Aragon Photonics manufactures a distributed acoustic DAS, but a high fidelity one based on a novel technology. The technology behind is based on Chirped-pulse Phase OTDR. HDAS can be perfectly used in standard single mode fibers (SMF) with many advantages of use.
- 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 base of the OTDR, a common tool to check fiber optic cables
- If injected light is coherent, the backscattered light shows an interference pattern.
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 trace, 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…
- Sensing length: Up to where the system maintains its sensitivity.
- Spatial Resolution: Ability to spatially resolve two close events.
- Trigger frequency
- 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)
HDAS unique technology differs greatly from other existing technologies. Following the operation of interferometry, a light pulse is sent to the fiber, the backscattered light of which will provide information on the state of the fiber. However, the proprietary pulse the HDAS makes use of is called “chirped pulse”, within which a frequency swept is performed. Thanks to this pulse nature, the variations sensed along the fiber are therefore transformed into time delays which, through the use of proprietary processing, are then converted into high-fidelity strain variations.
Among all the DAS technologies available in the market, there are some that stand out for their simplicity and low cost, such as the intensity Φ-OTDR, or for their performance, as would be the case of the dual pulse Φ-OTDR.
Both technologies, in terms of parameters concerning the quality and fidelity of the measurement, have been compared with the HDAS technology in the following graph. This comparison highlights the high fidelity the HDAS provides and how, thanks to its patented technology, it differs from existing technologies.
Linearity implies that the measurement changes proportionally to the changes in the input perturbations. The lack of it results in the introduction of harmonics and the loss of the signal original slope, then missing the real natural behavior of the sensed perturbations.
- HDAS is linear, does not introduce harmonics.
- The measurement changes in direct proportion to a change in the input perturbation.
- HDAS provides high-fidelity measurements, enabling better pattern analysis and reducing incorrect classifications.
A quantitative measurement provides the capability of quantifying the sensed perturbations, which means that each of those perturbations are detected as a concrete strain or temperature value and that their variations can be followed and characterized.
- The detected variations are measurable as strain or temperature values.
- Quantitative information is much more meaningful.
- Thresholding becomes much easier reducing false alarms.
The SNR (Signal to Noise Ratio) in non-linear DAS and phase DAS systems degrade with distance. Moreover, non-linear DAS technologies result in fading points: points that are temporary blinded from actual events.
- HDAS maintains SNR level for all the fiber (like FM vs AM radio).
- HDAS has no fading points.
Aragon Photonics provides the most advanced instruments to monitoring infrastructures using DFOS technology.
At Aragon Photonics we have conceived our products as an open platform to enable the development of applications for many end-users.