Numerical analysis of the azimuthal periodicity in the two lowest order modes of microstructured fibres applied to the precise determination of the cutoff wavelength

The azimuthal far-field analysis technique (AFFAT) is one of the only two methods reported to date, able to measure the cutoff wavelength λc in microstructured fibres (MOFs). The measurement is operated through a parameter D which is related to the modulation of the power crossing a rotating slit at the output of the tested fibre. In standard fibres, the criterion of the CEI/IEC 793-1-C7A normalised method (i.e., 2.25% of the output power carried by the second mode) is expressed by the unchanged −16.4 dB value of D(λc). In this paper, we show that this criterion is expressed in MOFs by a value of D which strongly depends on the geometrical parameters of the fibre and on the launching conditions at the input. However, we demonstrate that, with a slightly relaxed criterion corresponding to 10% of the output power carried by the second mode at λc, D remains close to −10 dB whatever the considered MOF. Thus, this constitutes a universal decision criterion allowing to determine λc in any classical MOF, by means of the AFFAT, with a very limited uncertainty.