Fig. 1. A model of interaction between water molecules and Si—O—Si bonds[21]. (a) Adsorption of water to Si—O bond; (b) concerted reaction involving simultaneous proton and electron transfer; (c) formation of silanol group Si—OH
Fig. 2. Sketch of potential energy vs. reaction coordinate for bond break without water (black line) and with molecular water (gray line)( and denote the energy barrier to be overcome without and with molecular water, respectively)[22]
Fig. 3. Dynamic TGA curves obtained for polyimide coating (solid line) and dual-acrylate coating (dashed line) in air at a heating rate of 0.5 ℃/min[23]
Table 1. Coating material and maximum long-term service temperature
Method
Material
Procedure and results
Reference
Flash evaporation
Al, Pb
Plating 80 nm aluminum and lead, respectively. Optical fiber is obtained good sensitization effect.
[27]
Magnetron sputtering
ZnO
Coating a layer of ZnO. FBG sensor has excellent surface quality and strain sensing performance.
[30]
Vacuum evaporation combined with electroplating
Au, Ni
Gold is evaporated first, then nickel is electroplated. Coating has good compactness.
[33]
Al, Cu, Zn
Vacuum evaporation aluminum plating and then electroplating of copper and zinc respectively. Coating has excellent performance
[34]
Electroless plating
Ni-P
Electroless plating is used to plate Ni-P alloy on the surface of bare fiber. Fiber surface can be welded to metal
[40]
Ni, Au
Electroless plating is used to form 2‒3 μm nickel coating and 0.3‒0.7 μm gold coating. Coating has a good bonding performance.
[41]
Ni, Ag
Processes of electroless nickel plating and electroless silver plating on the ends of various optical fibers were studied. Bonding strength of the coating and the substrate has been improved.
[42]
Electroless plating combined with electroplating
Ag, Ni
Adopt the method of combining electroless silver plating and electroplating nickel. FBG surface metallization.
[35]
Ag, Ni, Au
Palladium silver activation, electroless nickel plating followed by gold plating. Optical fiber with smooth surface, high tin soldering performance and high adhesion is obtained.
[44]
Freezing method
Al
A layer of 15‒20 μm aluminum is coated. Fracture stress is found to be independent of the strain rate, the metal coating is effectively blocking water from reaching fiber surface.
[58]
Cu
A layer of 5‒30 μm copper is coated. Produced optical fiber has high bending strength and tensile strength, and excellent welding performance, but when temperature exceeds 600 ℃, the loss increases significantly.
[59‒60]
Au
Reliability of gold-coated optical fiber in harsh environment is analyzed. Gold-coated fiber is suitable for industrial applications where temperature and thermal changes are slow and stable.
[10]
Table 2. Comparison between different coating methods