Self-written waveguides in photopolymerizable resins

Abstract

We study the optically-induced growth and interaction of self-written waveguides in a photopolymerizable resin. We investigate experimentally how the interaction depends on the mutual coherence and relative power of the input beams, and suggest an improved analytical model that describes the growth of single self-written waveguides and the main features of their interaction in photosensitive materials.Optical self-action effects occur when the beam induces a refractive index change in the medium through which it propagates. Such effects are usually associated with the generation, propagation, and interaction of spatial optical solitons -the self-trapped optical beams that exist due to the balance between diffraction and nonlinearity [1].One of the important concepts that drive the research on optical solitons is their possible use as steerable selfinduced waveguides that can guide other beams of different polarization or wavelength. However, in order to "freeze" the soliton-induced waveguides, one should use photosensitive materials which experience long-lasting refractive index changes in response to illumination at specific wavelength [2]. The main question then is if the properties of such self-written waveguides are similar to the properties of spatial solitons. This is especially important for writing multiple waveguides and optical components, such as Y-and X-junctions, based on the intersection of two (or more) self-written waveguides.In this Letter we study, first experimentally and then theoretically, the optically-induced growth and interaction of self-written waveguides in photopolymerizable resins, and demonstrate both differences and similarities between the interaction of spatial solitons and selfwritten waveguides.As a photosensitive optical material, we use a photopolymerizable resin (PR) where single and multiple self-written waveguides ("fibers") can be grown by a onephoton absorption process [3]. First, we reproduce the experimental results on the growth of single waveguides earlier reported in Ref. [3], and then we study experimentally different regimes of interaction between two self-written waveguides. For the experiments, we use PR in the form of a liquid urethane acrylate photopolymer (SCR-500: Japan Synthetic Rubber Co., Ltd). The PR is filled in a glass cell and the beam is focused on the entrance face of the cell. The PR initial refractive index (before the illumination) is 1.53, which increases to 1.55 gradually with the photo-polymerization reaction. In order to study the beam collision, a light beam operated from a He-Cd laser (λ = 441.6 nm) is split into two beams, which are then focused onto the resin, simultaneously or with a delay.The optical axes of two beams intersect inside the PR so that the two waveguides growing simultaneously from the two beam spots collide with each other inside the photosensitive material. The polymerized structure is observed by a charge-coupled-device (CCD) camera from a side of the sample cell.Figure 1(a) shows that when two self-written waveguides c...

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