In the power scaling process of fiber lasers, nonlinear effects are one of the most important limiting factors. In order to suppress the nonlinear effects in high power fiber lasers, researchers have proposed to use a gain fiber whose core diameter is variable along the length direction (called core-diameter-variable gain fiber) as the laser gain medium. Core-diameter-variable gain fibers mainly includes single-tapered fibers, spindle-shaped fibers, and saddle-shaped fibers. Core-diameter-variable gain fibers have many advantages over the ordinary gain fibers. This type of fiber used in lasers can make the laser have the capabilities of maintaining beam quality, suppressing mode instability, and mitigating amplified spontaneous emission (ASE) simultaneously, and has a broad application prospect in the field of high power lasers. In particular, the spindle-shaped gain fiber with an asymmetric structure may become the key device that breaks through the power limitation of the traditional gain fibers with uniform core diameters and obtains a high power laser output.
Core-diameter-variable gain fibers can effectively suppress nonlinear effects and maintain good beam quality, and have been widely used in single-frequency and narrow-linewidth amplifiers. Commercial polarization-maintaining tapered ytterbium-doped fibers are used in single-frequency fiber lasers with an output power of 550 W. They are also used in pulsed fiber lasers to break through the peak power output. Since 2014, the lasers based on this type of fiber have been extensively studied, and the power has been rapidly increased while maintaining the beam quality. In 2017, a femtosecond pulsed laser with an output peak power of 22 MW was realized based on the ytterbium-doped tapered fiber combined with the pulse compression technology. In 2021, the peak was increased to 97 MW with the diffraction limit beam quality in a femtosecond pulsed laser system using an ytterbium-doped tapered fiber. After the non-polarization-maintaining fiber technology matured, in order to achieve the high peak linear polarization, researchers began to study the tapered gain fiber lasers based on the linear-polarization-maintaining characteristics. There are two types of double-clad ytterbium-doped tapered fibers which can maintain the polarization: the Panda polarization-maintaining tapered fiber and the spun tapered fiber. The comparison between the spun tapered double-clad gain fiber and the Panda polarization-maintaining fiber shows that the spun tapered double-clad fiber has better extinction ratio retention characteristics. Based on these fibers, a linearly polarized pulsed laser output with an average power of 72.5 W and an extinction ratio larger than 17 dB was achieved. In 2021, based on the Panda polarization-maintaining ytterbium-doped tapered fiber, a linearly polarized pulsed laser with an average power of 150 W, a peak power of 0.74 MW, and an extinction ratio of 13.5 dB was realized. Narrow-linewidth pulsed lasers based on tapered gain fibers also attracted attention at the same time. In 2021, a narrow-linewidth pulsed laser with a linewidth of 167 pm and a peak output of 170 kW at a pulse duration of 3 ns was achieved. Core-diameter-variable gain fibers have also been widely used in the continuous-wave lasers. In 2020, the spindle-shaped ytterbium-doped fiber was used to achieve a continuous-wave laser with an output power of larger than 5 kW, and the saddle-shaped ytterbium-doped fiber was also used to achieve a 1313 W continuous-wave laser output. With the development of technologies, the application of core-diameter-variable ytterbium-doped fibers is becoming more and more extensive. At present, the development of core-diameter-variable ytterbium-doped fiber lasers shows that the output laser wavelength extends from the conventional wavelengths to some special wavelengths, the laser polarization characteristics extend from non-linear polarization to linear polarization, and the fiber structure changes from single-tapered to spindle-shaped and saddle-shaped. For new types of optical fibers, the concept of variable diameter has evolved from core to cladding. With the development of fiber manufacturing processes, core-diameter-variable ytterbium-doped fibers will be widely used in various fields of fiber lasers. The core-diameter-variable gain fibers have special requirements for the drawing process. In order to realize the fabrication of core-diameter-variable ytterbium-doped fibers, the method of preform form control and the method of drawing with a variable speed as well as the combination of these two methods can be used. Many institutions in the world have mastered the manufacturing processes of polarization-maintaining and non-maintaining core-diameter-variable fibers, and have launched some products. However, the domestic related researches mainly began at around 2016, and until now many institutions have already preliminarily mastered the manufacturing processes of non-polarization core-diameter-variable ytterbium-doped gain fibers.
The application of core-diameter-variable gain fibers in high power lasers has been widely studied. With the progress of technologies and the traction of high power laser demand, core-diameter-variable ytterbium-doped fibers have moved gradually from the laboratory to the market in recent years, greatly promoting the development of high power fiber lasers. In the near future, based on the technologies of core-diameter-variable ytterbium-doped fibers and new pumping sources, it can be expected that an industrial-grade, near single-mode fiber laser with an output power larger than 10 kW, beam quality (M2) smaller than 2, and stable operation for long time can be realized.
