Cut-off wavelength

Cut-off wavelength of single-mode or PM fibers

Cut-off wavelength

The cut-off wavelength λco is defined as the shortest ­wavelength for which the fiber is single-mode. The mode field can only have a Gaussian intensity distribution and ­rotational symmetry at wavelengths above λco.

If the wavelength of the guided radiation is shorter than the cut-off wa­velength, two or more modes are guided. The beam and intensity profile then differ significantly from a Gaussian distribution. The mode field distribution depends on bending or temperature variations (butterfly effect).

The wavelength range which the fiber can operate (operation range) depends on the fiber parameters and can reach 1.3 times λco . The operating wavelength range of fibers with a pure silica core is smaller.
If the wavelength is longer than 1.3 times λco, the guidance of the radiation becomes increasingly weaker. Even a slight bending of the fiber (as well as micro-bends) result in attenuation of the guided radiation (increased bending loss).
When more than one fiber can be used for a particular wavelength, the fiber with a cut-off wavelength closer to the operation wavelength should be chosen.
The measured cut-off wavelength λco of a fiber may be 10% less than the nominal value because of manufacturing tolerances. Carefully selected fibers with characterized values are available on request.

Mode field of a singlemode fiber used within the operation range

Gaussian mode field within a step index single-mode. The transmitted wavelength lies within the opertaion range of the fiber. 

Resulting intensity distribution at fiber exit

The resulting intensity distribution at the fiber exit is Gaussian.

Mode field of a singlemode fiber used below cut-off

For a transmitted wavelength below cut-off the mode field within the step index fiber shows multiple modes (butterfly effect).

Resulting Butterfly effect

The resulting intensity profile at the fiber exit is also non-Gaussian.