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Optical frequency domain reflectometry (OFDR) has been investigated for two decades as a way to replace the sensing based on fibre Bragg gratings (FBG) currently used in most industries for those applications, using the intrinsic Rayleigh scatter of fibres instead. [1] OFDR allows completely distributed strain and temperature measurements along a fibre. The increase of backscatter using UV laser exposition was recently reported, and was found to increase the sensitivity in both temperature and strain sensing. [2] We present a technique allowing to increase by over 50 dB the backscattered signal amplitude, based on the writing of a Random Optical Grating by Ultraviolet or ultrafast laser Exposure, i.e. a very weak, random grating over the entire length of the fibre. This improvement is, to the authors’ knowledge, over 25 dB higher than what was previously reported for UV exposure for the same exposition power. [2] This ROGUE is generated by inducing phase noise during the continuous writing of a FBG using a Talbot interferometer. This leads to a grating with a very broad bandwidth regardless of the exposure length and greatly increases the signal without limiting the scanning bandwidth, resulting in no loss in resolution. Using these enhanced fibres, we obtained a noise level over an order of magnitude lower than using regular unexposed fibres, allowing measurements of smaller temperature variations. Fibres where such ROGUEs are inscribed also allow the use of a much smaller scanning bandwidth with similar accuracy, resulting in faster acquisition speed.
REFERENCES
[1] M. Froggatt and J. Moore, "High-spatial-resolution distributed strain measurement in optical fiber with rayleigh scatter," Appl Opt, vol. 37, no. 10, pp. 1735-40, Apr 1 1998.
[2] S. Loranger, M. Gagne, V. Lambin-Iezzi, and R. Kashyap, "Rayleigh scatter based order of magnitude increase in distributed temperature and strain sensing by simple UV exposure of optical fibre," Sci Rep, vol. 5, p. 11177, Jun 16 2015.
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