The development of photonic integration technology provides an effective approach to constructing communication, sensing, computing, and information processing devices with high performance, low cost, scalability, and reliability. Among various material platforms, lithium niobate (LN) has long been considered one of the most suitable materials for realizing photonic integrated circuits (PICs). It possesses superior optical properties, including a wide transparent window (0.35-5 μm), large nonlinear/electro-optic coefficients, and strong acousto-optic effects. Significant progress has been made in the fabrication process of thin film LN (TFLN) wafers, which has laid a material foundation for manufacturing photonic devices with high refractive index contrast and strong light field confinement. To date, researchers have achieved a wide range of photonic integrated functional bricks on TFLN, such as modulators, optical frequency converters, splitters, quantum light sources, and delay lines. These devices have demonstrated notable photonic characteristics, including low transmission loss, high-speed controllability, efficient optical frequency conversion, and low energy consumption. However, due to the lack of optical gain characteristics in LN crystals themselves, it is challenging to directly fabricate essential components for on-chip integration, such as micro-lasers and optical amplifiers, on TFLN wafers.

One approach to achieving optical gain on TFLN is by doping gain media within the TFLN film. Rare-earth ion-doped (REI-doped) TFLN has been employed to realize on-chip micro-lasers and optical amplifiers at different wavelengths, such as around 1550 nm and 1030 nm. The specific working wavelength is determined by the intrinsic optical spectra of the rare-earth-doped ions. Although active integration of TFLN photonic devices is still in its early stages, the exceptional optical properties of LN crystals, combined with low-loss photonic chip fabrication techniques and innovative device designs, will endow on-chip TFLN photonic devices with unparalleled scalability and exceptional functionality.

In recent years, the combination of commercial TFLN wafers and low-loss LN photonic device nanostructuring technology has resulted in a series of high-performance photonic device applications. In less than a decade, several important manufacturing techniques for LN photonic chips have been developed internationally, enabling the realization of practical high-quality photonic chip prototypes. These techniques include focused ion beam fabrication of high-performance LN nanostructures, as well as the use of electron beam lithography or ultraviolet photolithography combined with ion etching to produce high-quality LN photonic chips. Additionally, the photolithography-assisted chemo-mechanical etching technology (known as PLACE) has emerged as a promising micro/nanofabrication technique.

Thanks to the rapid advancements in high-repetition-rate and highly stable femtosecond lasers and large-stroke high-precision high-speed motion stages, the PLACE technique for fabricating active photonic devices on REI-doped TFLN has demonstrated both high processing efficiency while maintaining its inherent high-precision processing quality. Many corresponding advances have been achieved, which is important and necessary to summarize the existing research to guide the future development of this field more rationally.

The fabrication process of the PLACE technique is summarized (Fig. 1). The home-built ultra-high-speed high-resolution femtosecond laser lithography fabrication system is reported (Fig. 2). The demonstration of integrated active LN photonic devices such as on-chip tunable micro-lasers and waveguide amplifiers on REI-doped TFLN using the PLACE technique are comprehensively reviewed. Specifically, an erbium ion-doped LN waveguide amplifier with a maximum internal net gain exceeding 20 dB is achieved (Fig. 18), and an electro-optically tunable single-frequency laser in a high-Q LN microdisk is demonstrated with an ultra-narrow linewidth of 454.7 Hz (Fig. 5). We also achieve an electrically driven microring laser by monolithically integrating a diode laser (Fig. 10) and an erbium ion-doped TFLN microring laser (Fig. 3). A novel hybrid integration scheme of passive and active LN microdevices is performed using a continuous lithographic processing approach (Fig. 19 and 20). Lastly, we summarize the afore mentioned results and give an outlook on this vibrant and promising field of research.

The utilization of PLACE technology for the fabrication of TFLN active photonic devices holds great importance. This cutting-edge technique, with its high processing efficiency, intrinsic high precision, and wafer-scale integration capability, enables the production of cost-effective and high-performance photonic devices. This breakthrough has the potential to revolutionize the field of photonic science and technology, promoting sustainable development across various scientific disciplines and applications such as high-speed communication, artificial intelligence, and precision measurement.