Research On Optical Bandwidth Improvement Of Multimode Interference Devices

In recent years, with the rapid growth of requirements on opticalinformation processing, people are paying more attention to siliconphotonics due to significant breakthroughs they have made. Multimodeinterference (MMI) couplers based on self-imaging effect have been widelyused as optical couplers, optical splitters, or other functional devices, owingto their advantages of low excess loss, large fabrication tolerance, andcompact size. But multimode interference devices are wavelengthdependent, because of their intrinsic interference mechanism.In this thesis, working principles of basic multimode interferencecouplers are presented firstly, and general self-imaging rules are studied indetail. Then the performance variation with the operating wavelength iscarefully analyzed. At last, we summarize two main reasons that limit thebandwidth: first, the imaging points are wavelength dependent; second, theimaging quality can be greatly affected by the calculation accuracy.Based on the theoretical analysis, we present three methods that canimprove the optical bandwidth of multimode interference devices.Furthermore, we simulate these structures to verify their performances. Thesimulation results fit well with the theoretical analysis. Compared with theregular structure whose bandwidth at1dB excess loss is only60nm, theproposed structures have broader bandwidth in the range of80nm to150nm.Finally, we combine multiple design approaches in one structure, and getsignificantly improved device performance after optimization. The1dBbandwidth is as high as300nm, which is5times larger than the regular one.Finally, we give the applications of the designed broadband multimodeinterference couplers. In fact, the wavelength dependence of certain optical devices are greatly alleviated by taking advantages of the designed MMIstructures.