A Study On High Performance On-chip Polarization Beam Splitting Based On Subwavelength Gratings Structure

Since the beginning of the 21 st century,emerging technologies such as cloud computing and the Internet of Things,along with the rapid proliferation of various smart terminals,have led to an increasing demand for data communication and increasingly higher requirements for transmission speeds in both work and daily life.Integrated silicon photonic chips exhibit numerous advantages,such as compatibility with complementary metal-oxide-semiconductor(CMOS)technology,compact structure,low cost,high reliability,and the ability to achieve optoelectronic hybrid integration,which has attracted growing attention and research in many fields,including optical communications interconnects,data centers,5G wireless communications,new radar systems,and high-performance computing.Polarization diversity systems are commonly used to address polarization sensitivity issues in optical interconnects,utilizing a polarization beam splitter to separate two polarization modes.In this paper,we will optimize the critical performance parameters of polarization beam splitters and conduct relevant research work.The main content of the paper is as follows:(1)By combining subwavelength gratings with traditional straight waveguides,we design an asymmetric directional coupler-type polarization beam splitter.Compared with traditional directional coupler-type polarization beam splitters,this device retains the advantages of simple structure while exhibiting better transmission characteristics.Meanwhile,we adopt a threewaveguide parallel structure to reduce device dimensions.The subwavelength grating structure in the middle forms an asymmetric coupling region,coupling with the TE polarization mode and mismatching with the TM polarization mode.The TE polarization mode is coupled to the output straight waveguide after passing through an appropriate length in the grating,while the TM polarization mode,mismatched with the grating,is confined in the input straight waveguide,thus achieving polarization beam splitting.(2)By combining subwavelength gratings with different duty cycles and uniform width variations with tilted subwavelength gratings,we design a polarization beam splitter with ultrahigh bandwidth and a compact size.First,we utilize the effective refractive index difference between subwavelength gratings with different duty cycles to match the upper conical subwavelength grating with the TE polarization mode and mismatch with the TM polarization mode,allowing the input TE polarization mode to be coupled upward.The tilted subwavelength grating produces distinct effects on the two different input modes: it blocks coupling of the TE polarization mode but does not affect the TM polarization mode,allowing the input TM polarization mode to be coupled downward.This double-asymmetric structure gives the device a significant advantage in size and bandwidth.(3)In order to further improve the device performance,we introduce a grating offset to enhance the transmission efficiency of the TM polarization mode.In addition,we also redesign the transition structure from the subwavelength grating to the straight waveguide with a new conical structure that makes the refractive index transition more uniform,reducing polarization mode loss.The slot waveguide in the new structure filters out unwanted polarization modes at the output port,thereby improving the extinction ratio.After discussing and optimizing the device parameters,we analyze the operational bandwidth and manufacturing tolerances.