High-speed optical fiber connectivity has revolutionized how we live, work, and communicate. The ever-growing global appetite for bandwidth and system reliability drives the increasing adoption of hyperscale technologies, with scalable, full-fiber networks facilitating seamless data flow at peak demand. Before delving into the mechanics of fiber optics, let’s briefly touch on the advantages of transitioning from traditional copper-based infrastructure to modern optical fiber technology.
In comparison to the 25-300Mbps data transmission speeds of legacy copper cabling, optical fiber cabling offers speeds of 10Gbps and beyond. That’s not all. Unlike traditional copper cabling, optical fibers transmit data as light, not electricity, minimizing heat concerns in compact cabling ducts and high-density networks. Also, a single optical fiber can transmit signals over 60+ miles (100 kilometers), whereas attenuation – or signal degradation – occurs in copper cabling at around 100 meters.
Undoubtedly, optical fiber technology is the backbone of tomorrow’s high-speed, low-latency, ultra-connected world. To explain how fiber optics work, and to ascertain what makes light stay in the fiber, this blog introduces the essential features of optical fiber technology, bringing together the pertinent factors, processes, and scientific principles underpinning the complementary technologies driving the future landscape of optical fiber connectivity.
Optical fiber cables: structure and composition
Optical fiber cables comprise three critical components. First, the light-carrying core. Next, the cladding, and finally, the protective outer coating (also known as the jacket). Each component – or cylindrical layer – of the optical fiber cable serves a specific purpose in the efficient propagation of data as light signals. Terminology such as refraction, the refractive index, and total internal reflection help to describe the function and purpose of the materials used in optical fibers and are explained in greater detail later in this blog.
Optical fiber cable structure:
- Core
Light signals travel through the core. The core is composed of highly purified silicon dioxide (SiO2) with very small trace amounts of “dopants” (such as Germanium), added to adjust the index of refraction for optimum optical transmission. Different diameter cores are available for different purposes. For example, the relatively narrow-diameter of single-mode fibers (typically around 8-10 microns) limits transmissions to a single, focused pathway, helping retain long-haul signal fidelity. Alternatively, multi-mode fibers carrying various light signals over short distances (e.g., within a building or on campus) necessitate diameters of 50+ microns.
Whether single-mode or multi-mode, the core’s comparatively higher refractive index in relation to the cladding is a factor in achieving total internal reflection.
- Cladding
Cladding surrounds the core. While double- and triple-clad fibers serve specialized high-power applications (e.g., industrial laser systems) standard, single-clad optical fiber cables serve in everyday applications such as telecommunications and data networking. The cladding’s primary purpose is to confine light within the core. This is done by providing a lower refractive index to achieve total internal reflection.
- Outer layer (or jacket)
The outer layer does not directly interact with the light transmitting through the core. Instead, the outer layer offers mechanical strength and physical protection against environmental factors that could potentially degrade the refractive indices of the fiber’s internal materials. These factors include weather-related water ingress and temperature extremes, as well as pulling, bending, and twisting during installation and moves. In this way, robust cable jacketing helps to ensure efficient and reliable light transmission.
To better understand how light stays in the fiber, we must begin linking the key concepts of total internal reflection, the critical angle, and the refractive index. The following paragraphs give a clear understanding of the science behind these concepts, while the dedicated section on Snell’s Law gives a foundational, scientific explanation of refraction.
What is refraction?
Refraction describes the change in light’s direction when crossing between media with different densities. For example, consider shining a flashlight into a large glass bowl filled with water. Because the light above the water line is viewed through comparatively less dense air than the light seen shining into the denser water below, the angle of the light’s path appears to change at the point of entry. This change in light’s direction when traversing media with different densities is called refraction (see Snell’s Law below).
What is the refractive index?
Continuing the example of the flashlight shining into water, we might ask the question, “Does light always refract at the same angle?” The answer is no. Light refracts at a calculable angle in accordance with the refractive index. By knowing the refractive indices of, for example, both water and air, the optical parameters dictating the angle of refraction can be entered into an equation that shows the exact angle of expected refraction at room temperature (in some cases, temperature extremes impacting media density must be considered).
How does the refractive index relate to the core and cladding?
Light moving from a denser medium to a less dense medium bends away from the ‘normal line’ (i.e., the imaginary line perpendicular to the interface between the two media at the point of entry). Going back to the flashlight example, we might consider submerging the flashlight such that the light must now travel from the denser water into the less dense air, mimicking light traversing the core and entering the cladding. Manipulating the flashlight’s beam angle to exceed the ‘critical angle’ will reflect the light into the water. In the same way, the angle of the light traversing the core must exceed the critical angle, while the cladding must provide a lower refractive index than the core.
The critical angle and total internal reflection
Light traveling from a denser medium to a less dense medium at an angle exceeding the critical angle will experience total internal reflection. This is where the light reflects into the denser primary medium and does not enter the secondary less dense medium. Understanding the principles of refraction, the refractive index, the critical angle, and total internal reflection enables engineers to select core and cladding materials for optimal optical fiber performance.
Snell’s Law: Understanding refraction
The scientific principles underpinning reliable modern optical fiber transmissions can be traced to 1621, when Willebrord Snellius, a Dutch astronomer and mathematician, first demonstrated and popularized the equation governing the law of refraction. However, notable historical mathematicians had previously advanced their own theories of refraction and deserve mention here, including Ptolemy of Ancient Greece, who lived approximately between 100-170AD, and Ibn Sahl, a Persian scientist who may have demonstrated the equation in the year 984.
Snell’s Law:
Explanation: Snell’s Law accurately determines the angle of refraction that will occur when light travels between media with known refractive indices. Below is a simple breakdown of the mathematical components represented within the equation:
n1 | The refractive index of the first medium. |
n2 | The refractive index of the second medium. |
Θ1 | The angle of incidence (i.e., the measurable angle that exists between the angle of the light source and the imaginary line drawn perpendicular to the plane connecting two media at the point of intersection). |
Θ2 | The angle of refraction (i.e., the measurable angle that exists between the refracted light entering the second medium and the imaginary line drawn perpendicular to the plane connecting the two media at the point of intersection). |
What about ‘sin’? Short for ‘sine’, sin is a function from trigonometry used to relate both the angles of incidence and refraction to the respective refractive indices of the appropriate media. More simply, sin helps to calculate consistent angular relationships and wavefront behaviors, with engineering applications across telecommunications, imaging systems, and any scientific instrument requiring accurate predictions of light’s behavior. For more information and a detailed breakdown of the topic, see Snell’s Law.
Conclusion
Due to its high bandwidth, unparalleled speed, and performance reliability in the efficient transmission of vast amounts of data, optical fiber technology is found across many common applications. For example, optical fiber connectivity underpins high-speed internet, telephone networks, cloud-based services, and mobile communications. As we look to the future, the growing demand for data will drive even greater innovation and advancement in how we weave optical fiber solutions into the fabric of a more connected society.