  We analyse the limitation of the amplitude modulation rejection due to the
spatial modulation of the output beam of an acousto-optic modulator used in an
active laser beam stabilisation system when a frequency modulation of a few
megahertz is applied to this modulator. We show how to overcome this problem,
using a single mode optical fibre at the output of the modulator. A residual
amplitude modulation of 10-5 is achieved.
