We discuss our Polychromatic Laser Guide Star (PLGS) end-to-end model which relies on the 2-photon
excitation of sodium in the mesosphere. We then describe the status of the setup at Observatoire de Haute-
Provence of ELP-OA, the (PLGS) concept demonstrator. The PLGS aims at measuring the tilt from the LGS
without any NGS. Two dye laser chains locked at 589 and 569nm are required. These chains, are similar to those
of our PASS-2 experiment at Pierrelatte (1999). The two oscillators, preamplifiers and amplifiers are pumped
with NdYAGs. Both beams are phase modulated with a double sine function. If required, a third stage can
be added. It is expected that beams will deliver an output average power of 34W each, so that 22W will be
deposited into the mesosphere. If it is not enough, there is enough power supply to twofold it.
These lasers are being settled in the building of the OHP 1.52m telescope, partly at the first floor, and partly
at the top of the North pillar. Beams will propagate from there to the launch telescope attached to the 1.52m
one through a train of mirrors fixed with respect to the beam, so that incident angles are constant.
The coudé focus of the 1.52m telescope will be equipped with an adaptive optics device, closely derived from
the ONERA's BOA one. The Strehl ratio at 330nm for the differential tilt measurement channel is expected to
be 30-40% for r0 = 8 - 10cm. Telescope vibrations will be measured with pendular seismometers upgraded from Tokovinin's prototype. The full demonstrator is planned to run in 2010.