Introduction to fiber lasers

The optical fiber amplifier developed by the optical fiber doped with rare earth elements has brought revolutionary changes to the field of light wave technology. Since any optical amplifier can form a laser through an appropriate feedback mechanism, fiber lasers can be developed on the basis of fiber amplifiers. The currently developed fiber lasers mainly use rare-earth-doped fibers as gain media. Because the fiber core in the fiber laser is very thin, it is easy to form a high power density in the fiber under the action of the pump light, which causes the 'population inversion' of the laser energy level of the laser working substance. Therefore, when a positive feedback loop (to form a resonant cavity) is properly added, laser oscillation can be formed. In addition, because the fiber matrix has a very wide fluorescence spectrum, fiber lasers can generally be made tunable, which is very suitable for WDM system applications.

　　 We can classify fiber lasers from different angles. For example, according to the structure of fiber laser resonant cavity, it can be divided into Fabry-Perot cavity and ring cavity. It can also be divided into single wavelength and multi-wavelength according to the number of output wavelengths. For the characteristics of different types of fiber lasers, the following points should be considered: (1) The threshold should be as low as possible; (2) The linearity of output power and pumping optical power should be better; (3) Output polarization state; (4) Mode structure ; (5) Energy conversion efficiency; (6) Laser working wavelength, etc.

　　 Compared with semiconductor lasers, the advantages of fiber lasers are mainly reflected in: fiber lasers have a waveguide structure, which can tolerate strong pumping, have high gain, high conversion efficiency, low threshold, and good output beam quality , Narrow line width, simple structure, high reliability and other characteristics, easy to achieve coupling with optical fiber.

　　 Compared with CO2 lasers, CO2 lasers are used in more stable, larger, and faster machine tools, while fiber lasers are mainly used for surface processing below 2mm. The main advantages of fiber lasers are: 1. The wavelengths of fiber lasers and CO2 lasers differ by an order of magnitude. CO2 lasers cannot be transmitted by optical fibers, while fiber lasers can be transmitted by optical fibers, which greatly increases the flexibility of processing; 2. The photoelectric conversion rate of fiber laser is as high as 25%, while that of CO2 laser is only about 10%. Fiber laser has obvious advantages in terms of electricity consumption and supporting cooling system.