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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-15

Definition: optical fibers supporting only a single guided mode per polarization direction

Single-mode fibers (also called monomode fibers) are optical fibers which are designed so that they support only a single propagation mode per polarization direction for a given wavelength. They 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.

For 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 [1]. This requires that the core radius is small, particularly for fibers with high NA.

Figure 1: Mode function of a single-mode step-index fiber. The refractive index change is 0.002 in that case, and the core radius is 4 μm. This leads to a V number of 1.95 at a wavelength of 1 μm.

Figure 2: Mode functions of a multimode step-index fiber, having the same index contrast as above, but a larger core radius of 10 μm. The V number is 4.87. This fiber supports 4 modes, disregarding different polarization states.

Single-mode propagation is easy to recognize in an experiment: changing the launch conditions only affects the launched power, whereas the spatial distribution of the light exiting the fiber is fixed.

Efficiently launching light into a single-mode fiber usually requires a laser source with good beam quality and precise alignment of the focusing optics in order to achieve mode matching. This means that a beam focus has to be formed at the fiber's input end, and that the beam radius must match that of the fiber's propagation mode.

Typically, a fiber has single-mode characteristics only over a limited wavelength range with a width of a few hundred nanometers. The limit towards smaller wavelengths is given by the single-mode cut-off wavelength, beyond which the fiber supports multiple modes. This transition is very sharp and can easily be seen e.g. when tuning the launched wavelength around the cut-off wavelength: the shape of the transmitted beam varies rapidly in the multimode regime but stays constant in the single-mode regime. The long-wavelengths limit of the useful single-mode region is usually given by excessive bend losses or sometimes by absorption of the material.

ITU Standards for Single-mode Fibers

The International Telecommunications Union (ITU) has developed a number of standards for various types of fibers as used for optical fiber communications. Some of the most important of those standards concerning single-mode fibers are:

Large Mode Area Single-mode Fibers and Effectively Single-mode Fibers

For some applications, single-mode fibers with relatively large core diameters of tens of micrometers (→ large mode area fibers) are required. This can be achieved in different ways, e.g. by making a large core with a small index difference (small numerical aperture), or with a photonic crystal fiber. In general, single-mode fibers with large mode areas tend to be more sensitive to bend losses, compared with multimode fibers, because the guiding is relatively weak.

In some cases, strictly single-mode guidance is not required; it is possible to use effectively single-mode fibers, having a few transverse modes, where however all higher-order modes have relatively high losses, and mode coupling from the fundamental mode to higher modes is weak. Some bending of the fiber is often used to more efficiently suppress higher-order modes, if they exhibit higher bend losses.

Applications

Single-mode guidance is important for many applications. Examples are:

• In fiber lasers and amplifiers, single-mode guidance is the basis for achieving a high beam quality of the output.
• In optical fiber communications systems, single-mode guidance avoids the problem of intermodal dispersion, which (in multimode fibers) would lead to the occurrence of multiple copies of the input signals at the receiver.
• In measurement setups, the fact is often exploited that the output of a single-mode fiber has a fixed spatial shape, independent of the launch conditions.

As a standard single-mode fiber in the 1.3-μm or 1.5-μm wavelength region, the SMF-28 of Corning (or the enhanced version SMF-28e) is common. The mode field diameter is ∼9.2 μm at 1310 nm, or 10.4 μm at 1550 nm. The single-mode cut-off is at 1260 nm.

Bibliography

See also: fibers, numerical aperture, V number, mode radius, multimode fibers, waveguides, mode matching, effective mode area

Category: fibers and other waveguides