Design and development of high speed optical modulator on silicon-on-insulator platform
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Optical communication has transformed information technology and enabled the present-day data explosion. The new era of data centric society is primarily driven by the ability to communicate data at high-speed. Migration from copper cables to optical fibres has created basic infrastructure for the revolution of optical interconnect. One of the important functionalities in an optical communication network is light modulation that imprints electrical signal information on to an optical carrier. Optical modulators are the devices that do the conversion. Optical modulator changes either the phase or intensity of the carrier light in accordance with the electrical signal. Broadly, modulators are classi fied as electro-refractive; where the phase of light is changed and electro-absorptive; where the intensity is changed based on the electrical signal. Both types of modulators rely on the refractive index of the optical material. The research work in this thesis is primarily focused on the design, fabrication and characterization of an electro-optic (EO) modulator in Silicon and electro-optic materials on Silicon-on-Insulator (SOI) platform for the implementation of high-performance electro-optic functionalities. The dominance of silicon in commercial CMOS for electronic applications led to the investigation of silicon photonic integrated circuits and enabled the implementation of photonic integrated circuit in silicon for many applications, including, high-speed communication and on-chip sensing. In addition, high-index contrast of silicon-silicon dioxide structures enabled the fabrication of complex and compact integrated circuits. Since silicon has a very-poor linear electro-optic coefficient, modulation is achieved through plasma dispersion effect, where the concentration of free carriers alters the real and imaginary part of the refractive index. The modulation speed of silicon optical modulators is limited by carrier mobility. In this research work, we demonstrated for the fi rst time carrier-injection based electro-optic modulation using a diffusion doped electro-optic modulator. In order to develop high-speed modulation, ion implanted EO modulators are developed and demonstrated with an electro-optic bandwidth of 25-50 GHz. Using the high-speed silicon modulators, we have demonstrated various applications including, 4-channel silicon photonics-based transceiver with 200 Gbps capacity, frequency doubling, pulsed-RF signal generation and an integrated approach of single sideband generation from double sideband signal. Though the high-index contrast platform of SOI offers compact device and circuit platform, the power density in a submicron waveguide result in undesirable two-photon absorption. A detailed study of non-linearity due to higher optical power and the ways of mitigating the non-linearity in silicon waveguides are also analyzed with possible trade-offs. The electro-optic bandwidth of a silicon modulator is limited by the carrier dynamics and associated electronics. Hence we investigated electro-optic materials compatible with silicon photonics. For the purpose of developing small footprint electro-optic switching and modulation (few GHz), devices with phase change materials (PCM) can be used. Of all the PCMS, vanadium dioxide is chosen because of its ability to transit from optically transparent monoclinic insulating phase to optically opaque tetragonal rutile phase. Pulsed laser deposited vanadium dioxide (VO2) on silicon-on-insulator is studied for material, electrical and optical properties. Electro-optic modulation using VO2 tab on silicon waveguides is also demonstrated by thermally tuning the phase of VO2. Furthermore, to achieve a high-speed phase modulation and overcome the bandwidth limit of silicon, we attempt to develop an electro-refractive type modulator using complex oxide such as barium titanate (BaTiO3) on SOI platform. Of all the complex oxides, BaTiO3 has the highest Pockels coefficient (1000 pm/V). To reduce the lattice mismatch with silicon, magnesium oxide (MgO) is used as the buffer layer, and BaTiO3/MgO/Si stack is characterized for material and electrical properties. Second-harmonic generation experiments are conducted on the material to characterize electro-optic characteristics of BaTiO3. Preliminary transmission measurements of BaTiO3-silicon waveguides were also demonstrated, and possible solutions to reduce the loss in the devices are also discussed.