What are the Cladding Modes and Leaky Modes?

   As it is known to us, not all light aimed into a optical fiber cable is guided along the core. Some enters the cladding at the end of the fiber; other light escapes from the core by hitting the core-cladding boundary at greater than the confinement angle. This light excites cladding modes, which can propagate in the cladding.
   Total internal reflection at the boundary between cladding and the surrounding material can guide light in the cladding just as it guides light along an unclad fiber. This can happen as long as the surrounding material—air or a plastic coating—has a lower refractive index than the cladding. This might sound like a good way to maximize light transmission, but it's usually undesirable.    It can introduce noise in communication fibers and crosstalk between adjacent fibers in an imaging bundle. To prevent this, manufacturers often coat fibers with a plastic having a higher refractive index than the cladding, so light striking the cladding-coating boundary leaks out. The fibers in rigid bundles sometimes are separated by "dark" glass, which absorbs light so it can't pass between claddings.
   In multimode fibers, the boundary between modes guided in the core and modes confined to the cladding is not sharp. Some light falls into intermediate leaky modes, which propagate partly in both core and cladding. These modes travel much farther than cladding modes but also are prone to leakage and loss.
   Both cladding and leaky modes can lead to spurious results in fiber measurements, so mode strippers have been developed to remove them. These devices work by surrounding part of the fiber with a material having a refractive index equal to or larger than that of the cladding, preventing total internal reflection at the outer boundary of the cladding. Light that leaks into this material is absorbed and lost from the fiber. A long length of optical fiber cable also can serve as a mode stripper if the cladding attenuation is much higher than that of the core.