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High-power fiber lasers
High-power fiber lasers are extraordinary devices. The laser has been the most significant, after the transistor, technological invention since World War 2. Lasers and laser systems find widespread application in different fields of science, engineering, and technology. They can be used for scanning barcodes, machining and welding, reading compact discs, printing paper, precision surgery, enabling high-speed communications, finding distances, guiding precision munitions, and driving controlled nuclear fusion. Pulsed fiber lasers are applied in elevated tasks such as the production of controlled nuclear fusion and laser eye surgery (ultrafast lasers), down to the mundane, such as cosmetic hair removal. The impressive increase in lasers’ peak power has been overwhelming over the last 50 years: the peak power attainable in a laser pulse has increased by roughly a factor of 1,000 every 10 years. The ability to produce high powers with lasers stems from the quantum mechanics that enable their operation.
How lasers work: the basics
The easiest way to determine the laser is to characterize it as an amplifier. The laser works by pumping energy into electrons of atoms in a substance, called the gain material. These atoms can be assembled in a number of forms, and many different media suitable for lasers have been developed. The active atoms or molecules in laser media can be in gaseous forms, such as the neon atoms in the ubiquitous helium-neon laser. They can also be semiconductor materials, such as gallium arsenide, used in the diode or solid-state lasers. Or they can be embedded in crystals such as the chromium ion in ruby. High-power fiber lasers capable of continuous output powers ranging from hundreds of watts to thousands of watts present exciting opportunities for rapid, directed delivery of energy.
Advantages of high-power fiber lasers
Beam quality and efficiency
High-power fiber lasers are much more progressive and promising than traditional lasers using solids or gases as the active medium in many aspects. In order to ensure the high beam quality of fiber lasers, it is necessary to select the appropriate core diameter and difference of relative refractive indices, which can reduce the number of transverse modes. The CO2 laser also provides a high beam quality. In addition to this, fiber lasers and fiber laser systems based on a thin optical fiber with a diameter of several hundred micrometers as an active medium can easily be cooled and therefore attain high power output while maintaining the laser beam quality. Also, high-power fiber lasers have very low loss of pump and laser light because they are both confined and guided in the low-loss fiber core. The high quantum efficiency of ytterbium serving as the active element leads to 60-70% efficiency in energy conversion from pump light to laser light.
Compact design and reliability
In virtue of these factors, laser systems on the basis of the optical fiber achieve a high output power with a high energy conversion efficiency while maintaining a high beam quality. Owing to the very high energy conversion efficiency and a resonator consisting of fine fiber and small optical components, high-power fiber lasers have a far smaller heat dissipation mechanism and power supply, and thus far smaller overall dimensions and weight than traditional high-power lasers. These lasers, constructed by fusion-splicing optical fibers, are not influenced by vibration, shock, or temperature changes, and therefore have stable output power and stable high beam quality. Besides the above-mentioned factors, high-power fiber lasers are practically maintenance-free due to the fact that the paths of the beam are not exposed to the atmosphere.
The designers, developers, and users of high-power laser systems discuss design approaches, methods of enhancing performance, new applications, and user requirements.