For single mode fibers, the cutoff wavelength is the shortest wavelength
at which only the fundamental mode is capable of propagation. Wavelengths below this cutoff wavelength are cut off; this wavelength is called the cutoff wavelength.
The cutoff wavelength depends on production factors, mode dispersion and numerical a perture (NA) and is 1,250 nm for 10 µm monomode fibers. Therefore, monomode fibers can only be operated with wavelengths above 1,250 nm; i.e. only in the 2nd and 3rd optical windows.