Study On The Loss Characteristics Of Integrated Silicon Nitride Waveguides

The silicon nitride waveguide has large refractive index difference between the core and cladding,which can realize compact device with high density integration and excellent stability.Compared with the current SOI waveguide,the silicon nitride waveguide has the advantages of lower cost and much more simple fabrication process.In this paper,the loss characteristics and loss optimization of the silicon nitride waveguide were studied theoretically and experimentally.Based on the theory of optical waveguide and mode coupling,silicon nitride optical waveguides with rectangular cross-section(1?m×0.4?m and 3?m×0.1?m)for measuring the transmission loss and the coupling loss were designed and fabricated.And the cut-back and CCD scattering measurement systems were built to measure the loss of silicon nitride waveguides.The measured propagation losses of the silicon nitride waveguides with cross-sections of l?m×0.4?m and 3?m×0.1?m are 1.76dB/cm and 5.1dB/cm,respectively.And by using the cut-back method,the coupling losses with the high numerical aperture fiber(mode size of 4.8?m×4.8?m)are 2.6dB and 9dB,respectively.We find that the main source of the coupling loss is the mode field mismatch between the waveguide and fiber.Then a three-layer inverse taper spot-size converter was designed to reduce the mode field mismatch between the fiber and the silicon nitride waveguide(1?m×0.4?m),the coupling losses with the single mode fiber are 2.3dB(TE mode)and 3.4dB(TM mode),which are improved 8.7dB and 9.1dB respectively compared to direct butt coupling.For the large propagation loss,the main reason is the scattering loss caused by the roughness of the waveguide sidewall.Based on the empirical equation of the scattering loss by Payne-Lacey theory and the established kinetic theory of roughness,the laser annealing experiment was designed to reduce the sidewall roughness of the waveguide.The optimized annealing parameters for the silicon nitride waveguide were obtained by several experiments:laser spot diameter isl.6mm,laser power is about 30W?35W and the annealing time is about 30?40s.