Why does the coating on optical fibers need to be exposed to UV light to cure/harden?
The process of curing the outer jacket (i.e., protective coating) on an optical fiber from liquid to solid state by exposure to UV light is known as UV Curing / Photopolymerization in materials science and optical engineering.
Its physical and chemical mechanisms and reasons for engineering adoption are as follows:
I. Physical and Chemical Reaction Mechanisms
The primary/secondary coating materials on standard communication optical fibers (such as silica glass fibers) are typically UV-Curable Acrylate Resin. Before UV light exposure, this material is a liquid mixture of pre-polymers, primarily composed of:
- Pre-polymers/Oligomers & Monomers: Containing unsaturated double bonds (e.g., acrylate groups).
- Photoinitiators: Chemical molecules sensitive to specific wavelengths of UV light.
Curing Process (Completed in Seconds):
- Photon Absorption and Free Radical Generation: When UV light (UV-A / UV-V bands) irradiates the liquid coating material, the photoinitiators absorb high-energy photons, undergo photolytic cleavage, and instantaneously generate highly reactive free radicals.
- Chain Polymerization and Cross-linking Reactions: Free radicals attack the carbon-carbon double bonds (\text{C}=\text{C}) in the pre-polymers and monomers, initiating a continuous chain polymerization reaction.
- Formation of a 3D Network Polymer: Molecular chains form a dense, three-dimensional cross-linked network structure within milliseconds to seconds, transforming the liquid resin into a solid protective layer with high tensile strength and elasticity.
II. Why is UV Curing Essential in Fiber Optic Manufacturing?
- Instantaneous Curing and Ultra-High Production Line Speeds:
The drawing speed of optical fibers in a fiber drawing tower typically reaches tens to hundreds of meters per second (\text{m/s}). Traditional thermal curing (e.g., baking polyimides) requires minutes to tens of minutes, which cannot match high-speed continuous drawing; whereas UV curing requires only milliseconds to seconds of exposure to fully harden. - Avoidance of Residual High-Temperature Thermal Stress:
UV curing is a “cold curing” process driven primarily by light energy, eliminating the need to place the entire fiber in a high-temperature furnace (hundreds of degrees Celsius) for extended baking. This prevents the introduction of additional thermal stress and geometric distortions into the silica cladding and core, which have a diameter of only about 125\ \mu\text{m}. - Precise Protection of the Silica Glass Surface:
Bare glass fibers are highly susceptible to micro-crack formation and significant reductions in fracture strength due to moisture ingress from the air. Immediate UV coating and recoating after drawing or grating inscription provides a mechanical barrier that seals out moisture, prevents scratches, and buffers micro-bending losses.
III. Application in Fiber Bragg Grating (FBG) Manufacturing and Recoating
During the production of Fiber Bragg Grating (FBG) sensors, after grating inscription using a UV laser in the fiber core, the bare grating region (where the coating has been removed) requires UV recoating to restore its original mechanical strength and environmental protection.
Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers standard optical fibers and FBG products utilizing UV-curable acrylate coatings and corresponding recoating processes:
1. OFSCN® G.652D Optical Fiber
A standard G.652D single-mode optical fiber with a core diameter of 9\ \mu\text{m}, cladding diameter of 125\ \mu\text{m}, and coating diameter of 255\ \mu\text{m}. The coating material is standard UV-curable acrylate.
2. OFSCN® G.657 Optical Fiber
Bend-insensitive single-mode optical fiber (available in A2/B3 standards), with a core diameter of 9\ \mu\text{m}, cladding diameter of 125\ \mu\text{m}, and coating diameter of 255\ \mu\text{m}, using an acrylate coating.
3. OFSCN® Polyacrylate Fiber Bragg Gratings / FBG Strings (Bare)
Inscribed using the UV phase mask method. After inscription, the grating region is recoated with polyacrylate material by default, resulting in an outer diameter of 255\ \mu\text{m}, fully restoring the mechanical protection of the bare fiber.
