The femtosecond laser fabrication of waveguide structures in transparent crystals

An optical waveguide is the basic structure of quantum optics, which has a strong confinement effect on light. It can complete the low-loss transmission of light and the enhancement of the nonlinear effect. Direct writing of optical waveguide structures in transparent crystals can enhance the electro-optic, acousto-optic, and nonlinear optical properties of crystals, making them widely used in the optical field. Femtosecond laser direct writing technology is a non-contact processing technology without thermal damage. The direct writing accuracy is high, and it is easy to fabricate the three-dimensional optical waveguide structure in the crystal. It is the best choice of a direct writing optical waveguide structure in a transparent crystal. This paper reviews the recent progress of femtosecond laser direct writing of transparent crystal materials to form optical waveguide structures. The basic mechanism of optical waveguide fabrication by femtosecond laser direct writing in transparent crystal, the methods of waveguide fabrication combined with different techniques, and the obtained optical waveguide structures are introduced, including single-line waveguides, double-line waveguides, depressed-cladding waveguides, ridge waveguides, and photonic crystal waveguides. The applications of optical waveguide structures in lasers, quantum memories, and quantum telecommunication devices are also presented.