Special features of Multi-mode fibers

Special features of multi-mode fibers

Typical Spectra for UV/VIS or VIS/NIR Multi-Mode Fiber Cables

Most of the multi-mode fibers from Schäfter+Kirchhoff are ­offered in a UV/VIS (High OH-) and in a VIS/NIR (low OH-) version. They have a different attenuation profile due to their different concentrations of OH- groups. OH- groups cause attenuation at IR wavelengths but they are beneficial for UV transmission.

Attenuation of UV/VIS and VIS/NIR multi-mode fibers

UV/VIS fibers should be used for transmitting wavelengths in the UV and VIS,  VIS/NIR fibers are suitable for transmitting wavelengths in the VIS and NIR range.

Graded index vs. step index in Multi-Mode Fibers

Graded index vs. step index in Multi-Mode Fibers

While common single-mode fibers have a step-index profile for the refractive index, there are two types of multi-mode ­fibers: step-index and graded-index (gradient-index) . 

Step index

Step-index fibers have a step profile with one refractive ­index n2 for the core and one for cladding (refractive index n1) throughout the fiber. The core diameter of a ­multi-mode fiber is rather large (>50 µm), allowing multiple modes of light guidance.  

Graded index (Gradient index)

A graded-index fiber exhibits a gradual profile (almost ­parabolical in shape) for the refractive index, which ­results in a smaller modal dispersion because of the ­approximately sinusodial beam propagation along the ­fiber.

Intensity profile of multi-mode fibers

The intensity profile of a multi-mode fiber strongly depends on the type of radiation input.
For coherent light sources, the intensity profile exhibits speckle that arise due to interference between the ­multiple modes.

Intensity profile: Multi-mode fiber with coherent light source

Typical speckle due to interference between the multiple modes.

intensity Profile: Multi-mode fiber with low coherent light source

No speckle effect on the intensity profile.

Coupling, Collimating and Focusing mult-mode fibers

Coupling, collimating and focusing multi-mode fibers are described in detail here: