A 20-W all-solid-state continuous-wave single-frequency source tuned to the sodium D2a line at 589.159 nm has been developed for adaptive optical systems. This source is based on sum-frequency mixing two injection-locked Nd:YAG lasers in lithium triborate in a doubly resonant external cavity. Injection locking the Nd:YAG lasers not only ensures single-frequency operation but also allows the use of a single rf local oscillator for Pound-Drever-Hall locking both the injection-slave and the sum-frequency cavities. We observe power-conversion efficiencies in excess of 55% and a linearly polarized diffraction-limited output tunable across the sodium D2 line (589.156 to 589.160 nm) with no change in output power and with high amplitude and pointing stability.
ABSTRACT. Measurements of a laser-pumped sodium guide star produced over the Starfire Optical Range in 2002 November show that the brightness of the spot produced by 11.5 W of linearly polarized power on the sky was equivalent to a mag star. However, taking into account that the transmission through a V filter V p 8.0 is only 55% at the wavelength of sodium, its corrected magnitude, , was 7.4, or 800 photons s Ϫ1 cm Ϫ2 at the V 1 top of the telescope. In 2003 March, tests with linearly and then circularly polarized beams out of the telescope showed that a circularly polarized beam from 12 W of power out of the telescope produced a spot with V p 1 (1015 photons s Ϫ1 cm Ϫ2 at the top of the telescope), 0.7 mag brighter than a linearly polarized beam from 7.1 11.1 W of power out of the telescope. Over the 4 nights of experiments over two seasons, the apparent 2j width of the spot varied between 3Љ .6 and 4Љ .6, or 1.6 and 2.0 m at 92 km altitude, and its length through the sodium layer was 4.6-8.5 km, but no variation of spot size with power on the sky was seen.
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