Development Of Polymer-waveguide Asymmetric Directional Couplers For Mode-division Multiplexing Applications

The network traffic in optical fiber communication systems has increased rapidly, and new technology such as space-division multiplexing, mode-division multiplexing and multi-level modulation,aiming at realizing a much larger capacity, have received increasing attention.By utilizing MDM,wavelength division multiplexing and polarization-division multiplexing, the capacity of single-core transmission can be enlarged greatly.In MDM system, it is the most important thing to design a mode multiplexer/demultiplexer(MUX/DEMUX) as well as mode converter. To solve this problem, an asymmetric directional coupler is proposed and fabricated in this dissertation, which can multiplex and demultiplex E11 mode with E21 mode using EpoCore and EpoClad materials and a tunable laser as a light source.What’s more, the EpoCore and EpoClad polymer materials have good thermo-optic effect, according to this characteristics, the operating point and the coupling efficiency can be controllable.First, the operation principle of the asymmetric diretional coupler is studied and the thermal-optic effect performance is analyzed. By adding electrodes upon the upper cladding of the asymmetric coupler to control the index of the two cores respectively, we can tune the operation point and the coupling efficiency.Then, the structure of the asymmetric directional coupler is emphasized, the relations between the coupling gap and the coupling length, and the influence on coupling efficiency caused by wavelength, the width of the waveguide,the index value are simulated by Comsol Multiphysicsand BPM. It is indicated that the fabrication tolerance of the wider waveguide width is only 0.1 μm, and that of the waveguide height is more than 2μm, and the index value can changed by heating the cores. So it is important to control the waveguide width when fabricting the sample. More important the coupler we designed can achieve 98% coupling efficency during the whole C band(1530-1565nm). We also simulated the transimission loss of the S-wavegude and the taper waveguide, then we have all the device parameters done.Finally, the mask is designed and fabricated based on the device parameters done above. The mask is divided into two parts, one is for electrode and the other for waveguides.We fabricated markers on the corresponding place of the two parts respectively for the sake of adding electrode later. Then the fabrication processes of the core,cladding and the electrode as well as problems solved during the process are introduced. We investigated the mode conversion operation and the near field patterns showed that there is more than 97%power conversion near 1545 nm.