Design of an O-band thin-film lithium niobate edge coupler based on subwavelength gratings

To achieve efficient O-band fiber-waveguide coupling, this work presents an X-cut thin-film lithium niobate edge coupler based on a subwavelength grating structure. The device features a compact length of 72 μm. Key structural parameters, including grating period, duty cycle, and tip width, are optimized through parametric scanning. Simulation results show coupling losses of 0.93 dB for the TE mode and 0.89 dB for the TM mode at 1310 nm, corresponding to a polarization-dependent loss as low as 0.04 dB. The device exhibits a 1 dB bandwidth of approximately 80 nm across the 1260–1360 nm wavelength range. Under lateral and vertical misalignments of ±1μm, the coupling loss increases by less than 0.55 dB. The coupler also maintains strong fabrication tolerance: coupling loss increases remain below 0.23 dB under duty cycle variations of ±5% or tip width deviations of ±0.02 μm. With its high coupling efficiency and low polarization dependence, the proposed design offers a promising coupling solution for O-band thin-film lithium niobate waveguide systems.