Design And Research On Electrooptic Modulators With High Linearity And Tolerance

With an increasing number of terminals connecting to the network to offer people various kinds of services, the future communication systems are required to give people easy access to the network with wide bands and large capacities, which will be a great challenge in the advancement of communication technologies. The optical communications will be effective means to meet the future requirements due to its advantages of great capacities, wide bands and low loss.In the dissertation, we pay much attention to the Mach-Zehnder Modulators(MZM), a kind of widely used modulators in modern optical communication systems. As an electronic-to-optic device in the system, the modulator’s linearity is one of the most important factors affecting the performance of the system closely. We use the Spurious-Free Dynamic Range(SFDR) to measure the modulator’s linearity. If the value of SFDR is higher, the linearity of the modulator is better, leading to less nonlinear distortion. We analyze the SFDR value of MZM and the factors limiting the SFDR after the introduction of the theory and transfer function. The results show that the modulator has relatively high SFDR and the main factor limiting the further improvement of SFDR is the third order intermodulation distortion(IMD3). Then, we introduce the theories of the Ring-resonator Assisted Mach-Zehnder Modulator(RAMZM) and the Dual Parallel Mach-Zehnder Modulator(DPMZM), on the basis of which we analyze their performances. It is demonstrated that both kinds of modulators can prohibit most of the IMD3 and improve SFDR significantly when working at their optimized operating points. On the other hand, these structures are intolerant to external factors which may result in the deviation of the working points.With the consideration of good tolerance and high accuracy of multimode interference(MMI) couplers, the Dual Parallel Mach-Zehnder Modulator incorporating MMI couplers(MMI-DPMZM) is proposed in the dissertation. For the proposed structure, the Y splitters are replaced by the MMI couplers. Specifically, the Y splitter of the outer MZM is substituted by an adjustable MMI coupler and those of the inner MZMs are replaced by the dissymmetrical MMI couplers. Compared with Y splitters, the MMI couplers in the structure can split the input light more precisely. Analysis into the theory,linearity and the tolerance of MMI-DPMZM demonstrates that the highest SFDR is 129.89 d B when working at the optimized operating point, which is the same as DPMZM. Moreover, the new structure shows excellent tolerance to the deviation of the RF splitting ratio and manufacturing errors of MMI couplers. When the RF splitting ratio deviates from its optimal value in the range of ±10%, the reduction of SFDR can be restricted within 0.1d B through the adjustment of MMI coupler. The dissertation also gives the matched values of the optical splitting ratio when the RF splitting ratio deviates in the range of ±10%. With respect to the effects of manufacturing errors of MMI couplers on the linearity, the deterioration is negligible due to the tolerance of MMI couplers. The SFDR of MMI-DPMZM is 129.60 d B when the uniformity and accuracy of modern MMI couplers are taken into account.