Current Status and Applications of High-Power Fiber Laser Development

Abstract

Definition, Structure, and Classification of High-Power Lasers, with a Comparison to Traditional Lasers: A Summary of the Advantages and Disadvantages of Fiber Lasers

This article provides an expert overview of high-power lasers, beginning with their definition, structure, and classification. It contrasts high-power lasers with traditional lasers, highlighting the merits and shortcomings of fiber lasers.

Development of High-Power Fiber Lasers: A Global Perspective

The article proceeds to discuss the development of high-power fiber lasers both domestically and internationally, offering insights into the progress made in this field.

Key Technologies of High-Power Fiber Lasers: An In-Depth Study

A significant portion of the article is dedicated to studying the critical technologies of high-power fiber lasers. Particular emphasis is placed on the gain medium and pump coupling technology of fiber lasers. The gain medium includes double-clad doped fibers and photonic crystal fibers. Pump coupling technologies encompass end-pumping and side-pumping techniques.

Keywords

High power, fiber laser, gain fiber, pump coupling

1. Introduction.   

Fiber lasers are a category of lasers that utilize optical fibers as the host material, doped with various rare-earth ions such as ytterbium (Yb), neodymium (Nd), etc.[1]. Similar to other types of lasers, fiber lasers consist of a pump source, coupling optical system, gain medium (gain fiber), resonant cavity, and collimating optical system, as illustrated in Figure 1.The gain fiber in a fiber laser serves as the medium for photon generation, where the pump source functions as an external energy source to achieve population inversion in the gain medium. The optical resonant cavity, composed of two mirrors, is designed to provide feedback for photons and to amplify them within the working medium.Upon entering the gain fiber, the pump source is absorbed, leading to a population inversion of the energy levels within the gain medium. When the gain within the resonant cavity exceeds the losses, laser oscillation is established between the two mirrors, resulting in the generation of a laser output.

The structure of the fiber, the structure of the resonant cavity, the power level, the gain medium, and the characteristics of the output laser, as depicted in Figure 2.

From the perspective of fiber structure, fiber lasers can be classified into single-clad fiber lasers, photonic crystal fiber lasers, double-clad fiber lasers, and special fiber lasers. In terms of resonant cavity structure, they can be categorized into Fabry-Perot (F-P) cavities, ring cavities, figure-eight cavities, distributed feedback (DFB) fiber lasers, and distributed Bragg reflector (DBR) fiber lasers.

Based on power levels, fiber lasers are divided into high-power, medium-power, and low-power fiber lasers. Regarding the gain medium, the classification includes plastic fiber lasers, rare-earth-doped fiber lasers, nonlinear optical fiber lasers, and crystal fiber lasers. Finally, according to the characteristics of the output laser, fiber lasers can be classified into continuous-wave fiber lasers and pulsed fiber lasers.

The comparison of fiber lasers with traditional lasers in terms of characteristics is presented in Table 1. As indicated in Table 1, fiber lasers exhibit unique advantages in many aspects. They are characterized by high beam quality, long lifespan, high electro-optical conversion efficiency, high output power, compact size, and light weight [2] [3] [4]. However, high-power fiber lasers also have some drawbacks: one is the susceptibility to nonlinear effects, and the other is the fragility of the fiber, which serves as the gain medium and is prone to breaking.

Table 1. Comparison of fiber lasers and other lasers

2.The Development History of High-Power Fiber Lasers       

  2.1. Development Status Abroad

Laser technology originated in the 1950s and stands as a significant milestone in the history of human scientific inventions. The birth of the world’s first ruby laser in 1960 [5] ignited the research enthusiasm of numerous scientists. In 1961, Snitzer and colleagues in the United States observed stimulated emission in Nd3±doped glass fibers [6], marking the beginning of fiber laser technology research.

In the late 1980s, the concept of a double-clad gain fiber structure was proposed [7] [8] [9]. This innovative structure improved the coupling efficiency of the pump light, thereby increasing the intensity of the laser output and significantly enhancing the laser’s output power, which was instrumental in the development of high-power fiber lasers. In 1993, a high-power Nd3±doped double-clad fiber laser was reported, with a single-mode output power of 5 W [10]. In 1994, HM Pask and others reported the first Yb3±doped double-clad fiber laser, which produced an output of 0.5 W at a wavelength of 1040 nm [11].

