How Does Light Carry Data Across Optical Fiber?

When streaming a video or loading a website on your phone, there’s a good chance that some or all of the data that makes its way to you has travelled across an optical fiber as flashes of light. How does computer information get transformed into light and back again? How does that light travel across optical fiber?

Optical fibers are cables made from plastic or silica glass. They are used to transmit information in the form of light, often across large distances. (Photo used with permission).

The quick answer:
Computers (and other digital devices and circuits) like to think of information in terms of ones and zeroes. Every file stored on your hard drive or available over the Internet is actually composed of a large number of ones and zeroes, called bits.

When information needs to be transmitted from one place to the other (say, from Facebook’s servers to your phone), those bits are sent one-by-one along some kind of physical conduit, e.g. copper wiring in the case of a DSL connection, the air in the case of a Wi-Fi network, and optical fiber in the case of long-distance communications. Bits will travel across several different physical media on their way to your device.

In the case of copper cables (like phone lines), an electric charge carries the information from one end of the cable to the other. When an electric charge is present, a 1 is transmitted. When an electric charge is absent, a 0 is transmitted. By flipping the current on and off rapidly (we’re talking about millions of times per second or more), the bits in the file make their way along the cable.

Optical fiber operates on the exact same principle, but uses light instead of electricity. On one end of the cable, an LED will flash on and off. On the other end of the cable, a light sensor will record a 1 when a light is present, and a 0 when light is absent.

Optical fibers are cheap, and since light travels along them extremely fast, they make excellent conduits for transmitting digital information.

A little more: modulation and pigeons

The process of changing bits of data to better travel across a physical medium (air, copper cabling, optical fiber, etc.) is called modulation. The process of reversing modulation is called, unsurprisingly, demodulation.

The word “modem” is actually a combination of “modulator” and “demodulator”. A modem’s main function is to take a stream of ones and zeroes, and transform them into a format best suited for the physical medium being employed. You can’t easily hear what modulated data sounds like with cable or DSL modems, but if you were using the Internet back in the bad old days of dial-up access, and you had the misfortune of picking up the receiver while online, you’d hear something like this, the sound of two modems talking to one another.

Morse code is kind of like an early form of modulation. A written message is passed to a telegraph operator, who transforms it into a series of long and short pulses, transmits it along a cable to another operator, who then demodulates it back into a written message.

A now-famous April Fools joke paper from 1990, RFC1149 — Standard for the Transmission of IP Datagrams on Avian Carriers, argues for carrier pigeons to be used as a physical medium for the transmission of bits on the internet:

1. A file would be printed, and the printout would be cut into small strips. (Modulation)
2. Each strip would be attached to a carrier pigeon, that would then return to its owner. (Transmission)
3. The owner would wait to receive all of his pigeons, collect all of the strips of paper, and then scan them back into his computer. (Demodulation)

Although this was in jest, it nicely illustrates the separate processes of modulation, transmission and demodulation.

The Bergen Linux Group created the world’s only known implementation of the CPIP (Carrier Pigeon Internet Protocol) 11 years later. It ran over a distance of 5km. The birds lost more than half of the pieces of paper, and even over very short distances, data would flow at a rate of a few bits every hour. (If someone approaches you with an offer to obtain pigeon Internet for a low price of $19.95/month, run!)

An optical fiber wrapped around the world at the equator, on the other hand, could carry the same information there and back in about 450 milliseconds (less than half a second). I think we made the right choice in deciding to invest in optical fiber…