In many applicative fields, fiber-optic devices are bound to be replaced by their integrated counterparts, which offer large benefits in terms of costs, size, robustness, and suitability for mass-production. Indeed, integrated technologies make use of fabrication processes, similar to those that have been employed in electronics for several decades, enabling the realization of many different components and functionalities that can be gathered together on a small photonic chip.
However, a strong limitation of conventional integrated optical technologies is that they are inherently planar. This means that only architectures lying on a common photonic layer can be fabricated, whereas structures extending in a full 3D space are not generally allowed. To circumvent this limit, a discrete number of photonic layers can be stacked, as it happens in electronics, even though in integrated optics, vertical connections are much more tricky and only a few guiding layers can be reasonably overlapped.
Unfortunately, there are functionalities that cannot be mapped onto a planar 2D topology, but require a full 3D design. An example is the so-called photonic lantern (PL), a device of growing interest for astrophotonic applications. Here, the light emerging from a multimode (MM) waveguide has to be efficiently coupled into a 2D matrix of single-mode (SM) waveguides, resulting in a full 3D device. PLs have been realized in fiber optics by using fiber tapering techniques, based on either multiple SM fibers stacked inside a low-index capillary or on a single multicore fiber, but no efficient solutions have ever been found to realize such devices in integrated optics...until this last work by R. R. Thomson and co-workers.
R. R. Thomson et al. were able to realize 3D integrated optical transitions that efficiently couple light from a MM waveguide to a 2D array of SM waveguides and back. They used a pulsed 350-fs laser source at 1047 nm to locally modify the refractive index of a borosilicate glass substrate, and optical waveguides were directly written by translating the material in 3D through the laser focus. The entire structure was constructed by using multiple scans (up to 20 scans at a speed of 8 mm/s for each SM waveguide) along the full length of the sample; in this way all the sections of the PL were constructed simultaneously, with the MM waveguide directly resulting from a spatial combination of multiple SM waveguides. This is the trick they used to get a very small coupling loss in the transition from the MM to the SM region, a tight requirement for PLs to be used in astronomic applications. And to avoid shadowing effects in the writing process, the deepest scans were performed first; then the sample was moved upwards, layer by layer, toward the surface. In this first report, the refractive-index contrast is not very high, about 0.2%, but the index modification is not fully saturated and probably higher index contrast waveguides will be fabricated in the future.
What is really promising in the work by R. R. Thomson and co-workers is that they succeeded in giving a third dimension to integrated optic devices and in controlling the fabrication process with sufficient accuracy to be of real interest for practical exploitation. This is a significant step forward not only for applications in astrophotonics, remote sensing, and space photonics, as suggested in the paper, but can also have a strong impact in every field that integrated optics has been penetrating, such as optical communications, biophotonics, optical interconnects, and quantum optics.
Adding new dimensions always brings more potential. Designers can now start thinking about 3D extensions of many powerful and well-established integrated devices that today are inherently 2D in their topology, including, among others, multibranch power splitters, array waveguide gratings, switch fabrics, and interferometric lattice filters. True, it is difficult to predict the actual limits of the technique proposed by R. R. Thomson and co-workers, but this first-stage effort is extremely promising!
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