International audienceIn the framework of Laser Guide Star projects and more particularly of VASAO project of the Canada France Hawaii Telescope, we have developed a modeless laser at 330 nm and we have carried out photometry experiments in laboratory, in order to evaluate the feasibility of the sodium atom excitation at 330 nm. Our modeless laser is a DCM dye laser frequency doubled by a BBO crystal. A high efficiency acousto-optical crystal assures the modeless property. We present the experimental results of sodium atom excitation of 3S{1/2} to 4P{3/2} transition at 330.24 nm and 3S{1/2} to 3P{3/2} transition at 589 nm. The rate equation model gives a good interpretation of the experimental results
The possibility to produce diffraction-limited images by large telescopes through Adaptive Optics is closely linked to the precision of measurement of the position of the guide star on the wavefront sensor. In the case of laser guide stars, many parameters can lead to a strong distortion on the shape of the LGS spot. Here we study the influence of both the saturation of the sodium layer excited by different types of lasers, the spatial quality of the laser mode at the ground and the influence of the atmospheric turbulence on the upward propagation of the laser beam. Both shape and intensity of the LGS spot are found to depend strongly on these three effects with important consequences on the precision on the wavefront analysis.
International audienceAn increasing number of adaptive optics systems in astronomy uses a sodium Laser Guide Star (LGS). The analysis of the distorsion of the return fluorescence requires high power lasers. We expose here two modeless laser amplifiers, based on a longitudinaly pumped 1 cm-long dye cell: the first one is a 4-pass amplifier at 589 nm pumped at 532 nm and the second one is a 4-pass amplifier at 660 nm pomped at 511 and 578 nm. These systems correspond respectively to the monochromatic and the polychromatic laser guide stars
International audienceThe differential atmospheric tip-tilt can be measured using a Polychromatic Laser Guide Star. A two photon excitation has been proposed. It consists in exciting the 4D 5/2 level of mesospheric sodium atoms with two identical lasers operating at 589 nm and 569 nm. With two modeless lasers of 2×15W at the mesosphere level, this scheme will produce a returned flux at 330 nm of about 4×10 4 photons/s/m2. Thanks to our modeless laser, we propose a new method which consists in exciting directly the 4P 3/2 sodium level with one photon excitation, using a single laser operating at 330 nm. This solution was previously rejected probably because of strong saturation problems using single longitudinal mode lasers. We show that 1 W modeless laser at 330 nm can produce the same returned flux at 330 nm. This solution will save at least 400 k€ of equipment. Moreover, our new method is very promising in terms of simplicity but also in terms of flux because the returned flux above will probably be not sufficient for getting a good Strehl ratio. We propose very efficient solid state laser systems for the production of tens of watts at 330 nm
International audienceThe first pulsed solid-state frequency-shifted-feedback laser as polychromatic laser guide star is reported. A Ti:Sa frequency-shifted-feedback (FSF) laser delivering a broad continuous spectrum in order to cover (after frequency mixing) the hyperfine-Doppler width of the 3S1/2 rarr4P3/2 transition at 330 nm is developed. The developed Ti:Sa laser can operate either in cw or pulsed mode. This new laser source has been characterized in terms of dynamic and spectral properties
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