Conventional lenses and optics exhibit chromatic aberration with a focal length dependent on wavelength.
The coupling of combined polychromatic beams requires that chromatic aberration is corrected to the highest degree possible. For efficient coupling, all beams have to be focussed at the fiber entrance onto a common point, as slight deviations do lead to significant coupling losses. Any correction attempts using standard achromatic optics are really only suitable for two different wavelengths.
Schäfter+Kirchhoff has, therefore, developed their own special apochromatically corrected optics for the efficient coupling of combined polychromatic beams in the spectral range of 400–660 nm