Coherent Rayleigh OTDR
Our system utilizes a coherent Rayleigh-OTDR technique. Short laser pulses are generated at 1,55 μm wavelength, amplified and launched into the fibre with repetition rates up to 4 kHz.
The returning Rayleigh backscattering signal can be observed at the same fibre end in form of a reflectogram in which the time delay of signal incidence is shown as distance on the cable. A highly coherent laser is used in order to increase the sensitivity. Due to this high level of coherence, an interference pattern can be observed in the backscattered signal (fingerprint). Vibrations cause temporary changes in this interference pattern.
The challenge involved in detecting third-party interference is to understand and filter different events caused, for instance, by manual or machine digging. Every kind of incident causes certain vibrations and changes the backscattering signals accordingly. When the optical signals are received, the signal patterns are digitalized and analyzed in a computer unit.
The patterns pass through digital filters in which a special and very complex algorithm is implemented and are then compared with samples stored in the library. After classifying the event, the system sends an alarm to the system operator.