Measurement of cutoff wavelength of single-mode fibre with periodic perturbation of fibre axis

A new method for determining the cutoff wavelength of an optical single-mode fibre has been developed. This method is based on inducing strong coupling between the LP11 and the LP01 modes by distorting the fibre axis with a pair of corrugated plates, and measuring the beat length between the modes as a function of optical wavelength. At the point where the LP11 ceases to be guided (cutoff), an irregular change occurs in the beating length between the modes. Owing to the high mode selectivity of the coupling process, the technique is not affected by microbendings.