Abstract

It has been predicted theoretically [1-2] and confirmed experimentally [3] that the mutual interference of modes of a two-mode, weakly guiding, step-index optical fibre leads to a variable modulation of the source spectrum when the source coherence length is shorter than the optical path difference (OPD) between modes. In this contribution, this effect, which means that the mutual interference of modes of an optical fibre occurs no matter how long or short the OPD between modes may be. is demonstrated experimentally al the output of a weakly guiding, step-index optical fibre excited by a low-coherence source whose central wavelength is below the cut-off wavelength of a single-mode regime. As an example, a weakly guiding, step-index optical fibre of length z= 1.76 m having a cut-off wavelength of 920 nm and a core diameter of 4 μm is excited at a wavelength of 671.6 nm by a low-coherence source whose coherence length is 280 μm. The intensity distribution corresponding to two lowest-order linearly polarized (LP) modes is resolved at the output of the optical fibre and no mutual interference between both LP modes is observed. Fortunately, it has been confirmed experimentally (see both figures) that the spectral interference between both LP modes is present

© 1996 IEEE