Directional coupling of optical signals by odd dark beams with mixed phase dislocations

Numerical simulations on the evolution of stepscrew and edge-screw optical phase dislocations in bulk saturable self-defocusing nonlinear media are presented, with emphasis on their ability to induce steering waveguides for signal beams (pulses). Two schemes for directional coupling of such signals, both ensuring a reasonable coupling-to-crosstalk efficiency ratio, are investigated. The parameters useful for performance optimization of the couplers are discussed. PACS: 42.65.Tg; 42.65.Wi Dark spatial solitons (DSSs) formed in bulk self-defocusing Kerr-type nonlinear media (NLM) are able to guide copropagating probe beams/pulses [1–3]. The underlying physical mechanism [4] is the intensity-dependent refractiveindex change in a plane perpendicular to the propagation direction. Weak signal beams passing along these optically induced gradient waveguides [5] are subject to effective induced-phase modulation and are trapped. In photorefractive NLM [6], after the experimental generation of dark planar and vortex solitons in a quasi-steady-state regime [7], the situation is even more spectacular [8]. Because of the wavelength-dependent material response, DSSs generated at low powers but at a photosensitive wavelength are able to guide and steer much more powerful “signal” beams at non-photosensitive wavelengths. This is confirmed for photovoltaic [9], biased [10] and quasi-steady-state [11] photorefractive solitons and seems to hold also for incoherent dark solitons [12]. Both bright and dark beams can be forced to steer by introducing a spatial chirp to their transverse phase profiles with spatial light modulators [13–15]. When bright beams are guided by DSSs, the self-trapped dark beams create gradient waveguides that keep the bright beams narrow. This ∗Corresponding author. (Fax: +35-92/962-5276, E-mail: [email protected]) ∗∗(Fax: +49-89/3290-5200, E-mail: [email protected]) ∗∗∗(Fax: +49-89/2891-4142, E-mail: [email protected]) motivates the interest in investigating techniques for manipulating the transverse dynamics of dark beams. Arrays of onedimensional DSSs generated by two intersecting plane waves in the regime of adiabatic amplification (and the probe beams guided by them) can be steered by changing the relative intensities of the interfering waves [16]. The transverse velocity of an optical vortex soliton (OVS) has a radial and an angular component arising from the transverse phase and intensity gradients, respectively [17, 18]. Two practical ways to control the vortex rotation have their origin in the Guoy phase shift on both sides of the background beam waist [18, 19] and in the interaction of ordered structures of OVSs [20] controlled by the topological charges. OVS steering is demonstrated by superposition of a weak background field [21]. Operation of planar Y-junction splitters for signal beams is demonstrated in both Kerr-type [22] and photorefractive NLM [23] with pairs of grey DSSs born from even initial conditions. The possibility to branch a single input probe beam into ordered structures of sub-beams by quasi-two-dimensional DSSs is demonstrated numerically in [24]. Other branching and steering schemes can be realized by employing the inherent dynamics of ring dark solitary waves [25], eventual NLM saturation [26] and/or anisotropy [27]. In this work we present numerical results on how one bright signal beam entering an input data channel can be guided and linked to a selected output port by a dark-beaminduced steering waveguide. A high steering speed of the induced waveguide can be obtained by using odd dark beams (ODBs) of finite lengths [28, 29] with a suitable choice of the mixed phase dislocation. Special attention is paid to ensure high energy efficiency of the directional coupler and short length of the interaction zone. Reconfiguration of the coupler is proposed to be done by changing the type of the phase dislocation reproduced by a multiple, active, single-voltage controlled computer-generated hologram [30]. 1 Step-screw and edge-screw mixed phase dislocations The mixed phase dislocations considered consist of a onedimensional phase step of limited length, which ends, by