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How Does Light Travel Through Optical Fibers?


What is Fiber Optics?

Fiber optics is the science of transmitting data by the passage of light through thin fibers. It is the field of applied science and engineering concerned with the design and application of optical fibers.

What are Optical Fibers?

Optical fibers are long, thin strands of carefully drawn glass with diameters in the microscale. The strands are arranged in bundles or “optical cables” and they transmit light signals over varying distances. At the transmitting source, the light is encoded with data. The optical fiber then transmits the light signal to a receiving end, where the data is decoded. Therefore, the optical fiber is a transmission medium. It is a pipe or passageway that carries signals at very high speeds.

Optical fiber parts core cladding plastic coating
Figure 1: Parts of optical fiber: core, cladding, and plastic coating

Optical fibers are used most often in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths or data transfer rates than electrical cables. Fiber optic cables were originally developed for endoscopes in the 1950s, to help doctors view the inside of a human body without major surgery. In the 1960s, engineers found a way to use the same technology to transmit and receive telephone calls at the “speed of light” (c ≈ 3 x 108 m/s). In reality, the transmission slows to about two-thirds of c in a cable.

Fibers are used instead of metal wires because they offer less signal loss. Fibers are also immune to electromagnetic interference, while metal wires suffer from this problem. Other applications of fibers include illumination and imaging, where they are used to carry light into, or images out of confined spaces, as in endoscopy. Fibers are also used for specialized applications, some of them being fiber-optic sensors and fiber lasers.

How Optical Fibers work?

Light travels down a fiber-optic cable by bouncing repeatedly off the walls, that is, each photon (particle of light) repeatedly bounces down the pipe. The cable is mainly made up of two separate parts, the core, and the cladding. The central part of the cable—in the middle—is called the core and that is where light travels through. The core is made of glass and has mirror-like properties that keep the light within it. If light hits the glass at certain angles, it reflects back in again—as though the glass is really a mirror. This phenomenon is called total internal reflection and is what keeps light from leaking out of the core.

Wrapped around the outside of the core is another layer of glass called cladding. The cladding also helps keep the light signals inside the core. It is made of a different type of glass which has a lower refractive index (n) compared to that of the core. Light traveling across an interface from a higher n medium to a lower n medium will bend away from the normal. At the critical angle (θc), light traveling from a higher n medium to a lower n medium will be refracted at 90°, or along with the interface. If light hits the interface at any angle larger than θc with respect to the normal, it will not pass through to the second medium and all of it will be reflected back into the first medium, thus total internal reflection. θc is dependent on the refractive indices of the two mediums and can be calculated by Snell’s Law.

Optical fiber total internal reflection
Figure 2: Total internal reflection keeps the light inside the core

Optical fibers typically have a core surrounded by a transparent cladding material with a lower index of refraction. Technically, the cladding ensures that total internal reflection occurs so that light will be reflected back inside the core.

A fiber jacket or coating encloses the core and the cladding. The coating usually comprises one or more coats of plastic material to protect the fiber from moisture and physical damage. Sometimes metallic sheaths are added to the coating for more protection from the environment

Being able to configure or join optical fibers with minimal loss in data encoded in the light is important in fiber optic communication. Achieving this requirement involves a complex process; careful cleaving of the fibers, and precise alignment and coupling of the cores.

Types of Fiber Optic Cables

Single-mode fibers have the simplest structure. They contain a very thin core, and because the centerpiece is so small, light does not really bounce around; all signals travel straight through the middle without bouncing off the edges. Single-mode fibers are used for long-distance communication links. They are typically used for Community Antenna Television (CATV), Internet, and telephone applications, where the signals are carried by single-mode fibers wrapped into a bundle.

Fibers that allow many propagation paths are called multi-mode fibers. The light beams can travel through the core by following a variety of different paths, or in multiple different modes. Multi-mode fibers generally have a wider core diameter. They are used for short-distance communication links and for applications where high optical power must be transmitted. Multimode fiber optic cables are also used as patch cords or “jumpers” to interconnect data equipment such as computer networks.


Fiber cables are extremely secure. Optical fibers are immune to EM interference, and so data cannot be intercepted, slowed, or jumbled with other signals. The safety, speed, and security of fiber optics do come at a higher cost relative to other cable options on the market. But compared to the rising costs of copper, which is used in other cable technology, it remains reasonably priced. Moreover, metallic wiring is naturally thicker, which reduces load capacity and makes installation difficult in more challenging environments. Fiber optics provide the clarity and safety that modern homeowners and business leaders demand.

Fiber optic cable assemblies have numerous applications, and their uses are only growing. Aside from private and public networking for cable and internet, this technology is the backbone of military networking and medical imaging and laser practices. Although the majority of practical applications in the past used metallic wiring, fiber optic technology delivers a superior amount of quality, convenience, performance capability and durability currently unmatched in telecommunication.


As new inventions become commonplace, new needs for fiber optics will arise. The technology will continue to be in high demand for healthcare equipment, telecommunication companies, and the automotive industry, among others. Fiber optics is rapidly growing and its future is likely to outlast the next generation of devices and industrial requirements.

Authored By

Susie Maestre

Susie is an Electronics Engineer and is currently studying Microelectronics. She loves fictional novels, motivational books as much as she loves electronics and electrical stuffs. Some of her fields of interests are digital designs, biomedical electronics, semiconductor physics, and photonics.

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