Research On Silicon-based Mode-order Conversion With Scalability For On-chip Multiplexing Transmission

Silicon-based photonic integrated circuit(PIC)integrates all kinds of photonic devices on a mature silicon technology platform to form a highly integrated loop structure that can complete specific functions.This kind of optical communication loop has the advantages that the electronic integrated circuit does not have,such as large bandwidth,low power consumption and large capacity,etc.,which shows great potential in the communication system.Silicon material has a high refractive index and good transmission characteristics of light,is an excellent carrier for the production of optical transmission devices,a large number of different types of optical devices based on silicon have been designed,such as optical detectors,modulator,beam splitters and mode converters,etc.Since light has multiple reusable dimensions,multiplexing technology is often used in optical transmission systems to improve the transmission capacity of the system.In the mode division multiplexing technology based on different optical modes,the mode converter can realize the conversion between different modes and generate different mode channels,which is the basis of the mode division multiplexing system.The mode conversion types can be roughly divided into three categories:conversions between TE modes,conversions between TM modes and conversions between TE and TM modes,and the realization of the mode converter structure is diverse,such as:metasurface structure,etching structure,grating structure and the introduction of medium or metal structure,etc.In addition,based on the basic principle of mode conversion,various structural optimization algorithms,such as topology optimization,reverse design and deep learning algorithm,are proposed by using advanced computational forces and algorithms.In this paper,three scalable mode conversion schemes are proposed to realize different types of mode conversion by using shallow etching and introducing metal(mode-order conversion is mode conversion of changing mode orders).1.conversion between TM modes,TM₀-TM₂ conversion is realized by shallow etching on the silicon waveguide surface,which introduces a perturbation pattern on the silicon surface.Under the effect of equivalent medium and mode coupling,the initial TE₀ mode will accumulate phase difference during transmission and form three beams in TM₂.In addition,the shallow etching pattern used in the device has good extensiveness.By adjusting the structure,a TM₀-TM₁ converter is obtained,and the conversion efficiency is 85%.2.The conversion between TE and TM mode,using the strong effect of metal on light and plasma effect,the TE₀-TM₁ mode converter containing metal is proposed.Metal layer is deposited directly on the surface of the silicon waveguide,three segments using the appropriate thickness,suppress the metal light loss,has good performance,and the length is3.5?m.3.Conversion between TE modes.Based on metal tapers,a series of TE mode converters are proposed to realize the conversion of TE₀-TE_n.Only TM-polarized mode can exist in the surface between silicon and metal,which can be used to reduce the coupling between TE mode and TM mode with designed metal structure.however,the TE mode is still susceptible to the strong confinement of the metal,then the TE mode can be transformed into TE mode of higher order through the effect of mode evolution.The series of TE mode converter can be achieved with compact size,due to the existence of metal shortening the length of mode evolution.The above-mentioned three schemes are supplementary to different types of mode converters,the TM₀-TM₂ converter offers reference to the less-studied TM mode conversion,especially,the third scheme proposes a series of mode conversion,using the metal structure to reduce the device size,which is expected to use in the future photonic integrated circuits.