Fiber Optic Principles - An Introduction To How Fiber Optics Work


Few things have changed the world of communications as much as the development and implementation of optical fiber. In this article we will review fiber’s basic characteristics, types of fibers, connectors, jacketing types, top applications, and the advantages of using fiber optics in light sources.


Optical fibers are made from either glass or plastic, and most are about as thin as a human hair. Fibers may be many miles long and are considerably lighter than traditional metal wire.

While optical fibers absolutely transformed communication when the first all-fiber cable was laid across the Pacific Ocean in 1996, fiber technology continues to advance. Today, optical fibers are used in a wide variety of industries, including medical, military, and industry.


An optical fiber consists of three basic concentric elements: the core, the cladding, and the buffer.

  • Core: The core is typically made of glass or plastic and is the part of the fiber that carries the light.
  • Cladding: The cladding, usually made of the same material as the core but with a slightly lower refractive index, surrounds the core. This difference in refractive index causes total internal reflection to occur along the length of the fiber, ensuring that the light is transmitted within the core and does not escape through the sides.
  • Buffer: To protect the fiber from the surrounding environment, a buffer, or coating, is applied and is usually made of one or more layers of plastic. In some cases, metallic sheaths may also be added for additional physical protection.

Optical fibers are typically described by their size, which is indicated by the outer diameter of the core, cladding, and coating. For example, a measurement of 62.5/125/250 would refer to a fiber with a core diameter of 62.5 micrometers, a cladding diameter of 125 micrometers, and an outer coating diameter of 0.25 millimeters.

Snell’s Law

Snell's law explains how light bends or changes direction when it passes from one material to another, and it therefore enables us to predict how light behaves as it travels through optical fibers.

By applying Snell's law, we can determine the angle at which light enters and exits the fiber, as well as how it will interact with the fiber's core and cladding. By optimizing the refractive index, signal loss can be minimized while fiber optic performance can be maximized.

Snell's law plays a crucial role in fiber optics by providing insights into how light behaves within optical fibers, guiding the design and optimization of fiber optic systems for efficient and reliable signal transmission.

Snell’s Law Equation

An optical fiber is divided into two sections in the image above. The left side portrays a portion of the fiber with a ray of light passing through it, while the right side is a diagram that illustrates the index of refraction of the glass present in the fiber. The core glass is represented as n1, while the cladding glass is represented as n2. It is important to note that the core glass has a slightly higher refractive index, approximately 1.46, compared to the cladding, which has a refractive index of approximately 1.45.

The sine represents the relationship between the side of a right triangle and the hypotenuse, which refers to the longest side opposite the right angle. Meanwhile, the refractive index of glass (n) is defined as the ratio of the speed of light in a vacuum (c) to its actual speed in the material (v), expressed as follows:

Using Snell's Law, we can determine the angle at which total internal reflection occurs in an optical fiber. This phenomenon, illustrated in the accompanying diagram, takes place when the refracted ray aligns with the boundary separating the core and the cladding. Perform this calculation using the previously mentioned values of a core index of 1.46 and a cladding index of 1.45.

The equations must be manipulated slightly.

This implies that any ray deviating by 6.8 degrees or less from the axis (90 - 83.2 degrees) will experience total internal reflection. Another way of thinking about it is the acceptance cone of the fiber is twice that angle or about 14 degrees. This process also establishes the numerical aperture (NA) of the fiber, although its calculation involves a mathematical manipulation of the critical angle computation.

The numerical aperture (NA) of an optical fiber refers to the acceptance angle of the fiber. In the case of the fiber we have defined earlier, the equation for calculating the numerical aperture is as follows:

That's a half angle of 9.8 degrees when used to calculate the angle - the total cone angle is ~20 degrees.

Numerical Aperture (NA)

The Numerical Aperture (NA) of a fiber is a measure of the maximum angle at which an incident ray can enter the core of the fiber to experience total internal reflection. Rays that fall outside this angle will excite radiation modes within the fiber. A higher core index, relative to the cladding, results in a larger NA. However, increasing the NA leads to higher scattering losses due to greater concentrations of dopants. The NA of a fiber can be determined by measuring the divergence angle of the light cone emitted when all modes of the fiber are excited.

In qualitative terms, the Numerical Aperture (NA) serves as an indicator of a fiber's ability to gather light effectively. Additionally, it provides insights into the ease of light coupling into the fiber.

Types of Fibers

There are three main kinds of optical fiber: single mode fiber, polarization maintaining, and multimode, which includes both graded-index and step-index fibers.

Single Mode Fiber (SMF)
An SMF, also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light, the transverse mode.

Because the SMF only transmits one mode, modal dispersion (the primary cause of pulse overlap) is eliminated. As a result, the bandwidth is much higher with an SMF than that of a multimode fiber. 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, depending on wavelength.

Polarization Maintaining
A Polarization-Maintaining Fiber (PM Fiber, PMF) maintains two polarization modes by intentionally inducing uniform birefringence along the entire fiber length. It is known as the slow axis and fast axis and is important when the input polarization needs to be maintained in the fiber.

Multimode (graded-index and step-index)
Both graded and step index fibers use refracted or reflected light.

The graded index’s refractive index is higher at the axis of the core and then decreases gradually towards the core-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. 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.

The step-index operates on the principle of total reflection and causes light to travel across the core in the zigzag pattern.

