How light carries your call through glass

An ordinary phone call needs 64,000 a second in each direction. That is 8,000 measurements a second, each written with 8 bits, and a bit is a one or a zero.

By the time your call reaches the fibre, it is only numbers.

Sources: ITU-T G.711: pulse code modulation of voice frequencies (1988)

No. It is infrared light with a wavelength of about 1.55 micrometres, longer than the red light eyes can see. Glass loses the least light at that wavelength, so long links use it.

The drawing paints each wavelength in its own colour, so you can tell them apart. Your call's light is purple.

In 1988, the lasers on TAT-8, the first fibre cable across the Atlantic, sent 280 million ones and zeros a second, at a wavelength of 1.3 micrometres.

Sources: Nobel Prize: popular information, the physics prize (2009); Nobel Prize: scientific background, the physics prize (2009); IEEE: Milestone, TAT-8, the first fibre-optic cable across an ocean (2016)

The ITU, the United Nations agency for telecoms, sets the colours on a grid around 193.1 terahertz, a wavelength of about 1,552.5 nanometres. With the colours 50 gigahertz apart, the band where glass loses least, from 1,530 to 1,565 nanometres, holds about 80 of them. The drawing shows seven.

In 1988 a fibre carried one colour. All the calls on it took turns, a few ones and zeros at a time.

Sources: ITU-T G.694.1: spectral grids for WDM applications (2020); ITU-T G.652: single-mode optical fibre and cable (2024)

It is the same effect you see from under water. Light going from water into air at a shallow angle does not get out at all. It reflects back into the water. Light in the core meets the cladding at just such a shallow angle, so the edge of the core works like a perfect mirror.

The light is fast even in glass. It needs about 5 millionths of a second for each kilometre, so it covers about 200,000 km a second, two thirds of its speed in empty space. Over the 160 km in the drawing that adds up to 0.8 thousandths of a second. The drawing runs it 6,000 times slower.

Sources: Nobel Prize: popular information, the physics prize (2009); Nobel Prize: scientific background, the physics prize (2009); ITU-T G.652: single-mode optical fibre and cable (2024); ITU-T G.114: one-way transmission time (2003); ITU-T G.694.1: spectral grids for WDM applications (2020)

A regenerator. A photodiode turned the weak light into electricity, a circuit board read the ones and zeros and cleaned them up, and a new laser sent them on as fresh light. It worked on one colour, at 1.3 micrometres, where glass loses more light, so the huts stood about 40 km apart.

The erbium amplifier came out of a lab at the University of Southampton in 1987. There, 3 m of fibre with erbium in it made the light up to 630 times stronger. Amplifiers like it have since replaced most of the regenerators.

Sources: ITU-T G.652: single-mode optical fibre and cable (2024); ITU-T G.692: optical interfaces for systems with optical amplifiers (1998); ITU-T G.977: optically amplified submarine cable systems (2015); ITU-T G.957: optical interfaces for SDH equipment and systems (2006); AT&T Technical Journal: lasers and photodetectors for submarine lightwave cables (1985); Electronics Letters: a low-noise erbium-doped fibre amplifier (1987); Nobel Prize: scientific background, the physics prize (2009)

Light carries far more. It can carry tens of thousands of times more information than radio waves.

Glass also shrugs off the weather. Unlike copper, it is not affected by lightning, and unlike radio, it is not affected by bad weather.

The same light, in the same kind of glass, carries calls under the oceans too, with amplifiers in pods on the seabed.

Sources: Nobel Prize: popular information, the physics prize (2009); AT&T Technical Journal: lasers and photodetectors for submarine lightwave cables (1985)