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. 2019 May 17;19(10):2294. doi: 10.3390/s19102294

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© 2019 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

PMC Copyright notice

Schematic illustrations of: (a) a fibre Bragg grating (FBG) and (b) a long period grating (LPG). Reprinted from [37] under creative common free licence (http://creativecommons.org/licenses/by/4.0/).