Self-trapped beams for fabricating 3D integrated optical components

Abstract

Extreme integration represents a popular trend in the construction of a wide range of technical devices. Optoelectronic and photonic components are no exception, as evidenced by the amount of research work devoted to writing optical microstructures inside dielectric (transparent) media. Advances in this area make it possible to envision more compact devices, consisting of 3D waveguides (see Figure 1), capable of functionalities that are out of reach with conventional technology. Moreover, this approach offers an advantageous substitute to standard planar integrated optics, which locates waveguides at the surface of substrates. Photoinscription is essentially the only simple, low-cost way of locally modifying a refractive index inside a medium in a controlled manner. The most common use of the technique involves scanning a tightly focused beam, often in the femtosecond regime, which modifies the refractive index through a combination of nonlinear processes: see Figure 2(a). Both optical waveguides and more complex photonic circuits can be made in this way.1 An alternative approach to this point-by-point inscription process—and the basis for our work—is to employ self-trapped beams, which can form a waveguide in a single step: see Figure 2(b). Beam self-trapping occurs when natural light diffraction is compensated by an appropriate nonlinear change (increase) in the refractive index induced by the propagating beam. The basic science of this phenomenon, which can be seen as the beam forming its own single-mode waveguide, has been widely studied over the past decades. In recent years, we have induced waveguides inside the ferroelectric material lithium niobate (LiNbO3). We chose this crystal for its strategic Figure 1. Schematic of an integrated optical component consisting of 3D optical circuits.