With the development of emerging applications, such as artificial intelligence (AI), automatic drive, and high-speed computing, the information transmission capacity and processing speed required by human society are increasing exponentially. Hence, photonics is expected to be a new approach to information transfer and processing in the future for the multiple-encoding degrees of freedom, low power consumption, ultrahigh-speed information transmission, and high parallelism in information processing. Compared with traditional bulky optical systems, photonic integrated circuits (PICs) have a low loss, multifunctionality, high degree of integration, and compact size. These advantages have attracted attention from scientists worldwide. Apart from the significant progresses made in two-dimensional (2D) PICs, a rapid development trend in three-dimensional (3D) PICs consisting of 2D and 3D waveguide devices is being witnessed.

With its 3D processing capability, femtosecond-laser direct writing facilitates the transformation of PICs from 2D into 3D. This transformation provides not only a direct solution for the improvement of chip integration in the physical space but also new physical degrees-of-freedom design to achieve more complex on-chip photonic manipulation methods. Compared to 2D optical waveguide devices, 3D optical waveguide devices offer a new three-dimensional architecture for photonic chips, enabling three-dimensional integration. By spatially combining waveguide devices, multiple degrees-of-freedom of photons can be fully achieved. By writing designed optical waveguide arrays, it is possible to construct Hamiltonians, enabling the direct simulation of quantum and topological phenomena in optical systems. With these advantages, 3D PICs have important applications in various fields, including optical communications, integrated quantum optics, topological optics, astrophotonic, and optical sensing.

We reviewed the research progress of femtosecond-laser direct writing for 3D PICs. First, we introduced the mechanism of interaction between the femtosecond laser and transparent materials. On this basis, four types of femtosecond-laser direct-writing waveguides were introduced. Finally, we extensively reviewed the important applications and recent progress in 3D PICs in various fields.

3D femtosecond-laser direct-writing PICs provides a crucial solution for manipulating optical information. However, significant scientific and technical problems must be addressed before practical applications can be achieved. For example, the spherical aberration and Kerr nonlinearity that hinder the high-quality fabrication of 3D waveguides need to be addressed. In addition, the dynamic modulation efficiency of 3D PICs is limited by the large spacing between the surface and internal components. If these problems are solved in the near future, the femtosecond-laser direct-writing technique can be fully utilized in fabricating 3D PICs, with significant economic benefits to human society.