Ytterbium-doped large-mode-area photonic crystal fibers (LMA PCFs) have attracted extensive attention owing to their important applications in high-peak-power ultrafast laser amplifiers. Fiber lasers are widely used in advanced manufacturing, medicine, national defense, and scientific research owing to their compact structure, high conversion efficiency, high reliability, and low cost. However, with the development of fiber lasers, particularly the development and application of ultrafast lasers in the field of fine processing in recent years, higher requirements have been placed on the output power and beam quality of fiber lasers. Currently, the output power of internationally commercialized fiber lasers has reached 100 kW. IPG Photonics Corporation uses a double-clad fiber with a core diameter of 30 μm to achieve a 10 kW single-mode single-fiber laser output. However, owing to the limitations of the physical mechanisms, such as nonlinear effects, optical damage, and thermal damage, it is very difficult to further increase the output power of a single laser module. The nonlinear effect of the optical fiber is related to the mode field area of ​​the optical fiber. The larger the mode field area, the weaker the nonlinear effect of the optical fiber, and the higher the threshold of the nonlinear effect. Therefore, large-mode field fibers are one of the most direct and effective ways of overcoming nonlinear effects and fiber laser damage to further increase laser power. However, an increase in the core diameter of large-mode field fibers inevitably causes competition among multiple transverse modes, degrading the beam quality of the laser.

Consequently, various fiber structure designs have been proposed to maintain a satisfactory beam quality with large core diameters, such as rod-type photonic crystal fibers, photonic bandgap fibers, leakage channel fibers, large-pitch fibers, chirally coupled-core fibers, and other microstructure fibers. Among them, the Yb-doped PCF has the most classic architecture, with an ordered array of microscopic air holes. These microscopic air holes favor convenient regulation of the effective refractive index of the cladding. Nevertheless, it is difficult to manipulate the refractive index so that it is close to that of pure silica glass cladding and maintain good uniformity in Yb-doped silica core glass. The commercial method of modified chemical vapor deposition (MCVD) combined with solution doping has some limitations in terms of the core size, refractive index uniformity in the radial and axial directions, and ultralow numerical aperture. Other non-MCVD fabrication technologies have also been developed and reported, including direct nanoparticle deposition (DND), reactive powder sintering of silica (REPUSIL), and sol-gel methods. Heraeus Quarzglas made great progress in the preparation of Yb³⁺/Al³⁺/F^--co-doped silica bulk glasses with the F^--doping-induced refractive index (RI) reduction being evident.

The sol-gel technique is a well-known method for producing centimeter-sized long glassy silica rods. Our group has committed to the preparation of large Yb³⁺-doped silica glass rods with a low refractive index and high optical homogeneity using a modified sol-gel method combined with high-temperature sintering. The sol-gel process ensures dopant mixing in the solution and consequently high doping uniformity, and high-temperature powder sintering allows the preparation of large-sized bulk glass. Fluorine incorporation during the sol-gel process is used to compensate for the increased refractive index caused by ytterbium and aluminum co-doping, and phosphorus is used to suppress the formation of Yb²⁺ and photodarkening. The sol-gel method combined with high-temperature sintering provides a cost-effective method for fabricating the core glass of a Yb-doped LMA PCF.

In this paper, we briefly introduce the progress of research on ytterbium-doped LMA PCF at home and abroad, as well as the design of ytterbium-doped LMA PCF. The effects of thermal history on the refractive index of the Yb/Al/P/F co-doped silica glass and the beam quality of the PCF are demonstrated. For comparison, the design and preparation methods of the polarization-maintaining ytterbium-doped PCF are presented. This paper focuses on the progress of research on Yb-doped LMA PCF in the past ten years at the Shanghai Institute of Optics and Fine Mechanics (SIOM) (Table 1 and Fig. 20). This includes accurate control of the refractive index value, homogeneity of the fiber core glass, and structure of the PCF. The output laser beam quality is significantly improved owing to the optimization of the Yb-doped core-glass rod.

With the rapid development of the domestic ultrafast laser processing industry, the demand for domestically produced Yb-doped LMA PCF by domestic ultrafast laser companies has increased. This paper summarizes the progress of ytterbium-doped large-mode-field photonic crystal fibers in SIOM over the past decade. Ytterbium-doped large-mode field photonic crystal fibers with core diameters of 40 μm, 50 μm, 75 μm, and 100 μm were prepared. Using a 40 μm /200 μm polarization-maintaining ytterbium-doped photonic crystal fiber, we independently designed and prepared an all-fiber amplification module, and achieved picosecond pulse amplification with an average power exceeding one hundred watts and high beam quality. The beam quality factor M² was less than 1.5, the polarization degree was greater than 12 dB, and the power fluctuation was less than 1.3% in 2 h under a 100 W amplification power operation. Using ytterbium-doped LMA PCF with a core diameter of 100 μm as the gain fiber, picosecond pulse amplification with a beam quality factor M²<1.3 and polarization degree greater than 95% was achieved. In the future, the performance of LMA PCFs should be further optimized to meet the requirements for high-average-power ultrafast fiber lasers.