Optical fiber is short for optical fiber, usually a cylindrical waveguide of light. It uses the principle of total reflection to restrain the light wave in the fiber core and guide the light wave to travel along the axis of the fiber. Replacing copper wire with quartz glass changed the world.
As a medium for conducting light waves, optical fiber has been widely used since it was proposed by Charles Kao in 1966 due to its advantages of large communication capacity, strong anti-interference ability, low transmission loss, long relay distance, good secrecy, strong adaptability, small size, light weight and abundant raw material sources. Dr. Kao, known as the "father of the optical fiber," was awarded the 2009 Nobel Prize in physics for his work. With the increasingly perfect and practical performance of optical fiber, optical fiber has revolutionized the transformation of the telecommunications industry, and it has basically replaced copper wire as the core component of modern communication.
Optical fiber communication system is a kind of communication system with light as the information carrier and optical fiber as the guided wave medium. When optical fiber transmits information, the electrical signal is converted into optical signal and then transmitted inside the optical fiber. As a new communication technology, optical fiber communication has shown great superiority from the very beginning, which has aroused great interest and wide attention of people. The extensive application of fiber in communication also promotes the rapid development of fiber amplifier and fiber laser . In addition to the communication field, fiber optic systems have a wide range of applications in medicine, sensing and other fields.
The gain medium of fiber laser is active fiber. According to its structure, it can be divided into single mode fiber, double cladding fiber and photonic crystal fiber.
Single-mode fiber Single-mode fiber is composed of fiber core, cladding and coating layer. The refractive index of the fiber core material, n1, is higher than that of the cladding material, n2. When the incident Angle of the incident light is greater than the critical Angle, the beam will be fully emitted in the fiber core, so the fiber can restrict the light to propagate in the fiber core. The inner cladding of the single-mode fiber cannot constrain the multi-mode pump light, and the numerical aperture of the fiber core is low. Therefore, the laser output can only be obtained by coupling the single-mode pump light into the fiber core. Early fiber lasers all use this single-mode fiber, resulting in low coupling efficiency, laser output power is only milliwatts.
Double clad optical fiber
In order to overcome the limitations of conventional single-mode single-clad Ytterbium (Yb3+) -doped fiber on conversion efficiency and output power, R.Moeller first proposed the concept of double-clad fiber in 1974 . After that, it was not until 1988 when E.Snitzer et al. proposed cladding pumping technology  that high-power ytterbium-doped fiber lasers/amplifiers were rapidly developed.
Double cladding fiber is a kind of fiber with special structure. Compared with conventional fiber, it adds an inner cladding layer, which is composed of coating layer, inner cladding layer, outer cladding layer and doped fiber core. Cladding pump technology is based on double cladding fiber, the core of which is to transmit multi-mode pump light in the inner cladding and laser in the fiber core, so that the pump conversion efficiency and the output power of fiber laser can be greatly improved. The structure of double cladding fiber, the shape of inner cladding and the coupling mode of pump light are the key points of this technology.
The core of the double-clad fiber is composed of silicon dioxide (SiO2) doped with rare earth elements, which is not only the laser medium but also the transmission channel of laser signal in the fiber laser. The V parameter of the corresponding working wavelength is generally reduced by designing its numerical aperture and core diameter to ensure that the output laser is a fundamental transverse mode. The inner cladding has a much larger transverse size (tens of times the diameter of the conventional core) and numerical aperture, and a smaller refractive index than the core, which can limit the laser propagation completely in the core. In this way, an optical waveguide with large cross section and large numerical aperture is formed between the fiber core and the outer layer, which can allow the high-power pump light with large numerical aperture, large cross section and multi-mode to be coupled to the fiber, and be confined to the inner cladding for transmission and non-diffusion, which is conducive to maintaining high power density optical pumping. The outer layer is composed of polymer materials with a lower refractive index than the inner cladding. The outermost layer is a protective layer made of organic material. The coupling area of the double-clad fiber to the pump light is determined by the inner cladding size, unlike the traditional single-mode fiber which is only determined by the core. On the one hand, the power coupling efficiency of the human fiber laser is improved. When the pump light conducts in the inner cladding layer, it will pass through the fiber core many times to excite the doped ion to emit the laser. On the other hand, the output beam quality is determined by the nature of the fiber core. The introduction of inner cladding does not destroy the output beam quality of the fiber laser.