Entering the 21st century, high-power fiber lasers have ushered in a new era. In 2000, IPG Photonics was the first to achieve an all-fiberized continuous wave fiber laser with an output power in the hundreds of watts. In 2002, J. Limpert and colleagues reported on a double-clad fiber laser capable of producing hundreds of watts of laser output [12]. Y. Jeong achieved kilowatt-level laser power output from a fiber laser in 2004 [13]. In 2005, V. Gapontsev developed a Yb3±doped large-mode-area double-clad fiber laser, which achieved a single-mode output of 2000 W [14].

In 2006, IPG Photonics achieved a single-fiber, single-mode continuous output of 3 kW. Then, in 2009, they realized the world’s first 10-kilowatt-level single-mode fiber laser output. IPG Photonics became the leader in the global high-power fiber laser industry, outpacing other laser manufacturing companies. Subsequently, in 2012 and 2013, experimental results were reported with single-fiber, single-mode power reaching 20 kW and multi-mode output power reaching 100 kW, respectively. By 2019, the highest output level for fiber lasers produced by IPG Photonics was 500 kW for multi-mode and 20 kW for single-mode. The company has developed a variety of fiber lasers and occupies a significant share of the high-power fiber laser market. The output power of early fiber lasers did not increase rapidly, but after 2002, the output power of fiber lasers has seen significant growth [15].

Several research institutions abroad have contributed to the development of high-power fiber lasers, including the University of Southampton and SPI in the UK, IPG Photonics in Germany, and Bell Labs in the United States, among others.

2.2. Domestic Development Status

Research on high-power fiber lasers in China began in the late 1980s on a large scale. To date, several institutions have been involved in the research of fiber lasers, including the Shanghai Institute of Optics and Fine Mechanics, Tsinghua University, among others. In the corporate sector, companies such as Wuhan Raycus, FiberHome, and AviOpto have been engaged in the research and development of fiber lasers.

3. Key Technologies of High-Power Fiber Lasers 3.1. Double-Clad Fibers In 1989, Tsinghua University reported on a tunable Nd3±doped fiber laser with a tuning range of 1077 to 1138.6 nm and a slope efficiency of 9.2% [16]. In 1990, C. Y. Chen and colleagues in Taiwan reported a tunable Er3±doped fiber laser with a tuning range of 1522 to 1567 nm [17].

In 2003, the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences reported a laser setup using Yb3±doped double-clad fibers, achieving a continuous laser output of 107 W. In 2005, in collaboration with FiberHome in Wuhan, they developed a Yb3±doped double-clad fiber laser that achieved a continuous laser output of 440 W from a single fiber [18].

In 2006, Tsinghua University utilized domestically produced mirror-doped double-clad fibers to achieve a continuous output power of 714 W from a fiber laser. In the same year, in August, the 11th Research Institute of China Electronics Technology Group Corporation successfully developed a high-power fiber laser with an average output power reaching 1207 W [20].

In 2010, the Shanghai Institute of Optics and Fine Mechanics reported a cladding-pumped Yb3±doped fiber laser with an output power of 1750 W [21]. In 2012, Wuhan Raycus, in collaboration with HuaGong Laser, developed an all-fiber laser with an output power of 4 KW. In 2015, the China Academy of Engineering Physics achieved continuous laser output of 5 kW from a single fiber using imported technology. In 2017, Liu Zejin and colleagues from the National University of Defense Technology obtained a combined laser output of 5.02 kW [22]. In 2018, the China Academy of Engineering Physics achieved a kilowatt-level all-fiber laser output [23] [24]. In 2020, Yang Baolai and colleagues from the National University of Defense Technology achieved an output power breakthrough of 6 KW from an all-fiber laser oscillator [25].

