2.1         Optical fiber

2.1.2        Types of optical fibers

The optical fibers can be classified as a function of the number of modes supported at a given wavelength :

-         Single-mode fiber : only the fundamental mode is possible, with two orthogonal polarizations admitted

-         Multi-mode fiber : several modes are supported by the waveguide, each one exhibiting a different field distribution and propagation constant

Standard single-mode fibers are of step-index type, that is, there is a discontinuity of refractive index between the core and the cladding. For single mode operation in the range 1300 - 1550 nm, the fiber core has a diameter between 4 and 9 mm (the fiber cladding diameter is 125 mm for fibers used in telecommunications). Multi-mode fibers are sometimes of step-index type but often they are of graded index type. In graded index fibers, the refractive index varies continuously between the core and the cladding in order to compensate the modal dispersion in multimode fibers (the shortest path has the highest index, and therefore the propagation velocity will be slower for the light following this path than for the zigzag rays propagating in lower-index regions).

Fig. 2-2 Polarization maintaining fiber of bow-tie type (left) and main regions (right)

Single-mode fibers with a cylindrical symmetry can be considered as two modes structures, since two orthogonal polarizations are permitted. Both modes are degenerated. Small perturbations in the fiber geometry or material properties (structural or induced by external conditions) lead to birefringence that changes the velocity of both polarization modes and removes the degenerency. Due to nearly identical propagation constants, important cross-talk between the modes induces non negligible polarization mode dispersion (PMD). To avoid this dispersion, polarization-maintaining (PM) fibers are used where both modes have well separated propagation constants. The separation is obtained by modifying the fiber geometry (elliptical core) or by inducing a refractive index anisotropy in the transverse plane of the fiber (birefringence induced by stress applying regions inside the cladding). For example in this work, we have used PM fibers of the bow-tie type. A micrograph of the cross-section of this fiber is presented in the left part of Fig. 2-2. The schematic drawing in the right picture fits precisely the left micrograph.

The bow-tie region is made of borosilicate. The silica and the borosicate glasses have different thermal expansion coefficient and then, residual stresses are created into the fiber perform during the cooling-down processus. Typical effective refractive index differences of 4×10^-4 are observed for our bow tie PM fiber. We observe in Fig. 2-2 that the core geometry is also modified, as it appears elliptical.