Characterization of carbon nanolayer of optical fibers via near-field Raman spectroscopy

 S.V. Saparina^a*, S.S. Kharintsev^a,b**

^aKazan Federal University, Kazan, 420008 Russia

^bTatarstan Academy of Sciences, Institute of Applied Research, Kazan, 420111 Russia

E-mail: *sveta.saparina@yandex.ru, **skharint@gmail.com

Received December 4, 2017

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Abstract

Optical fibers are commonly used for distributed sensing in oil wells. In typical down-hole environment fibers are subjected to a significant mechanical stress at high temperatures and pressures. To prevent mechanical destruction of the fiber surface, optical fibers are coated with a thin carbon layer. Although the considerable advance has been achieved in coating technologies, there is still no full understanding of the causes of microscopic cracks on the surface of the protective layer, which contribute to hydrogen penetration into the fiber core. In this work, we have characterized the surface structure of hermetic carbon coatings of different thicknesses, from 1 to 100 nm, using atomic force microscopy (AFM) and far- and near-field Raman spectroscopy. Based on the obtained results, we have determined the optimal composition, thickness, and morphology of the carbon layer that ensure the best hermetic properties of the layer with sufficient mechanical strength. In addition, the formation of ca rbon allotropes – nanotubes, graphene, soot, and fullerenes – in the protecting carbon layer has been revealed by near-field Raman spectroscopy. These allotropes can serve as additional pathways for diffusion of molecular hydrogen through the carbon layer onto silica glass.

Keywords: optical fiber sensor, carbon-coated optical fibers, carbon allotropes, atomic force microscopy, near-field Raman spectroscopy, optical antenna

Acknowledgments. The study was performed using the equipment of the Center of Collective Use ``Microsystems Technology and Electronic Component Base'' (Moscow Institute of Electronic Technology) and supported by the Ministry of Education and Science of the Russian Federation (agreement no. 14.575.21.0149 of September 26, 2017, project identifier RFMEFI57517X0149).

Figure Captions

Fig. 1. Schematic representation of the experimental unit for TERS and RS measurements.

Fig. 2.  a) AFM image of the carbon coating surface;  b) dependence of the roughness of the carbon coating surface on its thickness.

Fig. 3.  a) RS spectrum of the 6.2 nm-thick carbon layer, the sum of four profiles of the Lorentzian shape (G, D₁, D₂, D₄) and the profile of the Gaussian curve (D₃);  b) the dependence of the relations of line intensities (G*/D*, G/D₁) on the coating thickness.

  Fig. 4.  a) The dependence of the relations √GD₁/D₃ and √G*D*/Δ on the thickness of carbon coating;  b) RS spectrum of the 33.5 nm-thick carbon layer in the long-range field (lower curve: exposure time 20 s), in the near-range field with the use of AFM-cantilever (the curve is in the middle: exposure time 20 s) and golden needle (upper curve: exposure time 2 s).

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For  citation:  Saparina  S.V.,  Kharintsev  S.S.  Characterization  of  carbon  nanolayer of  optical  fibers  via  near-field  Raman  spectroscopy.  Uchenye  Zapiski  Kazanskogo Universiteta. Seriya Fiziko-Matematicheskie Nauki, 2018, vol. 160, no. 1, pp. 126–134. (In Russian)

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