Mode coupling in single-mode fiber: the advantage of using higher-order spatial-distortion-function harmonics

It is demonstrated that light power may be converted from the guided LP(01) mode to the radiative LP(11) mode in single-mode optical fiber by using a periodic square-shaped distortion function. Coupling between these modes is measured directly as a function of distortion wavelengths and amplitudes for different optical wavelengths. The beating length between the modes (approximately 0.5 mm) becomes practical (1.5 mm and more) through the use of the higher harmonics of the distortion function.