A Raman amplifier structure (121, 221) for optically amplifying an input optical signal comprises an optical means (22) through which the optical signal is propagated, a first pump optical source (10) for generating a first pump radiation and at least one second pump optical source (24, 27) for generating a second pump radiation. The first and second pump optical radiations are combined and propagated in optical transmission means (22) for supplying an optical amplification of the signal through the Raman effect. The first pump optical source (10) comprises a first laser source (12) for generating a radiation with relatively low noise and relatively low power and a Raman amplifier (13) for amplifying the radiation coming from the first laser source for generating the first pump radiation. The Raman amplifier (13) comprises a second laser source (14) for generating an optical radiation having relatively higher power and noise than the first laser source and the radiation coming from the second laser source is used for counter-pumping the radiation coming from the first laser source (12) for generating the first pump radiation. This limits the amount of noise transferred from the second source (14) to the first pump radiation.