Multi-stage raman amplifier

A method for amplifying an optical signal having frequency in a signal frequency range, comprising:

• introducing the optical signal respectively into at least a first and a second optical path disposed in series with each other, each optical path comprising a Raman-active material having a predetermined Raman shift, the second optical path being downstream from the first optical path with respect to a direction of propagation of the optical signal;
• introducing into the first optical path a first pump radiation including at least one first pump frequency;
• introducing into the second optical path a second pump radiation including at least one second pump frequency;
• a lowest frequency of the first pump radiation being higher than a highest frequency of the second pump radiation;
• preventing an entry into the first optical path of a residual of the second pump radiation having a power lower than the first pump radiation by less than 10 dB;
• preventing an entry into the second optical path of a residual of the first pump radiation having a power lower than the second pump radiation by less than 10 dB;
• the lowest frequency of the first pump radiation differing from a highest frequency of the first pump radiation by at most 70% of the Raman shift;
• and
• a lowest frequency of the second pump radiation differing from the highest frequency of the second pump radiation by at most 70% of the Raman shift.