  Starting from our puzzling on-sky experience with the GIANO-TNG spectrometer
we set up an infrared high resolution spectrometer in our laboratory and used
this instrument to characterize the modal noise generated in fibers of
different types (circular and octagonal) and sizes. Our experiment includes two
conventional scrambling systems for fibers: a mechanical agitator and an
optical double scrambler. We find that the strength of the modal noise
primarily depends on how the fiber is illuminated. It dramatically increases
when the fiber is under-illuminated, either in the near field or in the far
field. The modal noise is similar in circular and octagonal fibers. The Fourier
spectrum of the noise decreases exponentially with frequency; i.e., the modal
noise is not white but favors broad spectral features. Using the optical double
scrambler has no effect on modal noise. The mechanical agitator has effects
that vary between different types of fibers and input illuminations. In some
cases this agitator has virtually no effect. In other cases, it mitigates the
modal noise, but flattens the noise spectrum in Fourier space; i.e., the
mechanical agitator preferentially filters the broad spectral features. Our
results show that modal noise is frustratingly insensitive to the use of
octagonal fibers and optical double scramblers; i.e., the conventional systems
used to improve the performances of spectrographs fed via unevenly illuminated
fibers. Fiber agitation may help in some cases, but its effect has to be
verified on a case-by-case basis. More generally, our results indicate that the
design of the fiber link feeding a spectrograph should be coupled with
laboratory measurements that reproduce, as closely as possible, the conditions
expected at the telescope
