Research On Subwavelength On-chip Optical Mode Manipulation Devices And Applications

With the development and popularity of Internet technology,the explosive growth of global information data has become a trend,and the amount of data generated globally has exceeded tens of billions of gigabytes per day,and this number is still growing.The growth rate of data has exceeded the storage and processing capacity of traditional servers,so new data processing technologies and tools need to be continuously explored and developed to meet the growing data demand.In this context,integrated optical chips are one of the solutions for massive data processing due to their high bandwidth,high integration and low power consumption.Optical mode multiplexing technology can enhance the communication bandwidth in a limited spectral range.Optical mode modulation can be achieved by precisely controlling the geometry,material properties and coupling structure of the waveguide,as well as by combining external modulation,etc.The maturity of advanced semiconductor processes makes it possible to precisely modulate the modes in the waveguide.Subwavelength structures allow flexible control of the guided waves because they can achieve different equivalent refractive indices for electromagnetic waves with wavelengths larger than the subwavelength feature size on the same material,giving the devices efficient optical field modulation in a compact footprint.First,this paper investigates the mechanism of subwavelength grating structure on mode field modulation,based on which two subwavelength grating reconfigurable mode converters are designed;then,in order to enhance the integration of subwavelength devices,an ultra-compact multi-channel and multi-polarization subwavelength super-surface mode switcher is realized by the inverse design method;after that,in order to enhance the flexibility and non-volatility of mode field modulation of subwavelength super-surface devices.phase change materials are introduced in the optimization process to design a non-volatile mode switch;finally,this paper proposes a gradient-based photonic inverse design algorithm,which improves the efficiency of the algorithm using a gradient-driven approach and enhances the optimization efficiency of the superconfigurable surface to provide strong support for the development of on-chip optical communication systems.The details of the research are as follows:（1）In order to achieve highly flexible on-chip mode control,this paper uses a one-via wavelength grating with a special cladding design to achieve flexible modulation of the output amplitude and output mode of the mode converter.The multi-state mode converter can achieve three modes of TE₀,TE₁ and TE₂ at 1550 nm with 59.1%,73.9%and 56.7%output efficiency respectively using different cladding layers.The multimode tunable optical attenuator achieves stepless adjustment of multimode output amplitude from 0.5-96.4%within the temperature rise range of 400 K.（2）In order to compress the device size of the subwavelength optical mode modulation device and achieve more complex functions,this paper introduces a two-dimensional degree of freedom subwavelength superconfiguration surface to realize the modulation of mode and polarization.The mode-switching device is capable of TE₀-TE₂,TE₁-TE₃ interchange with an area of 4×3μm.The insertion loss range is less than3.6 d B and 5.1 d B in the bandwidth of 1525-1565 nm,and the inter-mode crosstalk is less than-10.7 d B and-13.1 d B,respectively.The dual polarization mode switch enables interchange between TE₀-TE₁ and TM₀-TM₁ with an area of 4×1.6μm.The insertion loss range is less than 2.3 d B and the crosstalk are less than-11.5 d B over the bandwidth range.（3）In order to make the subwavelength super-surface device tunable and non-volatile,phase change materials are introduced in the device design in this paper.In this paper,we design different structures of mode switches for three different phase change materials,Ge₂Sb₂Se₄Te₁,Ge Se and Sb₂S₃,to achieve non-volatile switching of multiple modes.The minimum device size is 5μm,and the mode switching is achieved in the bandwidth range of 1500 nm-1600 nm with insertion loss below 1.6 d B and crosstalk below-11.7 d B.（4）To further improve the design scale and design speed of multi-degree-of-freedom free-form subwavelength devices and to improve process compatibility.A gradient-based inverse design algorithm is proposed to improve the design efficiency of super-surface devices by more than two orders of magnitude relative to the conventional binomial tree search method.The algorithm improves the optimization efficiency by global gradient optimization,selecting the pixel with the highest weight in each iteration for flipping,while eliminating the need for an additional binarization process and maintaining good critical size constraints.The algorithm can help achieve fast and efficient photonic device design.