Initially, the inner cladding structure of double cladding fiber is cylindrical symmetry, its production process is relatively simple, and easy to pump laser diode (LD) tail fiber phase coupling connection, but the perfect symmetry of the presence of large amounts of the pump in the inner cladding spiral light, the light even reflected enough times can never reach the fiber core area, Thus, it is impossible to be absorbed by the fiber core, so even if the longer fiber is used, there will still be a lot of light leakage, which makes it difficult to improve the conversion efficiency. Therefore, the cylindrical symmetric structure of the inner cladding must be destroyed.
Photonic crystal fiber
In common double-clad fiber, the output laser power is determined by the geometric size of the core. The numerical aperture determines the beam quality of the output laser. Due to the limitations of nonlinear effects, optical damage and other physical mechanisms in the fiber, the single method of increasing the core diameter can not meet the needs of single-mode operation of double-clad fiber with large mode field at high power output. The emergence of special optical fiber, such as photonic crystal fiber (PCF), provides an effective technical way to solve this problem.
The concept of photonic crystal was first proposed by E. Yablonovitch "1 in 1987, that is, dielectric materials with different dielectric constants in 1D, 2D or 3D space are composed of periodic structures with the order of optical wavelength, in which photonic conduction band (PBG) allowing light propagation and photonic band gap (PBG) preventing light propagation are generated. By changing the arrangement and distribution period of different media, many changes in photonic crystal properties can be caused, so as to achieve specific functions. PCF is a two-dimensional photonic crystal, also known as microstructure fiber or porous fiber. In 1996, J.C.Knight et al. developed the first PCF, whose light conduction mechanism was similar to the total internal reflection of traditional optical fiber. The first PCF that uses photonic bandgap principle to guide light was born in 1998. After 2005, the design and preparation methods of large-mode field PCF began to diversify, and various shapes of structures appeared, including leakage channel PCF, rod-shaped PCF, large-spacing PCF and multi-core PCF. The area of the fiber's mode field is also increased.
In appearance, PCF is very similar to the traditional single-mode fiber, but it exhibits a complex pore array structure in microstructure. It is these structural characteristics that give PCF unique and incomparable advantages, such as no cutoff single-mode transmission, large mode field area, dispersion and low limiting loss performance, can overcome many difficulties of traditional lasers. For example, PCF can achieve single-mode operation under large mode field area, and significantly reduce the laser power density in the fiber, reduce the nonlinear effect in the fiber, and improve the damage threshold of the fiber while ensuring the beam quality. Large numerical apertures can be achieved, which means more pump-optical coupling and higher power laser output can be achieved. These advantages of PCF have caused a series of research upsurge in the world, which makes PCF become a new research highlight in the application of high power fiber lasers and play an increasingly important role.
The invention of the fiber laser
A laser with fiber as the laser gain medium is called a fiber laser. Like other types of lasers, it is composed of three parts: gain medium, pump source and resonator. Fiber lasers use active fiber doped with rare earth elements as gain medium. Generally, semiconductor lasers are used as pump sources. The resonator is usually composed of reflector, fiber end face, fiber ring mirror or fiber grating.
According to the time domain characteristics of fiber laser, it can be divided into continuous fiber laser and pulsed fiber laser. According to the different resonator structure, it can be divided into linear cavity fiber laser, distributed feedback fiber laser and ring cavity fiber laser. Depending on the gain fiber and pumping mode, they can be divided into single-clad fiber lasers (core pumping) and double-clad fiber lasers (cladding pumping).
In 1961, Snitzer discovered laser radiation in a neodymium-doped (Nd) glass waveguide. In 1966, Charles Kao made a detailed study of the main causes of optical attenuation in optical fibers, and pointed out the main technical problems to be solved for the practical application of optical fibers in communication . In 1970, Corning developed optical fiber with attenuation less than 20 dB/km, laying the foundation for the development of optical communication and optoelectronic technology industry . This technological breakthrough has also greatly promoted the development of fiber lasers. In the 1970s and 1980s, the maturity and commercialization of semiconductor laser technology provided a reliable and diverse pump source for the development of fiber lasers. At the same time, the development of chemical vapor deposition reduces the transmission loss of optical fiber. Fiber lasers are also developing rapidly in the direction of diversification. The fiber is doped with many rare earth elements, such as Erbium (Er3+), ytterbium (Yb3+), neodymium (Nd3+), samarium (Sm 3+), thulium (Tm3+), holmium (Ho3+), praseodymium (Pr3+), dysprosium (Dy3+), bismuth (Bi3+), etc. Depending on the ions doped, different wavelengths of laser output can be achieved. Meet different application requirements.
