Large-core Multimode Fibers
Large-core multimode fibers have a core diameter which is well above the typically used 50 μm or 62.5 μm of telecom fibers – for example, 100 μm or even 400 μm. The core is often not much smaller than the fiber cladding (see Figure 1). The cladding diameter may substantially go beyond the usual 125 μm in order to realize even larger cores. The number of guided fiber modes is then usually very high.
Frequently, such fibers have a pure silica fiber core (rather than e.g. a germanosilicate core) and a fiber cladding with somewhat reduced refractive index, e.g. achieved with fluorine doping. For realizing a high numerical aperture, however, one may still use fibers with germanosilicate core.
Large-core Single-mode Fibers
If a large-core fiber is a single-mode fiber, it also has a large effective mode area. Here, the term large mode area fiber is more common and more appropriate, since the large mode area is a particularly important property: it results in reduced nonlinear effects combined with a high beam quality.
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