Capacitively-Loaded Thin-Film Lithium Niobate Modulator With Ultra-Flat Frequency Response

Traveling-wave electro-optic (E-O) modulators based on thin-film lithium niobate (TFLN) have attracted much attention recently, as low half-wave voltage (
$\text{V}_{\pi }$
) and wide modulation bandwidth can be realized with a small footprint. A flat E-O response of the modulator relies on low microwave loss, perfect velocity match and suitable terminal resistance. In this Letter, we present velocity-matched TFLN modulator based on low-loss capacitively-loaded traveling-wave electrodes (CL-TWEs) with an on-chip terminal resistor. The effect of termination resistance on the E-O frequency response is theoretically analyzed and experimentally confirmed. The TFLN modulator with 6-mm modulation length exhibit a
$\text{V}_{\pi }$
of 2.7 V and a 6.4-dB electrical bandwidth over 160 GHz. By adopting a termination resistance slightly lower than the characteristic impedance of the CL-TWEs, an ultra-flat E-O response has been demonstrated with fluctuation less than 1-dB up to 50 GHz, consistent with the theoretical prediction.