Modulator

A modulator is a small component that puts data onto a beam of light. It works as a lightning-fast light switch: bright light is a 1, dark is a 0, and this alternates billions of times per second. The result travels through optical fibre as a light signal.

How does it work?

The laser burns all the time. The modulator splits the light into two halves, which each follow their own path and come together again at the end. Light is a wave, with peaks and troughs. If both halves arrive exactly in step, peak on peak, they reinforce each other and bright light comes out. If one half lags behind by half a wave, every peak falls on a trough and they cancel each other out.

To switch, you put an electrical voltage on one of the two paths. That makes the silicon there a fraction slower for light, so that half starts lagging behind by half a wave on command. That effect is very weak, which is why standard modulators are 2 to 4 millimetres long.

Why it matters

In AI data centres, thousands of chips work together and constantly send each other data. Over more than about a metre, optical fibre is needed, and so is a modulator that converts the electrical signals into light. The smaller and more economical it is, the more connections fit on one chip.

Japanese researchers built a modulator a third of a millimetre long, in which small holes slow down the light (link). It transmits up to 200 gigabits per second, enough for about five high-quality films per second. According to the university, the switch itself uses about a hundred times less energy than the standard one. The team is now working on losing less light, and on sending several colours of light through one modulator at the same time.