Ultra-low-loss slow-light thin-film lithium-niobate optical modulator

Electro-optic modulators for next-generation optical interconnects require low loss-efficiency products, compact footprints, high modulation efficiency, broad bandwidths, and low losses. Here we propose and demonstrate a low-loss high-efficiency thin-film lithium-niobate Mach Zehnder modulator enabled by a novel ultralow-loss slow-light structure based on apodized gratings in cascade. The present loss-engineered slow-light structure achieves excess losses as low as 0.6 dB/mm experimentally, which is tens of times lower than conventional slow-light structures, and a high modulation bandwidth up to 320GHz in theory is achieved with optimally-designed capacitively-loaded traveling-wave electrodes. Experimentally, the fabricated slow-light modulator with a 2.8-mm-long modulation region has an ultra-low loss-efficiency product of 7.4 VdB and a flat electro-optic response up to 67 GHz, enabling 100-Gbps on-off keying with high ERs of 4.5 dB at a low driving voltage of 2Vpp, while 200-Gbps PAM4 and 150-Gbps PAM8 signals are also generated to show great promise for advanced modulation formats. In particular, it has also achieved the highest figure-of-merit(FOM) of 182 for high-speed optical modulation , including the bit rate, the extinction ratio normalized with respective to Vpp, the modulation efficiency. The outstanding performance of the present apodized-grating-based slow-light modulator shows great potential and paves the way for developing high-speed optical interconnects for both data-centers and high-performance computing systems.