For coherent light sources, the intensity profile exhibits speckle that arise due to interference between the multiple modes.
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.
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: