How Fiber Optics Function

Fiber-optic cables are often mentioned in discussions about the telephone network, cable TV, or the internet. This technology can be defined as the process of transmitting data, voice, and images through the passage of light across thin fibers, as per Encyclopedia Britannica.

Fiber-optic cables are incredibly thin strands of optically pure glass, about the width of a human hair, that transport digital information over vast distances. They're also used in medical imaging and mechanical engineering inspections, having largely replaced copper wiring in telecommunications.

In this article, we will explain how these tiny glass fibers transmit light and reveal the intriguing process of how these fibers are created.

What Are Fiber Optics?

Fiber optics (optical fibers) are long, slender strands of highly pure glass, roughly the size of a human hair. They are grouped together into bundles known as optical cables, used to carry light signals across great distances.

If you take a closer look at a single optical fiber, you'll notice that it consists of the following components:

Hundreds or even thousands of these optical fibers are grouped together in bundles within optical cables.

Optical fibers are available in two varieties:

Single-mode fibers feature smaller cores (around x 10 inches or 9 microns in diameter) and carry infrared laser light (wavelength = 1,300 to 1,550 nanometers or nm). On the other hand, multi-mode fibers have larger cores (roughly 2.5 x 10 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) using light-emitting diodes (LEDs).

Certain optical fibers can be constructed from plastic. These fibers have a larger core (0.04 inches or 1 millimeter in diameter) and are compatible with silicon chips. Glass fiber, however, doesn’t perform well with silicon and is expensive to integrate.

Now, let's explore how an optical fiber operates.

How Does an Optical Fiber Transmit Light?

Imagine you are shining a flashlight down a long, straight hallway. Simply point the beam straight ahead — light travels in straight lines, so it’s easy. But what if the hallway has a bend? You could place a mirror at the corner to reflect the beam around it. If the hallway is winding with several turns, you might line the walls with mirrors and angle the beam so it bounces side-to-side all the way down. This is precisely how light behaves in an optical fiber.

The light within an optical fiber moves through the core (hallway) by continuously bouncing off the cladding (mirror-lined walls), a process known as total internal reflection. Since the cladding doesn't absorb any light from the core, the light wave can travel vast distances.

However, some of the signal within the fiber is lost as it travels over longer distances. The degree to which the signal weakens depends on the glass's purity, the number of bends or splices in the fiber, and the wavelength of the transmitted light.

For instance, with multimode fiber, the loss is 850 nm = 3 dB/km; 1,300 nm = 1 dB/km. For single-mode fiber, it is 1,310 nm = 0.5 dB/km; 1,550 nm = 0.4 dB/km.

A Fiber-Optic Relay System

To grasp how optical fibers are applied in communication systems, let's imagine a scene from a World War II movie or documentary. Two naval vessels, navigating under radio silence or battling through rough seas, need to exchange messages. One ship approaches the other. The captain of one ship passes a message to a sailor stationed on the deck. The sailor encodes the message into Morse code (dots and dashes) and utilizes a signal light (floodlight with a Venetian blind-like shutter) to convey the message to the second ship. A sailor aboard the second vessel decodes the Morse code and relays the message to the captain.

Now, envision this scenario with the ships located on opposite sides of the ocean, separated by vast distances, yet equipped with a fiber-optic communication system linking them. A fiber-optic relay system consists of the following elements:

Transmitter

The transmitter functions similarly to the sailor stationed on the deck of the sending ship. It receives a signal and guides the optical equipment to toggle the light between "on" and "off" in a precise sequence, thus creating a light signal.

Located in close proximity to the optical fiber, the transmitter may even feature a lens to focus the light into the fiber. Lasers offer more power than LEDs but are more temperature-sensitive and tend to be more costly. The most common wavelengths for light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible sections of the spectrum).

Optical Regenerator

As previously mentioned, some signal loss takes place when light travels through the fiber, especially over great distances such as with undersea cables. Therefore, one or more optical regenerators are incorporated along the fiber to amplify the weakened light signals.

An optical regenerator comprises optical fibers that are coated with a special material (doping). The doped section is "excited" by a laser. When the attenuated signal reaches the doped area, the energy from the laser prompts the doped molecules to turn into lasers themselves. These molecules then emit a more powerful light signal that maintains the same characteristics as the incoming weak signal. In essence, the regenerator serves as a laser amplifier for the incoming signal.

Optical Receiver

The optical receiver functions like the sailor aboard the receiving vessel. It captures incoming digital light signals, interprets them, and transmits the electrical signal to the recipient's computer, TV, or telephone (the captain of the receiving ship). The receiver employs a photocell or photodiode to sense the light.

Benefits of Fiber Optics

What makes fiber-optic technology a breakthrough in telecommunications? When compared to traditional metal wiring (copper wire), optical fibers:

Are more affordable. While fiber-optic cables may cost more initially than copper wire, they require less maintenance over time, leading to long-term savings for both you and your internet provider.

