Fiber Optics: Understanding the Basics (2022)

Nothing has changed the world of communications as much as the development and implementation of optical fiber. This article provides the basic principles needed to work with this technology.

Engineering and Marketing Staff, OFS


Optical fibers are made from either glass or plastic. Most are roughly the diameter of a human hair, and they may be many miles long. Light is transmitted along the center of the fiber from one end to the other, and a signal may be imposed. Fiber optic systems are superior to metallic conductors in many applications. Their greatest advantage is bandwidth. Because of the wavelength of light, it is possible to transmit a signal that contains considerably more information than is possible with a metallic conductor — even a coaxial conductor. Other advantages include:

• Electrical Isolation — Fiber optics do not need a grounding connection. Both the transmitter and the receiver are isolated from each other and are therefore free of ground loop problems. Also, there is no danger of sparks or electrical shock.

• Freedom from EMI — Fiber optics are immune to electromagnetic interference (EMI), and they emit no radiation themselves to cause other interference.

• Low Power Loss — This permits longer cable runs and fewer repeater amplifiers.

• Lighter and Smaller — Fiber weighs less and needs less space than metallic conductors with equivalent signal-carrying capacity.

Copper wire is about 13 times heavier. Fiber also is easier to install and requires less duct space.

Applications

Some of the major application areas of optical fibers are:

• Communications — Voice, data, and video transmission are the most common uses of fiber optics, and these include:

– Telecommunications
– Local area networks (LANs)
– Industrial control systems
– Avionic systems
– Military command, control, and communications systems

• Sensing — Fiber optics can be used to deliver light from a remote source to a detector to obtain pressure, temperature, or spectral information. The fiber also can be used directly as a transducer to measure a number of environmental effects, such as strain, pressure, electrical resistance, and pH. Environmental changes affect the light intensity, phase and/or polarization in ways that can be detected at the other end of the fiber.

(Video) Fundamentals of Fiber Optic Cabling

• Power Delivery — Optical fibers can deliver remarkably high levels of power for tasks such as laser cutting, welding, marking, and drilling.

• Illumination — A bundle of fibers gathered together with a light source at one end can illuminate areas that are difficult to reach — for example, inside the human body, in conjunction with an endoscope. Also, they can be used as a display sign or simply as decorative illumination.

Fiber Optics: Understanding the Basics (1)

Figure 1. An optical fiber consists of a core, cladding, and coating.
Construction

An optical fiber consists of three basic concentric elements: the core, the cladding, and the outer coating (Figure 1).

The core is usually made of glass or plastic, although other materials are sometimes used, depending on the transmission spectrum desired.

The core is the light-transmitting portion of the fiber. The cladding usually is made of the same material as the core, but with a slightly lower index of refraction (usually about 1% lower). This index difference causes total internal reflection to occur at the index boundary along the length of the fiber so that the light is transmitted down the fiber and does not escape through the sidewalls.

Fiber Optics: Understanding the Basics (2)

Figure 2. A beam of light passing from one material to another of a different index of refraction is bent or refracted at the interface.
The coating usually comprises one or more coats of a plastic material toprotect the fiber from the physical environment. Sometimes metallicsheaths are added to the coating for further physical protection.

Optical fibers usually are specified by their size, given as the outer diameter of the core, cladding, and coating. For example, a 62.5/125/250 would refer to a fiber with a 62.5-µm diam core, a 125-µm diam cladding, and a 0.25-mm diam outer coating.

Principles

Optical materials are characterized by their index of refraction, referred to as n. A material’s index of refraction is the ratioof the speed of light in a vacuum to the speed of light in the material. When a beam of light passes from one material to another with a different index of refraction, the beam is bent (or refracted) at the interface (Figure 2).

