Research On Polarization Properties Of Subwavelength Gratings And Its Application For Photodetectors In Optical Communication Systems

Subwavelength Gratings (SWGs) with a periodic structure in optical communication system has many excellent characteristics such as small size, low cost, high diffraction efficiency, etc. When gently designed by modifying structure parameters, SWGs can be applied in many scenes in optical communications:grating filter, broadband mirror reflector, polarization beam splitter, grating couplers and so on. In these applications, the SWG based on silicon-on-insulator (SOI) structures with the good characteristics of Si-based materials can be easily integrated into the lasers, photodetectors, and other optical devices. Therefore, the research on the subwavelength grating is of great significance for the development of optical communication systems.In this dissertation we have discussed the polarization properties of SWG and make analysis using relative theory. Different simulation software is used to make verification for the designed grating structure. A novel quantum efficiency enhanced photodetectors based on the SWG reflector structures is also proposed. The main content of the work can be summarized as follows:1. The theories and methods of grating analysis, including rigorous coupled-wave analysis(RCWA) theory and modal theory are introduced. We summarize and compare the characteristics of the two methods, which are discussed in different polarized situation. At the same time, the theoretical model described by Eigen equation of equivalent refractive index is made to explain the formation mechanism of the high diffraction efficiency on grating surface. By analyzing the values of grating period, duty cycle and fill factor, An optimization method for high reflective mirror grating is designed.2. RCWA methods are used to designed a polarization insensitive SWG with high reflectivity based on SOI structure. This SWG works in the range of the incident light from1520nm to1560nm, for the TM polarization its reflectance is higher than95%, and for the TE polarization reflectance more than94%. This range meets the requirements of the optical mirror in the C-band communications.3. A polarization insensitive high reflective grating structure is fabricated. The results of the experiments show that its reflectivity spectrum reaches from1480nm to1610nm, experiments and simulation results are consistent with reaching more than90%for TM polarization of the incident light, while high reflective area appeared an offset for TE polarization. An explain for this is given by analyzing the situation.4. A quantum efficiency enhanced photodetectors based on highly reflective subwavelength grating are proposed by using BCB glue. Then we make the photodetector performance experiments of quantum efficiency and high-speed response under the laboratory test systems.3dB bandwidth of the photodetectors is more than10Gb/s and the quantum efficiency is increased by more than10%because of the high reflectivity SWG.