Research On Microring Resonators And Their Applications In All-Optical Signal Processing

As the fast development of the optical fiber communications, and the continuous increasing of the communication capacity, all-optical communication technology has been paid a lot of attention because of its ability of solving the bottleneck, which exists in the convensional optical-electrical-optical (OEO) switching in the optical packet switching (OPS) module. As the key of the all-optical communication, all-optical signal processing technologies, such as all-optical buffering, wavelength conversion, format conversion and so on, have been gaining more and more studies and attentions. On the other hand, the fast developing semiconductor fabrication technologies, as well as the cost factor considered in the market, are making the optoelectronics on their way of integration. Thus all-optical signal processing based on integrated optoelectronics has been one of the brightest starts in the all-optical signal processing technologies with high-speed,high capacity and low-cost.Microring resonator (MRR) is one of the most important devices in integrated optoelectronic technologies. Thanks to fact that forming of its resonance does not need cleaving facet, microring is born with the advantage of integration. Its micro/nano size supports large-scale optoelectronic integration on chip. MRRs based on silicon material, III-V active materials and polymer materials have been greatly developed, and applied in researches on all optical buffer, wavelength conversion, optical parametric oscillation and so on. This dissertation will first introduce the basic theoretical knowledge, design method, as well as the fabrication technics of the MRR. After that, set in the all-optical processing, the novel optical devices, such as polarization control devices and bandwidth/wavelength tunable optical bandpass filter (OBPF), as well as all-optical signal processing applications, such as wavelength devision multiplexing (WDM) return-to-zero (RZ) to non return-to-zero (NRZ) format conversion, WDM NRZ-differential phase-shift keying (DPSK) signal demodulation,640Gbit/s NRZ signal generation, and all-optical memory unit, based on MRRs are studied both theoretically and experimentally. The major research achievements and contributions of this dissertation are summarized as follows:(1) Starting from the theory of optical waveguide, the basic principle of the MRR is deeply discussed. Using the parametric model, the expressions of transmission characteristics and some important parameters are derived for both all-pass and add/drop MRRs. Based on the theory, and associated with the methods of finite-difference time-domain (FDTD), three dimensional full vectorial mode-matching (3D FVMM), and three dimensional coupled mode theory (3D CMT) in space, the design of MRR is introduced in the aspects of coupling coefficient, as well as dispersion and losses of the bending waveguide. The fabrication technics of silicon miroring resonator are also studied. Furthermore, the accuracy of the design method is verified experimentally.(2) A bandwidth and wavelength tunable OBPF based on silicon add/drop microring-Mach Zehnder (ADMRR-MZI) structure is proposed and further demonstrated for the first time on silicon-on-insulator (SOI). Compared with a single MRR with transmission of Lorentzian-shape, the proposed device exhibits significant improvement of shape factor and extinction ration. Simultaneous bandwidth and wavelength tunability in-band ripple control is demonstrated by thermally tuning the resonance offset between the two MRRs.(3) Based on air-clad asymmetrical directional coupler, a polarization diversity chip with polarization independent through transmission of a single microring is proposed and demonstrated on SOI platform. Compared with the previous schemes, this proposal can be fabricated in a single step of exposure and etching, which greatly simplifies the complexity of the fabrication process. Only<1 dB polarization dependent loss is demonstrated. A significant improvement of the polarization dependence is obtained for 20 Gbit/s NRZ-DPSK demodulation using the polarization diversity circuit. Additionally, using the polarization dependency of a normal silicon MRR, a polarization converter based on integrating two identical MRRs and a phase shifter is proposed and analyzed in detail. By simply tuning the phase shifter, a polarization conversion between any two of linear, circular, and elliptic polarization states can be obtained. The performances of the device is detailly discussed using 3D FVMM and 3D CMT..(4) WDM applications based on a single MRR is studied theoretically and experimentally. First, RZ-NRZ format conversion based on MRR is discussed with optimizations in detail. Using the periodic transmission of the fabricated silicon MRR(free spectral range (FSR):100 GHz, Q:7900), WDM RZ-NRZ format conversion, with single channel bitrate of 50 Gbit/s, is successfully demonstrated. Experimental results show good conversion performance of the scheme. Secondly, NRZ-DPSK signal demodulation based on MRR is comprehensively analyzed. Using the fabricated device(FSR:100 GHz, Q:6700), WDM NRZ-DPSK demodulation, with single channel bitrate of 40 Gbit/s, is successfully demonstrated. Experimental results show very good performance for both through and drop port demodulations for all channels, and the drop port demodulation exhibits better wavelength detuning tolerance than for demodulation using a Mach-Zehnder delay interferometer (MZDI).(5) Optical time division multiplexing (OTDM) applications based on a single MRR is studied theoretically and experimentally. A 640 Gbit/s NRZ OTDM signal has been successfully generated for the first time by format conversion of a 640 Gbit/s OTDM signal from RZ to NRZ using a single silicon MRR (FSR:1.28 THz, Q:638). Experimental results show that the generated 640 Gbit/s NRZ OTDM signal has good quality. Moreover, compared to the wavelength converted 640 Gbit/s RZ signal, the generated 640 Gbit/s NRZ signal exhibits improved dispersion spectral efficiency.(6) An all-optical memory unit concept based on optical integration is proposed for the first time. First, an all-optical memory unit based on two coupled active microring optical integrator is proposed. The detailed performances of the memory unit are theoretically analyzed. Based on the memory unit, an all-optical digital register is proposed and simulated in detail. Second, an optical memory unit realized by an active microring optical integrator associated with electroabsorption modulators is proposed. Based on the two energy-band model of the III-V active materials, and considering the influences of the carrier density change as well as the noises of the side lasing modes, Detailed simulations of the memory unit performances are carried out.