Gain control in Raman amplifiers

A method of controlling gain in a Raman amplifier comprising at least two optical pumps supplying power in wavelength λi, said method including the steps of:

• (i) for two pumps at one wavelength determining pump driving currents Ci for each of said optical pumps so that the amount of power provided by the same wavelength pumps is about same;
• (ii) for two pumps at one wavelength determining the ratios of said driving currents with respect to one another;
• (iii) for two pumps at one wavelength driving the optical pumps with said pump driving currents, while maintaining said ratios;
• (iv) iteratively determining total pump powers Pi to be provided in each of said pump wavelengths λi;
• (v) adjusting the driving currents Ci proportionally, with respect to one another, to provide said pump powers Pi.

A Raman fiber amplifier comprising

• (i) a transmission fiber;
• (ii) at least one pump power detector;
• (iii) at least one pump optically coupled to said transmission fiber and to said pump power detector;
• (iv) at least one signal detector, optically coupled to said transmission fiber so as to detect signal power propagating through said transmission fiber;
• (v) a controller (a) operatively connected to said pump power detector and said signal detector, and (b) utilizing signals provided by said pump power detector and said signal detector, said controller adjusting the pump power provided by said at least one pump, so as to adjust gain provided by said Raman fiber amplifier or to keep the signal power level detected by said signal detector within a predetermined range;
• (vi) backscatter monitor, said backscatter monitor measuring amount of back-scattered pump signal and determining whether the amount of backscatter is within a predetermined range;
• and
• (vii) wherein said controller is connected to said at least one optical pump, said controller turning off said at least one optical pump when the amount of measured backscatter is outside a predetermined level.

A Raman fiber amplifier comprising

• (i) a transmission fiber;
• (ii) at least three optical pumps of wavelengths λ1, λ2, λ3, optically coupled to said transmission fiber;
• (iii) at least one pump power detector associated with each of said optical pumps;
• (iv) at least one signal detector, optically coupled to said transmission fiber so as to detect signal power propagating through said transmission fiber;
• (v) a controller (a) operatively connected to said pump power detectors and said signal detector, and (b) utilizing signals provided by said pump power detectors and said signal detector, said controller adjusting the pump power provided by said at least one pump, so as to adjust gain provided by said Raman fiber amplifier or to keep the signal power level detected by said signal detector within a predetermined range;
• (vi) at least one backscatter monitor measuring the amount of backscattered pump signal and providing input about the amount of measured backscatter to said controller, wherein said controller adjasts pump power of at least one pump when the amount of measured backscatter is outside a predetermined level, wherein said Raman fiber amplifier is a distributed fiber amplifier that includes, as its gain medium, more than 5 km of single mode transmission fiber.