Significance As a contactless and mask-free 3D micro-nano processing technology, femtosecond laser direct writing technology can efficiently and flexibly manipulate the interaction between light and matter with its ultra-high peak power and ultra-short pulse duration, which is conducive to the research and development of highly integrated multi-functional photonics devices, and has great significance for micro-nano photonics and integrated photonics. Glass, crystals and polymers represented by photoresists can be used as integrated photonic platforms for femtosecond laser direct writing due to their unique and excellent properties. In recent years, the research on integrated photonics devices based on femtosecond laser direct writing has developed rapidly. This paper summarizes several mature materials and processing processes suitable for the preparation of high-quality integrated devices by femtosecond laser, introduces the research progress and various applications of integrated devices on femtosecond laser direct-to-chip in recent years, and looks forward to its future research direction.

Progress The development of modern science and technology puts forward higher requirements for optical communication, optical transmission and other technologies. The large and complex optical systems in the past could no longer meet the convenient and efficient needs, and how to achieve various optical functions at the micro-nano level on-chip size has become a key technical bottleneck that needs to be broken through urgently. Therefore, integrated optics has been extensively researched and developed. Since the concept of photonic integrated circuits was proposed, achieving high-performance, low-loss photonic integrated chips has always been the goal pursued in processing technology. Traditional processing methods mainly include extreme ultraviolet lithography, electron beam resist, and focused ion beam etching on silicon-based platforms, which have the advantages of mature manufacturing technology and batch preparation. However, these methods require pre-preparation of masks, resulting in a more complex processing process. This planar processing could only perform structural preparation on the surface of the sample, which is lack of flexibility. In addition, the range of materials applicable to these processing methods is narrow, which limits the development of photonic integrated chips. Due to the defects and limitations of the above processing technologies, simple and efficient photonic integrated circuit processing methods have been continuously studied. After continuous development in recent years, femtosecond laser direct writing has been a mature technology. With the excellent performance of extremely short pulse width and high peak power, it can significantly reduce the generation of thermal effects to achieve cold processing when interacting with materials, and can achieve three-dimensional processing inside transparent materials with processing accuracy that breaks through the diffraction limit. In addition, thanks to its non-contact and mask-free nature, femtosecond laser direct writing technology provides a simple and efficient method for the preparation of on-chip integrated photonic devices. At present, femtosecond laser direct writing technology has enabled the preparation of integrated photonic devices with multiple functions in a variety of materials and can be combined with other processes to optimize processing performance. The application of these integrated photonics devices on the chip is not limited to light transport and optical field manipulation, but has also expanded to emerging fields such as optical topology and quantum optics. In summary, this paper briefly introduces the common materials, processing methods and mechanisms of on-chip integrated devices written by femtosecond laser, summarizes the recent research progress and application of these integrated devices, and looks forward to its high-potential research directions in the field of micro-nano optics and integrated optics.

Conclusions and Prospects This paper reviews the latest advances in femtosecond laser direct writing technology in on-chip passive integrated photonics devices, covering material platforms, processing processes, and their physical mechanisms, and focuses on the application of this technology in various fields such as light transmission, light field regulation, optical topology, and quantum optics. With the continuous breakthroughs in ultrafast optical research, femtosecond laser direct writing has become an important means to promote the rapid development of micro-nano optics and integrated photonics. This technology is suitable for various material platforms such as glass, crystal, and polymer, and can be combined with diversified processing methods to achieve high-precision and high-flexibility device preparation. Through multiple scanning of femtosecond lasers, waveguide structures with low transmission loss and low bending loss can be prepared, laying the process foundation for high-density photonic integration. In terms of functional applications, femtosecond laser direct writing can not only be used to organize complex waveguide structures, realize the conversion of Gaussian beams to vortex beams and other exotic beams, but also realize the filtering function with the integration of grating and waveguide, and realize beam splitting and multiplexing based on multi-mode interference and self-image effects. In topological optics, the waveguide array written directly by a femtosecond laser provides an effective platform for topological phase change observation and topological protection. In the field of quantum optics, this technology further supports important research progress such as quantum walking regulation, quantum overosmotic research, and quantum simulation of photosynthesis. In short, femtosecond laser direct writing technology has shown significant advantages in the preparation of on-chip integrated photonics devices by virtue of its processing flexibility, material adaptability and structural design freedom, and has continuously expanded its application boundaries in emerging optical research directions, and has broad development prospects in the field of integrated photonics in the future.