Research Of Polarization-Insensitive Silicon-Based Devices Based On Multimode Interference

With the continuous development of optical communication networks,silicon-based devices based on the photonic integrated circuits(PIC)have received widespread attention.However,most of the silicon-based devices are polarization-sensitive,which will limit their applications.Therefore,the research on polarization-insensitive silicon-based devices has significant values.As an important optical waveguide coupler in the PIC field,the multimode interference(MMI)coupler can realize many functions such as the splitting of optical power and the separation of different wavelengths.In addition,the MMI coupler also has advantages of simple structure,low loss and good tolerance.In summary,this paper has done the following works around the design of polarization-insensitive silicon-based devices based on MMI couplers:(1)A polarization-insensitive 1×3 MMI optical power splitter is designed to realize 1×3 uniform power splitting at 1550 nm wavelength.The device is designed by symmetrical interferences.The polarization insensitivity is obtained by adjusting the refractive index of Si N_x in the sandwiched structure.A new combination structure of trapezoidal MMI waveguide and tapered waveguide is adopted.The upper width and waist angle of the trapezoidal MMI waveguide are adjusted to realize a broader bandwidth and a better splitting uniformity.The results show that the size of the designed splitter is only 13.2?m;the excess loss and imbalance of the splitter are low less 0.07 d B and 0.03d B respectively.In addition,the 0.5 d B bandwidth is higher than 255 nm,which can cover S,C,L,U and part of E bands.(2)A polarization-insensitive MMI demultiplexer based on the silicon-on-insulator platform is designed to separate the 1310 nm wavelength from 1550 nm wavelength.The device is designed by paired interferences.The Si₃N₄ material has been used instead of traditional Si O₂ material as the cladding layer,which can further reduce the loss of the device.The thickness of the single-layer Si waveguide is adjusted to realize the polarization insensitivity.The width and length of the MMI waveguide are optimized,so that two different wavelengths can be separated through mirrored image and direct image respectively.The results show that the insertion loss and crosstalk are as low as 0.31d B and-20.7 d B respectively,and the 3 d B bandwidth can reach 70 nm.(3)An ultra-compact and polarization-insensitive MMI demultiplexer based on Si₃N₄/Si O₂platform is designed to separate the 1310 nm wavelength from 1550 nm wavelength.The device is designed by general interferences.The polarization insensitivity is obtained by adjusting the refractive index of Si N_x in the sandwiched structure.The relationship between the mirrored and quasi-state images is applied and a cascaded MMI waveguide is introduced to realize the separation of two wavelengths.The results show that the size of the demultiplexer is only 3.4×74?m²,the insertion loss and crosstalk are as low as 0.45 d B and-27.2 d B respectively.Besides,the demultiplexer has a large 3 d B bandwidth up to 190 nm.(4)A compact and polarization-insensitive MMI triplexer is designed by general interferences and the principles of photonic band gaps.The triplexer can realize the demultiplexing of 1490 nm and 1550 nm downstream wavelengths as well as uploading the 1310 nm upstream wavelength.The polarization insensitivity is obtained by adjusting the refractive index of Si N_x in the sandwiched structure.A triangular lattice array air-hole photonic crystal embedded in the MMI waveguide can transmit and reflect downstream and upstream signals respectively.Furthermore,a cascaded MMI structure is used to separate two downstream wavelengths.The results show that the size of the triplexer is only 5.5×234.5?m²,the insertion loss and crosstalk are as low as 0.45 d B and-27.2 d B respectively.Besides,the triplexer has a large 3 d B bandwidth up to 105 nm.The MMI power splitter designed in this paper can be applied to multi-channel optical switches,silicon-based optical phased arrays and other fields,and the MMI demultiplexer and triplexer can be applied to passive optical networks.