Bend losses of higher-order modes in dispersion-flattened multiple-clad optical fibres

The bending losses and effective cutoff wavelengths of the disturbing LPX1 and LP02 modes in dispersion-flattened, multiple-clad, quasi-single-mode fibres have been evaluated numerically. Even when the theoretical cutoff wavelengths of the LPn and LP02 modes are within the usual interval of operating wavelengths (1·3 < ¿ < 1·7¿m), because of the extremely large bending losses of these modes, the effective cutoff wavelength can be below 1·3¿m. These fibres are thus effectively single-moded.