bend-insensitive multimode fiber (BIMMF)
When laying optical fibers, the specifications for the bending radius are extremely important because certain FO characteristics such as attenuation deteriorate if the specifications are not met. For corresponding applications, there are gradient fibers with an extremely small bending radius: bend-insensitive fibers, Bend-Insensitive Multimode Fiber (BIMMF).
To improve bend radii and maintain standardized specifications, manufacturers rely on an innovative core design that has an additional glass cladding of the core glass of a few microns. This additional glass layer is called a trench, refractive index trench or separation zone and has a much lower refractive index than the cladding glass. As a result, modes with a wavelength of 850 nm, which are less strongly guided in the classic multimode fiber, are more strongly refracted and reflected towards the core glass; even in the case of strong bending. This measure means that characteristic values such as attenuation or numerical aperture are hardly affected in multimode fibers, so that OM classes are still met and full compatibility with installed optical fibers is ensured. Since manufacturers use different core designs, there are also differences in transmission performance.
For example, Bend-Insensitive Multimode Fiber (BIMMF) uses a refractive index for the core design in gradient fibers at which modes cannot escape from the core glass even at a tight bend radius. At the same time, the design ensures important design parameters such as the core diameter, numerical aperture and bandwidth.
As for the bending radius in general, a rule of thumb is that the smallest bending radius should be ten times the cable diameter. Accordingly, a 5 mm thin FO cable would have a smallest bending radius of 5 cm. The modified core design achieves bending radii between 2.5 mm for single- mode fibers and 10 mm for multimode fibers.
In addition to bend-insensitive gradient fibers, there are also bend-insensitive monomode fibers. These are specified in the ITU standardG.657A.