An optical transmission system comprises means for modulating the optical output of a laser of an optical amplifier module with a transmitted dither; means for measuring an incoming optical signal of the optical amplifier module for detecting a received dither; and means for processing the received dither and said transmitted dither for determining the ratio of the energy in the received dither to the energy in the transmitted dither. When the optical transmission system has a plurality of optical amplifiers, the optical output of each optical amplifier is modulated with a respective transmitted dither which is uncorrelated to the dithers transmitted by the other optical amplifiers in the transmission system. A method for monitoring the performance of an optical transmission system is also disclosed, the method comprising the steps of generating at an optical amplifier a transmitted dither of a known energy; measuring an incoming signal for detecting a received dither; and determining the ratio of energy in the received dither to the energy in the transmitted dither.