Dr. Pavel Cheben, Senior Research Officer at the National Research Council Canada (NRC) has been supervising the PhD work of graduate student Daniel Benedikovič, who won the prestigious Nature Photonics award at the IEEE’s Group IV 2014 conference in Paris this summer. “This award recognizes Daniel’s outstanding talent, systematic work and creative thinking, but also the excellent potential of this unique NRC proprietary technology of subwavelength refractive index engineering in integrated photonics,” says Cheben.
Benedikovič, from the University of Žilina in Slovakia, worked with Cheben and the NRC photonics integration research team on a project related to a fundamental challenge in integrated optics: how to couple the light from the outside macroscopic world to microscopic waveguides in a tiny optical integrated circuit.
This coupling issue is of critical importance in the field of silicon photonics because a typical cross-section of a silicon waveguide is several hundred times smaller than the optical fiber core. “If we connect an optical fiber with a silicon waveguide directly face-to-face—without a coupler—less than one per cent of the light would be coupled to the silicon chip and the rest would get lost to the surrounding environment,” explains Cheben. “Such low coupling efficiency would render silicon photonic circuits of little practical use. That’s why a high-efficiency fiber-chip coupler is a hot topic in silicon photonics.” Cheben and his NRC colleagues were first in the world to exploit subwavelength grating structures to make efficient silicon photonic couplers.
Benedikovič visited the lab for five months last year and was able to achieve a remarkably high coupling efficiency with a simple fabrication process using only a single-etch step. He also used an SOI (silicon-on-insulator) wafer with standardized thickness of layers and made sure that the feature sizes were large enough for compatibility with 193-nanometre deep-UV lithography.
The measured coupler loss is about -2.2 dB at a wavelength near 1550 nanometres. It is one of the lowest loss rates yet reported for a simple (single-etch) surface grating coupler without a bottom mirror. These results are comparable to those achieved in complex multiple-step grating couplers that require thicker and non-standard silicon waveguides.
Benedikovič also demonstrated how to make ultra-high-efficiency couplers by adopting the same concept to a double–SOI platform, obviating the need for any backside processing or mirror deposition, with a record calculated efficiency of -0.41 dB.
Cheben is an Adjunct Professor at Carleton University, where he collaborates with Winnie Ye on other promising subwavelength grating structures. Ye’s graduate student Bryce Dorin, was also recognized at the prominent Group IV Photonics conference in 2014. Cheben says CMC Microsystems plays an important role in exploring the potential of this emerging field and enabling researchers in Canada to develop advanced integrated photonic structures. “Indeed this is just one of many examples of excellent service CMC provides to our photonics community.”