Different methods have already been developed to measure the magnetic field with fiber Bragg gratings (FBGs). They are based on the use of a magnetic fluid or magnetostrictive materials. In addition to these methods, a direct measurement of the magnetic field is also possible by determining the circular birefringence created by the magnetic field inside the fiber. In standard optical fiber, this circular birefringence is of the same order as the intrinsic fiber birefringence or even below. The polarization properties of FBGs are therefore used to perform such measurement since they allow to determine weak birefringence with higher accuracy than standard read-out techniques. However, the obtained accuracy is usually low due to the influence of the intrinsic fiber birefringence. To mitigate this issue, we study in this work the use of the diattenuation vector. This parameter is obtained from the Mueller matrix and we show that it evolves in response to a magnetic field. In practice, we analyze its response by both simulation and experiment. In our simulations, we solve numerically the coupled mode equations of the FBG. For the experiments, the Mueller matrix is measured by an optical vector analyzer for the gratings connected in transmission. We apply an increasing magnetic field on different Bragg gratings photo-written in SMF28 fibers. The rotation of the diattenuation vector is then used to retrieve the magnetic field induced circular birefringence. A linear increase of the reconstructed circular birefringence is reported for increasing magnetic field values in the range 0-1T.
The application of transversal strain on an optical fiber leads to an asymmetry of the fiber section that locally induces
fiber birefringence. In this paper, we show that it is possible to use FBG as a probe to obtain the amount of this induced
birefringence. We describe two techniques able to get the birefringence value. They are both based on differential group
delay (DGD) measurement. The first technique makes use of the comparison between the measurement and the
simulation of the wavelength dependency of grating DGD. Considering that all grating parameters are known, we adjust
the birefringence value to obtain the best fit between experiment and simulation. For the second technique, we first
demonstrate that the wavelength dependencies of chromatic dispersion (CD) and DGD parameters differ in their
amplitude by a quantity proportional to the birefringence. We exploit this relationship to get the birefringence value by
separately measuring the DGD and CD spectral evolutions. The birefringence value is adjusted to obtain the best fit
between the two measurements. These two techniques are discussed and experimental results are finally reported.
Birefringence in fiber Bragg gratings (FBGs) can result from two distinct effects that combine with the intrinsic fiber
birefringence: the birefringence induced by the UV photo-writing and the birefringence due to a transversal load. In both
cases, it leads to polarization dependent loss (PDL) and differential group delay (DGD) inside gratings. Although these
parameters are not desired for optical telecommunications applications, we demonstrate here that they can be
advantageously used to obtain temperature-insensitive transverse strain measurements with uniform FBGs written into
standard single mode fiber, which is not possible through conventional amplitude spectral measurements. Simulated
results obtained by means of the coupled mode theory and the Jones formalism as well as experimental results measured
by means of a tunable laser source and a polarimeter are presented.
Birefringence in fiber Bragg gratings can result from two distinct effects that combine with the intrinsic fiber birefringence: the birefringence induced by the UV photo-writing and the birefringence due to a transversal load. In both cases, it leads to polarization dependent loss and differential group delay inside gratings. This paper aims to provide a characterization, both theoretically and experimentally, of the polarization dependent loss and the differential group delay generated by uniform Bragg gratings written into single mode optical fibers. We demonstrate that the measured polarization properties can be accurately reconstructed by means of the coupled mode theory and the Jones formalism. We also demonstrate that the PDL and DGD evolutions contain information about birefringence and can thus be used for transverse force sensing purposes. Experimental results obtained on fiber Bragg gratings transversally loaded by an external force confirm the simulated evolutions.
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