Review of Metal-Coated Methods for Optical Fiber

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

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Fig. 2. Sketch of potential energy vs. reaction coordinate for $S i — O$ bond break without water (black line) and with molecular water (gray line)( $E_{s t r e s s}$ and $E_{s t r e s s}^{'}$ denote the energy barrier to be overcome without and with molecular water, respectively)^[22]

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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]

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Fig. 4. Schematic of the aluminum-coated and gold-coated optical fiber structure produced by Fiberguide Industries^[57]

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Table 1. Coating material and maximum long-term service temperature

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Table 1. Coating material and maximum long-term service temperature

Coating material		Maximum long-term service temperature $T$ /℃
Polymer	Standard acrylate	85^［6］
	High-temperature acrylate	150^［1］
	Silicone	200^［1］
	Polyimide	300^［1］
Metal	Aluminum	400^［16］
	Copper alloy	600^［17］
	Gold	700^［17］

Table 2. Comparison between different coating methods

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Table 2. Comparison between different coating methods

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］
Vacuum evaporation combined with electroplating	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］
Electroless plating combined with electroplating	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］