There are two possibilities to create a sensor array modeling a probe.
The first one is to use a standard single mode fiber with FBGs lasered in. These FBGs have to be spectrally placed concerning the temperature and strain related shift and the filter edge of the data acquisition unit. This method provides a huge ability to multiplex several probes within one fiber line.
For example a probe for three phases current monitoring consists of only six FBGs. The drawback of this method is creeping of the squeezing device, which is needed to make the FBGs birefringent.
The more elegant way is to make use of FBGs shoot in high birefringent fibers (PM fibers).
These probes don’t suffer from creeping and the peak separation is also much more pronounced. Going one step more, a spliced in piece of single mode fiber having a high Verdet constant between two FBGs will increase the sensitivity of the probe, but also weaken the stability of the fiber due to the splicing. We know that the Faraday Effect can only satisfactorily be observed in a fiber medium with a perfect circular symmetry about the fiber axis, which means that the fiber must have a vanishingly small modal birefringence.
This makes it necessary to splice a single mode fiber in between the two birefringent FBGSs.
Due to the relatively small Verdet constant of glass, large rotation angles only can be realized by long path length or doping with terbium. This makes two alternative probe designs possible for current sensing, one with a fiber coiled around the conductor and the other with the conductor wrapped around the straight fiber.
A possible spectral response of the FBGs is shown below.
The birefringent FBGs show very pronounced peak separation.
Another very promising approach is the use of sensor arrays manufactured in planar waveguide technology.
The figure above illustrates a planar magnetic field sensor with birefringent FBGs taking the encoding of state of polarization.
Planar technology combined with the basic read out concept has great advantages like compact multiplexed sensor array with almost no influence on the surrounding magnetic field, what makes it very attractive for application in the field of magnetic resonance imaging (MRI). Further researches should be made, setting up a magnetic field sensor array manufactured in planar waveguide technology having high Verdet constant.
Such a waveguide material would be magneto optical glass MOS-04 / MOS-10, Terbium Gallium garnet (TGG) or Ytrium Irion garnet (YIG). Doping waveguides with terbium is possible from process technology view, but also a hybrid integrated optical device emedding YIG garnets is possible.
The figure above shows a planar sensor array 64 sensor regions embedded. Such a device is supposed to perform high speed and high accuracy measurements with large fields of application.