A multiple stage Raman amplifier includes an interstage
wavelength-selective pump combiner. Wavelength-selection characteristics
are such that the pump combiner substantially blocks the passage of the
pump signal of the first stage to the second stage and/or vice versa
while allowing the passage of the transmission signal from the first
stage to the second stage. The pump combiner substantially blocks
different portions of the wavelength spectrum of the pump radiation which
is fed into the pump combiner from different ports. The pump combiner
preferably includes thin-film filters. The pump combiner has the multiple
function of coupling the pump signal of the first (or second) pump source
and of isolating from one another the pump signals of the two
amplification stages. This allows the reduction of the number of passive
components present in a multistage optical amplifier, which in turn leads
to a decrease of the overall insertion loss in the amplifier and to
shorter assembly processing.