Browsing by Author "Sfar Zaoui, Wissem"

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Efficient coupling between optical fibers and photonic integrated circuits
(2014) Sfar Zaoui, Wissem; Berroth, Manfred (Prof. Dr.-Ing.)
The progress that photonic integration is undergoing may be compared to that of electronic integration nearly half a century ago. Its development will not only enable the transmission of huge amounts of information – particularly in optical data communication – but will also pave the way for large scale fabrication, the minimization of assembly processes, and the reduction in energy consumption. The benefits of photonic integration can even be increased by harnessing the salient properties of the silicon-on-insulator platform. In fact, silicon photonics can leverage the existing complementary metal-oxide-semiconductor infrastructure, and hence can offer a low-cost solution for the more and more complex sender and receiver architectures. Another advantage of the silicon-on-insulator platform is the possibility for high-density integration owing to the offered large index contrast between silicon and silicon dioxide. This property certainly enables the realization of compact circuitries with numerous functionalities on very small areas; however, it also creates a barrier to the connection with available optical fibers. While the integrated waveguide structures on the chip have cross sections in the order of 0.1 µm², external optical fiber cores possess dimensions of more than 50 µm². This large mismatch can lead to extreme insertion losses, and hence the advantage of miniaturization turns into a problem of coupling with the existent conventional fibers. At first view, the issue highlighted may be seen as trivial since several standard coupling techniques, such as tapered fibers or lensing systems, are available. Nevertheless, the stringent requirements for high efficiency, compact dimensions, and more flexible coupling in industrial applications indicate that better performing configurations have to be implemented. For this purpose, a variety of approaches starting from three-dimensional tapers to photonic crystals and plasmonic structures have been proposed. Each of these techniques, however, offers more cons than pros, and thus none of them have yet made the leap into practical application. Within the scope of this thesis, two different coupling approaches are investigated. The first method deals with metamaterials, which allow for the realization of effects not seen in nature. The second method is based on more application-oriented structures, known as Bragg gratings. The common purpose of both topics is the concrete realization of highly efficient couplers that alleviate the size difference between conventional optical fibers and integrated single-mode silicon waveguides. As a benchmark, the coupling efficiency has to exceed the value of –1 dB, whereas the 1 dB bandwidth has to be larger than 35 nm in order to cover the whole C-band. The investigation of focusing metamaterial structures is done first at millimeter wavelengths owing to the fabrication and characterization convenience. The main target of this approach is to create a negatively refracting material that can focus an input beam into a much smaller spot size at a short distance. Furthermore, the negative index metamaterial has to exhibit low reflection and absorption losses, and hence high transmissivity in a large frequency range. Thereafter, the dimensions of the focusing metamaterial lens are scaled down in order to analyze their applicability at telecommunication wavelengths. The metamaterial functional layer is designed based on the dielectric-metallic fishnet structure and fabricated using conventional etching techniques. The designed metamaterial stack exhibits a high transmissivity of nearly –0.5 dB with a negative refractive index of –1 at the operating frequency 38.5 GHz and a 1 dB bandwidth of 0.8 GHz. The measurement results are shown to be in good agreement with the theoretical calculations. Thereafter, in order to achieve a focusing metamaterial lens, the shape of the stack is modified to form a plano-concave configuration. This structure shows good focusing ability with a reduction of the launched beam waist by a factor of 2.2 at a distance of only 6 λ0. In comparison, a fabricated aspheric dielectric lens exhibits twice the beam waist at a distance of more than 12 λ0. The negative index lens, therefore, is a good candidate to replace conventional lenses at radio frequencies owing to its better focusing performance and more compact dimensions. Indeed, scaling the dimensions of the lens down to infrared wavelengths theoretically shows a similar behavior with a beam width reduction by a factor of 3.8 at a distance of 8.7 λ0, which is advantageous for nanocoupling between optical fibers and integrated waveguides. However, the considerable metal losses decrease the total efficiency to lower than –2 dB. Hence, the target efficiency cannot be achieved, and alternative solutions have to be used in the future in order to compensate for these absorption losses at optical frequencies. The second coupling method investigated in this thesis relies on Bragg diffraction gratings. In comparison to the first method, these structures have the advantage of being directly integrated with the waveguides on the chip, and thus they can be realized more cost-effectively. Moreover, this procedure allows out-of-plane coupling and wafer-scale testing without the need for edge cleaving and polishing. These advantages make grating couplers good candidates to compete with the in-plane coupling spot size converters, which require a much larger footprint, provided that the efficiency is enhanced to the same order of magnitude. As the coupling efficiency of standard diffraction gratings is relatively low, the loss sources have to be analyzed, and possible improvement methods have to be implemented. In fact, there are two main factors that limit the performance of grating couplers: directionality and modal overlap with the fiber profile. In this work, the first issue is tackled using a metal mirror at an adequate distance underneath the grating; the second factor, meanwhile, is rigorously optimized by reshaping the diffracted field profile based on a home-made algorithm. The theoretical results show efficiencies better than –0.3 dB with a 1 dB bandwidth larger than 40 nm. The designed grating couplers, including the metal mirrors, are fabricated cost-effectively using a complementary metal-oxide-semiconductor compatible technological process at IMS CHIPS. Placed at different positions on the wafer, around 75% of the fabricated structures exhibit a better coupling efficiency than –0.75 dB. The highest value reaches –0.62 dB at 1531 nm, which is, to the best of knowledge, the highest measured efficiency on a grating coupler reported so far. Furthermore, the achieved 1 dB bandwidth amounts to 40 nm and exceeds the predefined target value. This work, therefore, can be seen as a milestone in the field of silicon photonics and a bridging gap between optical fibers and photonic integrated circuits.