High-Performance 2 × 2 3 dB Power Splitter on Thin-Film Lithium Niobate via Numerical-Assisted Inverse Engineering

In this paper, we propose and experimentally demonstrate a high-performance 3 dB power splitter based on thin-film lithium niobate (TFLN). The waveguide widths and separations are designed using invariant-based inverse engineering. A mathematical model is developed to characterize the bandwidth of the splitter, enabling evaluation of splitting ratio (SR) error without additional simulations. The results show that the proposed splitter exhibit a highly balanced SR of 0.475-0.525 and a low excess loss (EL) below 0.2 dB over the entire S-,C-,Land U-bands (1400-1700 nm). The minimum waveguide gap is 400 nm, which significantly reduces fabrication difficulty and can be reliably achieved using high-resolution lithography, thereby facilitating large-scale manufacturing of TFLN devices. Unbalanced Mach–Zehnder interferometers (MZIs) and cascaded 3 dB couplers were fabricated for experimental validation. The measurement results confirm that the power splitter achieves an SR of 0.48-0.52 and an EL below 0.4 dB across the wavelength range of 1520 nm to 1630 nm