(Video) Fiber Optic Basics for Field Techs

Refraction is described by Snell’s law:

Fiber Optics: Understanding the Basics (3)

where nI and nR are the indices of refraction of the materials through which the beam is refracted and I and R are the angles of incidence and refraction of the beam. If the angle of incidence is greater than the critical angle for the interface (typically about 82° for optical fibers), the light is reflected back into the incident medium without loss by a process known as total internal reflection (Figure 3).
Fiber Optics: Understanding the Basics (4)

Figure 3. Total internal reflection allows light to remain inside the core of the fiber.
Watch a video definition of total internal reflection.

Modes

When light is guided down a fiber (as microwaves are guided down a waveguide), phase shifts occur at every reflective boundary. There is a finite discrete number of paths down the optical fiber (known as modes) that produce constructive (in phase and therefore additive) phase shifts that reinforce the transmission. Because each mode occurs at a different angle to the fiber axis as the beam travels along the length, each one travels a different length through the fiber from the input to the output. Only one mode, the zero-order mode, travels the length of the fiber without reflections from the sidewalls. This is known as a single-mode fiber. The actual number of modes that can be propagated in a given optical fiber is determined by the wavelength of light and the diameter and index of refraction of the core of the fiber.

Attenuation

Signals lose strength as they are propagated through the fiber; this is known as beam attenuation. Attenuation is measured in decibels (dB) with the relation:

Fiber Optics: Understanding the Basics (5)

where Pin and Pout refer to the optical power going into and coming out of the fiber. The table below shows the power typically lost in a fiber for several values of attenuation in decibels.

The attenuation of an optical fiber is wavelength dependent. At the extremes of the transmission curve, multiphoton absorption predominates. Attenuation is usually expressed in dB/km at a specific wavelength. Typical values range from 10 dB/km for step-index fibers at 850 nm to a few tenths of a dB/km for single-mode fibers at 1550 nm.

Fiber Optics: Understanding the Basics (6)

There are several causes of attenuation in an optical fiber:

• Rayleigh Scattering — Microscopic-scale variations in the index of refraction of the core material can cause considerable scatter in the beam, leading to substantial losses of optical power. Rayleigh scattering is wavelength dependent and is less significant at longer wavelengths. This is the most important loss mechanism in modern optical fibers, generally accounting for up to 90% of any loss that is experienced.

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• Absorption — Current manufacturing methods have reduced absorption caused by impurities (most notably water in the fiber) to very low levels. Within the bandpass of transmission of the fiber, absorption losses are insignificant.

• Bending — Manufacturing methods can produce minute bends in the fiber geometry. Sometimes these bends will be great enough to cause the light within the core to hit the core/cladding interface at less than the critical angle so that light is lost into the cladding material. This also can occur when the fiber is bent in a tight radius (less than, say, a few centimeters). Bend sensitivity is usually expressed in terms of dB/km loss for a particular bend radius and wavelength.

(Video) Free 2 Hour Fiber Optic Training

Fiber Optics: Understanding the Basics (7)

Figure 4. Numerical aperture depends on the angle at which rays enter the fiber and on the diameter of the fiber’s core.
Numerical aperture

Numerical aperture (NA), shown in Figure 4, is the measure of maximum angle at which light rays will enter and be conducted down the fiber. This is represented by the following equation:

Fiber Optics: Understanding the Basics (8)

Dispersion

As the optical pulses travel the length of the fiber, they are broadened or lengthened in time. This is called dispersion. Because the pulses eventually will become so out of step that they begin to overlap each other and corrupt the data, dispersion sets an upper limit on the data-carrying capabilities of a fiber. There are three principal causes for this broadening:

• Chromatic Dispersion — Different wavelengths travel at different velocities down the fiber. Because typical light sources provide power over a series or range of wavelengths, rather than from a single discrete spectral line, the pulses must spread out along the length of the fiber as they proceed. The high-speed lasers used in communications have very narrow spectral output specifications, greatly reducing the effect of chromatic dispersion.

