Research On SOI-Based Beam-Control Devices And Their Polarization-Insensitivity

Photonics has been found important applications in rail transit,such as the optical transport network and optical sensing.Optical sensing technology is helpful to the track state detection and the train positioning.The optical transport network plays an important role in the communication of the various systems of rail transit.Both of them are the key to ensure the train run safely.Integrated optics can provide the possibility of highly integrated,high speed,large bandwidth,low power,low cost optical networks and optical sensing.With the development of integrated optics,the Silicon-on-Insulator(SOI)platform has become the preferred platform due to its high refractive index difference,high integration,low loss and low cost.Tunable passive beam-control devices play important roles in integrated optics,including optical power splitters,wavelength filters and modulators,and so on.However,many optical devices are limited by the polarization dependence which is caused by the high refractive index difference of the SOI platform.Thus,the stability of the optical systems will be inevitably reduced.Nowadays,research on the polarization-independent integrated optical devices attracts the attention of worldwide researchers.However,achieving polarization independence,tunability and the basic performance of optical devices in a simple scheme remains to be further studied.In this thesis,two schemes about tunable passive beam-control devices are proposed and verified by numerical simulation,including a polarization-independent and wide-band tunable optical splitter,and a polarization-independent filter with flat-topped pass-band and tunable central wavelength.The details are as follows:(1)Based on the symmetric Mach-Zehnder(MZ)structure and the square cross-section waveguides,a polarization-independent power splitter with wideband and tunable splitting ratios is designed and analyzed.Moreover,the traditional 3-dB directional coupler is optimized by introducing delay to further improved polarization-independent performance and operating bandwidth of splitter.For both TE and TM modes,the splitter has a bandwidth of 63 nm and a low polarization dependent loss of 0.001-0.089 dB.This scheme not only has a simple structure,but also realizes three characteristics of polarization-independence,tunable splitting ratios and wide operating bandwidth.In addition,the tolerance of proposed power splitter is further analyzed to verify the feasibility of machining.(2)A polarization-independent cascaded MZ filter with a tunable central wavelength and flat-topped pass-band utilizing square waveguides is proposed.The simulation results show that the proposed filter has a free spectral range of 6.09 nm,a 3-dB bandwidth of 2.8 nm for both TE and TM modes,and a polarization dependent loss of 0.05 dB.At the same time,based on the thermo-optic effect,the central wavelength of the filter can be adjusted equally for both TE and TM modes.Finally,the tolerance of proposed filter is analyzed to demonstrate the feasibility of machining.