Optical method and system for measuring in-band crosstalk in Raman amplifiers

A system for measuring in-band crosstalk in an optical device under test, the system comprising:

• an input optical modulator for generating a modulated optical signal, the input optical modulator having an off-state and an on-state;
• a variable attenuator, optically coupled to the input optical modulator;
• a first optical coupler operable to couple the modulated signal optically to the device under test;
• an output optical modulator;
• a second optical coupler operable to couple the device under test optically to the output optical modulator;
• an optical spectrum analyzer, optically coupled to the output optical modulator;
• and
• a timing synchronizer, coupled to the input and output optical modulators and the optical spectrum analyzer, the timing synchronizer being operable in a first mode to control the optical spectrum analyzer to measure an output of the device under test when the input modulator is in the off-state and in a second mode to control the optical spectrum analyzer to measure an output of the device under test when the input modulator is in the on-state,
• wherein the variable attenuated is operable to attenuate the modulated optical signal by a factor equal to the extinction ratio of the input optical modulator when the timing synchronizer is operated in the second mode.