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Sensing & Measurement

Magneto-optical fiber Bragg gratings for magnetic field sensors

The Faraday effect in specialized fiber Bragg gratings enables photonic devices with polarized behavior that can be stimulated by external magnetic fields.
3 January 2012, SPIE Newsroom. DOI: 10.1117/2.1201112.003889

Sensors based on fiber Bragg gratings (FBGs) provide significant advantages, such as small size, geometric flexibility, wavelength multiplexing, simplicity of fabrication, and distributed detection. These devices satisfy requirements for a number of physical parameters related to sensing, including strain, temperature, pressure, and magnetic field.1–3 In recent years the technology has generated substantial interest because of its many industrial applications, particularly in monitoring the structural integrity of buildings, bridges, and advanced composite materials.4, 5 Most of these devices are wavelength-encoded, which means that they detect wavelength shifts induced by parameters such as strain and temperature. By the same token, measuring only wavelength shift makes it hard to distinguish the two effects.

Several different types of magnetic sensors have been reported based on the Faraday effect, the Lorentzian force, and magnetostrictive effects.6–8 Most of these approaches, too, are based on detecting shifts in wavelengths and thus also suffer from cross-sensitivity. Here we describe a new approach based on monitoring the polarization properties of light in FBGs that attempts to get around this problem.

The polarization state of an electromagnetic wave is described by three normalized Stokes parameters (s1, s2, s3). A magnetic field incident on an optical fiber will induce a circular birefringence (i.e., different refractive index) for the optical component traveling parallel with the field. The birefringence causes the right and left circularly polarized light to undergo different couplings through the grating. At a certain wavelength, the amplitudes of the polarizations are unequal. Consequently, their combination results in elliptical polarization, and the Stokes parameters, azimuth angles, and ellipticities are wavelength-dependent. The amplitudes of these parameters are linked to magnetic field strength. The Faraday magneto-optic effect can be enhanced by means of rare-earth-doped FBGs or writing FBGs on yttrium iron garnet (YIG) fibers,9 so-called magneto-optic fiber Bragg gratings (MFBGs).

Figure 1. (a) Wavelength dependence of power transmission and the third normalized Stokes parameters (s3). Evolution of s3 peak amplitudes as a function of the magneto-optic-to-grating coupling ratio kr. For bismuth yttrium iron garnet (Bi-YIG) and YIG materials, the magnetic field strengths corresponding to kr=1are 6250 (A/m) and 73,770 (A/m), respectively, with index modulation of 0.6×10−4. A/m: Ampere/meter. T: Total transmission.

Our simulations show that the Faraday effect induces mode conversion between light components having the same direction.10 Figure 1(a) shows the wavelength dependency of total transmission T and the third normalized Stokes parameter s3. Here, the parameter kr, known as the magneto-optic-to-grating coupling ratio (MGR),11 represents the magnetic field. For small magnetic field values, the Faraday effect is difficult to detect in the transmitted spectrum, whereas the amplitude of s3 is clearly linked to the magnetic field. As the field increases, the original single main rejection band (area with power transmission <1) presents two main lobes, and the s3 maximum amplitude tends toward a saturated value. Figure 1(b) shows the evolution of s3 peak amplitudes as a function of kr. For kr values less than 1.2, the maximum amplitude increases monotonically, which means that s3 evolutions could be used to derive the magnetic field values. The sensitivity and measurement range of the sensor depend on the physical parameters and material characteristics of the fiber grating.

Figure 2. (a) Evolution of polarization-dependent loss (PDL) curves and (b) maximum PDL amplitude evolution as a function of magnetic field value.

Polarization-dependent loss (PDL) is defined as the maximum change in the transmitted power when the input state of polarization is varied over all polarization states. Figure 2 shows the influence of the magnetic field on PDL. In Figure 3, we plot the influence of this field on the ellipticity of transmitted light. The evolutions are similar to those of s3 and can also be used to sense the magnetic field.

Figure 3. (a) Evolution of ellipticity curves and (b) maximum ellipticity amplitude evolution as a function of magnetic field value.

Figure 4. Schematic diagram of magnetic field sensor measurement system. MFBG: Magneto-optic fiber Bragg grating. PC: Polarization controller.

Figure 4 shows a schematic diagram of this type of sensor. A polarization controller (PC) is used to modify the polarization state of the light emitted by a light source such that the state at the input of the MFBG is linear. A longitudinal magnetic field is applied to the fiber grating. The signal transmitted from the grating is directed to a polarization analyzer, which characterizes the signal's polarization properties. These properties contain information about the magnetic field.

We are currently working toward improving our device by increasing its sensitivity for applications related to environmental monitoring. Future plans include developing a special grating structure for pulsed magnetic field measurements and building a setup for low-cost, high-speed polarization monitoring. At the same time, we are working on building an FBG multiparameter sensor based on evolution of both polarization and wavelength.

This research was supported by the National Natural Science Foundation of China under grant 60871075 and a China Postdoctoral Science Foundation funded project (20090451500).

Yang Su, Baofu Zhang, Yong Zhu, Yuquan Li
Institute of Communications Engineering
PLA University of Science and Technology
Nanjing, China

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