Femtosecond laser-induced optical waveguides in crystalline garnets: Fabrication and application

Femtosecond laser direct writing (FLW) is recognized as a powerful tool for generating optical waveguides for complex photonic integrated circuits in various transparent materials. Crystalline garnets doped with rare earth elements are a group of materials that have excellent physical and chemical stability and are often used as laser gain media. FLW allows for localized modification of the refractive index within crystalline garnets, resulting in the fabrication of optical waveguides with diverse configurations. To fully take advantage of FLW technique, special 3D writing schemes have been designed. Further, the development of femtosecond laser enhanced wet etching process provides the ability to create arbitrary shape hollow channel, thereby expanding the potential of on-chip photonic devices. Some intriguing devices and applications have been achieved, including waveguide arrays, mode modulators, beam splitters, directional couplers, and well-developed waveguide lasers. This work provides a concise overview of femtosecond laser induced optical waveguides in crystalline garnets, including recent advancements in fabrication and application studies.