Research On Mode Conversion And Multiplexing With Vertical Polymer-Waveguide Coupler

People continue to open up new technologys, including dense wavelength division multiplexing(DWDM), polarization division multiplexing(PDM), and etc, to improve single-mode fiber(SMF) transmission capacity. However, with the rapid rise of computer technology, people's demand for information shows an explosive growth.Meanwhile, the transmission limit of a SMF is estimated at about 100 Tbps. SMF can not meet the demand for this kind of information growth in the near future. Thus it is necessary to seek new technology to further enhance the transmission capacity of optical transmission network. Mode division multiplexing(MDM) technology is one of promising solutions to the problem.Mode (de)multiplexer and mode converter are key devices in MDM system.Currently, mode conversion and multiplexing can be realized with photon lanterns, Y branches, long period gratings, directional couplers and etc. Compared to plannar directional coupler is constrained by the equal height of two cores, the design of vertical directional coupler is more flexible. In this thesis, the LP01-LP21a? LP01-LP21b and LP01-LP02 mode converters, which are based on vertical directional coupler, are designed and fabricated by using the polymer materials EpoCore and EpoClad as core and cladding respectively because the polymer material has the advantage of easy processing. The core content of the thesis is organized as following.Firstly, studing the LP01-LP21a mode conversion. The LP01-LP21a mode converter is designed with coupled-mode theory and fabricated by micro-processing technology. A typical fabricated LP01-LP21a mode converter is weakly polarization correlated. TE(TM)polarization reaches optimal conversion efficiency 98.2%(94.9%) at wavelength 1530 nm, the conversion efficiency gradually decreased with the wavelength to the long wavelength direction in the C-band, TE(TM) polarization reaches minimal conversion efficiency 90.1%(88.8%) at wavelength 1530 nm. The parameters of the fabricated device are nearly identical with the theoretical design.Secondly, studing the LP01-LP02 and LP01-LP21b mode conversion. Different from the design of LP01-LP21a mode converter, LP01-LP02 and LP01-LP21b mode converter can not be realized by direct coupling between the two modes because of the degeneracy of LP21b and LP02. In the dissertation, LP01-LP02 and LP01-LP21b mode conversion are realized by coupling LP01 to E13 at first and then evolving into LP21b or LP02 through the tapered waveguide. The typical fabricated LP01-LP02 mode conerter is polarization correlated due to the stress of the multilayer structure. This device realizes LP01-LP21b mode conversion for TE polarization and LP01-LP02 mode conversion for TM polarization. TE(TM) polarization reaches Optimal conversion efficiency 98.0%(97.7%)at wavelength 1530 nm, the conversion efficiency sharply decreased with the wavelength to the long wavelength direction in the C-band, TE(TM) polarization reaches minimal conversion efficiency 65.5%(55.5%) at wavelength 1560 nm. The conversion efficiency of fabricated LP01-LP21b mode converter is extreme low, because the parameters of the fabricated device are greatly different from the theoretical design,which make the modes don't meet the phase matching condition precisely.