Globally, the deployment of fiber optic cables has been rapidly increasing as the demand for high-speed and reliable data transmission, via optical fiber, continues to grow. Fiber has become the technology of choice for delivering internet and network services, as well as bridging the “last-mile” gap from operator’s equipment to homes, businesses, and enterprises.

Fiber optics is a technology that uses optical fibers to transmit data as light signals, delivering high bandwidth, electromagnetic immunity, and low signal loss. Both indoors and outdoors, fiber optic cables are deployed to serve a wide range of applications, from communications to medical imaging.

Dgtl Infra provides an in-depth overview of fiber optics, including the major components of an optical fiber and how they are bundled together into a fiber optic cable. Additionally, we explain how fiber optics works and we compare the different types of optical fibers. Finally, Dgtl Infra examines the advantages and disadvantages of fiber optics, as well as the main applications that it is used for.

What is Fiber Optics?

Fiber optics, also known as an optical fiber, are thin strands of glass that data can be transmitted over, via optical equipment that transforms data signals into light. This thin, flexible glass strand has a similar diameter to that of human hair, around 125 microns (µm) or 0.125 millimeters (mm).

What are the Components of an Optical Fiber?

The main components of an optical fiber are the core, cladding, and jacket.

  • Core: cylindrical glass that forms the central part of the optical fiber, where light signals are transmitted. The core is made of high-purity glass (silicon dioxide) and has a higher refractive index than the cladding, so that light passes only through the core
  • Cladding: an outer coating layer that surrounds the core and reflects light back into the core, preventing the light from leaking out of the core. The cladding is made from less pure glass and has a lower refractive index than the core, which helps to keep the light signals in the core, ensuring that they travel along the fiber with minimal loss or dispersion
  • Jacket: an outer protective layer of the optical fiber made of a tough, flexible polymer material such as PVC (polyvinyl chloride) or plastic. The jacket, also known as a sheath, protects the core and cladding from mechanical damage, moisture, and abrasion
Optical Fiber Diagram – Schematic and Cross-Section
Optical Fiber Diagram Schematic View and Cross-Section

Overall, the diameters of the core, cladding, and jacket can vary, depending on the type of optical fiber. As an example, a single optical fiber can have the following diameters: core of 9 microns (µm), cladding of 125 microns (µm), and jacket of 250 microns (µm).

Plastic Optical Fiber

An optical fiber core and cladding can alternatively be made from transparent plastic, which is not as clear as glass. The main advantages and disadvantages of plastic optical fibers, as compared to glass optical fibers, are as follows:

  • Advantages of Plastic Optical Fiber: lower cost and more flexible, making them easier to install
  • Disadvantages of Plastic Optical Fiber: greater dispersion of light, which leads to a weaker signal and limited distance of transmission. Also, plastic optical fibers have lower bandwidth, meaning that their data transmission rate is less than that of glass optical fibers

Overall, plastic optical fiber is used in more consumer-focused applications, often in electronic systems for data communication between components. In these scenarios, plastic optical fiber’s signal loss and lower bandwidth are not as important. For example, plastic optical fiber is used in lighting, automobiles (data communication between different parts of the vehicle), music systems, video game consoles, and home automation systems.

What is a Fiber Optic Cable?

Fiber optic cables consist of one or more (usually many) strands of optical fiber bundled together. These optical fibers are protected by features of the fiber optic cable including central strength wires, gel filling, as well as inner/outer jackets and armor.

Fiber Optic Cable Structure – Outdoor Example
Fiber Optic Cable Structure Outdoor

The purpose of a fiber optic cable is to protect the optical fibers inside from mechanical and environmental damage, such as tensile stress (e.g., pulling), bending, third-party construction equipment, animals, and water.

The major types of fiber optic cable are: outdoor buried cable, outdoor aerial cable, indoor cable, air blown fiber (used both indoors and outdoors), and subsea cable.

How Does Fiber Optics Work?