Graded-index multimode fibers are used for data communications and networks carrying signals across medium distances - typically no more than a couple of kilometers, while step-index multimode fibers are mostly used for imaging and illumination (i.e., short distances).

A fiber optic connector, also known as a terminator, is used to join two ends of fiber optic cables together or to connect to a specific system. A connector should provide a reliable, low-loss contact in the plugged-in state.

Ferrule Connector (FC)

FC connectors have a threaded body that make it suitable for high-vibration environments. FC connectors are used in datacom, telecommunications, measurement equipment, and single-mode lasers but are now starting to get outpaced by SC and LC connectors.

Angle-Polished Connectors (APC)
APC connectors minimize back reflection and should only be paired with another APC connector. Reflected light cannot travel back up the fiber thanks to the polished angle at the end.

APCs are almost always green and are typically identified by adding /APC to the name of a connector. For example, an angled FC connector would be labelled an FC/APC.

Physical Contact (PC)
The PC fiber connector is polished in the PC contact style and is the most common polish type found on multimode fibers. PC fiber connectors address the air gap between two surfaces from small imperfections in the original flat fiber connector.

The PC connector has a cylindrical cone head to eliminate the air gap, making the expected return loss in SMF applications about -40dB, which is higher than the return loss of the original flat polish style (-14 dB or roughly 4%). Up to now, this polish style has been outdated and the evolution - UPC polish style has come into being.

Standard/Subscriber Connectors (SC)
SC connectors are versatile, push/pull snap-in connectors for SMFs. They have a ceramic ferrule and controlled fiber orientation with a key. Unlike rounded FCs, SCs have a rectangular shape that ensures defined rotations. The snap-in mechanism makes handling easier and less sensitive. SCs are increasingly used in network applications, particularly with SMFs, and are available in duplex versions.

SubMiniature version A (SMA)
SMA fiber optic cable connectors were created in the 1960s and are the oldest type of optical connector still in use. They have a threaded plug and socket design that was originally developed for coaxial cables, and they now also work with optical fibers. These connectors have a fiber holder with a 3.175 mm ferrule built into them.

It's important to note that SMA connectors are not twist-resistant, meaning that if the fibers of
two connectors touch, they could get damaged. To prevent this, the ferrules are brought close
without the fiber ends touching. Although SMA connectors are commonly used for multi-mode
fibers, they can also connect single-mode fibers. Their sturdy construction makes them popular
in metrology, medical, and military applications.

Common Connector Pairings
Many connectors combine two types of connectors into one, as indicated in their name. DAYY Photonics’ light sources typically come with an FC/APC connector to reduce back reflections with its angled polish, but other connector types are available on request.

  • FC/APC: Ferrule head with an angled polish
  • FC/PC; Ferrule head with a PC polish to minimize the air gap
  • SC/PC: Push/pull snap-in connector with a PC polish
  • SC/APC: Push/pull snap-in connector with an angled polish

Jacketing Types
Bare fiber is delicate and easily broken, so an exterior fiber optic cable jacket is needed to protect the fiber inside. The cable jacket is the first line of defense against moisture, heat, chemicals, and other wear and tear. Different jackets are most suitable for different environments, and the table below contains some of the most common fiber cable jacket materials and where they should be used.

Color Coding
Fiber optic cables are also color coded for different fiber types. For SMFs, the jacket color is typically yellow while PM fiber uses blue. For multimode cable, the jacket color can by orange, aqua, or purple. And for outside cables, the standard color is black.

Popular Applications

Optical fiber communications have become a crucial part of information technology. They allow fast and affordable transmission of digital data for things like phone calls, video signals, regional data networks, and computing. With the rise of fiber to the home technology, it's even more important for providing speedy internet access to businesses and homes, outperforming copper cables. Fiber optics is gaining popularity for short-distance transmissions within buildings or devices, including the use of plastic optical fibers.

Different types of fiber lasers are now significant light sources, not just for low-power applications but also for high output powers. They compete with traditional bulk lasers and can be a better choice depending on the circumstances.

Fiber-optic sensors are used to measure things like temperature, stress, strain, rotation, and chemical compositions. They are utilized in various fields such as aircraft and space technology, oil exploration, and monitoring structures like large bridges and pipelines. These sensors can be either localized or distributed, relying on different physical principles to function effectively. In many cases, fibers are used to transport light from a source to an application. For example, they carry light from a high-power laser diode setup to a bulk laser or from a laser diode to a light-powered sensor system on a high-voltage transmission line. They are even employed to connect a high-power fiber laser to a welding robot in a car factory.

The Fiber Optic Advantage

Fiber’s metal and copper counterparts stand no chance against fiber’s versatility, durability and ease of use when it comes means of communication. Some of fiber’s top advantages include:

  • Electrical Isolation - Fiber optics don't require a grounding connection. Both the senderand the receiver are separate from each other, which means they don't have issues with ground loops. Additionally, there's no risk of sparks or electrical shock.
  • Protection from EMI - Fiber optics are not affected by electromagnetic interference (EMI), and they don't emit any radiation that could cause interference in other devices.
  • Minimal Power Loss - This allows for longer cable distances without needing as many signal boosters.
  • Lighter and Smaller - Fiber is lighter and takes up less space compared to metal conductors that can carry the same number of signals. Copper wire, for example, is about 13 times heavier. Fiber optics are also easier to install and require less duct space.

Fiber optics is an exciting field because it is constantly evolving with the times. As our communications, medical, mechanical, and industrial needs change, fiber is there to help make it all possible.