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

摘要

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