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Intermodal Dispersion

Intermodal Dispersion . When an optical pulse is launched into a fiber, the optical power in the pulse is distributed over all of the modes of the fiber . Each of the modes that can propagate in a multimode fiber travels at a slightly different velocity .

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Intermodal Dispersion

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  1. Intermodal Dispersion • When an optical pulse is launched into a fiber, the optical power in the pulse is distributed over all of the modes of the fiber. • Each of the modes that can propagate in a multimode fiber travels at a slightly different velocity. • This means that the modes in a given optical pulse arrive at the fiber end at slightly different times, thus causing the pulse to spread out in time as it travels along the fiber. • This effect is known as intermodal dispersion Prepared by Edmond Fernandes

  2. Intermodal Dispersion • It can be reduced by using a graded-index profile in the fiber core. • This allows graded-index fibers to have much larger band-widths (data rate transmission capabilities) than step-index fibers. • Even higher bandwidths are possible in single-mode fibers, where intermodal dispersion effects are not present. Prepared by Edmond Fernandes

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  4. Single – Mode Step-index fiber Advantages • There is a minimum dispersion because all rays propagating down the fiber take approximately the same path; they take approximately the same amount of time to travel down the cable. • Consequently, a pulse of light entering the cable can be reproduced at the receiving end very accurately. Prepared by Edmond Fernandes

  5. Single – Mode Step-index fiber Advantages • Because of the high accuracy in reproducing transmitted pulses at the receive end, larger bandwidths and higher information transmission rates are possible with single-mode step-index fibers than with the other types of fibers. Prepared by Edmond Fernandes

  6. Single – Mode Step-index fiber Disadvantages • Because the central core is very small, it is difficult to couple light into and out of this type of fiber. The source-to-fiber aperture is the smallest of all the fiber types. • Again, because of the small central core, a highly directive light source such as a laser is required to couple light into a single-mode step-index fiber. • Single-mode step-index fibers are expensive and difficult to manufacture. Prepared by Edmond Fernandes

  7. Multimode Step-index fiber Advantages • Multimode step-index fibers are inexpensive and simple to manufacture. • It is easy to couple light into and out of multimode step-index fibers; they have a relatively large source-to-fiber aperture. Prepared by Edmond Fernandes

  8. Multimode Step-index fiber Disadvantages • Light rays take many different paths down the fiber, which results in large differences in their propagation times. Because of this, rays traveling down this type of fiber have a tendency to spread out. • Consequently, a pulse of light propagating down a multimode- step-index fiber is distorted more than with the other types of fibers. Prepared by Edmond Fernandes

  9. Multimode Step-index fiber Disadvantages • The bandwidth and rate of information transfer possible with this type of cable are less than the other types. Prepared by Edmond Fernandes

  10. Multimode graded-index fiber • Essentially, there are no outstanding advantages or disadvantages of this type of fiber. • Multimode graded-index fibers are easier to couple light into and out of than single-mode step-index fibers but more difficult than multimode step-index fibers. Prepared by Edmond Fernandes

  11. Multimode graded-index fiber • Distortion due to multiple propagation paths is greater than in single-mode step-index fibers but less than in multimode step-index fibers. • Graded-index fibers are easier to manufacture than single-mode step-index fibers but more difficult than multimode step-index fibers. • The multimode graded-index fiber is considered an intermediate fiber compared to the other types Prepared by Edmond Fernandes

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