Fiber optic systems consist of a transmitter, an optical fiberwhich is the transmission medium, and a receiver. The following are five simplified steps as to how fiber optics work:

  1. Electrical data input enters data into the fiber optic system
  2. Transmitter accepts and converts input electrical signals to optical (light) signals and then sends the optical signal by modulating a light source’s output (either an LED or a laser) by varying its intensity
  3. Optical fiber, a thin strand of glass, is the transmission medium. Light signals travel through the core of the fiber, from one end to the other, by a property known as total internal reflection. Simply put, the light signal bounces down the core of the fiber, to the other end of the glass strand, by a series of reflections on the boundary of the cladding
  4. Receiver is the optical (light) to electrical converter at the end of the glass strand. Here, the optical signals are received by a photodiode (photodetector), which converts the optical signals back into an electrical signal
  5. Electrical data output results, which can be decoded and processed by a router or network switch

Types of Optical Fibers

The two distinct types of optical fibers are multimode (multiple paths) and single-mode (single path). As shown below, multimode fiber has a relatively large core, enabling multiple modes (or paths) for light to travel down through the fiber. Whereas single-mode fiber has a much smaller core and thus there is only one effective mode (or path) through which light can propagate.

Multimode fibers are limited in terms of speed and distance, given that the multiple modes (or paths) tend to interfere with each other in these fibers. Therefore, optical networks requiring the highest speeds and spanning the greatest distances use single-mode fiber.

Multimode Fiber vs Single-Mode Fiber

Below is a comparison of light propagation in a multimode and single-mode fiber.

CharacteristicMultimode FiberSingle-Mode Fiber
Light Wave ModesMultiple pathsSingle path
Core Diameter62.5 microns (µm) most common;
range of 50 to 100 microns (µm)
8.3 to 10 microns (µm)
Wavelength of Operation850 to 1300 nanometers1310 to 1550 nanometers
Light SourceLight-emitting diode (LED)Laser light
Power DistributionIn all of the fiber core and into the claddingOnly in the center of the fiber core
DistanceShort distancesLong distances

For both multimode and single-mode fibers, the diameter of the cladding is 125 microns (µm) and, including the protective jacket, the diameter of a single optical fiber reaches 250 microns (µm).

Multimode Fiber

Multimode fiber is a type of optical fiber that has a larger core, typically 50 or 62.5 microns (µm) in diameter, that allows multiple paths of light to propagate. In other words, a multimode fiber can carry more than one frequency of light at the same time.

Multimode fiber utilizes blinking light-emitting diodes (LEDs), which are cheaper, to transmit signals. When the light signals hit the cladding boundary, they are reflected back into the core.

Multimode fiber has higher loss (as compared to single-mode fiber) and is therefore only used for communications over short distances (i.e., up to a couple of miles) and less bandwidth-intensive applications. For example, multimode fiber is used in local area networks (LANs), such as within a building, corporate network, or on a campus.

Multimode fiber can further be classified into multimode step-index fiber and multimode graded-index fiber. They differ in terms of how light propagates down through the different paths.

  • Step-Index: has a uniform refractive index across the core’s diameter
  • Graded-Index: gradually increases the refractive index across the core’s diameter, reaching a high-point in the middle of the fiber and then gradually decreasing toward the outer edge of the core
Multimode Step-Index Fiber

In multimode step-index fiber, light signals hit the cladding at a shallow angle and bounce back to hit the opposite wall of the cladding, which causes the light signals to zigzag down the core. As such, the light signals take alternative paths down the core, causing different groupings of light signals to arrive separately at the end of the glass strand, where they are converted by the receiver.

Multimode Step-Index Fiber
Multimode Graded-Index Fiber

Multimode graded-index fiber has a higher refractive index at the core and a lower refractive index in the cladding, enabling light signals from the paths to arrive at the end of the glass strand simultaneously.

Multimode Graded-Index Fiber

Single-Mode Fiber

Single-mode fiber is a type of optical fiber that has a small core, typically 8.3 to 10 microns (µm) in diameter, that allows only one path of light to propagate. The small size of the core nearly eliminates the occurrence of light bouncing off the cladding, even when the fiber is bent or curved.

