There does not seem to be any relation between an optical fiber carrying light and a wire through which an electric current is flowing. But as far back as 1845, Michael Faraday had demonstrated that the magnetic field generated by a current flow through a wire influences the plane-polarization of light waves.
Optical fibers are typically known for their usefulness in data links. In fact, these links can span not only intra-board and short distances but are extremely useful from inter-chassis links to those covering thousands of kilometers. Moreover, being immune to RFI/EMI and external electronic interferences, optical fibers are a good fit for linking data in high-interference situations. But then, this claim goes against the earlier observations of Michael Faraday.
As it is, a special arrangement and the right circumstances are necessary to make optical fibers immune to external electromagnetic influence. Meanwhile, engineers and scientists are taking advantage of the Faraday effect—passing light through a magnetic field rotates its optical polarization state. An electric current can induce this magnetic field. A large electric current can generate a strong magnetic field, and this can change the polarization significantly.
The Verdet constant is a proportionality constant relating the strength of the magnetic field to the angle of rotation. Although not easy to mix optics and electromagnetic physics, scientists use the Verdet effect to measure the current value in a current-carrying wire by covering it with an optical fiber. One of the advantages of this implementation is the high-voltage value of galvanic isolation obtained—very important in power-related applications.
However, there are other details to take care of, when sensing current using the Faraday effect. For instance, thermal fluctuations or minor vibrations can affect the polarization state in the fiber. Therefore, it is necessary to isolate the fiber from these effects, at the same time allowing it to remain sensitive to the magnetic field inducing the polarization.
Scientists have developed a solution for the above problem. They use a type of fiber different from the conventional ones for data links. This special fiber is an advanced type of optical fiber, an SHB or spun-high birefringent fiber. Although on a microscale, the SHB fiber maintains its polarization, on a macroscale, it offers a net-zero birefringence.
To make such a fiber, the manufacturer spins the glass to create a constant rotation of the polarization axis. They twist the fiber once every few millimeters. This allows the fiber to maintain circular polarization despite mechanical stresses on it, while still allowing it to remain sensitive to the Verdet effect.
A careful balance of the spin pitch of the fiber overcomes the effect of stress due to bending during the coiling process yet allowing the fiber to maintain its sensitivity to the Faraday effect. As a result, scientists can use the spun fiber in longer lengths and with smaller coil diameters, resulting in higher sensitivity.
Of course, this one complex subtle step based on optical fiber is not enough to build a current sensor. The input laser beam must have a stable circular polarization before it enters the fiber, requiring the use of polarization-control methods.
