Coherent technology has been employed in long-haul transmission systems in the past decade, with growing demand for capacity at ever-lower costs per bit. High-performance coherent modulators with high data rates, wide bandwidth, small footprint, and low power operation are highly desired. Toward this end, we propose a folded thin-film lithium niobate (TFLN) dual-polarization in-phase quadrature modulator featuring a low half-wave voltage of 1 V and a compact footprint of 4 × 8 mm2. To suppress RF wavefront distortion and optimize high-frequency electro-optic performance, we utilize air-bridge structures in the U-turns of the traveling-wave electrodes. As a demonstration of the long-haul transmission capacities with our device, we present driverless 703 Gb/s/λ line-rate transmissions, with a subcarrier modulation scheme, over a 1120 km single-mode fiber link. Here, for the first time, to our knowledge, our device allows for attojoule-per-bit level electrical energy consumption over transmission distances above 1000 km. The device opens opportunities for much lower-cost and capacity-intensive coherent systems that consume ultra-low power, support high data rate, and work in small spaces.
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