What is Optical Fiber Dispersion?

February 8, 2009
By Colin Yao

Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion in optical fiber includes model dispersion, material dispersion and waveguide dispersion. Each type is discussed in detail below.

optical-fiber-dispersion

Model Dispersion in Multimode Fibers

Multimode fibers can guide many different light modes since they have much larger core size. This is shown as the 1st illustration in the picture above. Each mode enters the fiber at a different angle and thus travels at different paths in the fiber.

Since each mode ray travels a different distance as it propagates, the ray arrive at different times at the fiber output. So the light pulse spreads out in time which can cause signal overlapping so seriously that you cannot distinguish them any more.

Model dispersion is not a problem in single mode fibers since there is only one mode that can travel in the fiber.

Material Dispersion

Material dispersion is the result of the finite linewidth of the light source and the dependence of refractive index of the material on wavelength.  It is shown as the 2nd illustration in the first picture.

Material dispersion is a type of chromatic dispersion. Chromatic dispersion is the pulse spreading that arises because the velocity of light through a fiber depends on its wavelength.

The following picture shows the refractive index versus wavelength for a typical fused silica glass.

refractive-index-versus-wavelength-in-silica-glass-fiber

Waveguide Dispersion

Waveguide dispersion is only important in single mode fibers. It is caused by the fact that some light travels in the fiber cladding compared to most light travels in the fiber core. It is shown as the 3rd illustration in the first picture.

Since fiber cladding has lower refractive index than fiber core, light ray that travels in the cladding travels faster than that in the core. Waveguide dispersion is also a type of chromatic dispersion.  It is a function of fiber core size, V-number, wavelength and light source linewidth.

waveguide-dispersion

While the difference in refractive indices of single mode fiber core and cladding are minuscule, they can still become a factor over greater distances. It can also combine with material dispersion to create a nightmare in single mode chromatic dispersion.

Various tweaks in the design of single mode fiber can be used to overcome waveguide dispersion, and manufacturers are constantly refining their processes to reduce its effects.