Polychromatic laser guide star using a single laser at 330 nm

Abstract

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

VASAO: visible all sky adaptive optics

“…At this point, it is envisioned to work with a single laser at 330nm, using a scheme proposed by J.P. Pique 20 . The excitation of the mesospheric sodium is done as shown on Figure 3.…”