Optical fiber magnetic field and electric current sensors have been known for several decades and are based on the Faraday effect in the fiber itself [1], [2], [3]. Because of the lower Verdet constant of silica, the sensitivity of the all-fiber current sensors, independently of the sensor arrangement, is enhanced by a simple increase of the number of fiber coils. The problem which arises is the increased linear birefringence [4] which limits the sensitivity and affects linearity of the response. Large current measurement range of up to hundreds of kA and 0.1% accuracy can be achieved with spun optical fibers. Both the polarimetric and interferometric detection arrangements offer advantages and suffer from disadvantages like complicated optical schemes, costly components and sophisticated interrogation. To overcome these complications a variety of other approaches such as the use of magneto-strictive, magneto-optical and magnetic fluid materials in combination optical fibers [5], [6], [7] have been tested. In the case of magneto-strictive transducer Fiber Bragg gratings (FBG) were used to detect the magnetic field induced deformations [5] and thus measurements are converted in the spectral area. The detection schemes using high Verdet constant magneto-optical materials such as Yb Bi: YIG, BGO crystals, TG20 paramagnetic glass, SF2 and others are versions the standard polarimetric scheme with a laser or LED source, polarizer/analyzer and a photodetector. To measure magnetic field with magnetostrictive fluids the latter are in combination with a Fabry Perot interferometer and a broad-band source and the measurements are in the spectral domain [6], [7]. These sensors allow measurements of typically up to 50–60 mT with a relative error of less than 2% to be performed. Specialty photonic crystal fibers and optical fiber Fabry Perot interferometers, tapers or gratings are needed for the operation of this type of highly sophisticated sensors with interrogation in the spectral domain which is why they are still away from commercialization.

Among the many magneto-optical materials of interest featuring high Verdet constant are the undoped and Cr, Mn and Cu doped Bi12SiO20 (BSO) crystals [8], [9] and Bi12SiO20:Ni [10]. Polarimetric fiberized BSO based magnetic field sensors have been known for over two decades and the wavelength dependence of the optical activity ρ and the Verdet constant VB have been studied in detail for both undoped and doped BSO and BGO crystals. However, in such sensors polarimetric signals at a single wavelength have been used which translate into sine non-linear responses and need a reference signal at a second wavelength. Yet, a simpler polarimetric scheme employing wavelength dependent interrogation that avoids these disadvantages is possible but has not been suggested so far.

In this paper we present a simple fiber optic polarimetric scheme based on a BSO crystal which makes use of a low cost white light LED broadband source and spectral interrogation using a simple spectrometer in the visible. In Section 2 we present the theoretical description and simulation of the wavelength dependent performance of the sensor. Section 3 is devoted to the experimental study of the proposed scheme and the results are discussed. The conclusions summarize the basic features and advantages of the proposed sensor.