The CUTOFF WAVELENGTH of a single mode fiber is the wavelength above which the fiber propagates only the fundamental mode. Below cut-off, the fiber will transmit more than one mode. An optical fiber that is single-moded at a particular wavelength may have two or more modes at wavelengths lower than the cutoff wavelength. The effective cutoff wavelength of a fiber is dependent on the length of fiber and its deployment and the longer the fiber, the lower the effective cutoff wavelength. Or the smaller the bend radius of a loop of the fiber is, the lower the effective cutoff wavelength will be.
