Mode coupling characteristics of refractive index-modulated tilted waveguide gratings

We present the detailed mathematical derivation and systematically analyze the mode coupling characteristics of refractive index-modulated tilted waveguide gratings (RI-TWGs). In this paper, we have simulated the mode coupling between a guided mode and a cladding mode/radiation mode of RI-TWGs. Compared with tilted fiber Bragg gratings (TFBGs), the noncircular symmetric structure of RI-TWG made it separate into two transverse dimensions, which greatly simplified the analyzing process of the tilted grating vector. Unlike TFBGs, the RI-TWG only excited the TE/TM0m. The simulation results have shown that the mode coupling coefficient between the guided mode and the cladding mode has a Sinc-like relationship, and the profile of the mode coupling behavior could be tuned by changing the tilt angle of grating, which as increasing of tilt angle, the maximum coupling of the cladding mode is moving to the higher cladding mode order. For radiation mode coupling, the RI-TWG has a similar transmission spectrum, which has a wide bandwidth and polarization-dependent attenuation. The results in this work give theoretical guidance and prediction for using the extra dimension of grating tilt to expand the functions and applications of RI-TWGs. (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.