Why do optical fibers typically have two coating layers, with the inner layer being soft and the outer layer being hard?
In the field of optical waveguide manufacturing and materials engineering, standard fused silica glass optical fibers must be immediately coated with a polymer during the drawing process to protect them from environmental damage and mechanical stress. Typically, standard polyacrylate-coated fibers (e.g., those with an outer diameter of 255\ \mu\text{m}) employ a Dual-Layer Coating System, featuring a “soft inner, hard outer” physical architecture.
This design is a synergistic optimization based on mechanics, material science, and optical transmission theory. The specific physical and engineering reasons are as follows:
1. Role of the Inner Layer (Primary Coating/Soft Coating)
The inner coating is applied directly over the silica cladding (with an outer diameter of 125\ \mu\text{m}). It is usually composed of a soft, elastomeric material (such as a soft acrylate resin) with a very low Young’s Modulus (typically E \approx 1\ \text{MPa} to 3\ \text{MPa}) and a very low glass transition temperature (T_g \approx -40^\circ\text{C} or lower).
- Stress Buffer: The soft inner layer acts as an “elastic cushion,” effectively absorbing and buffering external thermal expansion and contraction stresses caused by drastic temperature changes, as well as lateral mechanical squeezing. This prevents these external stresses from directly impacting the micrometer-scale glass cladding.
- Microbending Loss Mitigation: When optical fibers are cabled, deployed, or subjected to uneven lateral physical pressure, the soft inner layer absorbs deformation energy through localized shear deformation. This prevents the formation of extremely small-amplitude (micrometer-scale) and high-spatial-frequency perturbation bends in the quartz glass core. This effectively suppresses microbending scattering losses (a_{\text{micro}}) during signal transmission.
2. Role of the Outer Layer (Secondary Coating/Hard Coating)
The outer coating tightly encases the inner coating, forming the outermost surface of the coated fiber. It is typically made of a rigid polymer material with a high Young’s Modulus (typically E \approx 500\ \text{MPa} to 1000\ \text{MPa}) and a high glass transition temperature (T_g > 60^\circ\text{C}).
- Abrasion Resistance: During fiber drawing, reeling, stranding, and deployment, the fiber passes over various guides and rubs against other components. The hard outer layer provides extremely strong surface protection against abrasion, scratches, and cuts.
- Environmental Barrier (Moisture and Chemical Resistance): The surface of silica glass contains microcracks. Under the combined action of trace moisture (\text{H}_2\text{O}) and stress, slow chemical reactions occur, leading to crack propagation (stress corrosion) and subsequent fatigue failure of the glass. The hard outer layer, acting as a dense protective barrier, slows the penetration of moisture and corrosive agents like acids and alkalis, thereby extending the long-term mechanical life of the optical fiber.
- Processability for Cabling: The surface friction coefficient of the hard outer layer can be precisely controlled. It provides sufficient hardness to prevent adhesion during cabling processes, yet allows for clean and efficient separation of the inner and outer coatings from the glass interface during mechanical stripping, ensuring cleanliness and efficiency during splicing.
OFSCN® Official Product Application Relevance
In standard optical fiber communication and Fiber Bragg Grating sensor production, the standard polyacrylate-coated fibers provided by Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) strictly adhere to this dual-layer high-performance coating structure. This ensures excellent optical performance while guaranteeing long-term mechanical stability.
Refer to the following official specification products:
- OFSCN® G.652D Optical Fiber:
Produced from standard G.652D optical rod, with a core diameter of 9\ \mu\text{m}, cladding diameter of 125\ \mu\text{m}, and a coated fiber outer diameter of 255\ \mu\text{m}. Its coating employs dual-layer polyacrylate materials with high resistance to microbending and abrasion.
- OFSCN® G.657 Optical Fiber:
Standard bend-insensitive single-mode fiber (available in G.657 A2 or G.657 B3 levels), also featuring a 255\ \mu\text{m} dual-layer polyacrylate coating. It is suitable for environments with extremely small bend radii, leveraging the high-performance dual-layer coating to achieve ultimate bend loss resistance.
