Input and output waveguide grating couplers: Theory, fabrication and testing.

Abstract

This dissertation investigates the modeling, design, fabrication and testing of the input and output coupling properties of waveguide grating couplers. It is motivated by the application of waveguide gratings to integrated optics. The coupling in both normal incidence and oblique incidence are described, but more attention is paid to the normal incident case. The perturbation transmission line theory is chosen as a theoretical model to design grating couplers in this work. The fundamental coupling parameters were carefully measured by many experiments. The comparison of experimental results to theoretical results is also discussed in this dissertation. For the case of normal incidence, the emphasis is on the optimization of the input coupling efficiency. The maximum input coupling efficiency carried out from our experiment was 75% by means of blazed grating in dielectric waveguide at a wavelength 0.6328 $\mu$m, which is very close to the maximum theoretical value of 80%. This coupling efficiency is significantly larger than that having been reported (40-50%). To achieve this goal, some parameters, such as, output coupling efficiency (branching ratio) and coupling distance (leakage factor), have been measured to test the theoretical model. The effect of the angular divergence and the wavelength dispersion of the incident beam on the input coupling efficiency is taken into account, which was modeled by coupled mode equation and tested by experiment. In addition, the design and fabrication procedure is also discussed in detail. In the case of oblique incidence the emphasis is on the output coupling only, i.e. the coupling from guided wave to radiated wave. In this part of the work the important features of the radiated wave-mode coupling efficiency and mode-conversion efficiencies, the phase difference between TE and TM components and the polarization state--were revealed and compared with the theory. These parameters were measured using one shallow rectangular grating and one deep blazed grating. These measurements were compared with the theoretical model.