Engineers at Wits University have found that magnets placed next to a common type of optical fibre can change the light travelling inside it. According to TechCentral, the effect went unnoticed for decades because of a shortcut in fibre theory from the 1970s.
The work was done by MSc student Mohammed Raiyan Sharif and Mitchell Cox. Cox heads the Wits Optical Communication Lab. Their findings were published in the journal Optics Letters.
“Often these things are right under our noses, hidden in the assumptions we have made for decades,” Cox said.
The university said the discovery could lead to magnetic sensors that are easier to install. In the longer term, it could also open new ways of computing with light.
Optical fibre is best known for carrying internet traffic. It is also used to measure magnetic fields, such as those around power lines and substations.
These sensors use an effect Michael Faraday discovered in 1845. A magnetic field running along a fibre slightly twists the direction in which the light wave vibrates. By measuring that twist, engineers can tell how strong the current is.
The problem is that the fibre usually has to be wrapped around the conductor. This can be hard to install. The new effect, called the transverse Faraday effect, responds to a field coming from the side.
That means a future sensor could simply lie next to the source, such as a fibre placed alongside an electrical cable in a trench. Wits cautioned that magnetic fields near most cables are much weaker than those used in the lab. It is not yet clear how sensitive a real sensor could be.
The team tested a multimode fibre, where light travels along many paths and bounces off the edge of the glass core. At the far end, the light forms a random pattern of bright and dark spots, a bit like the view through a kaleidoscope.
Sharif and Cox filmed this pattern while removing magnets, pair by pair, from a rail beside the fibre. The pattern changed as the magnets were taken away.
“It wasn’t hard to 3D print a rig to test our intuition. Working out the theory for what we were seeing was the challenging part,” Sharif said. He said the key was to go back to the basic equations and question assumptions usually taken for granted in fibre optics.
According to the report, the effect was missed because of the simple model engineers use to describe light in fibre. Light in a fibre vibrates almost fully across the strand. A tiny part vibrates along it, and the standard model leaves that part out.
Where the glass of the core differs sharply from the glass around it, that small part matters near the edge of the core. It is exactly the part a sideways magnetic field acts on.
The experiments matched the team’s theory. In the strongest result, the before and after patterns were only about 78% alike. When the light was set to vibrate in line with the magnetic field, there was no measurable change, as the theory predicted.
If an electrical coil replaced the permanent magnets, the way light moves through the fibre could be adjusted electrically. Cox said this could unlock a new way to do optical computing.
“If we change our perspective on those models, we can actually use light to do mathematical computations for us, for free,” Cox said.
The lab already uses light-based computing to extend the range of wireless optical links, which send data through the air using lasers. The research was supported by the CSIR.
Source: this article is based on reporting by TechCentral. Image: TechCentral. Written with the help of AI and published by the Tzaneen Voice Technology desk. See our Editorial Standards.
Werner Jacobs covers technology for Tzaneen Voice. Werner reports on mobile networks, data prices, internet access, smartphones, apps, AI, cybersecurity and South African tech companies. Each story explains what a new product, price change or policy means for ordinary users in South Africa.