To obtain a good transfer performance, the beam must be optimally coupled into the fiber.
This requires that the focus on the fiber core generally should 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 that have a rectangular beam profile. If the laser beam couples not in the core only, but also in the cladding, the modes are also guided in the cladding. In order to remove them from the cladding, the specially developed for this purpose mode stripping is used. This mode stripper is installed in the connector body and removes 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 removed cladding modes are converted into the heat in the connector body. If, in this case, the connector is not sufficiently cooled, the fiber might burn off in the connector.
For this reason, different requirements may be imposed on the length of the mode stripper. This is why FCC has developed its own software to determine the mode stripper length.