Patent ID: 7145716

Claim:
A multiple stage Raman optical amplifier for amplifying an optical transmission signal within a transmission wavelength band comprising: a first amplification stage comprising a first length of Raman-active optical fibre and a first pump source for generating a first pump signal within a first pump wavelength band, said first pump signal being coupled to one end of the first length of Raman-active optical fibre; a second amplification stage comprising a second length of a Raman-active optical fibre and a second pump source for generating a second pump signal within a second pump wavelength band different from the first wavelength band, said second pump signal being coupled to a first end of the second length of Raman-active optical fibre, whereby a residual pump signal appears at a second end, opposite to the first end, of said second length of fibre; and a wavelength-selective pump combiner interposed between the first and the second amplification stages adapted for substantially transmitting the optical transmission signal between the first amplification stage and the second amplification stage, the pump combiner including a first port coupled to said one end of the first Raman-active fibre, a second port coupled to the second end of the second Raman-active fibre, and a third port coupled to the first pump source, wherein the pump combiner is arranged for substantially passing the first pump signal coupled to the third port to the first port and for substantially blocking the passage of the residual pump signal coupled to the second port into the first amplification stage.