MAVIS: 2 for 1, the art of LGS multiplication

Laser units to provide artificial laser guide stars (LGS) for adaptive optics (AO) are delivering high power on sky today, and more powerful ones are even on the way. At the same time, AO-assisted instruments are requiring increasing numbers of LGSs to deliver atmosphere correction with higher performances and on larger fields-of-view. This comes of course at a cost, financially and in terms of complexity. We present here a way to generate two (or more) LGSs from one single laser unit in a very simple and efficient way, using diffractive optics elements (DOEs). This study has been done in the frame of the MAVIS (MCAO Assisted Visible Imager and Spectrograph) instrument developed for the UT4 telescope at Paranal, where eight LGSs are required to reach the AO correction specifications, while only four are available with the installed Adaptive Optics Facility (AOF). A complete laboratory characterisation allowed to demonstrate the very good performances and compliance of these devices for implementation in laser units. The characterisation covers the transmission, power balance between the beams, beam profiles, polarisation, and wavefront quality. An on-sky validation has been performed on UT4, completing the validation of the specifications with precise measurement of the on-sky separation between the two generated LGS and with measurement of their differential tip-tilt.