Patent ID: 7652245

Claim:
Traffic sensing and monitoring apparatus comprising a length l of optical fibre, a light source arranged to introduce a series of pulse pairs into an input end of the length, each of the pulse pairs comprising first and second pulses of radiation having a frequency difference Ω and the second pulse being delayed with respect to the first by a delay τ, and the apparatus further comprising a photodetector arranged to detect radiation which is Rayleigh-backscattered within the length of fibre towards the input end and to generate an output signal in response thereto said apparatus further comprising an interrogation system arranged to obtain a set of signal samples by sampling the photodetector's output signal and to process the signal samples to generate an output signal corresponding to the frequency or phase of a signal composed of said samples, wherein sets of signal samples are obtained by sampling the photodetector output signal at times t 0 −Δt, t 0 and t 0 +Δt after the first pulse of each pulse pair enters the length l of optical fibre and to process the set having samples corresponding to the largest signal with a frequency of approximately Ω.