Ultrahigh‐Efficiency Dual‐Band Thin‐Film Lithium Niobate Modulator Incorporating Low‐k Underfill for 390 Gbit s⁻¹ PAM8 Transmission

Abstract High‐performance electro‐optic (EO) modulators are critical components in modern telecommunication networks and intra‐datacenter interconnects. Low driving voltage, wide EO bandwidth, compact device size, and multi‐band operation capability are essential for various application scenarios, particularly energy‐efficient high‐speed data transmission. However, it is challenging to meet all these requirements simultaneously. Here, a high‐performance dual‐band thin‐film lithium niobate EO modulator with a low dielectric constant (low‐k) underfill is demonstrated to achieve overall performance improvement. The low‐k material helps reduce the RF loss of the modulator and achieve perfect velocity matching with a narrow electrode gap to overcome the voltage‐bandwidth limitation, extending EO bandwidth and enhancing modulation efficiency simultaneously. The fabricated 7‐mm‐long modulator exhibits a low half‐wave voltage V π of 1.9 V in the C‐band and 1.54 V in the O‐band, featuring a low half‐wave voltage‐length product of 1.33 and 1.08 V·cm, respectively. Meanwhile, the novel design yields an ultrawide extrapolated 3 dB EO bandwidth of 220 GHz (218 GHz) in the C‐band (O‐band). High‐speed data transmission in both C‐ and O‐bands using the same device has been demonstrated for the first time by PAM8 with data rates up to 390 Gbit s −1 , achieving a record‐low energy consumption of 0.69 fJ bit −1 for next‐generation cost‐effective ultrahigh‐speed optical communications.