  Fiber modal noise is a performance limiting factor in high-resolution
spectroscopy, both with respect to achieving high signal-to-noise ratios or
when targeting high-precision radial velocity measurements, with multi-mode
fiber-fed high-resolution spectrographs. Traditionally, modal noise is reduced
by agitating or "shaking" the fiber. This way, the light propagating in the
fiber is redistributed over many different modes. However, in case of fibers
with only a limited number of modes, e.g. at near-infrared wavelengths or in
adaptive-optics assisted systems, this method becomes very inefficient. The
strong agitation that would be needed stresses the fiber and could lead to
focal ratio degradation, or worse, to damaging the fiber. As an alternative
approach, we propose to make use of a classic optical double scrambler, a
device that is already implemented in many high-precision radial-velocity
spectrographs, to mitigate the effect of modal noise by rotating the
scrambler's first fiber end during each exposure. Because of the rotating
illumination pattern of the scrambler's second fiber, the modes that are
excited vary continuously. This leads to very efficient averaging of the modal
pattern at the fiber exit and to a strong reduction of modal noise. In this
contribution, we present a prototype design and preliminary laboratory results
of the rotating double scrambler.