Single-mode fiber utilizes expensive laser light to transmit signals, which travels in a straight path down the narrow core of the optical fiber.

Single-Mode Fiber

In terms of purpose, single-mode fiber is used primarily for communications over long distances, as it can transmit data several miles with minimal signal loss, because there is no interference from adjacent modes (paths).

Single-mode fiber provides greater bandwidth for transmitting information due to its ability to maintain the integrity of each light signal over longer distances, without dispersion (spreading out of light) caused by multiple modes (paths). Additionally, single-mode fiber experiences lower attenuation (loss of optical power) compared to multimode fiber, allowing for the transmission of more information in a given amount of time.

Advantages and Disadvantages of Fiber Optics

Fiber optics offers several advantages over other forms of wired communications, such as copper telephone wires and hybrid fiber-coaxial (HFC) networks.

Advantages of Fiber Optics

The advantages of fiber optics are high bandwidth, electromagnetic immunity, low signal loss / attenuation, security, and less weight.

1) High Bandwidth

Fiber optics provide significantly greater bandwidth compared to copper wires, which results in faster data transmission. This is because an optical fiber uses light signals to transmit data, which have much higher information-carrying capacity, as compared to copper wires, which use electrical signals.

More specifically, light signals have frequencies measured in terahertz (THz), while electrical signals, in the form of analog or digital signals, have frequencies that range from megahertz (MHz) to gigahertz (GHz). Since bandwidth is proportional to the frequency range of the optical signals, optical fiber is able to carry significantly more information, which translates into terabits per second (Tbps) of capacity.

2) Electromagnetic Immunity

Optical fiber is made of dielectric material, meaning it is immune to electromagnetic interference (EMI) from electricity. In contrast, EMI is a major issue in copper wires because they are metal and conduct electricity, which can lead to signal degradation or corruption due to electrical “noise”.

Advantages of optical fiber having electromagnetic immunity are lower bit error rates (BERs), elimination of ground loops, reduction in signal distortion, and strong resistance to crosstalk interference. Additionally, because of optical fiber’s electromagnetic immunity, fiber optic cables can be placed near high-voltage power transmission lines, generators, or railway lines without any effect on data transmission, whereas copper wires cannot.

3) Low Signal Loss / Attenuation

Optical fiber has low signal loss (attenuation) because light signals can travel longer distances with minimal degradation, as compared to copper wires, which carry electrical signals. The reason for this characteristic is that light signals in optical fibers are much less susceptible to interference and degradation than electrical signals in copper wires – which suffer from issues such as electrical resistance and electromagnetic interference.

4) Security

Optical fibers are more secure from potential malicious interception due to their composition of dielectric material, rendering it challenging to tap into the fiber without disrupting communication. Although tapping into optical fibers is possible, it results in signal loss (attenuation), which is detectable.

Importantly, fiber networks can be constantly monitored for increases in signal loss, which could indicate the presence of taps. On the other hand, copper wires, which radiate electrical signals, are more vulnerable to unobtrusive tapping.

5) Less Weight

Fiber optic cables are significantly lighter than copper wires, weighing about 4 pounds per 1,000 feet, as compared to copper wires, which weigh nearly 10 times that amount. This light weight of fiber is particularly important during installation, as smaller fiber optic cable reels can be easily carried by the installation crew, allowing for more fiber to be installed in a shorter span of time. Furthermore, fiber optic cables can be run above drop ceilings and attached to a building’s slab without causing structural damage or impairing the load factor of the building.

Disadvantages of Fiber Optics

The disadvantages of fiber optics are installation and repair difficulty, bending, and environmental damage.

1) Installation and Repair Difficulty

Fiber optic cable installation and repair can be challenging, as it requires specialized labor and equipment. This can increase the cost and complexity of network deployment and maintenance, especially in challenging terrains.

In particular, joining fiber optic network sections together, such as residential access networks to customer’s homes, needs to be done in a way that minimizes signal loss (attenuation). The two most common ways used to join optical fibers together are through fiber optic connectors and fusion splicing.

