Reflection and transmission characteristics of silicon photonic wire Bragg gratings

In this work, we optimize Bragg gratings covered with SU-8 for TM polarized light at a center wavelength of 1550 nm with respect to high reflectivity and large wavelength range employing 3D FDTD simulations. Three different types of lateral grating modulation were studied: I) complete interruption of the waveguide, II) corrugation within the waveguide width, and III) corrugation exceeding the waveguide width. The wavelength response was analyzed with a discrete Fourier transformation algorithm for a Gaussian pulse source. The investigations resulted in a grating structure providing a reflectivity of >70% over a wavelength range of 50 nm. The transmission and the radiation losses amount both to approximately 10–15% each. Corresponding samples of these three Bragg grating structures with lengths of ~10 μm were fabricated employing e-beam lithography and reactive ion etching. In order to enable the experimental verification of the reflectivity a Y-branch separates the light paths of incoming and reflected light directly on the chip. The measured reflection and transmission spectra match well with the simulations and demonstrate the good performance of the optimized Bragg grating reflector.