Due to their superior performance and low maintenance cost at high average power, Yb-doped fiber lasers are one of the preferred choices for highly productive laser materials processing. To make use of their benefits in the precise fabrication of smaller microscale structures, the sizes of the focal volume and the heat-affected zone (HAZ) at high average power have to be reduced by operating the fiber laser with a shorter wavelength and femtosecond pulse duration. Since the Yb-based fiber gain medium lases fundamentally at 1.03 μm, using nonlinear crystals to shrink the laser wavelength by harmonic generations is unavoidable, and therefore proper management of the heat built up in the crystal is also key to high power operation. As reported in this Optics Letters article, by using a simple attachment of two thick sapphire plates at the front and rear surfaces of each β-BBO crystal, Rothhardt et al. cleverly dissipate the heat generated during second and third harmonic generation without affecting the beam quality of laser, and successfully demonstrate an Yb-doped femtosecond fiber laser system operating with 100 W average power at 343 nm. Moreover, as the authors note, it is expected that scaling up the beam diameters and crystal apertures can lead to further increases in the average power of this laser source.
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