Raman Amplification in Distributed Optical Fiber Sensing Systems

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A method of providing Raman amplification in an optical fiber sensing system 60, comprises generating a probe pulse of light and launching the pulse into a sensing optical fiber. A pump light 36 at a shorter wavelength is modulated by for example, an optical modulator 62 to produce a time-varying intensity profile and the pump light is launched into the sensing fiber. The intensity of the launched pump light during launch of the probe pulse is different from the intensity at other times. Raman amplification of backscattered light produced by the probe pulse as it propagates along the fiber is achieved, as is amplification of the probe pulse if the pump power is non-zero during launch of the probe pulse. The system may comprise; one or more rare-earth optical fiber amplifiers integrated into the sensing fiber, an optical detector for detecting the backscattered light, and a processor for processing signals to determine information and about the sensing fiber and/or an environment in which it is deployed.