3. Key Technologies of High-Power Fiber Lasers

The gain fiber, serving as the gain medium within fiber lasers, plays a crucial role in reducing the high numerical aperture of the pump light, which significantly impacts the quality of the laser’s output beam. With the advancement of fiber technology, gain fibers have evolved from the early single-clad structures to the current double-clad configurations, and the fiber core diameter has transitioned from smaller to larger cores. This section primarily discusses double-clad fibers and photonic crystal fibers.

3.1 Double-Clad Fiber

In the late 1980s, Snitzer and colleagues introduced the concept of a double-clad gain fiber structure for high-power fiber lasers [26]. The structure of double-clad fibers differs from that of conventional fibers [27] [28], comprising four main components: the core, the protective layer, the inner cladding, and the outer cladding, as illustrated in Figure 3.

Subsequent research revealed that the shape of the inner cladding affects pump efficiency. Consequently, inner claddings with various shapes have been developed [29] [30].

3.2 Photonic Crystal Fiber

In 1987, E. Yablonovitch introduced the concept of photonic crystals [33], and shortly thereafter, photonic crystal fibers (PCFs) emerged into the realm of optical fiber technology [34] [35] [36]. These fibers are characterized by the periodic arrangement of air holes at the wavelength scale within a silica fiber matrix.

Photonic crystal fibers can be categorized into two types based on their guiding mechanisms: index-guiding and photonic bandgap-guiding [37]. While PCFs may appear similar to traditional single-mode fibers in their external appearance, they differ significantly at the microscopic level. This uniqueness endows PCFs with properties that are not present in conventional single-mode fibers, such as endlessly single-mode transmission, exceptional nonlinear effects, and superior birefringence [38]. Consequently, PCFs play a pivotal role in the development of high-power fiber lasers.

3.3 Pump Coupling Techniques

Fiber pump coupling technology is one of the key technologies in fiber lasers, enabling the injection of pump light into the gain fiber, thereby enhancing the output power of fiber lasers. The following section primarily introduces two techniques: end pump coupling and side pump coupling.

3.3.1. Facet Pump Coupling

Facet pump coupling technology [39] is characterized by its simple structure and operation, making it the preferred coupling method for most fiber lasers. There are primarily two types of facet pump coupling:

Lens Direct Coupling Pumping The most common pumping method in laboratories is the lens direct coupling pump [40], as illustrated in Figure 5. To achieve higher coupling efficiency, it is necessary to match the numerical aperture of the lens group and the size of the focused spot with the double-clad fiber. This method can yield high-power laser output, but it tends to be less stable. Consequently, it is rarely used in commercial fiber lasers.

2）Fiber Facet Fusion Splicing

When utilizing an LD (semiconductor pump source) with a fiber-coupled output as the pump source for a fiber laser, the output fiber of the LD can be directly fusion-spliced to one end of the double-clad fiber. This setup, in conjunction with a fiber Bragg grating, forms an all-fiber-structured laser [41] [42]. This method results in a robust structure capable of high-power laser output. However, the high-power LD array used as the pump source requires semiconductor cooling. The emitted laser light needs to undergo beam shaping, collimation, and focusing with aspherical mirrors into a fiber with a diameter of a few hundred micrometers. As a result, the overall system tends to be larger in volume, more complex in construction, and higher in cost, as illustrated in Figure 6.

3.3.2. Lateral Pump Coupling

Lateral coupling technology involves the removal of the outer cladding and protective layer of a double-clad fiber, followed by the coupling of pump light into the inner cladding from one side [43] [44] [45] [46]. This section primarily introduces two coupling methods: V-groove lateral pump coupling and embedded mirror pump coupling.

1. V-Groove Lateral Pump Coupling The V-groove lateral pump coupling technique involves stripping the outer cladding from a double-clad fiber and then grinding a V-shaped groove on the inner cladding, as illustrated in Figure 7. The highest reported coupling efficiency using this method reaches 76%. This coupling approach is simple in principle and can achieve high output power. However, it has some drawbacks. For instance, it is challenging to fabricate a V-groove on the double-clad fiber, and poor execution can affect the fiber’s performance. Additionally, the V-groove can only be etched at the two ends. Consequently, this technology is still in the developmental stage in China.

