Design and fabrication of integrated on-chip silicon slow-light structures for optical delay line and eo modulator applications

Abstract

The emerging silicon photonics technology has enabled numerous compact and low-cost on-chip optical systems with various functionalities. In this thesis, the research focuses on silicon on insulator (SOI) based slow-light devices which have the optical signals propagating at a fraction of vacuum speed of light. Slow-light on-chip Bragg grating waveguides on Si/SiO2 are explored both theoretically and experimentally for applications in true-time delay lines and slow-light modulators. The theoretical study of this work focuses on Bragg grating waveguide for its power transmission, group delay, optical bandwidth and dispersion characteristics using the coupled-mode theory (CMT) and finite-difference time-domain (FDTD) methods. The strongest slow light effect is found at the wavelengths near the photonic band edge, while the strong reflection at the slow-light Bragg grating waveguide with regular Si waveguide interface gives rise to unwanted optical power oscillations. Two approaches have studied to suppress the oscillations: the cascaded apodized gratings and the mode-transition gratings. Apodized Bragg gratings have enabled smooth transition at the grating/waveguide interface by gradually modulating the effective index perturbation along the entire grating waveguide. It has been demonstrated that the optical power oscillations can be substantially suppressed using apodization method, thus increasing the delay-bandwidth product of the delay line. Mode-transition gratings, on the other hand, achieves impedance match at the grating/waveguide interface by inserting a short grating taper segment that has a group index smaller than the main section of the slow-light Bragg grating waveguide. By optimizing the taper structure, the spectral oscillations are successfully suppressed. It’s been also demonstrated that the mode-transition gratings exhibit higher group index than apodized gratings. Based on the mode-transition gratings, a dispersion cancellation scheme is designed. The group velocity dispersion (GVD) is negative at the right band edge of a Bragg grating and positive in the left band edge. By cascading two Bragg gratings with opposite signs in GVD, dispersion compensation will lead to much increased delay-bandwidth product. The slow-light grating waveguide has been incorporated in optical electro-optic (EO) modulators for analog photonic applications. Depletion-type EO modulators make use of the free carrier plasma effect to achieve intensity modulation. With light traveling at a slower group velocity, the plasma effect of the waveguide is enhanced, and thus the required modulator length is greatly reduced. The linearity of the modulators is also studied for analog applications. The nonlinear transfer function of modulators gives rise to distortion terms such as 3rd-order intermodulations (IMD3). By taking the transfer function for its third derivative, an optimal biasing point can be found, which gives the maximum spur-free dynamic range (SFDR) and improves the signal-to-noise ratio of the photonic link. In addition, EO crosstalk in densely integrated Mach-Zehnder modulators (MZMs) is studied and characterized, which provides a guideline to the trade-off between system integration and performance optimization.