In the structure of fiber optic cables, the filling compound is a layer that is easily overlooked yet critically important. It does not directly participate in optical signal transmission, nor is it as visibly apparent as the outer sheath, yet it directly affects the long-term reliability and transmission stability of the cable, making it an essential functional material for ensuring long-term cable operation.
I. What Is Filling Compound and Why Is It a “Necessity” for Fiber Optic Cables?
Fiber optic cable filling compound is not ordinary “grease” or “petroleum jelly,” but rather a semi-transparent paste-like functional material composed of base oils, thickening systems, water-blocking components, antioxidant systems, and other materials. The core of an optical fiber is an extremely fine quartz glass strand, which has three critical sensitivities: susceptibility to water, moisture, and mechanical stress. Once moisture intrudes into the surface area of the optical fiber, it can induce micro-cracks and lead to increased signal attenuation, potentially causing fiber failure over the long term. Additionally, there are numerous micro-voids within the cable structure, such as between loose tubes, in core gaps, and around strength members, which can form migration pathways for water and moisture.
The core functions of the filling compound are reflected in two aspects. First, water-blocking and moisture resistance: the compound fully fills the internal voids of the cable, forming a continuous hydrophobic barrier that effectively prevents longitudinal water migration, fundamentally protecting the structural stability of the optical fiber. Second, mechanical buffering protection: inside the loose tube, the compound coats the optical fiber to form a flexible support layer. When the cable is subjected to external forces such as bending, tension, or vibration, it effectively disperses stress and reduces the risk of microbending loss, thereby ensuring stable signal transmission.
II. Fiber Gel vs. Cable Jelly: Different Roles, Respective Responsibilities
In the fiber optic cable industry, filling compounds are mainly divided into two categories: Fiber Gel and Cable Jelly. There are significant differences in their application positions and performance requirements.
Fiber Gel is a functional material that comes into direct contact with the optical fiber, primarily filling the interior of loose tubes or backbone structures, maintaining long-term direct contact with the fiber. Therefore, its performance requirements are extremely stringent: it must have very high cleanliness with no mechanical impurities; good low-stress characteristics that do not cause microbending effects on the fiber; low or near-neutral acid value to avoid long-term chemical impact on the fiber coating; and critical control of hydrogen evolution performance, as hydrogen can cause OH-absorption loss in the optical fiber, leading to increased signal attenuation in the 1.38μm band. In terms of base oil selection, Fiber Gel mostly uses high-purity hydrogenated mineral oils or synthetic base oil systems, whose advantages include stable molecular structure and high batch-to-batch consistency, making them more suitable for high-reliability cable applications.
Cable Jelly is mainly used for filling core gaps, stranded structure voids, or outer layer structures of the cable. It does not come into direct contact with the optical fiber, and its core functions are overall water-blocking and structural filling. Therefore, its requirements for cleanliness and optical-grade performance are relatively lower, but it must have good water-blocking performance and long-term stability. Base oil systems mostly use naphthenic or intermediate-based hydrogenated mineral oil systems, achieving a balance between cost and performance, making them more suitable for outer layer protection.
From a material system perspective, filling compounds can also be divided into three types: mineral oil compound, synthetic oil compound, and silicone oil compound. Mineral oil compound offers high cost-effectiveness and is the most widely used. Synthetic oil compound is typically based on PAO (polyalphaolefin) as the base oil, offering excellent high and low temperature performance as well as oxidation stability. Silicone oil compound is suitable for extreme temperature environments, maintaining stable performance across a range from -70°C to 200°C, but its cost is higher and it is incompatible with mineral oil systems.
III. Common Issues and Countermeasures in Practical Applications
During the production, installation, and long-term operation of fiber optic cables, various performance issues may arise with filling compounds.
Oil separation typically manifests as the base oil separating from the compound system, leading to uneven distribution of the compound, which in turn causes uneven stress on the optical fiber and increased microbending loss. The root cause is usually related to the design of the thickening system or control of the dispersion process.
Low-temperature hardening is more apparent in cold regions. Conventional mineral oil systems experience a decrease in viscoelasticity at low temperatures, failing to provide effective buffering protection, which may lead to direct contact between the optical fiber and the tube wall. This should be optimized by selecting synthetic oil or silicone oil systems.
Compatibility issues mainly manifest as physical or chemical incompatibility between the compound and materials such as PBT loose tubes, fiber coatings, and water-blocking materials, which may lead to material swelling or performance degradation over the long term. Therefore, rigorous compatibility testing must be conducted in practical applications.
Hydrogen evolution issues primarily originate from trace unstable components in the compound system, which may slowly release hydrogen during long-term operation, resulting in increased additional attenuation of the optical fiber. Therefore, strict control of raw material purity and production environment humidity is necessary.
Filling process issues are related to the thixotropic properties of the compound and equipment control parameters, such as filling speed, temperature control, and uneven pressure distribution, all of which may affect the uniformity of compound distribution within the loose tube and consequently impact overall cable performance.
Conclusion
Although the filling compound occupies a non-prominent position in the cable structure, it is a key functional material affecting the long-term reliability and transmission performance of fiber optic cables. It plays an irreplaceable role in water-blocking, moisture resistance, buffering, and structural stability. As fiber optic communication networks continue to evolve toward higher speeds, larger capacities, and longer service lives, the performance requirements and process control demands for cable filling compounds are also steadily increasing.
Post time: Apr-29-2026