Ultra‐Low‐Loss Slow‐Light Thin‐Film Lithium Niobate Optical Modulator

Electro‐optic modulators for next‐generation optical interconnects require low loss‐efficiency products (αVπL), compact footprints, high modulation efficiency and broad bandwidths. Here it is proposed and demonstrated 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−1 experimentally, which is tens of times lower than conventional slow‐light structures, and a high modulation bandwidth up to 320 GHz 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 αVπL of 7.4 V dB 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 = BR×(ER/Vpp)/(VπL)) of ≈182 Gbps∙(dB/V)/(V∙cm) for high‐speed optical modulation. The outstanding performance of the present 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.