Abstract

In birefringent optical fibers, linearly-polarized solitons associated with the fast mode become unstable at a certain power level [1,2]. A similar instability exists in planar waveguides made from isotropic materials [3]. It has been shown for the first time in [4,5], that the physical reason for this instability in isotropic materials is the appearance of new elliptically-polarized solitons above a certain critical power. In this work we prove that new solitons also exist in media with cubic anisotropy. Moreover, we have found that in planar waveguides made from cubic crystals, the symmetry allows the existence of two pairs of families of mixed-mode spatial solitons above the instability thresholds for TE and TM-modes. These replace the single pair of the isotropic media case. In these mixed-mode solitons, both TE and TM components are nonzero, with the phase difference between them being 0, π (the pair of linearly-polarized solitons) or ±π/2 (the pair of elliptically-polarized solitons). They propagate without change in the state of polarization and exhibit no polarization rotation. One pair of new soliton states bifurcates from the TM-mode while the other pair bifurcates from the TE mode at a different value of the initial beam power. It is important to note that our present investigation reveals the stability of one pair of families of the mixed-mode solitons at any value of power above the critical one, in contrast to elliptically-polarized solitons in isotropic media, which are always unstable.

© 1996 Optical Society of America