Rare earth ions are doped into the core of the optical fiber. When the pump light passes through the optical fiber, the rare earth ions in the core absorb the pump light and transition to the upper energy level of the laser, resulting in a population inversion. The inverted particles transition to the lower energy level of the laser by stimulated radiation under the induction of spontaneous radiation photons or specially injected photons, and emit photons identical to the induced photons at the same time. This process occurs like an avalanche, and the laser is emitted. This is the basic principle of the fiber laser. The reason for choosing to dope rare earth ions in the optical fiber to form a fiber laser is partly because the absorption range of rare earth ions coincides with the radiation range of semiconductor lasers, so it is convenient to use low-cost and relatively mature semiconductor lasers as pump light sources.
The basic structure of the fiber laser consists of a gain medium, a resonant cavity, and a pump source. The gain medium is a fiber core doped with rare earth ions. The doped fiber is placed between two cavity mirrors with selected reflectivity. The pump light is coupled into the optical fiber from the left cavity mirror of the fiber laser cutting machine laser, and the output laser is obtained through the collimation optical system and the filter. Theoretically, only the pump source and the gain fiber are necessary components of the fiber laser, and the resonant cavity is not an indispensable component. The mode selection of the resonant cavity and the role of increasing the length of the gain medium can be ignored in the fiber laser of the laser cutting machine, because the long fiber itself can be very long, so as to obtain a very high single-pass gain, and the waveguide effect of the fiber can play a role in mode selection. However, in practical applications, people generally want to use shorter fibers, so in most cases, a resonant cavity is used to introduce feedback.
Advantages of fiber lasers
Compared with traditional gas lasers and solid lasers, fiber lasers with doped fiber as the working medium have the following advantages:
1. Large surface area/volume ratio of the gain medium
Fiber lasers use optical fiber as the gain medium, which has a large surface area/volume ratio, which makes it have very good heat dissipation performance. Therefore, even for very high-power fiber lasers, the gain medium will not be thermally damaged, and generally no special heat dissipation measures are required for the gain medium. For other types of lasers, the heat dissipation of the gain medium needs to be considered. Therefore, this feature is unique to fiber lasers.
2. Excellent dual-waveguide restriction mechanism
High-power all-fiber lasers use double-clad active optical fibers. This double-layer optical fiber is a dual-waveguide structure. High-power multimode pump light is confined to the inner cladding with a larger diameter for transmission, which provides conditions for the use of high-power and cheap multimode pump light. The signal laser is generated and transmitted in a core with a small diameter and a circularly symmetrical waveguide structure. Under the restriction of the small core waveguide, the signal laser can obtain ideal beam quality and extremely small light spot diameter, which is an important feature of the all-fiber laser. Among high-power lasers, no laser can currently surpass it. Excellent beam quality and extremely small light spot diameter are of great significance in laser applications, which can make the optical system of subsequent application equipment simpler, smaller in size, longer in working distance, smaller in laser focusing spot, higher in working efficiency, deeper in processing depth, better in processing quality, etc.
3. Inherent fully enclosed flexible optical path
The optical path of the all-fiber laser is entirely composed of optical fibers and optical fiber components. The optical fibers and optical fiber components are connected by optical fiber fusion technology, and the entire optical path is completely enclosed in the optical fiber waveguide. Once this natural fully enclosed optical path is formed, it can form a self-contained system without additional isolation measures, and can be isolated from the external environment. Because the optical fiber is small and has good flexibility, the optical path can be coiled and travel along a small pipe. Therefore, the all-fiber laser can work in a relatively harsh environment, and the output light can pass through a narrow gap or be transmitted over a long distance along a small pipe. These characteristics have great advantages in industrial applications. The laser can not only adapt to a relatively harsh working environment, but also make the laser principle light point, and can introduce the laser to places that were previously difficult to reach. It can be very easy to move and change the light point, so that the processing point can share a laser, which can make the design of laser processing equipment more flexible, etc.
4. The optical path is maintenance-free
As mentioned above, the key points of the all-fiber laser are all composed of optical fibers and optical fiber components. The optical fibers and optical fiber components are connected by optical fiber fusion technology. Therefore, once the optical path is completed, it forms a whole. Practice has proved that the link structure and connection parameters formed in this way will remain stable for a long time. If the optical fiber and optical fiber components themselves can have long-term stability, the entire optical path will be stable for a long time and no maintenance is required. It should be pointed out that this maintenance-free feature does not mean that it cannot be maintained and repaired. If necessary, the maintenance and repair of the entire optical path can also be carried out. Therefore, compared with the frequent maintenance and repair of gas and solid lasers, the maintenance-free feature of the optical path of the all-fiber laser is extremely excellent, and compared with the unmaintainability of semiconductor lasers, the maintainability and repairability of the all-fiber laser show obvious advantages.
5. Long life
The optical path of the all-fiber laser has long-term stability, so a long-life pump laser matching it is required to obtain the long life of the whole machine. The development of low-cost long-life multi-mode pump lasers is the focus of the development of long-life fiber lasers. A single wide luminous area multimode pump laser is such a long-life semiconductor pump laser. Its strip width is generally 100mm, which is basically close to the core diameter of 105/125 multimode optical fiber. Its active area strip width is dozens of times that of an array semiconductor laser. For a single luminous strip, under the same output power and the same injection current, its optical power density and current density will be reduced by dozens of times, and the temperature of the active area will also be reduced. Ignoring other factors, the role of a single wide luminous area semiconductor pump laser in reducing failure rate is extremely significant. At present, the average trouble-free working time of a single wide luminous area semiconductor pump laser with a pigtail output power greater than 5W has reached more than 500,000 hours. Using a single wide luminous area semiconductor pump laser as the pump source of a light laser, the all-fiber laser will have the characteristics of long life. Therefore, it is technically feasible to make a long-life fiber laser with hundreds of thousands of hours.
6. Small size and light weight
Since the optical path of the all-fiber laser can be coiled, the optical path occupies less space. When a single wide-light-emitting area semiconductor pump laser is used as the pump source, the pump laser can be installed in a dispersed manner and has good heat dissipation characteristics. When the installation density is not high, it can be cooled. When the installation density is high, only a small amount of water is needed to meet the heat dissipation requirements. Therefore, the volume of the all-fiber laser is smaller and lighter than that of the gas and solid laser systems with the same output power.