High-Speed and Power Efficient Integrated Electro-Optic Modulators
Communication networks worldwide are experiencing exponential growth in data traffic. The foundation of international communication is currently provided by fiber-optic communication networks,which manage more than half of the internet traffic. Increasing the capacity as well as reducing the energy consumption of the optical communication links is essential to meet the current demands. An electro-optic (EO) modulator, which translates the digital bit stream into an optical signal, is one of the essential components that determines the data rate of a fiber-optic link. The deployment of thousands of externally modulated laser links in a data center requires EO modulators with low energy consumption and small footprints, which enable high-density integration at a low cost. The standard silicon-on-insulator (SOI) platform and the more recently developed lithium niobate-oninsulator (LNOI) platform are the foundations of photonic integrated circuits (PIC), which have emerged as a promising technology to meet the needs of the optical communication industry. While effective optical modulators have been shown on both platforms, higher-speed and more compact devices are required to keep up with the increasing demand of internet traffic. Therefore, to enhance the bandwidth and to reduce the power consumption I demonstrated two SOI platform-based device architectures in this thesis: one based on the Mach-Zehnder interferometer (MZI) silicon modulator to enhance the bandwidth and the other on the microring resonator (MRR) silicon modulator to support higher order modulation formats. In addition to that, I also present a modulator with new device concept based on the LNOI platform which lowers the voltage needed to drive the state-of-the-art thin film lithium niobate (TFLN) modulators. I initially presented the silicon dioxide cladding capacitively coupled silicon (CC-Si) modulator in Chapter 2. I reported our observation of plasma dispersion effects for the first time in silicon strip waveguides fabricated in the multi-project wafer (MPW) run of a commercial foundry. In a proof-of-concept experiment, I generated sidebands up to 50 MHz around a carrier signal near telecom wavelengths by applying sinusoidal modulation signals at one arm of the MZI. Using a self-heterodyne measurement system I estimated a half-wave-voltage-length product (VπL) of ∼9.4 V.cm at 20 MHz modulation signal. The EO measurement of the proposed modulator can generate the first order sidebands up to 68 GHz of radio-frequency (RF) signals and almost a flat frequency response was observed. The modulator has an estimated 3-dB bandwidth (f3dB) of ∼9.3 GHz and with further optimization the bandwidth can be enhanced beyond 40 GHz. Later, I demonstrated a silicon strip waveguide-based modulator in a different configuration which is also known as the series push-pull (SPP) configuration. This modulator also shows a flat electro-optics response up to 68 GHz. A VπL of ∼87.8 V.cm is estimated from the power of generated sideband near a C-band optical carrier by a 15 GHz RF modulation signal. I also have demonstrated fabrication process of the CC-Si modulator with barium titanate cladding which was partially fabricated at a commercial foundry and the rest of the part was fabricated in several back-end-of-line (BEOL) post-fabrication processes at the university fabrication facility. I demonstrated an MZI based CC-Si modulator with SPP configuration. For a 2 mm long device I generated sidebands up to 67 GHz and observed a flat frequency response. I further demonstrated a VπL of ∼2 V.m at 15 GHz at a C-band carrier wavelength. In Chapter 3, I have demonstrated a SOI platform-based optical phase shifter by combining the through and drop ports of a silicon add-drop MRR modulator. The measured intensity spectra for different bias voltages show that, our proposed phase shifter combining the through and the drop ports show ∼4.29 dB less maximum intensity variation compared to the maximum intensity variation observed at the through port for ∼2π phase shift. I also extracted the phase spectrum by placing such an optical phase shifter in one arm of an integrated MZI structure. I also achieved a full 2π phase shift of a continuous wave (CW) carrier optical signal with maximum ∼4.38 dB of intensity variation by driving the silicon MRR modulator with a forward bias voltage which gives us a half-wave voltage (Vπ) of 250 mV for a telecommunication CW optical carrier. Finally, I created a constellation map of 8-ary phase shift keying (8-PSK) based on the extracted phase and the intensity at different forward bias voltages. In Chapter 4, I proposed a novel EO modulator based on the LNOI platform where a cladding material of very high dielectric constant i.e., barium titanate (BTO) is used around an etched lithium niobate (LN) waveguide. Lumerical Mode and Device simulations have been used to optimize a MZI based EO modulator that shows a VπL as low as ∼1.36 V.cm maintaining the optical loss due to metal absorption of ∼1 dB/cm. The 3 dB EO bandwidth of ∼132 GHz has been estimated using the data generated by the microwave simulations carried out in Computer Simulation Technology (CST) Studio Suite.
Bandwidth Enhancement, Capacitive Coupling, Lithium Niobate Modulator, Phase Shifter, Ring Resonator, Silicon Modulator
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