Self-trapped beams for fabricating 3D integrated optical components

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).

Figure 1. Schematic of an integrated optical component consisting of 3D optical circuits.

Figure 2. Configurations used to light-inscribe optical waveguides inside dielectric media. (a) Scanning a focused light beam and (b) beam self-trapping.

Figure 3. Self-trapping of a 10μm-diameter beam inside a 20mm-long lithium niobate (LiNbO₃) crystal subject to a 20 ^°C rise in temperature.

Figure 4. A 1×4 junction fabricated by self-trapping beams. (a) Schematic of the component and (b) observation at the exit face of the inscribed junction.

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 importance to the photonics industry, notably in producing key components such as high-speed modulators, sensors, and efficient frequency converters. Because LiNbO₃ is difficult to photostructure with a focused femtosecond beam, we use low-power self-trapped continuous-wave beams that modify the material's refractive index through the photorefractive effect.

Based on a light-induced nonuniform electric field, this effect can be precisely controlled to form stable, self-confined beams that carve out low-loss circular waveguides inside the medium.² Figure 3 shows the formation of a straight waveguide. The arrangement consists typically of focusing a low-power (100μW) visible beam onto a 10μm-diameter spot at the input face of a 20mm-long sample cut from a photonic-grade LiNbO₃wafer: see Figure 3(a). In a linear regime, the beam propagates inside the medium and naturally diffracts to attain a large diameter at the output face: see Figure 3(b). When the nonlinear regime is properly set, the beam gradually self-focuses and then propagates with a constant 10μm-diameter profile: see Figure 3(c). To control the nonlinearity, an initial solution is to apply a strong electric field to the crystal. But recently we found that uniformly raising the temperature of the crystal works just as well and is less cumbersome.³ Figure 3 shows results for a 20^°C crystal-temperature increase. Once this self-trapped regime is reached, a low-loss waveguide is present in the material. The writing beam and crystal heating can be turned off, and the written waveguide is memorized. The waveguides have a lifetime of up to a year.

Our low-cost writing technique is versatile enough to fabricate elaborate 3D components. To illustrate this potential, Figure 4 shows a 1×4 miniature 3D junction made by self-trapped beams.⁴ More generally, the technique provides a way to create original structures that are not feasible using other methods, such as waveguides with sharp turns formed by reflection at an interface.⁵ Beam trapping can also be induced at interfaces to form surface waveguides.⁶ We are currently developing fixing techniques to enable permanent index structures. In addition, we are working on fabricating elaborate microdevices, including sensors and interferometers, with potential application in telecommunications, biomedical, and environmental technologies.