We propose and demonstrate a high-performance asymmetrical multimode interference splitter on X-cut lithium niobate on insulator (LNOI) with an ultra-compact size of 5.8 μm × (26.4–35.6) μm. A rectangle with a small region is removed from the upper left corner of the multimode interference (MMI) coupler to achieve a variable splitting ratio. Here, we design and characterize MMIs in six different distribution ratios ranging from 50:50 to 95:5 on a 600 nm thick LNOI. Based on the cascade structure, the linear fitting method accurately shows the device loss (~0.1–0.9 dB). Our fabricated devices demonstrate robustness across a 30 nm optical bandwidth (1535–1565 nm). In addition, we numerically simulate the Z-cut LNOI, showing that the structure corresponding to the TM mode can also achieve a good variable splitting ratio.

With the rapid development of information technology, the global data volume has been continuously expanding, placing unprecedented demands on communication networks to accommodate precipitously increasing throughput. Thin-film lithium niobate (TFLN) modulators, characterized by their large theoretical bandwidth, low half-wave voltage, and suitability for high-density integration, show great application potential in high-speed optical modules and optical interconnection networks. However, the persistent issue of velocity mismatch between radio frequency (RF) signals and optical carriers invariably hinders the utilization of higher-frequency bands, which restricts the modulation speed of the fabricated devices. In this paper, an electrode co-loaded with square serrations and T-shaped stubs was utilized to achieve precise velocity matching and excellent impedance matching. Leveraging this approach, a TFLN modulator chip with an electro-optic bandwidth far exceeding 67 GHz and a return loss of greater than 12 dB was successfully fabricated on a silicon substrate. The velocity of RF signals can be tuned by altering the lengths of the slow-wave structures, which provides guidance for the design and optimization of broadband modulators.

A simple microwave photonic, reconfigurable, instantaneous frequency measurement system based on low-voltage thin-film lithium niobate on an insulator phase modulator is put forward and experimentally demonstrated. Changing the wavelength of the optical carrier can realize the flexibility of the frequency measurement range and accuracy, showing that during the ranges of 0–10 GHz, 3–15 GHz, and 12–18 GHz, the average measurement errors are 26.9 MHz, 44.57 MHz, and 13.6 MHz, respectively, thanks to the stacked integrated learning models. Moreover, this system is still able to respond to microwave signals of as low as −30 dBm with the frequency measurement error of 62.06 MHz, as that low half-wave voltage for the phase modulator effectively improves the sensitivity of the system. The general-purpose, miniaturized, reconfigurable, instantaneous frequency measurement modules have unlimited potential in areas such as radar detection and early warning reception.