TechWeb, November 12 — Researchers at the University of the Witwatersrand in South Africa have discovered that a magnetic field passing through an optical fiber from the side can still alter the propagation state of light within it. They have named this phenomenon the transverse Faraday effect.

The conventional Faraday effect requires a magnetic field to be applied along the direction of the optical fiber, causing the polarization direction of light to rotate. Leveraging this change, optical fibers can measure magnetic field strength and thereby determine the magnitude of current in power lines.

The researchers found that previous analyses of optical fibers had long relied on a simplification that ignored a tiny component of the electric field oscillating along the direction of light propagation—which meant the effect produced by a transverse magnetic field had also gone unnoticed.

In step-index multimode optical fibers, the refractive index of the core and the cladding changes abruptly at the boundary. That previously ignored electric field component does in fact exist, and light also propagates along many different paths, ultimately superimposing at the fiber output into a pattern of bright and dark spots.

In the experiment, the researchers placed a magnet beside a 1-meter-long optical fiber and observed the output pattern through a camera.

After applying a 0.24 Tesla transverse magnetic field, under a qualifying polarization direction, the correlation of the pattern with the original state dropped to about 0.84. When the polarization direction was changed to be parallel to the magnetic field, no obvious change was detected.

The experiment also showed that this effect depends on the angle at which light enters the optical fiber, the polarization direction, and the wavelength—and does not occur in all optical fibers. Graded-index multimode optical fibers and the few-mode optical fibers used in the experiment did not yield the same results.

If sensitivity can be improved in future work, optical fiber sensors might be laid parallel along power lines, without needing to be wound around the lines as in some existing solutions.

However, this experiment used a strong magnetic field of 0.24 Tesla. Whether it can detect the much weaker magnetic fields around ordinary power transmission lines still needs verification.