Compact and ultra-wideband 2 × 2 fast quasi-adiabatic 3-dB coupler on thin-film lithium niobate

3-dB couplers are key building blocks for on-chip optical switches, optical phased arrays, and photonic computing applications, for the ease of realizing balanced beam splitting and combining. Adiabatic 3-dB couplers ensure exclusive excitation and propagation of the fundamental eigenmode along the waveguide, characterized by low insertion loss, broad bandwidth, low power imbalance, and resilience to fabrication variations. However, conventional adiabatic designs need to extend the propagation length to achieve broadband performance. In this paper, we overcome such a length-bandwidth trade-off by employing fast quasi-adiabatic (FAQUAD) dynamics in the TFLN 3-dB couplers, thereby accelerating the mode evolution process. Theoretical analysis predicts that the proposed 2×2 FAQUAD 3-dB coupler exhibits an unprecedented operation bandwidth of 350 nm (1285 to 1635 nm) with a FAQUAD taper length of only 88.9 μm. Experimental characterization of the fabricated device demonstrates broadband 3-dB power splitting over 165 nm (exceeding the range of the used tunable laser: 1470 to 1635 nm), achieving the power imbalance of