Broadband Meandered Thin-Film Lithium Niobate Modulator With Ultra-Low Half-Wave Voltage

Compact thin-film lithium niobate (TFLN) modulators with low half-wave voltage $\text{V}_{\pi }$ are highly desirable for many applications. In particular, modulators with $\text{V}_{\pi }~\sim ~1$ V would allow direct driving by CMOS circuits, but they normally require a long modulation length of several centimeters. Folded TFLN modulators exhibit reduced $\text{V}_{\pi }$ together with a small device footprint. However, previously reported schemes suffer from complicated device fabrication procedure to maintain the relative direction of electric field with respect to the TFLN crystal polarization after electrode folding. In this work, a meandered X-cut TFLN Mach–Zehnder (MZ) modulator with specially designed capacitively-loaded traveling-wave electrodes (CL-TWEs) is demonstrated. Interdigitated T-rails are employed to reverse the electric field in the folded region for continuous accumulation of optical phase shift. Ultra-low $\text{V}_{\pi }$ of 1.08 V is achieved on an 8-mm-long modulator containing three-segments of 7-mm-long modulation sections, together with a 3-dB electro-optic bandwidth around 43 GHz. The broadband meandered modulator with ultra-low half-wave voltage as well as a compact footprint has the potential to be directly driven by CMOS electronics.