Double Axicons to maximize Optical Feeder Links transmission on conventional telescopes

In this paper we speak about the double-axicon unit built for the CaNaPy instrument, the LGS-AO backbone of the ESA ALASCA TRL6 facility, to demonstrate Optical Feeder Links for Satellite Communications in 2023 using Laser Guide Star Adaptive Optics technologies. The ALASCA system will co-propagate a guidestar laser (λ = 589 nm) and an infrared laser (λ = 1075 nm), using the monostatic approach through the entire 1-m Optical Ground Station (OGS) at Teide Observatory, Canary Islands (Spain). Provisions have been made to use either the full aperture or a section of the OGS primary mirror. The OGS telescope secondary mirror would introduce 30% central vignetting losses on the uplink laser beams. In order to minimise the losses, a double-axicon module shapes the gaussian beam as an annulus, thus optimises the laser profile to match the telescope pupil and nulls the losses, achieving the most efficient coupling to the OGS telescope. We present the CaNaPy axicon module design and analysis for the 589-nm laser as well as the ALASCA axicon module design for the OFL; we describe the tests done and the gain achieved in power transmission when shaping the laser compared to propagating a conventional Gaussian beam through a telescope with a non-negligible central obstruction.