Multimode Fibers | previous | next | feedback |
Definition: fibers supporting more than one guided mode per polarization direction
Figure 1: A single-mode fiber (left-hand side) has a core which is very small compared with the cladding, whereas a multimode fiber (right-hand side) can have a large core.
Multimode fibers are optical fibers which support multiple transverse guided modes for a given optical frequency and polarization. The number of guided modes is determined by the wavelength and the refractive index profile. For step-index fibers, the relevant quantities are the core radius and the numerical aperture, which in combination determine the V number. For large V values, the number of modes is proportional to V2. Particularly for fibers with a relatively large core (right-hand side in Figure 1), the number of supported modes can be very high. Such fibers can guide light with poor beam quality (e.g. generated with a high-power diode bar), but for preserving the beam quality of a light source with higher brightness it can be better to use a fiber with smaller core and moderate numerical aperture, even though efficient launching can then be somewhat more difficult.
Compared with standard single-mode fibers, multimode fibers usually have significantly larger mode areas, but also tentatively a higher numerical aperture of e.g. 0.2 to 0.3. The latter leads to robust guidance, even under conditions of tight bending, but also to higher propagation losses. The refractive index profile is usually rectangular (→ step-index fibers), but sometimes parabolic.
Launching light into a multimode fiber is comparatively easy, because there are larger tolerances concerning the location and propagation angle of incident light, compared with a single-mode fiber. On the other hand, the spatial coherence of the fiber output is reduced, and the output field pattern can be hardly controlled for reasons as explained below.
Figure 2: Electric field contour lines for all the guided modes of an optical fiber. In general, light launched into a multimode fiber will excite a superposition of different modes.
Figure 2 shows the electric field profiles of the guided modes of a step-index fiber, as calculated for one particular wavelength. There is a fundamental mode (LP01) with an approximately Gaussian intensity distribution, and a number of higher-order modes with more complicated spatial profiles. Each mode has a different propagation constant β. Any guided field distribution can be considered as a superposition of the guided modes.
Figure 3: Intensity pattern, as it evolves within a multimode fiber.
The total electric field distribution anywhere in a multimode fiber is a superposition of contributions from the different modes. The intensity profile depends not only on the optical powers in all the modes, but also on the relative phases, and there can be constructive or destructive interference of different modes at particular locations in the fiber. Both the powers and optical phases are initially determined by the launching conditions, and the relative phases evolve further due to the mode-dependent propagation constants. Therefore, the more or less complicated intensity pattern changes all the time, typically with significant changes occurring within a propagation length of well below a millimeter. Figure 3 shows an example in the form of animated graphics, where the different frames represent intensity distributions occurring at intervals of 2 μm. These interference conditions are also strongly affected by any change of bending or stretching of the fiber, and also by the temperature.
Note that for light with a broad optical bandwidth (e.g. for a white light source) such complicated intensity patterns are not observed, if only the intensity is detected without distinguishing different spectral components. This is because the shape of the intensity pattern is different for each wavelength component, so that contributions from different wavelengths are averaged out. The longer the fiber, the lower is the required optical bandwidth to achieve this averaging.
Due to intermodal dispersion, the group velocity depends on the mode, and ultrashort pulses propagating in a multimode fiber may be split into several pulses with different arrival times. This effect limits data rates and/or transmission distances achievable in optical fiber communications and thus basically excludes multimode fibers for long-haul transmission, even though intermodal dispersion can be reduced with a parabolic index profile. On the other hand, multimode fibers can be more convenient for shorter distances, because the demands on light sources and component alignment are lower.
Materials and Fabrication
Various materials can be used for multimode fibers. The most common multimode glass fibers are silica fibers, where either the core has additional doping e.g. with germania for increasing the refractive index, or a pure silica core is surrounded by a region which is doped with some index-lowering agent (e.g. fluorine). Particularly for large core fibers, the plasma outside deposition (POD) method allows the efficient fabrication of such "depressed index claddings".
There are also modified glasses e.g. for guiding light with longer wavelengths, and polymers (→ polymer optical fiber, POF). Both require adapted fabrication techniques.
Another possibility is to use photonic crystal fibers (PCF), which can e.g. have an air cladding to achieve a very high numerical aperture.
ITU Standards for Multimode Fibers
The International Telecommunications Union (ITU) has developed a number of standards for various types of fibers as used for optical fiber communications. The standards for multimode fibers are:
| Name | Title |
|---|---|
| G.651 (02/98) | Characteristics of a 50/125 μm multimode graded index optical fibre cable |
| G.651.1 (07/07) | Characteristics of a 50/125 μm multimode graded index optical fibre cable for the optical access network (pre-published) |
Applications
Some examples of applications of multimode fibers are:
- Multimode fibers are used for the transport of light from a laser source to the place where it is needed, particularly when the light source has a poor beam quality and/or the high optical power requires a large mode area. For example, light from stationary high-power lasers of various types can be sent to material processing stations for cutting or welding, where e.g. a robot moves a laser head mounted at the end of a fiber cable. Also, fiber-coupled high-power diode bars and diode stacks use multimode fibers, as their beam quality is far from diffraction-limited. Fiber coupling is useful because it allows e.g. to separate the pump diodes and their cooling arrangement from the laser head of a diode-pumped solid-state laser. However, fiber-coupled laser diodes are more expensive, and depending on the used beam shapers there can be some significant loss of brightness.
- Some high-power fiber amplifiers are based on multimode fibers, because these can have larger mode areas (→ large mode area fibers). It is often possible to obtain a nearly diffraction-limited output by launching into the fundamental mode and by minimizing mode mixing.
- Multimode fibers are also attractive for short-range optical fiber communications. For short distances, intermodal dispersion is not relevant, and the easier launching conditions are greatly relaxing the tolerances. For not too high data rates, it is even possible to use spatially incoherent light sources such as light emitting diodes (LEDs). Higher data rates are achieved with VCSEL transmitters, particularly when the multimode fiber has an optimized refractive index profile.
Bibliography
| [1] | A. W. Snyder and J. D. Love, "Optical waveguide theory", London: Chapman and Hall, 1983 |
| [2] | Standards of the International Telecommunication Union (ITU), see http://www.itu.int/ |
See also: fibers, single-mode fibers, fiber-coupled diode lasers, photonic crystal fibers, double-clad fibers, V number, intermodal dispersion, waveguides
Category: fibers and other waveguides
