Abstract: A method of operating Raman amplification pump lasers, especially in telecommunication systems, is described and claimed, in which the Raman pump laser output powers are modulated in characteristic fashions. Parameters of these characteristic modulations may be detected at remote locations (i.e. locations along the communications fibres) even in the presence of large amounts of noise. Thus, by detecting losses of signals indicative of the characteristic modulations, breaks in the communication fibres can be detected more reliably. The disappearance of these signals can then be used to shut down the typically high power Raman pump lasers, thereby reducing the possibility of high laser powers escaping from the broken fibres. The specification also described an improved method of detecting the disappearance of a data signal at an amplifier unit, using a periodic filter to split the incoming signal into two streams, the first stream comprising data and noise, and the second comprising noise only.