Tight buffered fibers or bare fibers in a loose tube? If loose tube, with or without gel? Does a corrugated steel armor attenuate the accustic energy?
The perimeter length is 40Km and the required spatial resolution is 10 meters. Does the answer depend on the sensing method (e.g. Rayleigh coherent back scattering, Brillouin scattering etc.)?
Hi, David! Yes, a tight buffer is better. Metal tubes and steel armor degrade the sensitivity of the cable to acoustic vibrations. In addition, the sound propagates along metal along the cable and this must be taken into account. Therefore, it is better to use a dielectric cable with Kevlar filaments and with tight packaging. It is advisable to choose the right materials. If you need to bury the cable, you need to understand that high frequencies are cut off a lot. I think that there is a difference in cable design for different methods.
Yes you are right ( the main point shall be to maximise the acoustic transmission to the fibre core, no? ). And in my experience of testing various systems in the laboratory and at the test site, the best results are shown in plastic cables with fiber optic cables made of tight buffer that are tightly packed in the body of the cable. In this case, the acoustic signal easily reaches the fiber and deforms it, and we happily fix the event in the right place.
Yes, I forgot to say, and with Brilliant cables it’s not so simple. A very specific topic is cable structures for special applications.
Victor, Vincent, thanks for your comments.
Victor, do you think that the acoustic transmission is different with aramid yarns (e.g. Kevlar) or glass yarns as mechanical strength members? Aramid yarns have no anti buckling effect while glass yarns under a tightly extruded jacket offer some contraction resistance
David, with the same cable design, the difference will not be noticeable. I think that the use of aramid yarns will make it possible to make a more reliable design of the acoustic cable. But my experience with modern materials for the cable industry is not as good as I would like.
I humbly add this reference to the discussion:
Joachim Hofmann, Massimo Facchini, and Mark Lowell "Analysis of the acoustic response in water and sand of different fiber optic sensing cables", Proc. SPIE 9491, Sensors for Extreme Harsh Environments II, 94910E (19 May 2015); https://doi.org/10.1117/12.2178282.
Furthermore, about materials ( polymers vs fimt), I may suggest to recover the old patents and papers of Bucaro and Lakagos about the acoustic sensitivitity of optical fibers that, in my opinion, are still valid for DAS technology.
Is it possible to increase the back-scattering coefficient of single mode fibers? A typical value is -82dB at 1550nm (for 1ns PW). Is there a technology to increase this value to around -70dB without increasing the forward attenuation above 0.2dB/Km at this wavelength?
Vincent
I thought about a possibility of somehow changing the spherical symmetry of the Rayleigh scattering inside a fiber and have it low in forward direction and high in the back direction but I guess this contradicts some basic conservation law...bad idea anyway
There are a number of recent papers on adding very weak fibre Bragg gratings that increase the backscatter while not increasing the loss in proportion, for example
P. S. Westbrook, K. S. Feder, R. M. Ortiz, T. Kremp, E. M. Monberg, H. Wu, D. A. Simoff, and S. Shenk, “Kilometer length, low loss enhanced back scattering fiber for distributed sensing,” Optical Fiber Sensors Conference (OFS), 2017 25th, 2017, pp. 1-5.
V. A. Handerek, M. Karimi, A. Nkansah, A. Yau, P. S. Westbrook, K. S. Feder, R. M. Ortiz, T. Kremp, E. M. Monberg, H. Wu, and D. A. Simoff, “Improved Optical Power Budget in Distributed Acoustic Sensing Using Enhanced Scattering Optical Fibre,” 26th International Conference on Optical Fiber Sensors, Lausanne, 2018, pp. TuC5.
There are also some patent disclosures on the fibre
T. Kremp, P. S. Westbrook, and T. Geisler, High Backscattering Waveguides, US20170192167, 2017.
and its application in this area
F. Englich, and A. Hartog, Fiber optic array having densely spaced, weak reflectors, US Patent App. 15/205,164,, 2016.
The basic idea is the Rayleigh scattering re-directs the extract light almost uniformly into 4 pi st rad, whereas the FBGs largely re-direct it backwards.
Dear René Eisermann ,
From the paper published in 2019 I understand that the separation between the dots was ~6.5 meters. Am I correct assuming that each dot in those experiments contributed a loss of ~0.0013dB?
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