Chapter 4: Optical fibers and their parameters

Graphic representation of three different types of how the refractive index change in the core of an optical fiber.

Modes

Graph of the Bessel function.

LP01 mode - the fundamental mode and cut-off

Numerical aperture

The acceptance cone of a fiber.

Group refractive index

Refractive index profile for a primary coated fiber including the refractive index for the acrylate. Diagram shows the refractive index and group refractive index versus wavelenght used.

Dispersion

Intermodal dispersion

Because the different modes follow different paths through the fiber, a light pulse is broadened in proportion to the length of the fiber.

Intramodal dispersion or chromatic dispersion

The chromatic dispersion is the sum of material- and waveguide dispersion.

Intramodal dispersion or chromatic dispersion

Pulse broadening through dispersion. In single-mode fiber, intramodal and PMD dispersion occurs; in multi-mode fiber, modal dispersion causes the greatest amount of pulse broadening.

Polarization mode dispersion, PMD

Pulse broadening through polarization mode dispersion, PMD.

Nonlinear effects

Because of the three evenly spaced wavelengths (channels) λ1, λ2 and λ3 in this example, some of the newly

generated signals occur at the original wavelengths.

Multimode fiber with rectangular index profile

Energy path in a step index, multimode fiber. Note that the angle (90 - α) < β.

Multimode fiber with graded index profile

Ray path in a graded index multimode fiber.

Standard single-mode fiber with rectangular indexprofile

Energy path in an ideal single-mode fiber.

Chromatic dispersion in a standard single-mode fiber for the interval 1150 – 1600 nm.

Dispersion shifted fibers

Standard dispersion shifted fiber

Chromatic dispersion for a dispersion shifted fiber and the refractive index profile.

Non-zero dispersion shifted fiber

Graph showing the different types of non-zero dispersion fibers compared with a dispersion shifted fiber (red), included is also the refractive index profile.

Graph showing the chromatic dispersion in a non-zero dispersion shifted fiber and a dispersion shifted fiber. New techniques have opened two new windows, (4 and 5) for WDM. Window 3 is traditionally used for DWDM.

Fiber with a continuous usable bandspectrum from 1285 nm to 1625 (1700) nm.

Attenuation versus wavelength for the fiber with reduced “water peak”. The fiber may be used continuously for 1285–1625 nm.

Dispersion-compensating fiber

The dispersion compensating modules can be used at the beginning, in the middle or at the end of a transmission link. They can preferable be combined with en EDFA.

The evolution of the optical fiber.