SALT) in support of the work. The ongoing research is focused on multicore fibers, offering the potential to deliver further superior performance in simpler and compacted systems.
The research was published in Optics Letters (www.doi.org/10.1364/OL.392843).