  We present a set-up dedicated to the measurement of the small scalar
directional anisotropies associated to the magnetochiral interaction. The
apparatus, based on a polarization-independent fiber Sagnac interferometer, is
optimized to be insensitive to circular anisotropies and to residual
absorption. It can thus characterize samples of biological interests, for which
the two enantiomers are not available and/or which present poor transmission.
The signal-to-noise ratio is shown to be limited only by the source intensity
noise, leading to a detection limit of Df = 500 nrad.Hz-1/2. It yields a limit
on the magnetochiral index nMC < 4 10-13 T-1 at 1550 nm for the organic
molecules tested.
