Polarisation Modulation Instability in Weakly Birefringent Fibres

Abstract

Modulation instability (MI) is a phenomenon exhibited by waves propagating in nonlinear dispersive media, in which weak amplitude perturbations at different frequencies are subject to gain. In the scalar description of wave propagation in optical fibres governed by the nonlinear Schrödinger equation (NLSE), MI can occur only in the anomalous dispersion regime, and this was the first form of MI to be observed [1]. Soon after this observation, however, it was predicted that the coherent interaction of two different circularly polarised waves of the same frequency propagating in a birefringent fibre would give rise to modulation instability in both the normal and the anomalous dispersion regimes [2,3]. To our knowledge this prediction has not been experimentally verified. Modulation instability has however, been observed in the normal dispersion regime by pumping highly birefringent fibres with intense red or green pulses [4,5]. This form of MI arises from the incoherent interaction of two linearly polarised pulses propagating down the fast and slow axes of the fibre simultaneously. In these highly birefringent fibres, the terms which give rise to the coherent MI can be neglected in view of the large wavevector mismatch between the orthogonal modes. We report here the first observation of "polarisation modulation instability" (PMI). This instability possesses different characteristics depending on whether the pump is on the fast or slow axis.