1.2 W Average Power Deep-UV Light at 229 nm Generated by a Frequency-Quadrupled Nd-Doped Fiber Laser

Pulsed laser sources emitting in the deep-UV (DUV) spectral range, at wavelengths
$< 240$
nm, are interesting for many scientific and industrial applications such as materials processing, micro-structuring or laser-induced spectroscopy. High power
$(>1\mathrm{W})$
, efficient laser sources in the UV are mainly based on infrared emitting laser sources followed by nonlinear frequency conversion stages [1], [2]. To access UV wavelengths below 240 nm using frequency quadrupling, a neodymium-doped fiber laser (NDF) operated on the
${ }^{4} \mathrm{F}_{3 / 2}-{ }^{4} \mathrm{I}_{9 / 2}$
three-level transition is particularly adapted with effective tunable emission in the IR spectral range between 875 nm and 935 nm [3]. The competition with the 4-level transition, corresponding to an emission wavelength close to 1050 nm, is however a serious drawback because the gain at 1050 nm in a NDF amplifier is considerably higher than the gain at 900 nm. To reduce the gain difference between the two transitions, we have developed Nd-doped large mode area (LMA) PM fibers with a small clad-to-core diameter ratio (~4) and a very low core numerical aperture. As for example, a record output power of 83 W at 910 nm was achieved using a NDF LMA fiber with a 30/125 geometry and NA of 0.05 [4].