The development of doped fiber originates from the research of fiber laser. As early as 1961, Snitzer found laser radiation in Nd doped glass waveguide. The concepts of fiber lasers and amplifiers were put forward successively from 1963 to 1964. However, due to the high fiber loss at that time and the inability of lasers to work continuously at room temperature, fiber lasers and doped fibers did not develop well for a long time after that. In 1966, Dr. Kao Kun studied in detail the main causes of light attenuation in optical fiber, and clearly pointed out the main technical problems that optical fiber actually needs to solve in communication. This problem was solved by Corning in 1970. They developed optical fiber with attenuation less than 20dB/km. This technological breakthrough not only laid the foundation for the development of optical communication and optoelectronic technology industry, but also provided advanced technical means for the development of special optical fiber. In the mid-1980s, Poole et al. took the lead in developing vapor phase doping and liquid phase doping technologies on the basis of MCVD, making the manufacturing process of rare earth doped optical fibers increasingly perfect.

After that, great progress has been made in the research of rare earth ion doped fiber and devices. Fiber lasers have been widely valued and gradually commercialized because of their low threshold, high efficiency, narrow linewidth, tunable and high performance price ratio. However, because the single-mode fiber core (diameter 4-6 µ m) is small, high pump power is difficult to effectively couple into the fiber core, so the power of the fiber laser is low. The rare earth doped double clad silica fiber technology, which emerged until the late 1980s, was proposed by Polaroid Corp. of the United States and the University of Southampton in the United Kingdom. Based on double clad fiber, cladding pumping technology is applied at the same time, which effectively solves the coupling efficiency problem between the pump power and the gain fiber in the fiber laser, so that the output power of the fiber laser is rapidly improved, which greatly promotes the development of high output power lasers. The output power has risen from hundreds of milliwatts to tens or even hundreds of watts, and has begun to be used in optical communication, printing and other fields. Therefore, the research technology of rare earth doped double clad silica fiber has become one of the key technologies in the research of high power fiber laser. In order to make the pump light transmitted in the inner cladding pass through the fiber core doped with rare earth ions more times and improve the pump efficiency, different inner cladding structures have been proposed. Firstly, the circular inner cladding structure is developed and used, but the circular symmetry makes a lot of spiral light in the inner cladding. This part of pump light does not pass through the fiber core and is not absorbed by rare earth ions, which greatly reduces the utilization rate of pump light. Later, different shapes of inner cladding were gradually developed, such as eccentric circle, rectangle, square, D, quincunx, hexagon, octagon, etc. The theory shows that the rectangular and D-shaped inner cladding structures have high pump efficiency. [1] 

Although the output power of double clad fiber laser has been greatly improved, because its fiber core still belongs to the traditional single mode, the fiber core diameter is small, the gain is large, the amplified spontaneous emission is easy to establish, and the nonlinear effect is strong, so it is difficult to obtain the pulse laser output of high pulse energy. In order to obtain higher power output, the conventional "small core diameter, large numerical aperture" optical fiber design is no longer suitable for high power output applications.

However, as the core diameter increases, the V value of the fiber increases, and the transmission modes in the core increase, the beam quality of the fiber output laser will become worse. Therefore, double clad optical fiber (LMA) with large mode field area has been developed. By increasing the core area, the nonlinear effect is overcome and the storage energy of the core is increased; At the same time, the relative refractive index difference between the core and the inner cladding is reduced to maintain the approximate single-mode transmission of the radiation laser, so as to achieve high pulse energy and high beam quality laser output in the fiber. Therefore, large mode area double clad active fiber has become a hot spot in active fiber research. Many western countries have invested a lot of manpower and material resources to actively carry out the research work on high-power, large mode area, double clad active optical fibers, and the variety and quality of optical fibers are increasing and improving. At present, high-power and large mode field area double clad active fiber and fiber laser have been sold abroad, but only NUFERN Company in the United States, LieKKI Company in Finland and other companies can provide such products. The double clad active optical fiber products of NUFERN mainly have an outer diameter of 400 μ m. Core diameter 20 μ M and 30 μ m. The absorption coefficient at 976nm is 2-4 dB/m. Double clad fibers with large mode area have been widely used in high-power continuous and pulsed fiber lasers and amplifiers.

