underground fiber optic cable
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Indoor and outdoor applications of fiber optic cables you should know

We people who practice Fiber optics are often familiar with the basics of fiber optic cables, such as the difference between singlemode and multimode fiber optic cables and their own advantages and disadvantages. However, when it comes to the usage scenario of fiber optic cables, even us can sometimes get confused about which types of fiber optic cables to use for some specific application.

Direct burial installation
Direct burial installation

Hence, this article will primarily focus on the types of fiber optic cables and their usage scenario classified by application. After reading this, you can definitely make the right choice of fiber optic cables to meet your specific demands.

What are Fiber Optic Cables?

Fiber optic cables transmit data using light pulses instead of electricity. The cables consist of silica glass fibers, which are as thin as a stand of human hair, and allow the light to travel back and forth along the length of the cable. To prevent the light from escaping, the glass core is covered with a thin layer of glass cladding with a lower refraction index, which is then protected by a primary coating to prevent excessive bending and provide physical protection. High-quality cables often have an outer layer of strength members made of different materials, such as kevlar or gel-filled sleeves, to reinforce protection and rigidity. The final layer is an outer jacket made of colored plastic to identify the cable type and offer fire resistance, with different ratings available for varying levels of protection.

Fiber optic cable Structure
Fiber optic cable Structure

Optical Fiber Structure

Optical fibers are composed of several layers, each with its own function, as mentioned above:

  • Core: the core is the central part of the fiber, made of high-quality optical glass. It carries light signals from one end to the other.
  • Cladding: the cladding is the layer of glass surrounding the core. It has a lower refractive index than the core, which helps to keep the light signal within the core by reflecting it back into the core whenever it tries to escape. This process is known as total internal reflection.
  • Coating: the coating is the protective layer coating the cladding, usually made of a polymer material. It protects the fiber from damage and makes it easier to handle during installation.
  • Strength Member: the outer layer of the coating used to reinforce protection and rigidity of the fiber optic cable.
  • Jacket: The outermost layer is the jacket, made of a tough, protective material such as PVC. The jacket provides additional protection to the fiber and also makes it easier to handle during installation.

Designs of Optical Fiber Structure by indoors/outdoors applications

There are two basic fiber optic cable structures divided in terms of usage scenarios, i.e., the tight-buffered fiber cable design and the loose-tube cable configuration.

The tight-buffered fiber optic cable structure is specifically designed for indoor use. In addition, the ribbon cable, where fibers are made into ribbons to allow small cables with the largest fiber count, is an extension of this type.

Construction of a simplex tight-buffered fiber cable module
Construction of a simplex tight-buffered fiber cable module

Correspondingly, the loosetube structure is ideal for long-haul outdoor applications.

Both the fibers themselves are composed of the normally manufactured glass core and cladding surrounded by a protective 250 μm diameter coating.

Designs of Indoor Optical Cables

Designed to interconnect telecommunication equipment, distribute the signal, and terminate with patch cords within LAN, three main types of indoor optical cables include:

Interconnect cable

This is a tight-buffered cable, flexible, compact, and lightweight, thus, it is suitable for light-duty low-fiber count indoor applications. It is widely used as fiber-to-the-desk links, patch cords, or point-to-point runs in conduits and trays. patch cords, also called jumper cables, belong to this type of indoor cables.

Breakout or fanout cable

A breakout or fanout cable contains a central strength member with up to 12 tight-buffered fibers stranded around it. These types of cables are useful for applications with low-to-medium fiber counts that require protection for individual jacketed fibers. The breakout cable makes it simple to install connectors on individual fibers within the cable. This type of cable configuration allows for easy routing of separately terminated fibers to various pieces of equipment.

Distribution cable

The distribution cable is composed of small groupings or individual tight-buffered fibers stranded around a central strength member. This cable is used for various network applications, including the transmission of data, voice, and video signals. It is designed to be placed within intra-building cable trays, conduits, and dropped-ceiling structures. One of the primary advantages of this cable is its ability to branch off (distribute) groupings of fibers within the cable to multiple locations.

Designs of outdoor Optical Cables

Outdoor cable installations come in various types, such as aerial, duct, direct burial, and underwater applications. These cables typically use a loose-tube structure and are available in different designs and sizes to suit the specific physical environment and application.

Direct burial cables

Direct burial cables are meant to be buried directly in the ground, while underwater cables are designed to be used in underwater environments. The loose-tube structure helps to protect the individual fiber-optic strands from damage caused by environmental factors such as moisture and temperature changes.

underground fiber optic cable
underground fiber optic cable

Aerial cable

Aerial cable is designed to be installed outside, either between buildings or on poles or towers. There are two common designs of aerial cable structures: self-supporting and facility-supporting. The self-supporting cable has an internal strength member that allows it to be strung between poles without any additional support mechanisms. On the other hand, for the facility-supporting cable, a separate wire or strength member is first strung between the poles, and then the cable is either lashed or clipped onto this member.

Armored cable

Armored cable is used for direct-burial or underground-duct applications and features one or more layers of steel-wire or steel-sheath protective armoring, which are located below a layer of polyethylene jacketing, as illustrated in Figure 2.31. This armored design not only enhances the cable’s strength but also offers protection against gnawing animals such as squirrels or burrowing rodents, which can cause damage to underground cables. In the United States, for instance, the plains pocket gopher (Geomys busarius) can destroy unprotected cables that are buried less than 2 meters (6 feet) deep.

Underwater cable

Underwater cable, also referred to as submarine cable, is utilized in ocean environments, rivers, and lakes. These cables are typically exposed to high water pressures, which means they must comply with more stringent requirements than underground cables. For instance, as illustrated in Figure 2.32, cables intended for use in rivers and lakes come with multiple water-blocking layers, one or more protective inner polyethylene sheaths, and a heavy outer armor jacket. On the other hand, cables that are installed under the ocean have additional layers of armoring and contain copper wires, which are necessary to provide electrical power for submersed optical amplifiers or regenerators.

Fiber types and typical specifications Reference

Fiber types and typical specifications
Fiber types and typical specifications

Source: Jim Hayes, FOA reference guide to fiber optics, The Fiber Optic Association, Inc. 2018, p. 56.

Miscellaneous Fiber Cable Types

duplex zipcord fiber

The duplex zipcord fiber consists of two fibers surrounded by strength members and an outer jacket. It has twin LC connectors on either end and can be separated by pulling the connectors apart.

Mode Conditioning patch cords (MCP)

MCP are duplex cables used for converting singlemode signal to multimode signal. MCP has multimode to multimode connectors on the receive side and singlemode to multimode connectors on the transmit side. Mode Conditioning cables help avoid expensive network upgrades to replace legacy Gigabit LX transceivers by converting and transmitting singlemode signal over multimode fiber.


  1. Keiser, Gerd. Fiber Optic Communications. Springer, 2021.
  2. Hayes, Jim. FOA reference guide to fiber optics, The Fiber Optic Association, Inc. 2018.

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