Bend Induced Loss
Bend induced loss (BIL) is the loss of light due to bending a fiber. When a fiber is bent or coiled, the light prefers to carry on in a straight line so tends to want to shoot right out the cladding at a bend. The makes the light more weakly confined in a coiled fiber than a straight one, causing some degree of loss. To get round this the fiber is designed to confine energy more tightly in the core, reducing the light that escapes at the bend. The way to do this is to increase the NA and bring the cut-off up to just below the operating wavelength - a smaller MFD tells you the confinement is greater, so generally speaking the bend loss reduces going from an MFD of, for example, 6.4 µm to 5.3 µm to 4.2 µm at a given wavelength in fibers of increasing NA. The light in the core is largely unaffected by how thick the cladding is, so bend induced loss (BIL) is the same for an 80 µm or a 125 µm fiber.