Coherently coupled optical spatial solitons in polymeric media

We show, for the first time, the existence and characteristics of coherently coupled optical spatial soliton pairs in biased photorefractive polymers under steady-state conditions. The theoretical formulation is set up using the coupled paraxial Helmholtz equations and the conditions for the existence of the bright, dark and gray coherently coupled soliton pair in polymeric media have been found. These soliton pairs can be established provided that the collinearly propagating carrier beams have the same polarization and wavelength, and are mutually coherent. We find that the coherently coupled soliton spatial profiles and soliton pair width are profoundly affected by the intensity ratio, phase difference between the incident coherent beams and the strength of the external bias field. The effect of these parameters is studied in detail for all three cases of bright, dark and gray coherently coupled soliton pairs. We study the stability of these coherently coupled soliton pairs against small perturbations by the numerical beam propagation method. The bright coherently coupled soliton pairs are found completely stable, while dark and gray coherently coupled soliton pairs exhibit instability on the uniform background.