As solid-state lasers develop toward higher power, better beam quality, miniaturization, and lower cost, they face such challenges as degraded optical performance caused by thermal effects (e.g., thermal lensing and thermal stress birefringence) induced by high-power pumping. Traditional gain media can no longer meet the demands for efficient energy conversion and specific wavelength output.

Single-crystal fiber (SCF), a new type of laser gain medium typically with a diameter of tens to hundreds of micrometers, combines the advantages of crystalline materials (high thermal conductivity, high damage threshold, and excellent mechanical properties) and traditional optical fibers (high aspect ratio and large surface area). It significantly enhances the thermal management performance of laser gain elements, offering a promising solution to the aforementioned issues. SCFs can be prepared through mechanical processing, which is only suitable for SCFs with a diameter of approximately 1 mm—with sharply increasing processing costs and technical difficulties as the diameter decreases. They can also be directly grown from melts, with the main methods including micro-pulling-down (μ-PD) and laser-heated pedestal growth (LHPG). Among these, LHPG, featuring ultrahigh heating temperature (>3000 ℃), large temperature gradient (>4000 K/cm), and crucible-free growth, stands out as the optimal method for growing flexible SCFs with a core diameter of less than 100 μm.

In recent years, rare-earth-doped SCFs have achieved remarkable progress in laser oscillators and amplifiers. In the field of laser oscillators, in 2012, Délen et al. demonstrated the high-power output capability of 1%Yb∶YAG SCF. Using a 600 W, 940 nm laser diode (LD) for end pumping, they achieved a continuous laser output of 251 W with an optical?optical conversion efficiency of 44% (Fig. 8), setting a record for SCF laser output power. In 2020, the Liu team from Shandong University increased the 1064 nm continuous laser power of Nd∶YAG SCF to 72.3 W, with an efficiency of 47.3%. In 2025, Tang et al. from Harbin Institute of Technology used a 788 nm fiber-coupled LD to end-pump a composite-structured Tm∶YAP SCF, achieving maximum continuous laser outputs of 11.9 and 20.6 W under single-end and double-end pumping, with corresponding slope efficiencies of 53.2% and 40.7% (Fig. 11). In 2024, the Zhao team from Jiangsu Normal University used two serially connected 0.5%Ho∶YAG SCFs as the gain medium, pumped by a 1907 nm Tm-doped fiber laser (153 W pump power), achieving over 100 W output at 2.1 μm with an optical?optical conversion efficiency of 67.6% (Fig. 14). Constrained by the special energy level structure of Er ions, traditional Er-doped oxide crystals require high Er³⁺ doping concentrations to ensure laser efficiency, but strong absorption from high doping hinders the high aspect ratio advantage of SCFs, limiting their application in mid-infrared lasers. Our team grew Er∶CaF₂ SCFs using the multi-microporous crucible method, realizing continuous laser operation of Er-doped SCFs at approximately 2.8 μm. The 3%Er∶CaF₂ SCF achieved a maximum output of 0.939 W at 2756.9 nm, with a slope efficiency of nearly 35%, reaching the Stokes limit (Fig. 16). In laser amplifiers, Nd∶YAG and Yb∶YAG SCFs show potential in amplifying the power and energy of ultrashort pulses, with further pulse energy amplification achieved using such technologies as chirped pulse amplification (CPA), division pulse amplification (DPA), and coherent beam synthesis (CBC). In 2025, Cao et al. from Xi’an Institute of Optics and Mechanics realized high-peak-power ultrafast lasers based on a three-stage end-pumped Yb∶YAG SCF single-pass amplified CPA system, obtaining a near-transform-limited pulse width of 323 fs and a peak power of up to 2.6 GW (Fig. 25). In addition, progress has been made in SCF structural design. In 2018, Dubinskii et al. from the U.S. Army Laboratory grew an approximately 120-μm-thick pure YAG single-crystal film on a 100 μm 1%Yb∶YAG SCF using liquid phase epitaxy (LPE). This cladded SCF had a transmission loss of only 0.011 dB/cm at 632 nm, achieving a 68.7% optical?optical conversion efficiency and approximately 50 W quasi-continuous output at 1030 nm (Fig. 30). In 2023, our team achieved axial gradient doping of Nd∶YAG SCFs through source rod concentration distribution design based on LHPG, obtaining a maximum output of 6.46 W at approximately 1.06 μm with a slope efficiency of 44% (Fig. 31).

SCFs, with excellent thermal/mechanical properties, weak nonlinear effects, and a wide transmission band, hold great promise in high-power ultrafast and mid-infrared lasers. Progress in growing low-phonon-energy sesquioxide and fluoride SCFs facilitates mid-infrared laser power breakthroughs; with matured preparation processes and optimized laser designs/water cooling, mid-infrared outputs exceeding 100 W are expected. Future directions include improving cladding quality and core?cladding matching for long-distance waveguide gain amplification and integrating doping concentration design during growth to suppress thermal effects, thereby advancing laser power and efficiency.