Quartz optical fibers mainly consist of a core and a cladding, both primarily made of fused silica glass. Nevertheless, fused silica glass features low ductility and is prone to cracking and breakage. Hence, a coating layer is applied externally to the cladding to effectively shield the fiber from mechanical damage, chemical corrosion and erosion by other environmental factors. A wide range of coating materials have been investigated to date, including polymers (e.g., polyethylene, acrylate), epoxy resin, silica gel and metals. Among these materials, acrylate is widely adopted in mass optical fiber manufacturing thanks to its low cost, yet it suffers from relatively poor mechanical properties, heat resistance and chemical stability. It is susceptible to damage under tension, bending or high-temperature conditions, and may undergo degradation reactions when exposed to certain chemicals.

By contrast, metal-coated optical fibers (metallized optical fibers) exhibit distinctive advantages for specific application scenarios, such as outstanding electromagnetic shielding performance, thermal conductivity, high hardness, compression resistance and corrosion resistance. These properties enable their extensive use in optical communication components and fiber-optic sensing devices. Especially in extreme environments featuring high temperature, intense radiation and corrosive media, hermetic packaging realized by welding metallized optical fibers to metal substrates delivers reliable protection for optical devices and guarantees their stable operation. Furthermore, in the field of fiber-optic sensing, metallic coatings can significantly boost the sensitivity of equipment.

Metallization Process of Optical Fibers

The fabrication of optical fiber metallic coatings generally comprises four sequential steps: stripping the original coating, clamping the optical fiber with fixtures, plasma cleaning, and magnetron sputtering coating. Notably, coating stripping, fixture mounting and plasma cleaning constitute the pre-treatment procedures prior to formal coating deposition. These pre-treatment steps are critically important, as their processing quality directly governs the bonding force between the thin metal film and the fiber substrate, and thereby exerts a profound impact on the tensile strength of the optical fiber. Figure 1.1 illustrates the detailed procedures of the overall fabrication workflow.

Figure 1.1 Flowchart of Optical Fiber Coating Fabrication

Plasma Cleaning

After batch laser stripping of the outer polymer coating, bare fiber sections often bear trace residual polymers, alongside dust and other contaminants adsorbed during subsequent handling and storage. For this reason, rigorous cleaning of the optical fiber is mandatory.

Conventional cleaning techniques fall into two main categories: wet cleaning and dry cleaning. Wet cleaning relies on liquid media, including vapor treatment, solution immersion, spin spraying and ultrasonic cleaning, all of which involve chemical reagents. Dry cleaning removes contaminants via pressure, suction or gaseous media, covering mechanical cleaning and plasma cleaning. Mechanical cleaning is technically viable but suffers from low efficiency, high costs and potential fiber damage. Taking all factors into consideration, plasma cleaning stands out as the optimal solution, capable of thoroughly eliminating surface residues and contaminants without introducing any chemical agents.

Plasma refers to ionized gaseous matter composed of ions, electrons, excited-state molecules and free radicals. The core mechanism of plasma cleaning lies in the bombardment and activation reactions induced by high-energy and reactive particles within plasma, which efficiently remove contaminants from substrate surfaces.

Low-temperature plasma cleaning is implemented prior to magnetron sputtering on optical fibers, achieving surface modification through a combination of physical bombardment and chemical reactions. The entire process operates below 60 °C, avoiding scorching of the fiber coated zones and damage to the fused silica crystal lattice, with three key effects as follows:

1. Physical sputtering bombardment: High-energy inert gas ions strike the cylindrical surface of the optical fiber in a directional manner, stripping nanoscale organic residues and inorganic dust. This process polishes micro unevenness on the fiber surface, improves substrate flatness and enhances the adhesion of deposited metal atoms.

2. Degradation via chemical reactions: Oxidative plasma free radicals trigger oxidative cleavage of hydrocarbon organics on the fiber surface, breaking down large polymer macromolecules into small gaseous molecules such as CO₂ and H₂O, which are directly evacuated by the vacuum system to achieve residue-free dry decontamination.

3. Activation of surface functional groups: Plasma disrupts Si-O bonds on the silica (SiO₂) surface, generating abundant hydrophilic hydroxyl (-OH) active functional groups. This raises the surface wetting tension of the optical fiber, allowing the formation of Si-O-M covalent bonds between the fiber and the Cr/Ti metal adhesion layers. The bonding mode is upgraded from simple physical adhesion to chemical bonding, drastically improving the adhesion strength of the metallic coating.