Advances in silicon photonics have ignited an intense interest in optical devices based on silicon (see pages 53, 62 and 68). An inherent problem with the application of this technology, however, is the low efficiency with which light can be coupled from conventional optical fibers into the new silicon devices. If not solved, this problem could nullify many of the advantages of silicon photonics because the penalty paid to get light into these devices could outweigh the advantages of getting it there.Recently, reseaerchers at the University of Surrey in Guildford, UK, Politecnico di Bari in Italy and the University of Southampton in the UK demonstrated a coupling technique that has an overall efficiency that could approach or exceed 80 percent.The low coupling efficiency of silicon results from two factors. First, the huge difference in the refractive indices between glass and silicon creates an impedance mismatch, which reflects a large percentage of the incident light. Second, the core of a single-mode optical fiber typically is 9 µm in diameter, while the dimensions of the silicon devices are usually on the order of 1 µm. Numerous solutions to these problems, such as antireflection coatings and tapered waveguides, have been evaluated. Although many of them are theoretically capable of efficient coupling, none has lived up to that promise in the laboratory.Figure 1. The dual-grating assisted directional coupler is capable of coupling light from the ~9-µm core of an optical fiber into an ~0.25-µm-thick silicon waveguide with efficiency as high as 80 percent. Images ©OSA.Dual gratingThe solution proposed by the European scientists is a device they call a dual-grating assisted directional coupler (Figure 1). The top SiON layer is designed to be ~5 µm thick, with a refractive index very close to that of glass so that light can be coupled from an optical fiber into the waveguide with a loss of less than 0.05 dB. The Si3N4 waveguide acts as a bridge between the two waveguides, in terms of both refractive index and thickness, and enables highly efficient coupling at both gratings. The bottom SiO2 layer acts as a lower-index cladding for the silicon waveguide, minimizing light leakage into the substrate.Two analytical techniques, the transfer-matrix method and the Floquet-Bloch theory, produced nearly identical results when applied to the dual-grating device: The coupling efficiency could be as high as 93 percent. But other coupling schemes had produced similarly high coupling efficiencies in theoretical predictions, so the investigators fabricated a dual-grating assisted directional coupler and tested its actual performance in the laboratory.Figure 2. A “double” dual-grating coupler allowed accurate measurement of the coupling efficiency.The device they fabricated was a “double” dual-grating coupler (Figure 2). They measured its coupling efficiency by calculating the ratio of its throughput to that of a SiON waveguide of the same dimension as the one in the coupler. Unfortunately, the fabrication process went slightly awry, and the dimensions of the fabricated device were different from those called for in the optimal design. They recalculated the coupling efficiency expected for their device and found that the fabrication discrepancies would lower the efficiency from 93 percent to 60 percent.In laboratory tests, the efficiency was measured to be as high as 55 percent, or within 5 percent of the predicted value (Figure 3). If an equally close-to-predicted performance were obtained from a device fabricated with the ideal design dimensions, its coupling efficiency would be ~80 percent. Based on this calculation, the scientists conclude that their dual-grating coupler is arguably the most promising method of coupling light into small silicon waveguides.Figure 3. The best coupling efficiency measured with the dual-grating coupler was 55 percent, just slightly less than the 60 percent predicted. The peaks at different wavelengths were achieved using gratings with different grating parameters.One potential drawback of the dual-grating coupler is the narrow (~4 nm) peak (Figure 3). The researchers are working on techniques to broaden the wavelength dependence of the coupling by chirping the gratings and varying their duty cycles.