Passive Silicon Nanowire Devices And Functional Integration

With the fast development of internet, the communication of information has become globalized, which in turn increases the demand for expanding the data transmission capicity of the network. As a unique technique to fullfill this requirement, optical communication networks have been developed rapidly. In recent years, the backbone networks have been gradually upgraded to 100G coherent systems and the next step is toward 400G or even 1T optical systems. Due to the compatibility with traditional complementary metal oxide semiconductor (CMOS) technology, integrated optical devices on silicon can be quite cost-effective and are believed to be one of the most promising technologies, and the corresponding research region for this hot topic is called silicon photonics. In this thesis, we focus on silicon nanowire based devices and their functional integration. Both theoretical and experimental studies are presented.Firstly, we make a lot of efforts on fabrication processes and measurements for silicon nanowire based devices. For the fabrication, we have tried to reach the best working status of our electron beam lithography (EBL) systems for highly efficient exposure. For the measurement, we have found a rule for normalizaton as the grating coupler is wavelength-sensitive. Based on these efforts, we have achieved silicon nanowire waveguide with about 1.5～4.2dB/cm propagation loss. We have also designed and fabricated a few passive devices based on silicon nanowire waveguide, including 3dB couplers, micro-reflectors and arrayed waguide grating (AWG) based wavelength division mulipexers. All the devices show quite good reproducibility.Secondly, we have explored Mach-Zehnder Interferometer (MZI)-based thermal tuning devices. The fabricated 8-channel reflective variable optical attenuators (VOAs) show more than 30dB dynamic extinction ratio. Besides, we have proposed ultra-broadband MZI thermal switch based on bent directional couopler, which shows more than 140nm bandwidth with less than 1dB insertion loss and more than 20dB extinction ratio. Further, by integrating two indentical AWGs with 8 ultra-broadband low-loss MZI thermal swithes, we have demonstrated a reconfigurable optical add drop multiplexer (ROADM), and the dynamic switching range is more than 20dB.Finally, based on a regular symmetrical AWG structure, we have utilized its bi-direcitonal transmission characteristic to realize three types of functional integrated modules. By integrating an optical interleaver with bi-directional AWG (bi-AWG), we have demontrated a 200GHz channel spacing demultiplexer with performance comparable to that of a 400GHz demultiplexer with a single AWG. By integrating polarization beam splitter (PBS) and polarization rotator with a bi-AWG, we have demonstrated 8-channel polarization-division-multiplexing (PDM) wavelength-division-multiplexing (WDM) hybrid demultiplexer. Besides, by cascading the regular PBS with a transverse magnetic (TM)-mode polarizer, we have improved the performance significantly, and the polarization extinction ratios for both polarizations exceed 20dB while the insertion losses are less than 1dB in about 60nm wavelength range. All these functional integrations pave a way for a larger scale intergration in the near future.