Characteristics of high power fiber lasers
The advantages of high power fiber lasers are shown as follows.
(1) Good beam quality. The waveguide structure of the fiber determines that the fiber laser is easy to obtain the single transverse mode output, and the influence of external factors is small, which can achieve high brightness laser output.
(2) High efficiency. The optical fiber laser can achieve high light-to-light conversion efficiency by selecting the semiconductor laser with matching emission wavelength and absorption characteristics of doped rare earth elements as the pump source. For ytterbium-doped high-power fiber lasers, 915 nm or 975 nm semiconductor lasers are generally selected. Due to the simple energy level structure of Yb3+, upconversion, excitation state absorption and concentration quenching rarely occur, and the long fluorescence lifetime, YB3 + can effectively store energy to achieve high-power operation. The overall electro-optical efficiency of commercial fiber lasers is up to 25%, which is conducive to reducing costs, energy saving and environmental protection.
(3) Good heat dissipation characteristics. The fiber laser uses slender rare-earth element doped fiber as the laser gain medium, which has a very large surface area to volume ratio. It is about 1000 times that of solid bulk laser, and has a natural advantage in heat dissipation. In the case of medium and low power, there is no need for special cooling of the fiber, and in the case of high power, water cooling is used for heat dissipation, which can also effectively avoid the degradation of beam quality and efficiency caused by the thermal effect commonly seen in solid state lasers.
(4) Compact structure, high reliability. Because the fiber laser uses small and soft fiber as the laser gain medium, it is beneficial to compress the volume and save the cost. Pump sources are used are also small volume, easy to modular semiconductor laser, commercial product can generally be tail fiber output, combined with optical fiber Bragg grating, packtized devices, as long as these devices are welding can be realized all packtized, immune to environmental disturbance ability is high, has the very high stability, and can save time and cost of maintenance.
High power fiber lasers also have some disadvantages that are difficult to overcome. First, they are easily restricted by nonlinear effects. Because of the geometric structure of the waveguide, the effective length of fiber laser is long, and the threshold of various nonlinear effects is low. Some harmful nonlinear effects, such as stimulated Raman scattering (SRS) and self-phase modulation (SPM), cause phase fluctuation, energy transfer in the spectrum, and even damage to the laser system, which limit the development of high-power fiber lasers. The second is the photon darkening effect. With the increase of pumping time, the photon darkening effect will lead to a monotonically irreversible decrease in the power conversion efficiency of the rare-earth element doped fiber, which restricts the long-term stability and service life of the high-power fiber laser, especially in ytterbium-doped high-power fiber laser.
With the development of high-brightness fiber-coupled semiconductor lasers and double-clad fiber technologies, the output power, opto-optical conversion efficiency and beam quality of high-power fiber lasers have been greatly improved. Driven by the huge demand for industrial processing, directed energy weapons, long-distance telemetry, lidar and other applications, Apache Photonics (IPG), Nufern (Nlight) and Germany's Transom Group are the main research and development units of CW and pulsed-wave high-power fiber lasers, and have launched a rich product line. Exciting results have also been reported by Tsinghua University, National University of Defense Technology, Shanghai Institute of Optics and Fine Mechanics under the Chinese Academy of Sciences and the Fourth Academy of China Aerospace Science and Industry Corporation.
Fiber laser power boosting technology
Due to the limitation of nonlinear effect, thermal effect and material damage threshold in fiber laser, the output power of single fiber laser is limited to a certain extent, and with the increase of power, the beam quality gradually decreases. It is necessary to use mode control technology and design a new type of fiber with special structure to improve the beam quality. J.w. Dawson et al.  theoretically analyzed the output power limit of a single fiber, and calculated that a single fiber can obtain a near-diffraction limit laser output with a maximum power of 36 kW in a wideband fiber laser, while for a narrow-linewidth fiber laser, the maximum power is 2 kW. In order to further improve the output power of fiber laser and amplifier, it is an effective method to synthesize the power of multiple fiber lasers by coherent synthesis technology. It has become an international research hotspot in recent years.