It is known that fiber optic cable has been the preferred medium for the data center backbone due to channel capacity and scalability, total immunity to electromagnetic interference (EMI) and radio frequency interference (RFI), elimination of the crosstalk inherent in copper cabling (like twinax cable), smaller diameter, and ease of installation, particularly when using pre-terminated modular trunk cables. Selecting the right type of fiber optic cable boils down to the applications and the installation. This post will introduce and compare two fiber optic cable types based on different cable constructions—loose-tube and tight-buffered fiber optic cable.
In loose-tube construction, the fiber is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fiber itself, which can protect the fiber from tension during laying and due to temperature changes. Loose-tube fiber may be “dry block” or gel-filled. Dry block offers less protection to the fiber than gel-filled, but costs considerably less. Loose-tube fiber cable comes in various fiber counts that typically range from 6 to 144, with some manufacturers offering higher counts up to 216 fibers. Except a 6-fiber cable, the fibers are grouped into sets of 12 for maximum density.
The following image shows the main components used to construct a Loose-tube cable.
- Multiple 250um coated bare fibers in loose tube.
- One or more loose tubes holding 250um bare fibers. Loose tube stranded around the central strength member.
- Moisture blocking gel in each loose tube for water blocking and protection of 250um fibers.
- Central strength member in the center of the cable and is stranded around by loose tubes.
- Aramid Yarn as strength member.
- Ripcord for easy removal of outer jacket.
- Outer jacket(PVC is most common for outdoor cables because of its moisture resistant, abrasion resistant and stable over wide temperature range characteristics).
- Extreme Temperature Resistant: Loose-tube cable establishes a strain-free environment for the optical fiber by mitigating the influence of external effects, which makes it perform well at extreme temperatures. Besides, this characteristic also enhances the performance of the loose-tube cable design under a variety of mechanical forces, such as tensile, flexure, twisting, crush, etc.
- Water-blocking Characteristic: As we all know, the core fiber of loose-tube cable is surrounded with a dry water-swellable tape and yarns, or with a gel, which can effectively stop the entry and migration of water.
- UV Protection: The outer jacket of loose-tube cable consists of carbon black, which can provide maximum ultraviolet protection, making the cable able to withstand direct exposure to ultraviolet sunlight in aerial installations.
Instead of a loose tube, the fiber may be embedded in a heavy polymer jacket, commonly called “tight buffer” construction. Tight-buffered cables are offered for a variety of applications, but the two most common are “breakout” and “distribution”. Tight-buffered cable comes in fiber counts ranging from 2 to 144 fibers, with larger fiber counts featuring fiber subunits of 6 or 12 fibers within the cable. For example, a 144-fiber cable usually has twelve 12-fiber subunits while a 36-fiber cable could have six 6-fiber subunits or three 12-fiber subunits.
The following image shows the structure of tight-buffered cable.
- Multiple 900um tight-buffered fibers stranded around the central strength member.
- Central strength member in the center of the cable.
- Aramid Yarn(trade name Kevlar) wrapped around the fibers for physical protection and cable pulling.
- Ripcord for easy removal of outer jacket.
- Outer jacket or sheath(usually PVC).
- Easy to Install: Tight-buffered cables are easier to install, because there is no messy gel to clean up and they don’t require a fan-out kit for splicing or termination. You can crimp connections directly to each fiber.
- Higher Survivability Standard: Based on military technology for survival under mechanical and environmental stresses.
- Flexibility: No stiff strength member is needed, making the cable more flexible. The cable is also “tight bound” allowing it to be pulled around multiple bends or hung vertically without causing “fiber axial migration”.
After we have introduced these two types of fiber optic cables, people may wonder which one is the best for their project. Actually, these two cable types are deployed in different applications.
Loose-tube cable is specifically designed for harsh environments. It has a water-resistant gel that surrounds the fibers, which helps the fibers from moisture, making the cable ideal for harsh, high-humidity environments where water or condensation can be a problem. The gel-filled tubes can also expand and contract with temperature changes. Loose-tube cable has a higher tensile strength than tight-buffered cable. But it is not the best choice when cable needs to be routed around multiple bends, which is often true in indoor applications. Excess cable strain can force fibers to emerge from the gel.
Tight-buffered cable is optimized for indoor applications. Because it’s sturdier than loose-tube cable, it is more available for moderate-length LAN/WAN connections or long indoor runs, and even direct burial. Tight-buffered cable is also recommended for underwater applications.
Choose the right cable types for your applications can effectively improve network performance and ensure the life span of the whole project. Think twice before choosing fiber optic cable.