Optical Time-Domain Reflectometry (OTDR)
An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize optical fibres. It injects series of optical pulses into the fibre that is being tested and detects at the same fibre end the returning light that has been scattered or reflected back from points along the fibre. It performs an efficient, precise and wide analysis of the fibre characteristics.
A laser diode launches light into one end of the optical fibre and a photo diode measures the returning light. The backscattered signal provides relevant information about events along the fibre. The signal is influenced by both the attenuation and reflections which the injected laser pulse experiences on its way through the fibre. If a certain area within the fibre has a higher degree of attenuation or reflection, for instance caused by a bending or a fibre connector respectively, this is detected by certain differences in the backscattered signals. The velocity of the light pulse is known and the exact location of the event can therefore be determined on the basis of the time difference between the injection of the light and the return of the backscattered signal.
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.