That radially polarised lasers can offer benefits, has been well known since the 1980s. However, such beams were also known to be inherently more unstable compared to conventional Gaussian beams. Much work has gone into addressing these instability problems, albeit mostly for low power applications such as quantum encryption, information capacity scaling, spectroscopy and nano-imaging. The high powers needed for metal machining pose a few additional challenges – the need for ultrashort pulses (for high peak powers), and the management of thermal instabilities.
This work is an impressive demonstration of the solution of all these interrelated problems. The authors use single-crystal fibers which provide both a cylindrically symmetric guiding medium – of necessity for generating cylindrical polarisation beams – as well as excellent heat sinking properties. The high powers themselves are obtained by using a MOPA configuration often used for power scaling lasers, and it is very encouraging that the performance they get out of a system seeded with a radially polarised beam is similar to that of a system seeded with a conventional Gaussian beam – in both cases they obtain approximately 100 W of output. Crucially, the degree of polarisation of the input doughnut beam (~95%) does not degrade after amplification – a key metric that suggests excellent thermal stability of their laser. Depolarisation of cylindrically symmetric beams due to propagation has, in fact, been one of the most difficult challenges to overcome, and the results in this paper provide proof of the fact that, even if there may be thermal instabilities in the single-crystal fiber, the design is such that (thermal) perturbations to the system are substantially cylindrically symmetric – an impressive engineering advance.
This paper paves the way for the realistic consideration and deployment of high-power radially polarised lasers in the rapidly growing application space laser-based industrial machining. Above and beyond that, the success of the authors in creating high-power ultrashort radially polarised beams may have other far reaching implications, perhaps in the fields of electron and particle acceleration.
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