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

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

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

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

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

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

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

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

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

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