Modal noise in few-mode fibers

Abstract: NIRPS (Near Infra-Red Planet Searcher) is an AO-assisted and fiber-fed spectrograph for high precision radial velocity measurements in the YJH-bands. NIRPS also has the specificity to be an SCAO assisted instrument, enabling the use of few-mode fibers for the first time. 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 present in this paper the result of a semi-empirical work that allowed to validate the scrambling device and strategies to mitigate modal noise. It is based at first on a complete set of lab measurements of the final fibers. Second, such measurements are injected in the spectrograph design to study in particular the impact of grating and optics illumination on derived RVs.