• Modal Dispersion — Different fiber modes reflect at different angles as they proceed down the fiber. Because each modal angle produces a somewhat different path length for the beam, the higher-order modes reach the output end of the fiber behind the lower-order modes.

• Waveguide Dispersion — This minor cause for dispersion is due to the geometry of the fiber and results in different propagation velocities for each of the modes.

Bandwidth

Bandwidth measures the data-carrying capacity of an optical fiber and is expressed as the product of the data frequency and the distance traveled (MHz-km or GHz-km, typically). For example, a fiber with a 400-MHz-km bandwidth can transmit 400 MHz for a distance of 1 km, or it can transmit 20 MHz of data for 20 km. The primary limit on bandwidth is pulse broadening, which results from modal and chromatic dispersion of the fiber. Typical values for different types of fiber follow:

Fiber Optics: Understanding the Basics (9)

Power transmission

The amount of power that a fiber can transmit (without being damaged) is usually expressed in terms of the maximum acceptable power density. Power density is the product of the maximum power output of the laser and the area of the laser beam. For example, a 15-W laser beam focused onto a 150-µm diameter spot produces a power density of

Fiber Optics: Understanding the Basics (10)

The output of a pulsed laser (typically specified in millijoules of energy per pulse) must first be converted to power per pulse. For example, a pulsed laser that produces 50 mJ in a 10-ns pulse provides an output power of
Fiber Optics: Understanding the Basics (11)

The power density then can be calculated from the spot size.

To transmit the absolute maximum energy levels down a fiber, the fiber end faces must be absolutely smooth and polished and be perpendicular to the fiber axis and the light beam. Also, the beam diameter should be no greater than approximately one-half of the area of the core (or the diameter of the core). If the beam is not appropriately focused, some of the energy may spill into the cladding, which quickly can damage polymer-clad silica fibers. For this reason, it is better to use silica-clad silica fibers in higher power density applications.

Fiber types

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There are basically three types of optical fiber: single mode, multimode graded index, and multimode step-index. They are characterized by the way light travels down the fiber and depend on both the wavelength of the light and the mechanical geometry of the fiber. Examples of how they propagate light are shown in Figure 5.

Fiber Optics: Understanding the Basics (12)

Figure 5. Modes of fiber transmission.
Single mode

Only the fundamental zero-order mode is transmitted in a single-mode fiber. The light beam travels straight through the fiber with no reflections from the core-cladding sidewalls at all. Single-mode fiber is characterized by the wavelength cutoff value, which is dependent on core diameter, NA, and wavelength of operation. Below the cutoff wavelength, higher-order modes may also propagate, which changes the fiber’s characteristics.

Because the single-mode fiber propagates only the fundamental mode, modal dispersion (the primary cause of pulse overlap) is eliminated. Thus, the bandwidth is much higher with a single-mode fiber than that of a multimode fiber. This simply means that pulses can be transmitted much closer together in time without overlap. Because of this higher bandwidth, single-mode fibers are used in all modern long-range communication systems. Typical core diameters are between 5 and 10 µm.

The actual number of modes that can be propagated through a fiber depends on the core diameter, the numerical aperture, and the wavelength of the light being transmitted. These may be combined into the normalized frequency parameter or V number,

Fiber Optics: Understanding the Basics (13)

where a is the core radius, λ is the wavelength, and n is the index of the core and the cladding. The condition for single-mode operation is that:
Fiber Optics: Understanding the Basics (14)

Perhaps more important and useful is the cutoff wavelength. This is the wavelength below which the fiber will allow propagation of multiple modes and can be expressed as:
Fiber Optics: Understanding the Basics (15)

A fiber is typically chosen with a cutoff wavelength slightly below the desired operating wavelength. For lasers typically used as sources (with output wavelengths between 850 and 1550 nm), the core diameter of a single-mode fiber is in the range of 3 to 10 µm.

Multimode graded index

The core diameters of multimode fibers are much larger than single-mode fibers. As a result, higher-order modes also are propagated.

