High Performance Thin-Film Lithium Niobate Modulator with Suppressed Slot-Line Mode on Quartz Substrate Fabricated by Photolithography

In this work, we report high performance thin-film lithium niobate (TFLN) modulators with capacitively loaded traveling-wave electrodes (CL-TWEs) on quartz substrate. Photolithography is employed for fabrication due to its high efficiency compared to electron beam lithography. By utilizing a folded design, the length of the modulator is reduced to a third. A theoretical model is proposed to analyze the electro-optic response of the designed modulators, which considers the impact of the folding design. Additionally, various bend structures of CL-TWEs are fabricated to analyze the dips in the EE response caused by the slot-line mode, and a narrow signal is employed in the bend part to suppress the EE response dips. By introducing the suitable-length delay line, the light wave and microwave in the bend part can match with each other, which further improves the bandwidth. Experiments show that the unfolded modulator with 10-mm modulation length exhibits a low fiber-to-fiber insertion loss of 3.9 dB, a low half-wave voltage of 2.2 V, and an electro-optic response with a 2-dB roll-off at 67 GHz. The modulator demonstrates a π-phase shift thermal power of 15 mW, with an extinction ratio close to 26 dB. The folded modulator with 12-mm modulation length exhibits a low half-wave voltage of 1.85 V and an electro-optic bandwidth exceeding 67 GHz. This work provides a solution for fabricating TFLN modulators with CL-TWEs on quartz substrate.