基于硅波导分布布拉格取样光栅的四通道Ⅲ-V/Si激光器阵列

A Four-channelⅢ-V/Si Laser Array Based on Silicon Waveguide Distributed Bragg Sampled Grating

提出了一种波长间隔0.8 nm的四通道Ⅲ-V/Si激光器阵列。在四通道硅波导表面设计了两组分布布拉格取样光栅,分别作为前反射镜和后反射镜,形成Ⅲ-V/Si激光器的谐振腔。优化设计分布布拉格取样光栅的参数,选择四个通道的取样光栅的一阶子光栅对应的波长进行振荡和输出。采用直接晶片键合技术,将图案化的绝缘体上硅晶片和Ⅲ-V外延晶片非集成在一起,实现了Ⅲ-V波导与硅波导的自对准和高效倏逝波耦合。在室温下连续波条件下,制备的四通道Ⅲ-V/Si激光器阵列的硅波导输出功率均大于0.7 mW@60 mA,阈值电流均小于25 mA,激射波长分别为1 569.64 nm、1 570.45 nm、1 571.27 nm和1 572.08 nm,波长间距为0.8 nm±0.2 nm。这种四通道Ⅲ-V/Si激光器阵列经优化后可应用于密集波分复用硅光学系统。

In modern optical communication,Dense Wavelength Division Multiplexing(DWDM)technology provides the possibility of simultaneous transmission of multiple signals and improvement of information transmission capacity,and the multi-wavelength monolithic integrated silicon-based laser arrays with small wavelength intervals in a dense DWDM silicon-based optical system has become a research hotspot and a difficult point.However,traditional multi-wavelength monolithic integrated silicon-based laser array with 0.8 nm wavelength spacing based on first-order uniform gratings are difficult to achieve due to the limitation of sub nanometer processing accuracy.In this paper,a four-channel monolithic integrated Ⅲ-V/Si laser array with 0.8 nm wavelength spacing based on silicon waveguide distributed Bragg sampling gratings is demonstrated.Firstly,it is proposed to design and manufacture a set of distributed Bragg sampling gratings at both ends of the silicon waveguides in four channels,as front and rear mirrors,to form the resonant cavity of each channel Ⅲ-V/Si laser.Secondly,by changing the micro-meter level sampling period of the silicon waveguide distributed Bragg sampling grating of four channels,four different wavelengths corresponding to the+1st sub-gratings of the four channel silicon waveguide distributed Bragg sampling gratings are selected to oscillate in the resonant cavity and emit.In the proposed laser,the silicon waveguide′s width,height,and ridge etch depth are fixed as 1.5μm,0.34μm,and 0.22μm,respectively.The Ⅲ-V layer stack has eight strained InAlGaAs Quantum Wells(QWs)with graded index separate confinement hetero-structure layers.The front and rear mirrors′length are set 100μm and 400μm,respectively.In the simulation design,the etching depth of the seed grating of the Bragg sampling grating of the silicon waveguide distribution of each channel is set to 30 nm,the duty cycle is 50%,the period of the seed gratingΛ0 is 266 nm,and the other parameters are the same as those in 1.1 of this paper,and the corresponding excitation wavelength of the seed grating is 1635 nm.Then the overall effective refractive index neff of the silicon waveguide distribution Bragg sampling grating of one of the channels is calculated,and the sampling period P of the silicon waveguide sampling grating of this channel is set to 6.3μm,the excitation wavelength corresponding to the positive first-order seed grating of the silicon waveguide sampling grating of this channel is calculated to be 1570 nm.According to the above method,keeping other parameters unchanged and only changing the sampling period P of the other three channels of silicon waveguide sampling gratings,three different wavelengths corresponding to the positive primary sub-gratings of the three channels of silicon waveguide sampling gratings are calculated.Finally,four different excitation wavelengths of 1569.2 nm,1570 nm,1570.8 nm,1571.6 nm are obtained for the four-channelⅢ-V/Si laser array based on the silicon waveguide distributed Bragg sampling grating.The four+1st wavelengths are within the gain spectrum of Ⅲ-V epitaxial material,while the corresponding 0st wavelength of the seed grating is outside the gain spectrum of Ⅲ-V epitaxial wafer,which satisfies the excitation conditions of the corresponding wavelengths of the positive 1st order sub-gratings of the sampling grating of the four-channel Ⅲ-V/Si laser array.Meanwhile,in order to improve the coupling efficiency between the Ⅲ-V active waveguide and the silicon waveguide,a two-stage Ⅲ-V tapered waveguide is designed at both ends of the Ⅲ-V active waveguide for each channel of the Ⅲ-V/Si laser.The first segment is the Ⅲ-V tapered waveguide 1,and the second segment is the Ⅲ-V tapered waveguide 2.The width of the Ⅲ-V tapered waveguide 1 decreases from 4μm to 1μm,and the width of the Ⅲ-V tapered waveguide 2 decreases from 1μm to 0.8μm.Simulation demonstrates that when the length of the Ⅲ-V tapered waveguide 2 is larger than 40µm,the evanescently coupling efficiency between the Ⅲ-V active waveguide and the silicon waveguide reaches more than 99%,and the light can be efficiently coupled from theⅢ-V active waveguide into the silicon waveguide.To fabricate such a device,firstly the distributed Bragg sampling and silicon waveguide are fabricated on the 100-oriented silicon-on-insulator wafer by photolithography,e-beam lithography and dry etching.Then,the Ⅲ-V epitaxial wafer is transferred onto SOI wafer with a low temperature directly wafer bonding technology.Ⅲ-V epitaxial wafer and SOI wafer need acetone isopropanol cleaning and HF aqueous solution surface treatment before directly wafer bonding.After physical connecting ofⅢ-V epitaxial wafer and SOI wafer,the wafers are put into a wafer bonding machine with a 1.5 MPa pressure and low vacuum,under the 150℃bonding temperature for hours.By solving the directly wafer bondingtechnology,the patterned silicon on insulator wafers andⅢ-V epitaxial wafers is heterogeneous integrated together,achieving self-alignment of four-channel Ⅲ-V waveguides and four-channel silicon waveguides without the sub-micrometer level passive or active alignment technology.Finally,after three-stepⅢ-V etching,4μm width and 1.7μm depth current channel is formed to prevent lateral diffusion of carriers,MQW taper is to couple light to the silicon waveguide effectively,N-InP etch is to remove N-InP on the grating,respectively.Ti/Au metal stack is deposited as contact metals for p-type and n-type electrodes.The fabrication of micrometer level sampling period avoids the sub-nanometer processing required for fabricating first-order uniform gratings.Ultimately,a four-channel Ⅲ-V/Si laser array is fabricated.Under continuous wave conditions at room temperature,the output power of the single-wavelength from each channel silicon waveguide is greater than 0.7 mW@60 mA,the threshold current is less than 25 mA,and the lasing wavelengths are 1569.64 nm,1570.45 nm,1571.27 nm,and 1572.08 nm,respectively,with a wavelength spacing of 0.8 nm±0.2 nm.This type of the Ⅲ-V/Si laser array is easy to integrate on a large area with high density,and can further achieve more channels of Ⅲ-V/Si laser arrays with wavelength spacing of 0.8 nm or even smaller.After further optimization,this type of Ⅲ-V/Si laser array can be applied in dense wavelength division multiplexing silicon optical systems.