Fiber bend loss
Optical fibers suffer radiation losses at bends or curves on their paths. This is due to the energy in the evanescent field at the bend exceeding the velocity of light in the cladding and hence the guidance mechanism is inhibited, which causes light energy to be radiated from the fiber. An illustration of this situation is shown in Figure 2.5. The part of the mode which is on the outside of the bend is required to travel faster than that on the inside so that a wavefront perpendicular to the direction of propagation is maintained.
Hence, part of the mode in the cladding needs to travel faster than the velocity of light in that medium. As this is not possible, the energy associated with this part of the mode is lost through radiation. The loss can generally be represented by a radiation attenuation coefficient which has the form:
Where R is the radius of curvature of the fiber bend and c1, c2 are constants which are independent of R. Furthermore, large bending losses tend to occur in multimode fibers at a critical radius of curvature Rc which may be estimated from:
It may be observed from the expression given in Eq. (2.8) that potential macrobending losses may be reduced by:
ü designing fibers with large relative refractive index differences;
ü operating at the shortest wavelength possible.
The above criteria for the reduction of bend losses also apply to single-mode fibers. One theory, based on the concept of a single quasi-guided mode, provides an expression from which the critical radius of curvature for a single-mode fiber Rcs can be estimated as
where λc is the cutoff wavelength for the single-mode fiber. Hence again, for a specific single-mode fiber (i.e. a fixed relative index difference and cutoff wavelength), the critical wavelength of the radiated light becomes progressively shorter as the bend radius is decreased.