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The limit towards smaller wavelengths is given by the single-mode cut-off wavelength, beyond which the fiber supports multiple modes. Typically, a fiber has single-mode characteristics only over a limited wavelength range with a width of a few hundred nanometers. This fiber supports four modes, disregarding different polarization states. Mode functions of a multimode step-index fiber, having the same index contrast as above, but a larger core radius of 10 μm. This leads to a V number of 1.91 at a wavelength of 1 μm. The refractive index change is 0.002 in that case, and the core radius is 4 μm. Mode function of a single-mode step-index fiber. This requires that the core radius is small, particularly for fibers with high NA. Conditions for Single-mode Guidanceįor step-index fibers, the condition for single-mode guidance can be formulated using the V number (normalized frequency), which can be calculated from the wavelength, the core radius, and the numerical aperture (NA): the V number must be below ≈ 2.405. This has a core diameter of 8.2 μm and a numerical aperture of 0.14. This is an important advantage for the application in optical fiber communications at high data rates (multiple Gbit/s), particularly for long distances.Įssentially for that reason, and partly because of their tentatively lower propagation losses, single-mode fibers are exclusively used for long-haul data transmission, and nearly always for outdoor applications even over shorter distances.įor short-distance indoor use, multimode fibers are more common, mostly because that allows the use of cheaper multimode data transmitters based on light-emitting diodes instead of laser diodes.Īs a standard single-mode fiber for use in optical fiber communications in the 1.3-μm or 1.5-μm wavelength region, the SMF-28 of Corning (or the enhanced version SMF-28e) is common. Intermodal dispersion can of course not occur in single-mode fibers. The launch conditions only influence the efficiency with which light can be coupled into the guided mode. However, there are also large mode area fibers with a relatively large mode area.Ī peculiar property of single-mode fibers is that the transverse intensity profile at the fiber output has a fixed shape, which is independent of the launch conditions and the spatial properties of the injected light, assuming that no cladding modes can carry substantial power to the fiber end. Single-mode fibers usually have a relatively small core (with a diameter of only a few micrometers) and a small refractive index difference between core and cladding the mode radius is typically a few microns.
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then do not exist – only cladding modes, which are not localized around the fiber core. Higher-order modes like LP 11, LP 20 etc. Single-mode fibers (also called monomode fibers) are optical fibers which are designed such that they support only a single propagation mode ( LP 01) per polarization direction for a given wavelength.
#SM CUT OFF WAVELENGTH HOW TO#
How to cite the article suggest additional literature Using our ad package, you can display your logo and further below your product description.ĭefinition: optical fibers supporting only a single guided mode per polarization direction