A configuration of acousto-optical tunable filters (AOTF) having stable
output characteristics so that the output does not significantly vary
with time. The configuration includes a plurality of AOTFs cascaded
together. Each AOTF generates a surface acoustic wave in an optical
waveguide in accordance with RF signals applied to the AOTF. A phase of a
beat generated by the RF signals applied to one of the plurality of AOTFs
is different from a phase of a beat generated by the RF signals applied
to a different AOTF of the plurality of AOTFs.