Macro bending losses occur when the fiber cable is subjected to a significant amount of bending above a critical value of curvature. Such losses are also called as large radius losses.
- 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 claddingand hence the guidance mechanism is inhibited, which causes light energy to be radiated from the fiber.
- The part of the mode which is on the outside of the bend is required to travel faster than that on the inside so thata wave front 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 αr=C1exp(−C2R)
Where R is the radius of curvature of the fiber bend and C1,C2 are constants which are independent of R.
In case of multimode fibers, at a particular value of radius of curvature; large bending losses occur.
Furthermore, large bending losses tend to occur in multimode fibers at acritical radius of curvature Rc which may be estimated from
a. Designing fibers with large relative refractive index differences;
b. Operating at the shortest wavelength possible.
An illustration of the radiation loss at a fiber bend is as shown in figure 2.12.
The part of the mode in the cladding outside the dashed arrowed line may be required to travel faster than the velocity of light in order to maintain a plane wave front. Since it cannot do this, the energy contained in this part of the mode is radiated away.
Minimize micro bending losses:
To minimize the losses following precautions are taken
- While manufacturing the cable; a precise control of core diameter is maintained.
- A compressible jacket is fitted over the fiber, so that when the external pressure is applied then the deformation of jacket takes place and there will not be creation of micro bends in the core layer of fiber.
- To avoid deterioration in the optical fiber transmission characteristics resulting from mode-coupling-induced microbending, it is important that the fiber is free from irregular external pressure within the cable.
- Carefully controlled coating and cabling ofthe fiber is therefore essential in order to minimize the cabled fiber attenuation.
- Furthermore, the fiber cabling must be capable of maintaining this situation under all the strain and environmental conditions envisaged in its lifetime.
- Minimal losses were predicted at operating wavelengths below 1.3 μm, with a rapid rise in attenuation at wavelengths above 1.5 μm.