Research On The Coupling Devices Between Silicon Chip And Fiber

Silicon-based photonic integrated chips play an important role in integrated optical communication systems with their unique advantages.In order to achieve high-performance transmission of optical signals between the silicon chip and the optical fiber,it is necessary to introduce a coupling device to bridge the transmission gap for the optical signals between the two mediums.In addition,with the increasing demand for data capacity,mode division multiplexing technology has been proposed to expand the communication capacity in both optical fibers and chips.Therefore,on the basis of a single-mode transmission link,how to design and implement a coupling device which can support higher-order modes in a mode division multiplexing system is of importance.However,limited by the mode field size and waveguide structure mismatch of higherorder modes between fiber and silicon chip,conventional coupling devices are difficult to achieve higher-order mode coupling,and few reports can be found on higher-order mode coupling schemes.Thus,we focus on the two common coupling structures of grating coupler and inverse taper coupler.Based on the analysis of coupling mechanism,the concept of "mode diversity" is proposed for the first time to solve the coupling difficulties of high-order modes.Besides,we propose the device design schemes for the coupling between fundamental modes and higher-order modes,respectively.Furthermore,the applications of the coupling devices in the mode division multiplexing system is explored.The main research contents of this thesis can be summarized as follows:(1)The theoretical basis of the grating coupler and the inverse taper coupler is deeply studied.The Bragg diffraction conditions of the grating coupler and the mode field propagation and coupling mechanism of the inverse taper coupler are theoretically derived.On this basis,the coupling models of the two coupling devices are established respectively,and the main performance indexes are characterized.The influence of the structural parameters of the devices on the coupling performance is analyzed in detail.(2)The device design schemes applied to the fundamental mode coupling are studied.The structure of the linear apodized grating coupler and the cladding-bounded double-arm inverse taper coupler are proposed respectively for the requirements of vertical and horizontal coupling schemes.By linearly control the slit etching widths of the linear apodized grating coupler,the matching degree of the eigenmode field between grating coupler and the singlemode fiber is optimized.The device design and fabrication process can be simplified while the coupling efficiency can be improved.As for the cladding-bounded double-tip inverse taper,we attempt for the first time to realize the deep trench etching method on the chip surface to restrain the mode field of the chip cladding,in order to improve the matching degree with the mode field in fiber,while the double-arm inverse tapers are introduced to improve the mode field collection and guiding capability.Both coupling structures have been demonstrated to achieve coupling efficiency improvement relative to conventional structures.(3)The device design schemes applied to higher-order mode coupling are studied.Based on the "mode diversity" concept,we propose and implement for the first time a compact double-part grating coupler and a double-tip inverse taper coupler based on a conventional silicon chip with the merits of simple structure and high fabrication compatibility,in order to meet the need for vertical and horizontal coupling scheme,respectively.By the experimental measurements,the high-performance coupling of the two structures for the first-order mode is verified,and the structures can be also compatible with the coupling of the fundamental mode.In addition,for the double-part grating coupler,we fulfill a compact structure in which the taper length is 20 times shorter than conventional grating coupler structures.(4)We propose and demonstrated for the first time to directly utilize a coupling device for the application of on-chip mode conversion.A fork-like inverse taper mode converter is proposed for the horizontal coupling scheme.With the higher-order mode coupling between fiber and chip,the mode conversion can be realized directly on the chip,without the need for additional higher-order mode excitation and multiplexing devices.Good coupling and mode conversion performance can be obtained under different switching cases.Furthermore,we propose a theoretical improvement scheme for the fork-like inverse taper mode converter,which can further improve the coupling performance of the fundamental mode,and fully realizes a complete conversion function for both the fundamental and the first-order modes between fiber and silicon chip.(5)We also study the novel design scheme of optical switch in the mode division multiplexing system.Crossing-free on-chip 2×2 mode/polarization-transparent switch is proposed.A polarization-transparent switch is constructed by introducing the transparent beam splitter/combiner in polarization dimension,and the mode converter is introduced to further achieve the expansion into the mode dimension.The scheme can simultaneously realize the switching and routing of the two-group mode division multiplexed signals between fiber and chip,while avoiding the waveguide crossings compared with the conventional structures.The expansion of the mode capacity is realized and a flexible manipulation for the signals of each mode channel can be achieved.By measuring the performance of the polarization/mode switch,good spectral characteristics and clear eye diagrams with low power penalties can be obtained under different switching cases.