基于硅基砖砌型亚波长光栅的紧凑型模式转换器

Silicon-based compact mode converter using bricked subwavelength grating

亚波长光栅可以等效为均匀介质,具备可控的双折射、色散和各向异性等优势,有利于设计高性能的光子器件.尽管目前传统的亚波长光栅结构只需要单步刻蚀,然而通常需要100 nm及以下的制造分辨率,这对当前主流的晶圆级硅光子芯片制造技术来说比较困难.亚波长光栅的各向异性可以通过引入砖砌型拓扑结构来进一步设计,从而在设计中提供额外的自由度,同时还可以降低制造分辨率需求(>100 nm).本文提出并研究了基于硅基砖砌型亚波长光栅的紧凑型TE_(0)-TE_(1)和TE_(0)-TE_(2)模式转换器,其中砖砌型亚波长光栅的最小特征尺寸为145 nm.实现了TE0模式到TE1模式和TE_(2)模式的转换,转换区域长度分别为9.39μm和11.27μm.测试结果表明,在68 nm(1512-1580 nm,受限于激光器调谐范围和光栅耦合器)带宽内,插损和串扰分别小于2.5 dB和-10 dB.

Facing the increasing capacity requirements of on-chip optical interconnects,mode division multiplexing technology(MDM),which fully uses the different spatial eigenmodes at the same wavelength as independent channels to transmit optical signals,has attracted tremendous interest.Mode-order converter that can convert the fundamental mode into high-order mode is a key component in MDM system.However,it is still very challenging to achieve compact mode-order converters with high performances.Subwavelength grating(SWG)can be equivalent to homogenous material,which has the prominent advantages such as controlling over birefringence,dispersion and anisotropy,thus making photonic devices possess high performance.Wheras the conventional SWG only needs single-etch step,but the implementation of SWG structure usually requires a fabrication resolution on the order of 100 nm and below,which is difficult for current wafer-scale fabrication technology.The anisotropic response of SWG can be further engineered by introducing bricked topology structure,providing an additional degree of freedom in the design.Meanwhile,the requirement for fabrication resolution can also be reduced(>100 nm).In this work,we experimentally demonstrate compact TE_(0)-TE_(1) mode-order converter and TE_(0)-TE_(2) mode-order converter by using a bricked subwavelength grating(BSWG)based on a silicon-on-insulator(SOI)with the BSWG having a minimum feature size of 145 nm.In the proposed mode-order converter,a quasi-TE_(0) mode is generated in the BSWG region,which can be regarded as an effective bridge between the two TE modes to be converted.Flexible mode conversion can be realized by only choosing appropriate structural parameters for specific mode transitions between input/output modes and the quasi-TE_(0) mode.By combining three-dimensional(3D)finite difference time domain(FDTD)and particle swarm optimization(PSO)method,TE_(0)-TE_(1) mode-order converter and TE_(0)-TE_(2) mode-order converter are optimally designed.They can convert TE_(0) mode into TE_(1) and TE_(2) mode with conversion length of 9.39μm and 11.27μm,respectively.The simulation results show that the insertion loss of<1 dB and crosstalk of<-15 dB are achieved for both TE_(0)-TE_(1) mode-order converter and TE_(0)-TE_(2) mode-order converter,their corresponding working bandwidths being 128 nm(1511-1639 nm)and 126 nm(1527-1653 nm),respectively.The measurement results indicate that insertion loss and crosstalk are,respectively,less than 2.5 dB and-10 dB in a bandwidth of 68 nm(1512-1580 nm,limited by the laser tuning range and grating coupler).