Objective The integration of Raman spectroscopy detection system is the current focus of Raman technology, especially combining waveguide with Raman, and effectively coupling excitation light into waveguide is particularly important for Raman spectroscopy sensing and signal collection. The process is mainly realized by waveguide grating couplers. However, the majority of the waveguide grating couplers are studied for the C band. In addition, the physical designs of the grating couplers are very complex and the preparation processes are difficult. The transparent window of the silicon nitride used in this paper is between 400 nm and 3500 nm. It has a wide range and a high refractive index, which can form a refractive index difference with the surrounding materials to bind the energy of the light in the waveguide layer, and it can reduce the transmission loss of light in the waveguide. The grating coupler designed here has a simple structure and preparation process. Besides, the wavelength of excitation light is 785 nm, which is also commonly used in Raman sensing. The grating couplers studied here can effectively couple the excitation light into the waveguide, and it is helpful for Raman spectroscopy sensing and signal collection. Methods Firstly, we have a theoretical analysis based on the principle of the grating coupler and analyze the interaction of parameters and the effect of each parameter on the coupling efficiency. Then the two-dimensional and three-dimensional models of the grating couplers are established, and the finite difference time domain (FDTD) simulation software is used to analyze them. With coupling efficiency as the main performance index, the influences of incident angle, grating constant, grating height, filling factor, and etching depth are analyzed to achieve the maximum of the coupling efficiency of the grating couplers. Electron beam lithography is used to prepare three-dimensional fully etched and focused waveguide grating couplers and an optical system including a supercontinuum source, tunable narrow bandpass filters which can select the wavelength of incident light, optical fibers which are used as the input and output fibers, and a spectrometer which collects output spectral signal is built. Then the prepared grating couplers are tested experimentally. In the test, direct waveguide devices of different lengths are prepared to test the loss factor of silicon nitride waveguide, and the test conditions such as light source, angle of incidence, and spectrometer are kept exactly the same. Next, the coupling efficiency of the grating coupler is tested. After the laser from the light source passes through the tunable narrow bandpass filter, the optical fiber couples the incident light into the grating coupler, the input light passes through the silicon nitride direct waveguide, then the grating coupler couples the light into the output optical fiber, and the output light is transmitted to the spectrometer for measurement. Finally, the coupling efficiency of the grating couplers is calculated according to the relationship of power of emergent light and incident light. Results and Discussions In the simulation analysis, for the two-dimensional grating coupler, the final optimization value for the grating constant, grating height, filling factor, and etching depth are 0. 455 mu m, 0. 260 mu m, 0. 514, and 0. 132 mu m, respectively. The coupling efficiency of the final two-dimensional grating coupler is about 39. 64% ( Fig. 2). The coupling efficiencies of three-dimensional direct waveguide grating coupler, three-dimensional half etched and focused waveguide grating coupler, and three-dimensional fully etched and focused waveguide grating coupler are 23. 43%, 37. 52%, and 21. 29% (Fig. 4), respectively. Compared to the direct waveguide grating coupler, the focused grating coupler has a smaller structural size, while ensuring a lower mode conversion loss. All the grating trenches of the focused waveguide grating coupler are focused on the junction between the waveguide and the grating coupler, which can focus the coupled light into the waveguide layer. The efficient coupling between the optical fiber and waveguide is realized. In the experimental test, firstly, the loss factor of the silicon nitride waveguide is tested. Then the light source and test conditions remain constant, and the light is transmitted to the spectrometer for measurement finally. The coupling efficiency of the three- dimensional fully etched and focused grating coupler can reach about 19. 91% (Table 3). The reason for the different coupling efficiencies between experimental test results and simulation results is that the surface of the actual grating couplers is not completely flat due to the matching error in the process of fabricating the grating couplers. So there is a little difference between the simulation models and the samples in the test, and thus the coupling efficiency of the grating coupler in the experimental is slightly lower. Conclusions In this paper, the finite difference time domain (FDTD) simulation software is used to analyze and optimize the two-dimensional and three-dimensional silicon nitride waveguide grating couplers. The three-dimensional fully etched and focused waveguide grating couplers are prepared by electron beam lithography, and the performance of grating couplers is tested. The results show that the performance of the two-dimensional grating coupler whose coupling efficiency is 39.64% is the best, and the coupling efficiency of three- dimensional fully etched and focused waveguide grating coupler in the experiment 19.91%, which can couple light into the waveguide effectively. The material selection and structure design of the grating couplers can also be optimized to improve the coupling efficiency of the grating couplers in the future.