In order to achieve coherent combination of laser power, it is necessary for many high-power fiber lasers and optical amplifiers to work in a stable linear polarization state. It has been reported that non polarization maintaining fiber is used to achieve polarization maintaining, but the use of polarization maintaining large mode area double clad fiber is undoubtedly the most ideal scheme for high-power fiber lasers to achieve linear polarization output. With the growing demand for industrial applications with an output power of more than 100kW (continuous), the demand for polarization maintaining large mode field area double clad fibers is also rising. Polarization maintaining fiber technology mainly uses the difference between the thermal expansion coefficient of the stress region and the doped core and cladding to produce birefringence characteristics. According to the shape of the stress region, polarization maintaining double clad fiber mainly has two structures: bow tie type and panda type. Kliner et al. reported for the first time that a polarization maintaining ytterbium doped double clad fiber amplifier was made with bow tie polarization maintaining fiber. However, due to the complex production process, poor stability and consistency of bow tie polarization maintaining double clad fiber, its birefringence characteristics are not as good as those of panda polarization maintaining double clad fiber. Therefore, panda polarization maintaining fiber is mainly used in high-power fiber lasers and amplifiers.

A kind of information fiber called crystal fiber (PCF) has been developed. According to different light guiding principles, PCF fiber can be divided into two types: one is TIR-PCF based on Total Internal Reflection (TIR), and the other is PBG-PCF based on Photonic Band Gap (PBG). PBG-PCF uses the photonic band gap effect of the cladding to limit the guided wave propagation in the air core of the fiber. However, TIR-PCF has a fiber core with a high refractive index, and the refractive index of the empty hole is generally 1. Therefore, the refractive index of the fiber cladding area where the empty hole is introduced is actually reduced, and its effective refractive index is lower than that of the fiber core. Therefore, the light can be constrained by the same total internal reflection as the traditional fiber. When the arrangement of empty holes is periodic, this fiber is called total internal reflection PCF. TIR-PCF is relatively simple in fabrication process, and can be made into active photonic crystal fiber through certain doping technology. Therefore, rare earth doped PCF is all this type of fiber at present. Like rare-earth doped double clad silica fiber, rare-earth doped PCF can also design double clad fiber structure, which is of great significance for the development of high power fiber lasers.

In the MCVD process of rare earth doped double clad silica fiber, single mode output can only be achieved by controlling the core diameter and numerical aperture. This method has two problems. One is that the increase of core diameter is limited by the process and other parameters (such as numerical aperture and fiber loss) and cannot be designed freely; Second, the refractive index difference between the core and the inner cladding cannot be accurately controlled. However, the guided wave properties of rare earth doped double clad PCF mainly depend on the structure of the optical fiber and have nothing to do with the material. For example, through the selection and design of the air hole size and spacing distance, single-mode transmission can be achieved within a wide range of core diameters and wavelength values. 

This single-mode transmission property has nothing to do with the size of the optical fiber. This can be used to make the mode field area of rare earth doped double clad PCF large enough, In order to reduce the power density in the optical fiber and control the nonlinear phenomenon of the optical fiber. In order to improve the coupling efficiency of pump light, the numerical aperture of inner cladding of rare-earth doped double clad PCF should be as high as possible. For this reason, rare earth doped double clad silica fiber uses a low refractive index coating as the optical fiber cladding, which can increase the numerical aperture to the level of 0.46-0.48, which is the limit level of this process. The rare earth doped double clad PCF easily breaks this limit. By increasing the air filling ratio of the inner cladding, the relative refractive index difference between the inner cladding and the fiber core is increased, thus increasing the numerical aperture of the inner cladding of the fiber, which can be as high as 0.9. At present, Yb doped double clad PCF with inner cladding numerical aperture of 0.8 has been reported.

The rare-earth doped double clad PCF with polarization maintaining properties is another development direction that deserves attention. By changing the diameter of the air hole near the core of the x and y axes, the difference in the effective refractive index of the two orthogonal axes can be caused, thus introducing birefringence into the fiber, which can be one order of magnitude larger than the ordinary polarization maintaining fiber, reaching the order of 10.