The core in a graded-index fiber has an index of refraction that radially decreases continuously from the center to the cladding interface. As a result, the light travels faster at the edge of the core than in the center. Different modes travel in curved paths with nearly equal travel times. This greatly reduces modal dispersion in the fiber.

As a result, graded-index fibers have bandwidths which are significantly greater than step-index fibers, but still much lower than single-mode fibers. Typical core diameters of graded-index fibers are 50, 62.5, and 100 µm. The main application for graded-index fibers is in medium-range communications, such as local area networks.

Multimode step index

The core of a step-index fiber has a uniform index of refraction right up to the cladding interface where the index changes in a step-like fashion. Because different modes in a step-index fiber travel different path lengths in their journey through the fiber, data transmission distances must be kept short to avoid considerable modal dispersion problems.

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Step-index fibers are available with core diameters of 100 to 1500 µm. They are well suited to applications requiring high power densities, such as medical and industrial laser power delivery.


FAQs

What are the basics of fiber optics? ›

Fiber Basics

Optical fiber is a highly-transparent strand of glass that transmits light signals with low attenuation (loss of signal power) over long distances, providing nearly limitless bandwidth. This technology enables telecommunications service providers to send voice, data, and video at ever increasing rates.

What is fiber optics answer? ›

Fiber optics, or optical fiber, refers to the technology that transmits information as light pulses along a glass or plastic fiber. A fiber optic cable can contain a varying number of these glass fibers -- from a few up to a couple hundred.

Is fiber optics hard to learn? ›

Learning fiber optics is much easier than learning the laws of electricity. The only problem is it's relatively new to electricians and technicians entering the datacom industry. Here's an overview of how we use light to send communication signals, and why this method works so well.

How fiber optics works simple explanation? ›

Fiber-optic cables transmit data via fast-traveling pulses of light. Another layer of glass, called “cladding,” is wrapped around the central fiber and causes light to repeatedly bounce off the walls of the cable rather than leak out at the edges, enabling the single to go farther without attenuation.

What are the 3 basic components of an optic fiber system? ›

The three basic elements of a fiber optic cable are the core, the cladding and the coating.

What are three types of fiber optic cables? ›

There are three types of fiber optic cable: single mode, multimode and plastic optical fiber (POF).

How many types of fiber optics are there? ›

Types of optical fiber

There are two primary types of fiber, each of which has a different application. These are multimode (MM) fiber, which has a large core and allows for multiple paths through the fiber, and single-mode (SM) fiber, which has only one path, through a much smaller core.

How many types of fiber optic cable are there? ›

There are two types of fibre optic cables – multimode and single-mode. Multimode optical fibre or OFC is capable of carrying multiple light rays (modes) at the same time as it has varying optical properties at the core. Single-mode fibre has a much smaller core size (9 microns).

What are the parts of fibre optics? ›

A fiber optic cable consists of five main components: core, cladding, coating, strengthening fibers, and cable jacket.

How long does it take to learn fiber optics? ›

Certification Programs

A typical program consists of around 25 credit hours, less than half of the requirement for an associates degree.

Is fiber optic a good career? ›

The Pay Is Great

Happily, pay isn't something fiber optics technicians need to worry about. According to Payscale, folks with a fiber optics certification make an average of $61,000 annually. Keep in mind that means you can earn significantly more than that if you stay in the field.

How do I get started with fiber optics? ›

If you're interested in pursuing a career in fiber optics, you might consider a fiber optics certification.
...
Certification through work experience:
  1. Verify your experience. ...
  2. Verify your knowledge. ...
  3. Submit application. ...
  4. Find a proctor and set a time and date for the exam. ...
  5. Take the exam.
26 Oct 2021

Why is fiber optic faster? ›

Fiber relies on light instead of electricity to transmit data, which facilitates much faster Internet connections that are capable of handling higher bandwidth. According to the FCC, fiber providers consistently offer 117 percent of advertised speeds, even during times of peak demand.

