DAS / DVS (Distributed Acoustic / Vibration Sensing)
These systems detect vibrations and capture acoustic energy along the optical fiber. The fiber functions as if there were thousands of microphones installed. Classification algorithms are used to detect and locate, for example, intrusion activities.
Various DAS / DVS technologies are used in the market. Depending on the application and project requirements AP Sensing is able to offer the most suitable technology, fully integrated into AP Sensing’s software suite.
C-OTDR (Coherent Optical Time Domain Reflectometry) utilizes Rayleigh scattering, allowing acoustic frequency signals to be detected over large distances. The interrogator sends a coherent laser pulse along a single mode optic fiber (sensor cable) and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length similar to the pulse length (e.g. 10 meters).
Acoustic disturbance on fiber generates microscopic elongation or compression of the fiber (strain), which causes a change in the phase relation.
Before the next laser pulse can be transmitted the previous pulse must have had time to travel to the far end of the fiber and for the reflections from there to return. For a fiber 40 km long the maximum pulse rate is at about 2.5 kHz. As a consequence acoustic signals can be measured that vary at frequencies up to the Nyquist frequency of about 1.25 kHz.
Shorter fibers clearly enable higher acquisition rates e.g. up to 10 kHz. For TPI (third party interference) events, the lower frequency range of typically < 500 Hz is of relevance.
In addition to the known C-OTDR technology AP Sensing offers a hybrid interferometer, working on a transmissive technology, with an extreme broadband frequency response and no degradation of signal strength over distance. This unique technology acts as a continuous microphone, up to 50km in fiber length, designed to “monitor” signals into the ultrasonic frequency range with good TPI (third party interference) locating capabilities.