To obtain a good transfer performance, the beam must be optimally coupled into the fiber.
This requires the focus on the fiber core generally not to be more than 85% to 90% of the fiber core diameter. At the same time its divergence angle must be less than numerical aperture of the fiber. This is especially important for diode lasers which have a rectangular beam profile. If the laser beam couples not only in the core, but also in the cladding, the modes are also guided in the cladding. In order to remove them from the cladding, the mode stripping specially developed for this purpose is used. This mode stripper is installed in the connector body and detracts the cladding modes over a certain length. The required length is defined according to the coupling properties (spot size and NA-difference). The mode stripper ensures that the beam profile looks perfect at the fiber output. The detracted cladding modes are converted into heat in the connector body. If, in this case, the plug is not sufficiently cooled, the fiber might burn off in the connector.
For this reason, different requirements may arise with respect to the length of the mode stripper. This is why FCC has developed its own program to determine the mode stripper length.