For more than 50 years, engineers have described light in optical fiber using a convenient shortcut, one that works so well it is rarely questioned. Engineers at the University of the Witwatersrand (Wits University) in Johannesburg, South Africa, have now shown that this shortcut hides a small but real effect. A magnet placed beside a multimode fiber, rather than along it, can change the light traveling inside.

Optical fibers are best known for carrying the light signals that connect us to the internet. These hair-thin strands of glass can also act as sensors, and for decades engineers have used them to measure magnetic fields, including those produced by the electrical currents in power lines and substations.

Such sensors rely on the so-called Faraday effect, first observed by Michael Faraday in a block of glass in 1845. When a magnetic field points along a fiber, it slightly twists the polarization of the light traveling through it. Polarization is the direction in which the light wave oscillates, much as a rope can be shaken up and down or side to side. Measuring the twist reveals the strength of the magnetic field, and therefore the strength of the current that produced it.