2）Embedded Mirror Pump Coupling

This technique represents an advancement on the V-groove side pump coupling technology. It involves grinding a square groove on the surface of the inner cladding of the double-clad fiber (care must be taken to ensure the depth of the groove does not damage the fiber core), followed by adhering a reflecting mirror to the square groove using optical cement. The specific structure is illustrated in Figure 8. This method offers the advantages of high coupling efficiency and low cost. However, similar to the V-groove side pump coupling technology, the etching results of the groove within the inner cladding can affect the performance and transmission characteristics of the fiber.

4. Applications of High-Power Fiber Lasers

Currently, due to advancements in industry and science and technology, high-power lasers are beginning to be widely used across many sectors, particularly high-power fiber lasers. Coupled with the advantages mentioned above, fiber lasers are poised to be deployed on a large scale.

4.1 Fiber Laser Marking

Laser marking technology involves the use of laser beams to irradiate the surface of the object to be processed, thereby engraving patterns, digits, or other designs with a certain depth and color onto the surface, leaving a permanent mark. The application of laser marking is extensive, encompassing fields such as computer accessories, automotive parts, and more. Fiber laser marking machines offer superior beam quality and higher photoelectric conversion efficiency when compared to traditional CO2 and YAG laser marking machines. As a result, fiber laser marking machines are gradually replacing the conventional marking equipment.

4.2. Laser Cutting

With the continuous deepening of research into optical lasers, various high-power fiber lasers have been developed, leading to an increasingly wide range of applications for fiber laser cutting machines. They are now commonly used in industries such as automotive, advertising, sheet metal processing, and server cabinet manufacturing.

4.3. Applications in the Medical Field

Currently, the lasers predominantly used in clinical medical settings are carbon dioxide lasers and YAG lasers. However, these devices suffer from several drawbacks, including their large size, low beam quality output, the need for a massive water cooling system, and difficulties associated with installation. With the advancement of fiber technology, the emergence of fiber lasers has addressed these issues perfectly. Consequently, fiber lasers are being widely utilized in medical applications. For instance, they are used in soft tissue surgical procedures, laser eye surgery for myopia correction, skin resurfacing operations, and more. Fiber lasers are more portable and are poised to gradually replace a significant portion of the existing light sources in medical applications. The small spot size of fiber lasers allows for more precise surgical procedures.

4.4. Applications in the Military Field

Due to their exceptional characteristics such as high brightness, a very small irradiation area, and ease of operation, high-power fiber lasers have long been a focus of research for the development of laser weapons in the military domain. The application of high-power fiber lasers to offensive weaponry has the potential to utterly destroy exceptionally robust targets. In the realm of laser weapon development, the United States has shown remarkable prominence. For instance, in recent years, the U.S. Air Force research staff has been dedicated to innovative technology research on fiber laser systems. It is evident that in the future of military applications, laser weapons will demonstrate a particularly significant development trend and provide a robust and effective guarantee for enhancing military combat capabilities.

5. Outlook

Fiber lasers, known as the third generation of lasers, boast numerous advantages such as high beam quality, high electro-optical conversion efficiency, and low maintenance costs. Consequently, fiber lasers are destined to be widely applied in industries like industrial manufacturing, medical treatment, and military operations. Looking at the current status of fiber lasers in terms of profit and sales revenue, it is likely that high-power fiber lasers will dominate the laser market in the future. As such, laser manufacturers both domestically and internationally have shown great interest in the development of high-power fiber lasers.

However, there is still a gap between China’s research and production of high-power fiber lasers and that of foreign corporations. It is hoped that China’s scientific research teams can achieve significant breakthroughs in fiber laser technology to narrow this gap with the rest of the world. The development of fiber lasers has been rapid, evolving from early low-power outputs to the current kilowatt-level laser outputs.

With the progress of industry and science and technology, fiber lasers are bound to be extensively applied across many sectors, particularly high-power fiber lasers. Considering the inherent advantages of fiber lasers, the following trends can be anticipated in the development of high-power fiber lasers over the next few years: