Abstract

Self-writing allows the generation of waveguides deep in the volume of crystals: the beam penetrates into the volume guided by the very waveguide it is forming, a form of irreversible spatial soliton [1], However, the effectiveness of self-writing for multi-waveguide structures is limited by interaction: the solitone, which remain distinct through the particle-like behavior, undergo elaborate dynamics, oscillating and spiralling. Even sequential self-writing is not a solution since the beams in succession respond dynamically to the prewritten pattern. We report the building block of a possible solution by demonstrating a technique to write a waveguide in the volume through only linear beam propagation. A λ=633 nm TEM₀₀ beam is focused onto the input facet of a photorefractive sample of potassium-lithium-tantalate-niobate. The beam diffracts from its initial intensity FWHM of 7 μm (Fig. 1(a)) to 11 μm (Fig.1(b)) after propagating 1 mm in the sample. The crystal is kept at T = 22°C, 9 degrees above its Curie temperature, and the voltage V=850 V is applied on lateral facets 2.4 mm apart along the x-direction. The 180 nW beam, polarized along the y-direction, i.e., orthogonal to the bias field, is allowed to separate charge for t*=600 s. Since the beam and the bias field are not parallel, the resulting index of refraction pattern is insufficient to alter beam propagation, and no nonlinearity intervenes to alter diffraction (Fig.1(c)). The linear waveguiding properties of this pre-written structure are now analyzed (readout phase): a weaker (or longer wavelength) signal beam is launched into the pattern but with a polarization along the x-direction, i.e., parallel to the bias field. The index pattern, now strongly enhanced by the geometry, leads to a quasi modal propagation(Fig.l(d) ). In comparison, if the writing stage is carried out with an x-polarized beam, the conventional nonlinear quasi-steady-state soliton phenomenon is observed, achieving self-writing at t = t* (Fig.l(f)-(i)).

© 2007 IEEE