  A fundamental parameter to determine how electromagnetic waves interfere is
their relative phase. Therefore, achieving a fine control over it enables a
wide range of interferometric applications. Existing phase control methods rely
on modifying the optical path length by either changing the path followed by
the light or by altering the thickness or index of refraction of an optical
element in the setup. In this work we present a novel method, based on
acousto-optic modulators (AOM), which allows adjusting the phase by shifting
the frequency of the light in a segment of its path. Since the amount of
phase-shift depends on the length of the segment, an optical fiber is used to
realize a \boldmath$2\pi$ shift. Two experimental implementations are described
which deal with different sources of phase fluctuations. The first addresses
fluctuations resulting from the optical fiber while the second also tackles
unwanted variations originating from the AOMs.
