A variable gain optical amplifier, in which homogeneous gain broadening is dominant,
has first and second fixed gain rare-earth doped optical waveguide amplifiers (21,
23) optically in series together with an intervening variable attenuation optical
attenuator (22). This arrangement circumvents the problem of gain tilt encountered
when operating such amplifiers under variable gain conditions. An alternative form
of the module has variable gain waveguide amplifiers, but these are co-regulated
so that the aggregate of their gain at a wavelength within the gain spectrum is
maintained constant. A further alternative form of module is employed in a concatenation
of such modules. In such a concatenation, the gain of individual modules is allowed
to vary, but the aggregate of the gain, at a wavelength within the gain spectrum,
of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.