Are more compact. Optical fibers can be produced with smaller diameters compared to copper wires.

Offer greater bandwidth. Due to their smaller size, more fibers can be grouped together in a cable of the same diameter than copper wires, enabling more phone lines or cable TV channels to be transmitted through the same cable.

Experience minimal signal loss. The reduction in signal quality is less severe in optical fibers than in copper wire.

Are free from interference from light signals. Unlike electrical signals in copper wires, light signals in optical fibers do not interfere with one another, leading to clearer phone calls or TV reception.

Use less energy. Since optical fibers experience less signal loss, they can operate with lower-power transmitters, reducing the need for the high-voltage electrical transmitters required for copper wiring. This ultimately cuts costs for both you and your provider.

Carry digital signals. Optical fibers are perfectly designed to transmit digital data, making them essential for computer networks.

Are fire-resistant. As no electrical current flows through optical fibers, they don't generate heat, significantly lowering the risk of fire.

Are lightweight. A fiber-optic cable weighs less compared to a copper cable (4 pounds or 2 kilograms per 1,000 feet vs. 39 pounds or 18 kilograms per 1,000 feet). Additionally, fiber-optic cables require less space in the ground.

Are flexible. Due to their flexibility and light-transmitting capability, fiber optics are used in a variety of flexible digital cameras for the following applications:

Thanks to their numerous benefits, fiber optics are widely used in industries such as telecommunications and computer networking. For instance, when calling Europe from the United States via a landline, the signal may bounce off a communications satellite, causing an echo. But with transatlantic fiber-optic cables, there's a direct connection, and no echo is heard.

How Are Optical Fibers Manufactured?

Now that we understand how fiber-optic systems operate and why they are essential, let’s explore how they are made. Optical fibers are crafted from highly purified optical glass. While typical window glass is transparent, its transparency decreases as it gets thicker, due to impurities. However, the glass used in optical fibers contains far fewer impurities than windowpane glass.

The process of creating optical fibers involves several key steps:

Creating the Preform Blank

The preform glass is produced through a method known as modified chemical vapor deposition (MCVD).

During MCVD, oxygen is bubbled through solutions containing silicon tetrachloride (SiCl4), germanium tetrachloride (GeCl4), phosphorus trichloride (PoCl3), and/or other chemicals. The precise composition of these chemicals determines the different physical and optical characteristics (like index of refraction, coefficient of expansion, melting point, etc.). The vapors are then introduced into a synthetic silica or quartz tube (cladding) using a specialized lathe. As the lathe rotates, a torch is moved along the outside of the tube. The intense heat from the torch leads to two outcomes:

The lathe rotates continuously to ensure a uniform coating and a consistent blank. The glass's purity is preserved by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by carefully managing the mixture's flow and composition. The preform blank production process is highly automated and takes several hours. Once the preform blank cools, it undergoes quality control testing (index of refraction).

Extracting Fibers from the Preform Blank

After the preform blank passes testing, it is placed into a fiber drawing tower.

The blank is lowered into a graphite furnace, which heats it to temperatures between 3,452 and 3,992 degrees Fahrenheit (1,900 to 2,200 degrees Celsius). The tip melts, and a molten glob drops under gravity. As it falls, it cools and solidifies into a thread.

The operator guides the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. The tractor mechanism steadily draws the fiber from the heated preform blank, with its diameter precisely monitored using a laser micrometer to adjust the tractor mechanism's operation.

The fibers are pulled from the blank at speeds of up to 66 ft/s (20 m/s), and the finished product is wound onto a spool. It's common for individual spools to contain several miles or kilometers of optical fiber.

Testing the Finished Optical Fiber

The finished optical fiber undergoes testing for the following attributes:

Once the fibers pass quality control, they are sold to telephone companies, cable service providers, and network suppliers. Many companies have upgraded their legacy copper-wire systems to fiber-optic systems for enhanced speed, capacity, and clarity.

Physics of Total Internal Reflection

When light transitions from a medium with one index of refraction (m1) to another medium with a lower index of refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As the angle of the light beam through m1 increases with respect to the normal line, the refracted light through m2 bends even more away from the line.

At a certain angle (critical angle), the refracted light will no longer pass into m2, but will instead travel along the surface between the two media (sine [critical angle] = n2/n1, where n1 and n2 represent the indices of refraction [with n1 greater than n2]). If the angle of the beam through m1 exceeds the critical angle, the refracted light will be fully reflected back into m1 (total internal reflection), even if m2 is transparent!

In physics, the critical angle is defined in relation to the normal line. In the context of fiber optics, however, the critical angle is referenced to the parallel axis running along the center of the fiber. Therefore, the fiber-optic critical angle is equal to (90 degrees - physics critical angle).

In an optical fiber, light travels through the core (m1, with a higher index of refraction) by constantly reflecting from the cladding (m2, with a lower index of refraction), as the angle of the light always remains greater than the critical angle. This reflection occurs regardless of how much the fiber is bent, even if it forms a complete circle!