Research Of Optical Coupler Based On SOI

Integrated photonics has attracted more and more attention because of its compatible fabrication process with integrated electronics.Silicon-on-insulator(SOI)is regarded as the most promising platform to perform integrated photonics due to the matured fabrication technologies and compatibility with complementary metal-oxide-semiconductor(CMOS).A 1 ×3 coupler with variable power splitting ratio is one of the most essential components in integrated photonics,because it plays a crucial role in complex photonic integrated circuits(PICs)applications such as power taps,switches,high-Q ring resonators,Mach-Zehnder interferometers(MZI),etc.Typically,1×3 couplers have been implemented by two types of methods,as directional coupler and multimode interference(MMI)coupler.Among them,directional coupler which is based on coupling theory has been widely used due to its simple configuration,flexible design for variable splitting ratio and easy fabrication on SOI platform.Compared to 1×3 directional couplers,1×3 MMI couplers have advantages of more compact size,better tolerance to fabrication parameters and most importantly broader bandwidth.Nevertheless,conventional 1×3 MMI coupler can only achieve even power splitting ratio of three outputs.Therefore,several approaches have been reported to obtain variable power splitting ratios by using a bent MMI coupler,a butterfly-shaped MMI with a taper-width,or cascading two short MMI sections,etc.This dissertation mainly focuses on the optical couplers,especially directional coupler and MMI coupler with variable power splitting ratio(PSR)on SOI.The main research efforts are summarized as follow.1.A simulation platform of 1×3 directional coupler based on SOI has been built.It was found in simulation that the choices of parameters affect the performance of couplers significantly,the parameters includ:the radius of bend waveguide,spacing gap,the coupling length of waveguide,etc.Simulation is carried out mainly in studing various features of 1×3 directional coupler with PSR of 1:2:1,such as insertion loss,operation bandwidth,electric field,magnetic fields and energy diagram.Simulation results show that the insertion losses of 1×3 directional coupler with PSR of 1:2:1 are lower than 0.1 dB.But the operation bandwidth of the directional couplers is very poor,so we design and simulate 1×3 adiabatic coupler of which coupling area is taper type in order to improve the operation bandwidth.Simulation results show that when wavelength changes from 1400nm to 1700nm,PSR of the middle output waveguide in adiabatic coupler changes by 56%,while the PSR in directional coupler changes by 74%,which indicates that the adiabatic coupler improves the bandwidth characteristics.2.We propose and demonstrate a novel 1×3 MMI couplers based on SOI with variable power splitting ratio for the first time,which has broadband operation,compact size,simple structure,and easy fabrication.We analyzed the performance of 1 X 3MMI coupler with rectangular gap with PSR of 1:2:1 by using 2.5 variational-the finite difference time domain method(2.5 Var-FDTD).This scheme is much smaller than previously reported 1 × 3MMI coupler,which is very comprtitive for smaller integration.Moveover,the scheme has not only compact structure,but also small insertion loss.Most importantly,its operation bandwidth is broad.Couplers with quasi-arbitrary PSR are obtained at the wavelength of 1550nm for transverse electric(TE)light.Simulation results show that PSR of middle output changes by 2.2%while the operation bandwidth changes from 1500nm to 1600nm,the insertion losses keep as low as 0.3 dB.We also design 1 × 3 MMI couplers with triangular gap,the simulation results show that from 1530nm to 1600nm,PSR of middle output waveguide changes by 3.9%,while the insertion losses keep as low as 0.21 dB.Although,the coupler with triangular gap does not improve the operation bandwidth,but improves the insertion loss.3.The 1×3 directional coupler and MMI couplers with SPR of 1:2:1 are fabricated and tested on a test platform with semi-automatic coupling system.Experimental results show that directional coupler has insertion loss of 0.1 dB,and PSR of 25.9%:48.3%:25.8%;while MMI coupler has insertion loss of 0.2 dB,and PSR of 25.4%:49.6%:25%.The PSR and insertion loss of the two types of couplers agree well with the design and simulation results,which verify the feasibility of our scheme.The simulation of fabrication tolerance of two types of couplers proved that the scheme can be made into semiconductor integrated device,and has great potential in practical application in the future.