These filling compounds—also known as thixotropic gels or water-blocking yarns and powders—are strategically introduced into the cable's core to fill gaps between fibers, tubes, and sheathing layers. Their main purpose is not to transmit data but to shield the infrastructure that. 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. During installation, all curvatures should be smooth. These gels are usually made from a mix of silicone and other polymers — kind of like a special glue that adds strength and. stallers should consider bend radius, tension, jamming, and fill ratio before performing any conduit pull.