Research On Mode Conversion Between LP₀₁ Mode And LP_11b Mode Based On Asymmetric Directional Coupler

Nowadays, the large transmission capacity of communication system is demanded, just a few decades after the single mode fiber appeared, thanks to the application of wavelength division-multiplexing (WDM), polarization mode multiplexing (PDM) and higher order modulation technology, communication capacity of optical fiber had been improved several orders of magnitude. Predictably, the demand for the transmission capacity will continue to ascend. However the transmission capacity of standard single-mode optical fiber (SMF) is limited to around 100Tbps according to the Shannon limit, optical fiber nonlinearities, as well as fiber fuse effect. In order to improve the carrying-capacity of optical fiber, new technologies constantly are being developed, mode division multiplexing (MDM) is considered to be one of most promising among such various technologies.Mode multiplexer is a critical device to achieve mode conversion, multiplexing and de-multiplexing in MDM system. At the moment, there are three technology platforms to realize the mode conversion:bulk-optic components, optical fibers, and planar waveguides. Mode converter based on planar waveguides has been attracted the attention because of its compact structure, high conversion efficiency and easy integration. Mode multiplexer based on planar waveguides could relized by multimode interference structure, Y branch, directional coupler, etc. Among them, directional coupler has the advantages of simple structure, easy realization and wide working bandwidth. It is easy to realize the coupling between two modes, e.g. LP11a and LP01 modes, so long as the phase matching condition is satisfied and the coupling coefficient is not zero. But for those modes whose field distributions are anti-symmetric, such as LP11b mode, it’s not easy to achieve coupling with LP01 through planar directional coupler because coupling coefficient is zero. To solve the problem, in this dissertation a novel asymmetric directional coupler which was fabricated with polymer material EpoCore and EpoClad is proposed, the proposed coupler has different waveguide heights, so it is able to achieve mode conversion, multiplexing and de-multiplexing between mode LP01 and LP11b.The dissertation is organized as following.At first, MDM system, the research status of the mode converter, the analysis method of the rectangular waveguide and coupled mode theory are introduced. The working principle of the proposed device is also analyzed.Next, the theroretical model and the design process of the device are presented, and the parameters of device are obtained. Then the designed parameters are confirmed by using BPM software simulation. The design of device mainly based on effective index method and finite element method; OptiBPM and Rsoft are used to simulate the designed device, and the simulation results were consistent with the theoretical analysis.Finally, the feasible fabrication techniques are studied, the fabrication procedure of the device is designed and improved according to the research on the performance of the device, and relized device are characterized.The measured results show that device realize the functions of mode convertion, multiplexing and de-multiplexing. The maximum of conversion efficiency of 99.8% at 1545 nm wavelength is obtained, and the conversion efficient is more than 95% over C-band wavelength. In addition, the central wavelength of the device can be tunable at the speed of 11 nm/℃ by using thermo-optic effect. Therefore, the fabricated device can operate over the entire C+L band under the tuning temperature from 24 ℃ to 28℃. The performances of realized device are consistent with its theoretical design, which further prove the validity of the theoretical analysis.