Technology – Fiber Optic Sensing

The DFOS (Distributed Fiber Optic Detection) system allows real-time measurements at thousands of points along a fiber optic cable, enabling continuous measurements of temperature, acoustic profiles and deformation profiles, both in time and space.

Advantages of using fiber optics as a sensor

The main advantages offered by this technology lie in the good properties of the optical fiber: low attenuation, immunity to electromagnetic noise and deflagrations, high transmission speed, small size and weight, flexibility, possibility of multiplexing, etc.

Distributed Fiber Optic sensors are very suitable for monitoring infrastructures in which the threat may appear at any point. Turning an existent or newly installed optical fiber into thousands of sensors, and then monitor tens of kilometers of asset with high accuracy and a single interrogator, makes it a very cost effective and non-intrusive solution that is being adopted by more industries every year.

What is the phenomenon of backscattering?

Backscattering is the process of sending an optical signal through a fiber optic cable and analyzing the reflected light to determine the location and characteristics of disturbances or changes along the fiber. In the field of distributed sensing, the exploitation of backscattering phenomena, including Rayleigh, Raman and Brillouin effects, opens up a range of possibilities. This technique is used to monitor and diagnose the state of the fiber optic network, detecting problems such as strain, temperature changes or physical damage with high spatial resolution.

Distributed Acoustic Sensing Technology

Aragon Photonics manufactures a distributed acoustic DAS, but a high fidelity one based on a novel technology. The technology behind it is based on Chirped-pulse Phase OTDR. HDAS can be perfectly used in standard single mode fibers (SMF) with many advantages of use.

How does DAS work?

Optical fiber reflects a small portion of the injected light as photons “rebound” on the fiber imperfection. This effect is known as Rayleigh Backscattering.
By using a pulsed light source, only a section of the fiber is lightened at a time, providing a distributed measurement of the scattering on the fiber. If injected light is incoherent, the backscattered light decays with fiber losses. This is the basis of the OTDR, a common tool to check fiber optic cables.
If injected light is coherent, the backscattered light shows an interference pattern.

How does DAS measure?

Typically, the 2D waterfall chart is used as a representation of the changes observed in the fiber:

If the fiber is at rest, the interference pattern is stable and the difference between consecutive traces only shows the measurement noise.
If a section of the fiber is subject to vibrations, the interference pattern changes for that section and the difference shows that concrete change.
By the proper calibration of the system is possible to measure the strain in a section subjected to physical changes, and thanks to the time of flight of the pulse is possible to extract separated information from all the different points of the fiber
Thus, if we represent the difference between traces, we get the acoustic energy and, by representing it as a function of time, we get the 2D chart called waterfall.

Depending on the sensed infrastructure or the concrete application, many other ways to analyze data can be implemented: Spectrogram, Filtered Waterfall, Acoustic traces…

Key specifications for distributed acoustic sensing models

Main parameter

Sensing length: Up to where the system maintains its sensitivity.
Spatial Resolution: Ability to spatially resolve two close events.
Trigger frequency.

Advanced parameter

Quantitative measurement: Capacity to quantify the measured variations.
Linearity: Proportionality between measured values and the stimulus applied over the fiber.
Signal-to-Noise Ratio (SNR): Ratio of the desired signal power to the background noise power. (Fading ≡ temporary loss of SNR)