Patent ID: 7865044

Claim:
A sensing system ( 100 ) comprising a sensing optical fiber, and a sensing system receiver ( 104 ) arranged to receive signals from the sensing optical fiber and determine conditions from the signals, and a fiber loss detector ( 800 , 820 , 830 , 835 , 840 , 845 , 850 , 860 , 870 ) comprising lasers ( 860 ) for sending loss detection signals comprising a wavelength spectrum matching Stokes and anti Stokes components of the received sensing signals and the fiber loss detector also comprising one or more loss detecting receivers ( 830 , 835 ) for receiving backscatter from the loss detection signals as they propagate along the sensing fiber, and comprising circuitry ( 840 , 845 , 850 ) for determining loss for different distances along the sensing fiber by optical time domain reflectometry (OTDR), and arranged to derive from the received backscatter signals a profile of differential loss along the sensing fiber.