Abstract

Semiconductors are attractive materials for the study of the dynamics of Kerr spatial solitons where, for example. Al_0.18Ga_0.82As has a nonlinear coefficient around 1000 times that of silica around the communication wavelength of 1.55 µm. Cubic semiconductors display an anisotropic third-order nonlinearity (although in terms of linear optics they are non-birefringent). In addition they are usually exploited in the vicinity of an optical resonance (e.g. just below the half-bandgap) so Kleinmann symmetry cannot be assumed. The most common orientation for semiconductor waveguides has TE polarisation parallel to [110] and TM polarisation parallel to [001] crystallographic directions. If the structurally induced birefringence is small then the nonlinear refractive effects of self-phase-modulation, cross-phase-modulation and four-wave-mixing need to be taken into account. For a slab waveguide the optical propagation can be described by the pair of coupled Nonlinear Schrodinger equations,¹ where u and v are the scaled electric field amplitudes for the TE and TM components respectively, γ is proportional to the (structurally induced) birefringence, σ is the nonlinear refractive anisotropy parameter and δ the induced birefringence parameter. For an isotropic medium with Kleinmann symmetry these material parameters take the values σ = 0 and δ= 1/3, but in Al_0.18Ga_0.82As at the half-bandgap measurements indicate σ = −0.54 and δ = 0.18.² Note that the anisotropy leads to an asymmetry in the self-phase-modulation terms for the two polarisation components.

© 1998 IEEE