  • Fiber Optic Connectors: devices that allow optical fibers to be plugged together and unplugged, but they are less reliable and cause signal loss
  • Fusion Splicing: the ends of two optical fibers can be fused by melting part of the glass in the two fibers together, a task that requires precision equipment. Fusion splicing is the most optimal way to join optical fibers together because it can create a connection point with very little signal loss (attenuation). However, fusion splicing is difficult to do well outdoors when temperatures are low, such as during winter

READ MORE: Fiber Optic Cable Installation Process – Connecting Homes

2) Bending

Microbends, which refer to a bend or kink in optical fiber, can cause signal loss. Light travels through the core of an optical fiber by reflecting off the boundary of the cladding, but only at the proper angle. If microbends move the angle of incidence beyond the critical angle, light can escape the core and leak into the cladding, which results in light signals being lost for information carrying purposes.

3) Environmental Damage

Optical fibers can suffer from environmental damage due to various factors, including:

  • Water: through tiny voids or air bubbles, water can penetrate into the fiber cladding and affect signal transmission. Water can absorb light and cause microcracks in the glass, both of which lead to signal loss
  • Ultraviolet (UV) Radiation: prolonged exposure to UV light, from the sun, can cause an optical fiber’s surface to degrade, breaking down the boundary between the core and the cladding of the fiber, which affects signal transmission. In particular, outdoor aerial fiber optic cables are susceptible to damage from solar UV radiation as it is very penetrating
  • Animals: rats, squirrels, gophers, rabbits, termites, and iguanas will all gnaw through fiber optic cable causing it to break

What are Fiber Optics Used For?

Fiber optics are used for a wide range of applications, beyond just communications systems, and these include industry and domains such as medical imaging, military, sensing, lighting, security, industrial automation, and energy.

Examples of Applications that use Fiber Optics
  1. Communications: fiber optics are widely used for high-speed data communication, including internet service, broadcast television, telecommunications, backhaul from cell towers, and data center networking
  2. Medical Imaging: fiber optics are used in medical imaging systems, such as endoscopes and cameras, to provide a real-time view of internal tissues and organs
  3. Military: fiber optics are used in the main branches of military (army, navy, air force) for communication and data transmission. For example, these include command-and-control links on vessels and airplanes, communication links between satellite ground stations and data centers, and connections for tactical command post communications
  4. Sensing: fiber optics are used in applications such as temperature sensors, pressure sensors, and vibration sensors, to provide real-time data and facilitate analytics. For example, structural health monitoring (SHM) sensors are used to monitor the vibration of structures, such as bridges and high-rise buildings
  5. Lighting: fiber optics and, in particular, plastic optical fiber, is used in lighting systems, such as fiber optic illuminations, to provide bright, cheap, and energy-efficient lighting. For example, fiber optic lighting can be used to illuminate the interior and exterior of residential buildings and hotels
  6. Security: fiber optics are used in physical security applications like the perimeter fencing of data centers, whereby optical fiber can sense if someone is nearby or touches the fence. While surveillance systems use fiber optics to transmit high-definition video over long distances
  7. Industrial Automation: fiber optics are used in industrial automation systems to transmit signals and control data, such as for a programmable logic controller (PLC)
  8. Energy: fiber optics are used in energy applications, such as connecting oil & gas platforms, to transmit data and monitor well conditions in real-time

READ MORE: Fiber Optic Network Construction – Process and Build Costs

Jonathan Kim covers Fiber for Dgtl Infra, including Zayo Group, Cogent Communications (NASDAQ: CCOI), Uniti Group (NASDAQ: UNIT), Lumen Technologies (NYSE: LUMN), Frontier Communications (NASDAQ: FYBR), Consolidated Communications (NASDAQ: CNSL), and many more. Within Fiber, Jonathan focuses on the sub-sectors of wholesale / dark fiber, enterprise fiber, fiber-to-the-home (FTTH), fiber-to-the-premises (FTTP), and subsea cables. Jonathan has over 8 years of experience in research and writing for Fiber.


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