Fiber Optic Cables: Sending Data with Light

In the news

How do data centers talk to each other across a whole country? On October 26, 2026, Tata Communications announced a plan to lay about 10,000 kilometers of new fiber optic cable across India. The new network would link the country's main data center and AI hubs. Following a flash of light through the cable shows how this kind of link works.

Look closer

Simplified diagram. Arrows show the main path; some parts are left out.

1. What does a fiber optic cable carry instead of electricity? (write or draw)
2. What happens to light pulses on a very long trip? (write or draw)

Start with the whole system

Think about sending a text message. Your words leave your phone and reach a friend in a moment. Something has to carry that information the whole way.

AI computers send huge amounts of information. AI means computer programs that learn patterns from examples. The computers that run AI live in data centers. They need to share information with each other and with people far away.

A fiber optic cable carries information as light pulses (quick flashes of light). Each fiber is a strand of glass about as thin as a human hair. A cable bundles many fibers inside a tough cover. Fiber links computers inside data centers. It also links data centers in different cities.

Follow the diagram from left to right: Laser, Core, Cladding, Amplifier and Receiver. The sections below explain each part.

Laser

Start at the sending end. Computers work with electrical signals. A laser turns electrical signals into light pulses. These light pulses can travel through glass.

The light pulses follow a code. In the simplest code, a flash means 1 and no flash means 0. Computers store all information as patterns of 1s and 0s. Real systems use faster and more complex codes, but the idea is the same.

Safety: Never look into a laser or into the end of a fiber. The light can hurt your eyes, even when you cannot see it.

Real-world example: Imagine flashing a flashlight to a friend across a dark yard. Long flash, short flash, no flash. If you both know the code, you can send a message with light.

Core

Next, the light enters the fiber. The core is the center of the fiber, where the light travels. It is made of very clear glass. Clear glass lets light pass through without blocking much of it.

Light in glass is fast. It travels about 200,000 kilometers each second. That is about two-thirds as fast as light moving through empty space.

Real-world example: Look through a clean window. Light passes right through the glass. The core is like a very long, very thin window that light travels along.

Cladding

A fiber is not a straight path. Cables bend around corners and over hills. So what keeps the light inside? The cladding is the layer of glass around the core. The cladding makes light reflect back into the core.

To reflect means to bounce back from a surface. When light hits the edge of the core, it bounces back inside. It keeps bouncing as it travels down the fiber. Very little light escapes.

Real-world example: Roll a ball down a bowling lane with bumpers. Each time the ball hits a bumper, it bounces back into the lane. The cladding acts like bumpers for light.

Amplifier

Even clear glass is not perfect. On a very long trip, the glass absorbs a little light. Some light also scatters away. So light pulses get weaker as they travel far.

An amplifier makes weak light pulses strong again. Long routes place amplifiers along the cable, often tens of kilometers apart. Then the light can keep going across a whole country, or even under an ocean.

Real-world example: Think of a relay race. One runner gets tired, so a fresh runner carries the baton. The message keeps moving at full strength.

Receiver

Finally, the light reaches the far end. A receiver turns light pulses back into electrical signals. Now the computers at that end can use the information.

The receiver reads the pattern of flashes and no flashes. It changes them back into 1s and 0s. The message arrives in the same pattern it started with.

Real-world example: Your friend across the yard watches your flashlight and writes down each flash. Your friend is the receiver.

Put the five parts together

Look back at the diagram. The laser makes light pulses. The core carries them. The cladding keeps them inside. The amplifier makes weak pulses strong. The receiver turns them back into electrical signals.

Why not use copper wire? Copper wires carry electrical signals, and many still do. But fiber can carry much more information than copper wire, and its signal stays strong over longer distances. That is why long links between data centers use fiber.

Try the math. Suppose a fiber cable is 2,000 kilometers long. Light in glass travels about 200,000 kilometers each second. 2,000 ÷ 200,000 = 0.01. So the light crosses the cable in about one hundredth of a second.