Modal noise in single-mode fibres operated slightly above cutoff

If a single-mode fibre is operated slightly above the theoretical cutoff frequency, at every discontinuity like a misaligned joint, a part of the LP01-mode will be converted to the LP11-mode. At a following discontinuity, the LP11-mode will be partly reconverted to the LP01-mode, e.g. a source wavelength shift will then cause modal noise.