The idea of information traveling in a variety of ways is something we’re already used to. When you speak into a landline phone, the sound from your voice is carried via a wire cable into a socket in the wall when yet another cable takes it to the local telephone exchange. Cell phones work in a different way: they use invisible radio waves to send and receive information. They use a technology known as wireless since it doesn’t use any cables.
Fiber optic technology is the third way we transmit information. It involves sending information coded in a beam of light down a plastic or glass pipe. It was originally developed in the 1950s for endoscopes to allow doctors to see inside a human body without the need to cut it open first. Engineers in the 1960s found a way to use the same technology to transmit telephone calls at the speed of light (usually that’s 186,000 miles or 300,000 km per second in a vacuum, but slow to about two-thirds this speed in fiber optic cable).
Fiber optic cable comprises extremely thin strands of plastic or glass referred to as optical fibers. A single cable contains anywhere from 2 to several hundred strands. Each strand is less than one-tenth of the thickness of a human hair and is capable of carrying up to 25,000 telephone calls, which means that the entire fiber-optic cable easily carries several million calls.
The current record for a “single-mode” fiber (explained below) is 178 terabits (trillion bits) per second, which is enough for 100 million Zoom sessions.
Fiber optic cables use purely light-based (optical) technology to carry information between two places. Suppose you want to use fiber optics to send some information from your own computer to your friend’s house down the road to people across the world via subsea connectors.
You could connect your computer to a laser, and this would convert electrical information from the computer into a series of light pulses. You would then fire the laser down the fiber optic cable. The light beams would then emerge at the other end after traveling down the cable.
Your friend would require a light-detecting component in the form of a photoelectric cell to turn the light pulses back into electrical information that their computer could understand. So, the entire apparatus would be like an advanced version of the kind of telephone that you can make out of 2 baked bean cans and a piece of string.
How Does Fiber-Optic Technology Work?
To move through a fiber optic cable, light signals bounce off the walls repeatedly. Each photon (tiny light particle) bounces down the pipe. You might assume that a beam of light traveling in a clear glass pipe would simply leak out of the edges. However, this isn’t the case.
If light hits the glass at a really shallow angle of below 42 degrees, it reflects back in again, as though the glass was actually a mirror. It is a phenomenon known as total internal reflection. It is one of the things responsible for keeping light inside the pipe.
The other thing responsible for keeping light in the pipe is the cable’s structure, which comprises 2 separate parts. The cables’ main part, which is in the middle, is known as the core, and it’s the bit that light travels through.
The cladding is yet another glass layer that’s wrapped around the outside of the core. It ensures that light signals remain inside the core. It is able to do this since it is made of a different type of glass to the core.
Types of Fiber Optic Cables
Light signals carried down optical fibers are what are known as modes. It might sound technical, but it simply refers to different ways of traveling: a mode is just the path followed by a light beam down the fiber. One mode is to do it straight down in the middle of the fiber. The other mode is bouncing down the fiber optic cable at a shallow angle. In other modes, light signals bounce down the fiber at other angles that are less steep or steeper.
Single-mode is the least complex type of optical fiber. It has an incredibly thin core with a diameter of about 5 to 10 microns (millionth of a meter). All the signals in a single-mode fiber travel straight down the middle of the cable and don’t bounce off the edges. Single-mode fibers that are wrapped together into a big bundle usually carry Internet, telephone, and cable tv signals. Cables such as this are able to send information over 60 miles (100km).
Multi-mode is another type of fiber optic cable. Here, each optical fiber is about 10 times larger than 1 in a single-mode cable. Therefore, light beams travel through the core by following various paths (blue, orange, cyan, and yellow lines). Multi-mode cables are only able to send information over relatively short distances, which is why they are often used for linking computer networks together.
A medical tool known as a gastroscope (a type of endoscope) uses even thicker fibers. The device is poked down the patient’s throat and is used to detect illnesses inside the stomach. A gastroscope is a thick fiber optic cable that consists of many optical fibers.
At the end of the gastroscope, there’s a lamp and eyepiece. The lamp shines light down part of the cable into the patient’s stomach. Once light hits the stomach walls, it is reflected into a lens at the cable’s bottom. It then travels back up another part of the cable into the doctor’s eyepiece.
Other types of endoscopes work the same way and can be used for inspecting different parts of the body. An industrial version of the tool is also available, which is known as a fiberscope that can be used for examining things such as inaccessible pieces of machinery in airplane engines.