Optical fibers suffer from macro-bending loss 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.
This is shown in the following illustration.
The part of 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.
So part of the mode in the cladding needs to travel faster than the velocity of light in that medium. Because this is impossible, the energy associated with this part of the mode is lost through radiation.
Thus it is absolutely critical that sharp bends, with a radius of curvature approaching the critical radius, are avoided when optical fiber cables are installed.
Of course, it is very important that the macro-bending with radius of curvature approximating to the fiber radius are not produced in the fiber cabling process. These macrobends can cause significant losses from cabled fibers.