What is the advantage of fiber optics? ›

Greater flexibility

Compared to copper cables, fibre optic cables are thinner and lighter in weight. Fibre can withstand more pull pressure than copper and is less prone to damage and breakage. Fibre is flexible, can bend easily and resists most corrosive elements that often attack copper cables.

What is 2 core fiber cable? ›

2 Core Fiber Optic Cable

The optical fiber unit is positioned in the centre. Two parallel Fiber Reinforced Plastics (FRP) are placed at the two sides.

What is the size of fiber optic cable? ›

Let's talk about fiber optic types and core sizes. There are three common core sizes: 9/125, 50/125, and 62.5/125. Each of those numbers stands for a measurement, and that measurement is in microns.

How do you calculate fiber optic cable? ›

A value of “002” means 2 fiber strands, “048” means 48 fiber strands. Valid fiber counts are: 002, 004, 006, 008, 012, 024, 036, 048, 72, 96 144, 288. “01” means 1 fiber per sub unit (tube).

What color are fiber optic cables? ›

Cable Jacket Colors
Fiber TypeColor Code
Multimode (50/125) (850 nm Laser-optimized) (OM5)Lime GreenUndefined
Multimode (62.5/125) (OM1)OrangeSlate
Multimode (100/140)OrangeGreen
Single-mode (OS1, OS1a, OS2)YellowYellow
4 more rows

What is OS1 and OS2 fiber? ›

OS1 fiber optic cable is designed for premises where the maximum distance is 2,000 metres with transmission speeds of 1 to 10 gigabit Ethernet. OS2 fiber optic cable is designed for larger transmission distances in the range of 5,000 to 10,000 metres with similar transmission speed of 1 to 10 gigabit Ethernet.

What type of light is used in fiber optics? ›

Lasers: the Best Light for Optical Fiber Communications

Laser light is used for optical fiber communications for the simple reason that it is a single wavelength light source.

What is frequency of optical fiber? ›

Optical fiber links are used in all types of networks, LAN and WAN. The frequency range of fiber optics is approximately 180 THz to 330 THz.

Which wire is used for fiber optic cable? ›

The world of telecommunications is rapidly moving from copper wire networks to fiber optics. Optical fiber is a very thin strand of pure glass which acts as a waveguide for light over long distances. It uses a principle known as total internal reflection.

What is fiber cable made of? ›

Fiber optic cable is made of many thin strands of coated glass fibers. Each measures about eight microns - that's smaller than a strand of human hair. Digitized information is “coded,” or placed on to light pulses for transmission. It travels along the glass fiber at the speed of light - 186,000 miles per second.

How strong is fiber optic cable? ›

The glass fiber's theoretical maximum (tensile) strength is about 2 million pounds per square inch, but the practical limit is 10 to 20 percent of that. The cross-sectional area is so small,about 20 millionths of an inch,that the actual maximum fiber strength is about five to 10 pounds of tension.

What is critical angle in optical fiber? ›

critical angle, in optics, the greatest angle at which a ray of light, travelling in one transparent medium, can strike the boundary between that medium and a second of lower refractive index without being totally reflected within the first medium.

Which type of fibre has highest bandwidth? ›

Fiber-optic bandwidth is high both because of the speed with which data can be transmitted and the distance that data can travel without attenuation. Optical fiber transmits data as pulses of light through glass wire, allowing data to travel at nearly the speed of light.

