Plate 42 · Information

Light in a Glass Thread

Shine a light into a stream of water and something strange happens: the light bends with the falling water and is carried all the way to where it lands. The idea behind that experiment of 1841 now turns your messages into flashes of light, traps them in glass threads as fine as a hair, and carries them across the floor of the Pacific.

Try it

Chapters · 7

01Light That Follows Water
02A Perfect Mirror
03A Thread of Glass
04Ones and Zeros in Light
05As Thick as a Garden Hose
06Across the Pacific
07Your Turn

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01

Light That Follows Water

In 1841, in Geneva, Jean-Daniel Colladon let water pour from a small hole in the side of a large tank and shone sunlight in from behind. The light did not carry on in a straight line. It stayed inside the stream and curved down with it.

Today a small waterproof laser, set in the tank and aimed at the hole, does the job. Look where the water lands in the bowl: the stream has carried the light all the way there.

Water leaving 22 cm below the surface at about 2 m/s · green laser light

02

A Perfect Mirror

Light passing from water into air bends as it crosses the surface, and the more glancing its path, the more it bends. Once it meets the surface more than 48.6° from straight on, it cannot get out at all: every bit of it is reflected back. This is total internal reflection.

It makes a better mirror than any silvered one: nothing is lost at each reflection. So the light inside the stream bounces off the surface again and again, trapped.

03

A Thread of Glass

An optical fibre works the same way, with extraordinarily pure glass in place of water, drawn as fine as a hair: 0.125 mm across.

At its centre runs a far thinner core, about 0.009 mm wide. A trace of another substance mixed into the glass makes light travel slightly more slowly in the core than in the glass around it, and where the two meet is the mirror that keeps the light in.

04

Ones and Zeros in Light

The information rides on flashes: light on for a 1, off for a 0, billions of times a second. Several colours of light can share a single core, each carrying its own messages.

The light is really infrared, invisible to the eye, with a wavelength of about 1.55 micrometres; the colours here are painted in.

05

As Thick as a Garden Hose

An undersea cable holds anywhere from a few to a couple of dozen pairs of fibres, wrapped in jelly, a copper tube, steel wires and plastic. Out in the deep ocean it is about as thick as a garden hose.

Even the purest glass swallows a little light. In the 1960s, only 1% of it survived 20 metres of glass fibre. Today’s fibre keeps 95% after a whole kilometre, but still only 0.6% after 100 km, so every 60 to 100 km along the seabed an amplifier brightens the light again.

06

Across the Pacific

The FASTER cable, opened in 2016, carries six pairs of fibres from the coast of Japan, in Chiba and Mie, about 9,000 km along the seabed to Oregon.

In glass, light travels about a third slower than in a vacuum, roughly 200,000 km a second, so each flash takes 0.044 s to cross: less than a twentieth of a second. More than 95% of the internet traffic between countries travels through cables like this one.

07

Your Turn

Change the speed of the stream (the deeper the water above the hole, the faster it leaves) and the angle of the laser. Aimed along the stream, the light stays in at any speed. Slant the laser and the light meets the surface more directly; once it comes within 48.6° of straight on, it escapes, and a slower stream, which curves more sharply, lets it out sooner. Watch where it leaks, and how much still reaches the bowl.

Try thisSet the laser angle to 40°: how much light reaches the bowl, slowest stream vs fastest?

What the model leaves out

Rays are traced with geometric optics in the vertical plane of the stream only. A real stream narrows as it falls and breaks into drops, and its surface ripples; all of these let some light escape. In the single-mode fibres used for communication the core is only a few times wider than the wavelength, so strictly the light travels as a “mode” of the core; bouncing between mirrors is an approximation. The glass is fused silica (Malitson 1965); in a real fibre a trace of added material makes the core’s refractive index very slightly higher than the cladding’s. Today’s ocean links do not simply switch the light on and off: they also code bits in the phase and polarisation of the light, several bits to a flash. And real flashes are far longer than drawn: at ten billion a second, each fills about 2 cm of fibre. The cable’s route on the globe is drawn as the shortest path between its landing points; the real cable bends round undersea mountains and trenches and is longer. Colours and enlargements are for clarity.

A common misreading

Light in a fibre is not “faster light”: in glass it travels about a third slower than in air. The fibre’s strengths are elsewhere: almost no light is lost, it follows a curving path, and one thread carries many colours of light flashing billions of times a second. And when your data crosses an ocean, it almost certainly travels through an undersea cable, not by satellite.

Sources

Data

Refractive index of fused silica: Malitson (1965), via refractiveindex.info. FASTER cable: landing points and length from the operators’ announcement (2016); the route on the globe is drawn as the shortest path between its landings. Earth: NASA Earth Observatory, Blue Marble Next Generation, October 2004 (Reto Stöckli); city lights, NASA Earth Observatory Black Marble 2016 (Joshua Stevens, Miguel Román, Suomi NPP VIIRS); clouds, NASA Blue Marble 2002 (Reto Stöckli, NASA GSFC). Stars: Yale Bright Star Catalogue, 5th revised edition (Hoffleit & Warren 1991), via CDS VizieR.