Modal dispersion

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Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes. Other names for this phenomenon include multimode distortion, multimode dispersion, modal distortion, intermodal distortion, intermodal dispersion, and intermodal delay distortion. [1] [2]

Contents

In the ray optics analogy, modal dispersion in a step-index optical fiber may be compared to multipath propagation of a radio signal. Rays of light enter the fiber with different angles to the fiber axis, up to the fiber's acceptance angle. Rays that enter with a shallower angle travel by a more direct path, and arrive sooner than rays that enter at a steeper angle (which reflect many more times off the boundaries of the core as they travel the length of the fiber). The arrival of different components of the signal at different times distorts the shape. [3]

Modal dispersion limits the bandwidth of multimode fibers. For example, a typical step-index fiber with a 50  μm core would be limited to approximately 20 MHz for a one kilometer length, in other words, a bandwidth of 20 MHz·km. Modal dispersion may be considerably reduced, but never completely eliminated, by the use of a core having a graded refractive index profile. However, multimode graded-index fibers having bandwidths exceeding 3.5 GHz·km at 850 nm are now commonly manufactured for use in 10 Gbit/s data links.

Modal dispersion should not be confused with chromatic dispersion, a distortion that results due to the differences in propagation velocity of different wavelengths of light. Modal dispersion occurs even with an ideal, monochromatic light source.

A special case of modal dispersion is polarization mode dispersion (PMD), a fiber dispersion phenomenon usually associated with single-mode fibers. PMD results when two modes that normally travel at the same speed due to fiber core geometric and stress symmetry (for example, two orthogonal polarizations in a waveguide of circular or square cross-section), travel at different speeds due to random imperfections that break the symmetry.

Troubleshooting

In multimode optical fiber with many wavelengths propagating, it is sometimes hard to identify the dispersed wavelength out of all the wavelengths that are present, if there is not yet a service degradation issue. One can compare the present optical power of each wavelength to the designed values and look for differences. After that, the optical fiber is tested end to end. If no loss is found, then most probably there is dispersion with that particular wavelength. Normally engineers start testing the fiber section by section until they reach the affected section; all wavelengths are tested and the affected wavelength produces a loss at the far end of the fiber. One can easily calculate how much of the fiber is affected and replace that part of fiber with a new one. Replacement of optical fiber is only required when there is an intense dispersion and service is being affected; otherwise various methods can be used to compensate for the dispersion.

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Dispersion may refer to:

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<span class="mw-page-title-main">Single-mode optical fiber</span> Optical fiber designed to carry only a single mode of light, the transverse mode

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<span class="mw-page-title-main">Transmission medium</span> Conduit for signal propagation

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<span class="mw-page-title-main">Birefringence</span> Property of materials whose refractive index depends on light polarization and direction

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<span class="mw-page-title-main">Multi-mode optical fiber</span> Type of optical fiber mostly used for communication over short distances

Multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus. Multi-mode links can be used for data rates up to 800 Gbit/s. Multi-mode fiber has a fairly large core diameter that enables multiple light modes to be propagated and limits the maximum length of a transmission link because of modal dispersion. The standard G.651.1 defines the most widely used forms of multi-mode optical fiber.

<span class="mw-page-title-main">Fiber Bragg grating</span> Type of distributed Bragg reflector constructed in a short segment of optical fiber

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<span class="mw-page-title-main">Double-clad fiber</span>

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<span class="mw-page-title-main">Fiber-optic communication</span> Transmitting information over optical fiber

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In fiber-optic communication, an intramodal dispersion, is a category of dispersion that occurs within a single mode optical fiber. This dispersion mechanism is a result of material properties of optical fiber and applies to both single-mode and multi-mode fibers. Two distinct types of intramodal dispersion are: chromatic dispersion and polarization mode dispersion.

<span class="mw-page-title-main">Polarization-division multiplexing</span> Method for multiplexing signals

Polarization-division multiplexing (PDM) is a physical layer method for multiplexing signals carried on electromagnetic waves, allowing two channels of information to be transmitted on the same carrier frequency by using waves of two orthogonal polarization states. It is used in microwave links such as satellite television downlinks to double the bandwidth by using two orthogonally polarized feed antennas in satellite dishes. It is also used in fiber optic communication by transmitting separate left and right circularly polarized light beams through the same optical fiber.

<span class="mw-page-title-main">Chromo-modal dispersion</span>

Chromo-modal dispersion (CMD) results from exciting various modes of a multimode waveguide with unique spectral components of a broadband optical signal. Modal dispersion during propagation in the waveguide then provides group velocity dispersion to the signal. The large modal dispersion inherent to multimode waveguides enables the dispersion per unit length of a chromo-modal dispersion device to be several orders of magnitude higher than that of diffraction grating or dispersion compensating fiber-based dispersive elements.

References

  1. ↑ "Multimode distortion". Federal Standard 1037C . August 7, 1996.
  2. ↑ "Multimode distortion". ATIS Telecom Glossary. Retrieved June 29, 2014.
  3. ↑ Mitschke, Fedor (2009). Fiber Optics: Physics and Technology. Springer. p. 20. ISBN   978-3-642-03702-3.
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