What are the main characteristics of optical fiber? ›

Benefits and Characteristics of Fiber Optic Cable
  • Extremely high throughput.
  • Very high resistance to noise.
  • Excellent security.
  • Ability to carry signals for much longer distances before requiring repeaters than copper cable.
  • Industry standard for high-speed networking.
8 Apr 2014

What are the main characteristics of optical fiber? ›

Benefits and Characteristics of Fiber Optic Cable
  • Extremely high throughput.
  • Very high resistance to noise.
  • Excellent security.
  • Ability to carry signals for much longer distances before requiring repeaters than copper cable.
  • Industry standard for high-speed networking.
8 Apr 2014

How many types of fiber optics are there? ›

Types of optical fiber

There are two primary types of fiber, each of which has a different application. These are multimode (MM) fiber, which has a large core and allows for multiple paths through the fiber, and single-mode (SM) fiber, which has only one path, through a much smaller core.

What is structure of optical fiber? ›

Optical fiber is composed of three elements – the core, the cladding and the coating. These elements carry data by way of infrared light, thus propagating signal through the fiber. The core is at the center of the optical fiber and provides a pathway for light to travel.

How many types of fiber optic cable are there? ›

There are two types of fibre optic cables – multimode and single-mode. Multimode optical fibre or OFC is capable of carrying multiple light rays (modes) at the same time as it has varying optical properties at the core.

What is fiber speed? ›

Fiber-optic internet, commonly called fiber internet or simply “fiber,” is a broadband connection that can reach speeds of up to 940 Megabits per second (Mbps), with low lag time. The technology uses fiber-optic cable, which amazingly can send data as fast as about 70% the speed of light.

What are the applications of optical fibers? ›

Applications. Optical fiber is used by telecommunications companies to transmit telephone signals, Internet communication and cable television signals. It is also used in other industries, including medical, defense, government, industrial and commercial.

What type of light is used in fiber optics? ›

Lasers: the Best Light for Optical Fiber Communications

Laser light is used for optical fiber communications for the simple reason that it is a single wavelength light source.

What is the size of fiber? ›

Fiber types are identified by the diameters of the core and cladding, expressed in microns. Multimode fiber is available in two sizes, 62.5 or 50 microns, and four classifications: OM1 (62.5/125 µm), OM2, OM3, OM4 (50/125 µm). The diameter of a single mode core is 9µm.

What is the color code for fiber optic cable? ›

Cable Jacket Colors
Fiber TypeColor Code
Multimode (62.5/125) (OM1)OrangeOM1, 62.5/125
Multimode (100/140)Orange100/140
Single-mode (OS1, OS1a, OS2)YellowOS1, OS1a, OS2, SM/NZDS, SM
Polarization Maintaining Single-modeBlueUndefined (2)
4 more rows

Which are the five main parts of optical fibre? ›

The cladding of the Optical fibre is made up of Glass / Plastic. Buffer: The Buffer is the moisture substance that is coated over the surface of the cladding. They are mainly used to provide flexibility to the optical fibre's core and cladding. Jacket: The jacket is the outer surface of the optical fibre.

What are the 4 types of optical fiber cable construction? ›

A fiber optic cable consists of five main components: core, cladding, coating, strengthening fibers, and cable jacket.

Which wire is used for fiber optic cable? ›

The world of telecommunications is rapidly moving from copper wire networks to fiber optics. Optical fiber is a very thin strand of pure glass which acts as a waveguide for light over long distances. It uses a principle known as total internal reflection.

How thick is Fibre optic cable? ›

Fiber-optic cabling consists of a signal-carrying glass core of 5 to 100 microns in diameter (a sheet of paper is about 25 microns thick and a human hair about 75 microns thick), surrounded by a layer of pure silica called cladding, which prevents light from escaping.

What is a good light level for fiber? ›

Single mode fiber is used to transmit 1270 - 1650 nm light over long distances and high data rates, most commonly at 1310 and 1550 nm.

Videos

1. Keeping Your Fiber Optic Skills Sharp: Understanding Fiber-optic communication
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2. Optical fiber cables, how do they work? | ICT #3
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3. Fiber Optic Fundamentals Pt 2
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4. Structure, basics and types of optical fibers (step index and graded index Optical Fiber)
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5. Fundamentals of Fiber Optics
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6. Fiber optic cables: How they work
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