Study On End-Face Optocoupler Based On Sub-wavelength Grating Structure

The advent of digitalization has led to a continuous increase in data communication bandwidth for data centers.To address this,silicon-based optoelectronic technology has gained prominence due to its compatibility with the complementary metal oxide semiconductor(CMOS)processing technology used in the current microelectronics industry.This technology is also cost-effective,providing technical support for the large-scale production of highly integrated,low-cost,and low-power integrated optoelectronic chips,which is the main direction of data communication development.Discrete devices and opto-couplers play crucial roles as important components of the system for achieving efficient coupling,which is a significant factor affecting the optical transmission efficiency of the entire optoelectronic system.In this thesis,we propose a low-loss,high-alignment-tolerance optical coupling structure between silicon photonic chips,optical fibers,and lasers using sub-wavelength grating(SWG)structures.Our research results and innovations are as follows:Firstly,we designed an end-face coupling structure of the fiber and silicon nitride(SiN)waveguide based on a single taper structure of the SiN waveguide mode spot size converter.The theoretical coupling loss is 0.5 dB/end face,and the experimental measured coupling loss is 1.2 dB/end face for a mode field diameter of 3.2μm tapered lens fiber.After optimizing the structure design,the theoretical coupling loss between the SWG-based SiN waveguide mode spot size converter and the above fiber is less than 0.2dB/end face,and the overall coupling loss of the structure is 0.3 dB/end face,which is the lowest reported coupling loss index.Secondly,we designed an optical fiber-chip coupling structure for a mode field diameter of 10.4μm single-mode fiber(SMF-28).A multi-layer waveguide structure based on different refractive indices of silicon oxynitride(SiO_xN)was proposed to achieve low-loss coupling.At a wavelength of 1550 nm and in TM mode,it achieved a98.45%overlap efficiency with SMF-28 fiber,and the theoretical coupling loss between the structure and the fiber is less than 0.1 dB/end face.This structure also has the advantages of large manufacturing tolerance,large alignment tolerance,wide operating bandwidth,and low polarization-related loss.This structure is one of the single-mode fiber coupling structures with the highest theoretical coupling efficiency reported so far.Lastly,we designed a silicon nitride waveguide chip-laser chip coupling structure for on-chip bonded lasers.For the laser output mode spot,a single taper coupling structure was proposed,which theoretically achieves a coupling loss of 1.21 dB/end face with the laser in TE mode.After structure preparation,the experimental coupling loss is about 3.72dB/end face.To address the reflection problem in the above coupling,we proposed a low-reflection structure based on the effective medium theory of sub-wavelength grating structures.A sub-wavelength waveguide grating structure with a gradually varying duty ratio was added at the end face to achieve the effect of gradually varying the equivalent refractive index.The structure reflection loss was reduced from-29 dB to-32 dB without increasing the coupling loss.