Research On Spot Size Converters For Silicon Nitride Photonic Chip Application

Silicon nitride（Si₃N₄）material has attracted much attention in the field of integrated optical waveguide devices because of its wide transparent window,low temperature sensitivity,and the advantages of achieving optical waveguides with high refractive index difference and low transmission loss.In recent years,various laboratory prototype silicon nitride optical waveguide devices have been successfully developed,which promotes the silicon nitride optical waveguide devices to become practical.However,the cross-section size of the silicon nitride optical waveguide is usually much smaller than that of the ordinary single-mode fiber.Therefore,the coupling loss caused by the mismatch of the mode field between the waveguide and the fiber is also large.The performance of the device can not be fully exerted,which hinders the practical application of silicon nitride optical waveguide devices.At present,the two main schemes to reduce the coupling loss between silicon nitride waveguide and fiber are the use of grating couplers and spot size converters,in which the spot size converters can usually achieve higher coupling efficiency,larger working bandwidth and lower polarization dependence.In this paper,the spot size converter applied to silicon nitride optical waveguide is studied,designed and fabricated for the coupling of standard single mode fiber and tapered single mode fiber.The coupling efficiency of devices are tested,and the technical route to improve the coupling efficiency is analyzed and discussed.The specific work of the paper is as follows:First of all,the current research status of spot size converters are introduced,and the research significance of silicon nitride optical waveguide size spot converter is explained.Then,for the theoretical design of the spot size converter,the related theoretical knowledge is introduced,including the mode solution of the strip waveguide,the coupling theory of the fiber and the optical waveguide,and the truncation method of device testing.And using a combination of theory and simulation,ideas of spot size converters applied to two kinds of fiber coupling situations are determined,which is mainly to design the silicon nitride waveguide in the horizontal and vertical directions.After a lot of simulations and optimizations,the device parameters are preliminarily determined.Finally,the fabrication and performance test of the spot size converters are studied.The manufacturing process research and design,mask design,device manufacturing and testing,and result analysis are carried out in sequence.The experimental results show that the value of end-face coupling loss is 14.98 d B when the strip straight waveguide is coupled with the standard single mode fiber,the silicon nitride inverted-taper waveguide spot size converter with horizontal tapered Epo Core as cladding can reduce this value to 12.39 d B;The value of the end-face coupling loss is 6.82 d B when a straight strip waveguide is coupled to a tapered single mode fiber with a mode field diameter of 3.2μm,and the silicon nitride composite-taper stepped waveguide spot size converter can reduce this value to 5.98 d B.The results show that the studied silicon nitride spot size converters can effectively improve the coupling effciency with the fibers.