Few-mode fibers and AO-assisted high resolution spectroscopy: coupling efficiency and modal noise mitigation

NIRPS (Near Infra-Red Planet Searcher) is an AO-assisted and fiber-fed spectrograph for high precision radial velocity measurements that will operate in the YJH-bands. While using an AO system in such instrument is generally considered to feed a single-mode fiber, NIRPS is following a different path by using a small multi-mode fiber (more specifically called "few-mode fiber"). This choice offers an excellent trade-off by allowing to design a compact cryogenic spectrograph, while maintaining a high coupling efficiency under bad seeing conditions and for faint stars. The main drawback resides in a much more important modal-noise, a problem that has to be tackled for allowing 1m/s precision radial velocity measurements. We study the impact of using an AO system to couple light into few-mode fibers. We focus on two aspects: the coupling efficiency into few-mode fibers and the question of modal noise and scrambling. We show first that NIRPS can reach coupling >= 50% up to magnitude I=12, and offer a gain of 1-2 magnitudes over a single-mode solution. We finally show that the best strategy to mitigate modal noise with the AO system is among the simplest: a continuous tip-tilt scanning of the fiber core.