The main output of the first ring resonator was fed via two lenses for mode matching to the second one, equipped with a 4 x 4 x 10 mm 3 Brewster-cut barium borate crystal in the focus, critically phase matched.
Approximately 10 mW of the 0.95 W green beam was split off to enable frequency stabilization by locking the laser frequency to an iodine transition, feeding back to a piezoelectric tuner. One mirror, which doubled as the output coupler, was shaped as a zero-power lens for the green beam. The angles of the 18 mm crystal were chosen such that, together with the 10° folding angle of the resonator, a round, nonastigmatic second-harmonic (green) beam was generated. Lithium triborate was chosen for its very high optical quality, high damage threshold, and the moderate temperature (89☌) required for noncritical type-I phase matching. The output was fed via an optical isolator to a first folded-ring resonator that contained a lithium triborate crystal for frequency doubling. After the settling of some initial polarization drift upon switching on the laser, polarization was set using one-quarter and one-half optics in the beam. 1) made by Koheras (Birkerød, Denmark) with a 1118 nm spectral line with a bandwidth of 200 kHz its 2 W of laser power is combined with an additional 1.2 W of amplified spontaneous emission (ASE) peaking between 10 nm.
The laser system was based on a commercial diode-pumped ytterbium-fiber oscillator-amplifier laser (see Fig. A fiber laser was chosen by the researchers because such lasers have very high beam quality, making them promising candidates for frequency conversion. 1Ĭonversion efficiency and intensity stability for frequency conversion into the UV are of major concern, requiring precise control of phase matching. The laser emitted 2 W at its 1118 nm fundamental wavelength the researchers achieved highly stable narrowband 280 nm radiation that reached the noteworthy power of 275 mW. To aid in experiments involving optical trapping of ions, researchers at Max-Planck-Institut für Quantenoptik (Garching, Germany) investigated two-step frequency quadrupling of a ytterbium-doped fiber laser.
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