Birefringent optical fiber

For measuring based on the principle of polarization modulation, one of the essential conditions is using fibers which make it possible to preserve the waves' states of polarization while they propagate. In theory, standard fibers should preserve the state of polarization but in practice, a wave which enters with a definite state of polarization will come out after having been through some centimeters with a completely random state of polarization, notably because of micro-disturbances which appear during the making. That is why people use birefringent optical fibers which have two main axes. These axes are called fast axis and slow axis, depending on how fast the waves propagate on each one. A guided polarized wave travelling on the axis which has a high refractive index (i.e. slow axis) will run more slowly than a guided polarized wave running on the other axis (i.e. fast axis).

The birefringence of a fiber is given by the difference between the refractive indexes of the two axes:

where ns and nf are respectively the refractive indexes of the fast and fast axes. The birefringence is generally defined as the beat time and LB, represents the length of fiber necessary for the phase difference between the two orthogonal axes to be   :

If the fiber is subjected to mechanical disturbances whose periods are similar to LB ,then there will be a strong coupling between both orthogonal polarizations. When making those fibers, such disturbances can appear that is why manufacturers pay a particular attention to it in order to avoid the fiber being curved or twisted while stretching it. The polarization will be maintained if the length of the beat time is shorter than about ten centimeters. If the light coming in have its polarization aligned on one of the main axes, it will be guided without any variation of its state of polarization.

There are two main methods to get birefringence in optical fibers:

1. 1.You can modify the core's guidance characteristics by adultering its geometry, for example by breaking its circular symmetry. We can mention the elliptical core fiber (cf Figure 14) which have some millimeters-long beat time length [41].

2. 1.Another possibility is to apply an asymmetrical strain on the core of the fiber, to get at the same time an asymmetry of the index profile. You can accomplish it by introducing highly doped areas around the core (cf Figure 15).

The bow-tie fiber has the higher birefringence value with 0.5mm. Recently with the development of microstructured fibers, new birefringent fibers have appeared with more or less possibilities regarding the birefringence values and especially the guidance characteristics (unimodality area, dispersion curve...).