Eli5: How does data move through fiber optic cables?

Question

I understand that there are 1’s and 0’s (on/off) in binary code to transmit data, but does the light inside the fiber optic flash billions of times per second to simulate the 1’s and 0’s? Im also curious if copper ethernet cables work in a similar way to transmit data.

In: Technology

Answer 1

That is exactly right for most optical fiber data transmission.

A modern 10 Gbps fiber network connection works by flashing a laser on-and-off 10 billion times per second. On means a binary 1, and off means a binary 0.

Typically a fiber network connection is 2 fibers. Computer A connects it’s laser to fiber 1, which then runs to computer B’s light detector. Computer B connects it’s laser to fiber 2, which then runs to computer B’s light detector.

It is possible to combine the upstream and downstream connections onto a single fiber, if computer A and computer B use different color lasers (e.g. computer A has a red laser and green detector, and computer B has a green laser and red detector). This is often how “fiber to the home” works – the laser/detector combos are more expnsive due to more complicated lenses/mirrors needed to connect a detector and a laser onto a single fiber, but it saves money on the fiber.

For even faster network connections, like 400 Gbps fiber connections, simply flashing the laser on and off isn’t good enough. A 400 Gbps connection will typically use “pulse amplitude modulation” and multiple lasers. Pulse amplitide modulation means that the laser isn’t just turned on or off, but it’s brightness is adjusted.

At 400 Gbps, PAM4 is used, which has 4 brightness levels – Off to transmit 00, 33% to transmit 01, 66% to transmit 10, and 100% to transmit 11. The laser is pulsed 50 billion times per second, allowing 100 gigabits per second to be transmitted with one laser. 4 separate lasers are then used to transmit the data in parallel. These can either be 4 standard color lasers, each connected to a separate fiber, running to a receiver with 4 separate detectors (This means a 400 Gbps fiber connection is often 8 fibers, 4 each direction). Alternatively, for long distance “wavelength division multiplexing” can be used – where there are 4 lasers of slightly different colors, connected to the same fiber, and 4 detectors sensitive to each laser color at the other end – this allows 400 Gbps over a standard 2 fiber connection.