Silicon Mode-related Devices And Their Applications In Multi-dimensional Multiplexing Systems

To meet the rapidly growing demands of bitrate and transmission capacity, optical communication network is well developed by taking advantage of its superiorities, including high speed, large capacity, low loss and many others. In order to obtain even higher transmission capacity and broader bandwidth, various multiplexing techniques are exclusively studied, such as wavelength-division multiplexing (WDM), time-division multiplexing (TDM), polarization-division multiplexing (PDM), and space-division multiplexing (SDM) etc. WDM technology regarded as the most mature multiplexing technology, has been well applied in some commercial optical networks. The researches on the multi-dimensional multiplexing system which utilizes different multiplexing technologies simultaneously to further enlarge the transmission capacity have attracted continuous attention. On the other hand, silicon based photonic integrated technology is widely regarded as the choice of future optical communication network thanks to its compact structure, low fabrication cost and low power consumption. Hence, the researches on multi-dimensional multiplexing system, relevant advanced control, and routing functionalities based on silicon are the key technologies for the development of high speed optical communication networks.In this thesis, mode related mechanism and theriotical model are analized, different kinds of silicon mode convertors/multiplexers are proposed and successfully fabricated. Based on these basic elements, several functional devices which can be utilized in the multi-dimensional multiplexing system are proposed. Meanwhile, the applications of these devices in transmitter, router and receiver are presented in detail. The major achievements of this thesis can be summarized as follows:(1) Three kinds of structures based on silicon-on-insulator platform are investigated, including a device based on the cascaded multi-mode interference (MMI) and phase shifter, asymmetrical directional coupler and micro-ring resonator (MRR). A novel high performance mode multiplexer is proposed, which achieves improved conversion efficiency and reduced mode crosstalk. This chapter provides solid theoretical support for the next chapters which focus on the applications in multi-dimensional multiplexing system with MDM technique involved.(2) A polarization controlled on-chip mode converter is designed and successfully fabricated. This device consists of two-dimensional photonic crystal grating coupler and MRR based mode converters. Signals in the single-mode fiber are coupled into the chip, the mode profile of the output few-mode fiber can be determined by controlling the polarization state of the input signals. The coupling problem between the two-dimensional photonic crystal grating coupler and few-mode fiber is further investigated. Besides, the fabricated device is measured, and the observed mode spots from the output few-mode fiber verified the functionality of the device. The transmission experiment with 4× 40 Gb/s NRZ-OOK signal input is also performed. The measured eye diagrams and bit-error-rate (BER) results show good performance of the fabricated device.(3) An on-chip MDM system with optional demodulation functionality is proposed, and demonstrated experimentally. This system can be utilized as the photonic interconnection between long-distance fiber communication network and short-distance chip-level communication network. The MRR based structure is utilized as the mode multiplexer. The waveguide structure and parameters of the MRR are specifically designed. Thus the desired spectrum characteristic for DPSK signal demodulation can be realized. Thermal-optic effect is introduced to realize the control unit. Modeling and analysis of the thermal tuning structure is also performed. Detailed measurements of the fabricated device are included, the measured insertion loss, crosstalk and transmission spectrum are quantitatively provided. The functionality for the signals with three modes and four wavelengths are demonstrated in the experiment, and demonstration with more modes and wavelengths can be expected in the future.(4) An on-chip data exchange circuit for MDM signals is proposed and fabricated, to be used as routing and switching component in advanced MDM network. Furthermore, the proposed circuit is compatible with the mature WDM technology, thus it can be utilized in WDM-MDM multi-dimensional multiplexing network. The proposed mode exchange circuit is composed by the combination of two mode converters based on the MRR. Adiabatic taper couplers are designed to connect waveguides with different widths so as to maintain the modes adiabatically transmitted. For demonstration, single and four wavelengths signals carried on different modes are successfully processed. Measured eye diagrams and BER results verified the data exchange functionality and show great performance.(5) A novel PDM-MDM converter, which is first reported to the best of our knowledge, is proposed. This device can perform the conversion between fiber-based PDM technology and chip-based MDM technology. This device is composed by the combination of two-dimensional photonic crystal grating coupler and cascaded MMI-based mode converter/multiplexer. The insertion loss of the two-dimensional grating coupler is reduced efficiently by introducing distributed Bragg reflector based gratings. Fabrication tolerance of the phase shifter can be improved by replacing the traditional butterfly shape phase shifter with anti-butterfly shape. The optimizations improve the performance of the fabricated device. A transmission experiment with 40 Gb/s NRZ-OOK signals is performed to measure the fabricated device. The measured eye diagrams and BER results show excellent performance of the device.(6) Structures based on mode interference for 90 degree hybrid are proposed and sucessfuly fabricated, including the structure with 4x4 MMI, and structure with cascaded 2x2 MMI and phase shifter. Both devices are investigated and fabricated on the SOI platform. Measurements are performed to characterize the device performance in detail. The proposed devices can be utilized in coherent detection of multi-multiplexing system.