With the rapid advancement of high-performance optical computing and high-speed optical communication, a demand for more efficient and compact photonic systems has driven the development of three-dimensional photonic integrated circuits (3D PICs). The existing approaches for the fabrication of 3D integrated waveguides primarily involve two methods, i.e., 3D stacking technique and femtosecond laser direct-writing technique. 3D stacked integrated waveguides suffer from limitations such as high cost and fabrication complexity. In contrast, femtosecond laser direct writing (FLDW) technique, which offers advantages in surpassing diffraction limit and providing flexible 3D processing capability, has emerged as a promising technique for high-precision 3D micro-nano fabrication of integrated 3D photonic optical circuits.

Femtosecond laser, characterized by ultrashort pulse width and extremely high peak power, can result in material modifications in focal regions of transparent materials. These modifications are typically categorized into two types, i.e., Type-I and Type-II, corresponding to positive and negative refractive index changes within the material, respectively. 3D optical waveguides can be fabricated in transparent materials by using the two types of modifications. However, despite these impressive capabilities, femtosecond laser direct-writing still faces some challenges like the difficulty of fabricating waveguides with controllable cross-sectional geometries and low propagation losses. These issues are due to spherical aberrations caused by refractive index mismatches between the waveguide and the surrounding air, as well as nonlinear effects that degrade the performance of waveguides. A conventional solution to these problems is beam shaping technologies, i.e., slit beam shaping, astigmatic beam shaping, astigmatic beam shaping and spatial light modulator beam shaping. However, these strategies have drawbacks of large energy loss, complex operation, high transmission loss, and low quality of generated light field, respectively. There is an ongoing need for innovative methods that can overcome these issues.

To address the challenges above, multi-scan laser direct-writing technology gradually becomes a research hotspot in the field of microstructure optical waveguide fabrication. In this approach, the laser trajectory with full positive refractive index is carefully controlled in three-dimensional space, allowing for a more refined modification of the refractive index. The multi-scan method facilitates a precise control over the cross-sectional geometry, refractive index profile, and mode field distribution of waveguides via adjusting parameters such as the offset distance between laser scans and the scanning time.

This review first systematically describes the latest research progress on direct-written optical waveguides inside transparent materials by femtosecond laser multiple scanning. General methods for fabricating waveguides with precisely controlled refractive index profiles and well-defined cross-sectional geometries in glass and crystal substrates using the multi-scan strategy are described in details. This review also focuses on the innovative applications of this technique. The multi-scan technique can be used to fabricate active optical waveguide devices, such as variable cross-section waveguide amplifiers with high gain. It is also utilized to prepare passive optical waveguide devices, including optical beam splitters, 3D mode conversion devices, and photonic lanterns, which can pave a way for applications in the field of integrated quantum optics and astronomical photonics. In addition, this review discusses the key technical challenges currently faced by the 3D integrated photonic circuits from the dimensions of precise control of waveguide cross-sectional morphology, optimization of low-loss curved waveguide structures, and cross-large-depth direct-writing technology, and looks forward to the development trend of realizing all-glass-based three-dimensional photonic chips.