Photometric observations of a polychromatic laser guide star

Ne report the photometric observation of a polychromatic laser guide star ( PLGS) using the AVLIS laser at the Lawrence Livermore National Laboratory ( LLNL). The process aims at providing a measurement of the tilt of the incom ing wave front at a telescope induced by atmospheric turbulence. It relies on the two-photon coherent excitation of the 4D(5/2) energy level of sodium atoms in the mesosphere. Ne used two laser beams at 589 and 569 Mit with a maximum total average output power of approximate to 350 W. For the purpos e of photometric calibration, a natural star was observed simultaneously th rough the same instrument as the PLGS at the focus of the LLNL 50-cm telesc ope. Photometric measurements of the 330-nm return flux confirm our previou s theoretical studies that the PLGS process should allow us at a later stag e to correct for the tilt at wavelengths as short as approximate to1 mum at good astronomical sites. They show also that, at saturation of two-photon coherent absorption in the mesosphere, the backscattered flux increases by a factor of approximate to2 when the pulse repetition rate decreases by a f actor of 3 at constant average power. This unexpected behavior is briefly d iscussed. (C) 2000 Optical Society of America [S0740-3232(00)01110-8] OCIS codes: 010.1080, 010.1330, 010.3310, 020.4180, 140.1340, 120.4640.