激光 器的典型光路结构和光纤功率合成技术 Typical optical path structure and fiber power combining technology of all-fiber laser
的光路示意图。 FIG. 1A is a schematic diagram of a typical single-ended pumped continuous single-mode high-power all-fiber laser . The single-ended pump has a simple structure, but the gain in the active area is exponentially attenuated, and the distribution is extremely uneven, which is suitable for situations where the power is not very high.
FIG. 1B is a schematic diagram of a typical bidirectionally pumped continuous single-mode high-power all-fiber laser. The bidirectional pumping structure is relatively complicated, but the uniformity of gain distribution in the active area is improved, which can achieve higher power output.
At present, the continuous output power of single-mode fiber lasers has reached 2000W.
Figure 2 is a schematic diagram of the optical path of a multimode fiber laser using fiber power synthesis technology. Using this technology, the output power of multiple single-mode continuous fiber lasers can be combined to obtain tens of thousands of watts or higher of optical power output and beam. Good quality.
The coherent synthesis technology that people are developing is expected to use this method to achieve coherent synthesis of fiber lasers and achieve single-mode continuous output on the order of 10,000 watts of fiber lasers.
Features and advantages
All-fiber lasers have a number of significant features that make them obvious advantages in many ways:
1) The surface area / volume ratio of the gain medium is large
Fiber laser uses optical fiber as gain medium, which has a large surface area / volume ratio, which makes it have very good heat dissipation performance. Therefore, even very high-power fiber lasers, the gain medium will not be thermally damaged, and there is generally no need to adjust the gain. The medium adopts special heat dissipation measures. 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 confinement mechanism
The high-power all-fiber laser uses a double-clad active fiber. This double-clad fiber is a dual-waveguide structure. High-power multimode pump light is limited to transmission in the inner cladding with a large diameter. Inexpensive multi-mode pump light provides the conditions. Signal lasers are generated and transmitted in a small-diameter core with a circularly symmetric waveguide structure. Under the restrictions of small-core diameter core waveguides, signal lasers can obtain ideal beam quality And extremely small light spot diameter, which is an important feature of the all-fiber laser's unique attractiveness. Among the high-power lasers, no laser can surpass it at present. Excellent beam quality and extremely small light spot diameter are of great significance in laser applications, which can make the optical system of subsequent applications simpler, smaller, longer working distance, smaller laser focusing spot, and more efficient High, deeper processing depth, better processing quality and so on.
3) Inherent fully enclosed flexible optical path
The optical path of an all-fiber laser is composed of optical fibers and optical fiber components. The optical fiber and optical fiber components are connected by fiber fusion technology, and the entire optical path is completely enclosed in the optical fiber waveguide. Once this natural fully enclosed light path is formed, it can be self-contained without additional isolation measures to achieve isolation from the external environment. Because the optical fiber is small and has good flexibility, the optical path can be coiled and passed along a small pipe. Therefore, all-fiber lasers can work in harsh environments, and the output light can pass through narrow gaps or remotely along small pipes. transmission. 设备的设计具有更高的灵活性等等。 These features have great advantages in industrial applications. The laser not only can adapt to the harsh working environment, but also can keep the laser away from the light exit point. It can introduce the laser to a place that was previously difficult to reach. It can easily move and change the light exit point. The processing point shares a laser, which makes the design of laser processing equipment more flexible and so on.
4) The optical path is maintenance-free
As mentioned earlier, the optical path of an all-fiber laser is composed of optical fibers and optical fiber components. The optical fiber and optical fiber components are connected by fiber fusion technology. Therefore, once the optical path is completed, it forms a whole. Practice has proved that the connection structure and The connection parameters will remain stable for a long time. If the optical fiber and the optical fiber components themselves can have long-term stability, the entire optical path will be stable for a long time without maintenance. It is important to point out that this maintenance-free feature is not unmaintainable and repairable. When necessary, maintenance and repair of the entire optical path can also be performed. Therefore, compared with the frequent maintenance and repair of lasers such as gas and solid, etc. The maintenance-free characteristics of the all-fiber laser's optical path are exceptionally excellent. Compared with the unrepairability of semiconductor lasers, the maintainability and maintainability of all-fiber lasers show obvious advantages.
5) Single long-life multimode pump laser with wide light emitting area
The optical path of an all-fiber laser has long-term stability. Therefore, a matching long-life pump laser is required to obtain the long life of the entire machine. The development of low-cost long-life multimode pump lasers is the focus of the development of long-life fiber lasers. Studies have shown that the failure rate of a semiconductor laser is proportional to the m-th power of the optical power density in the active area, and the value of m ranges from 2 to 3; it is proportional to the n-th power of the current density in the active area, The value ranges from 3 to 6; it has an exponential relationship with the temperature of the active area. Its mathematical expression is as follows:
Where F is the failure rate, I is the optical power density in the active region, J is the current density in the active region, EA is the activation energy, and the value is between 0.4 and 0.7, KB is the Boltzmann constant, and T is Source temperature.
It can be seen that reducing the optical power density, current density, and active temperature of the active area has significant significance for reducing the failure rate. A single wide-emitting region multimode pump laser is a long-life semiconductor pump laser designed based on this principle. Its strip width is generally 100mm, which is basically close to the core diameter of a 105/125 multimode fiber. The strip width of the source region is tens of times that of the array semiconductor laser. For a single light-emitting strip, with the same output power and the same injected current, its optical power density and current density will be reduced by dozens of times, and the temperature of the active region will also be reduced. It has been reduced, and under the premise of ignoring other factors, the effect of a single wide-emitting region semiconductor pump laser in reducing the failure rate is extremely significant. At present, the average trouble-free working time of a single wide-emitting region semiconductor pump laser with a pigtail output power greater than 5W has reached more than 500,000 hours.
6) Long life
From the previous discussion, we already know that using a single wide-emitting region semiconductor pump laser as the pump source of the fiber laser, the all-fiber laser will have the characteristics of long life. Therefore, the production of long-life fiber lasers with hundreds of thousands of hours in technology It is already feasible.
7) Small size and light weight
All-fiber lasers can be coiled and the optical path occupies less space. In the case of a single wide-emitting area semiconductor pump laser as the pump source, the pump lasers can be installed discretely, with good heat dissipation characteristics. In high cases, air cooling can be used. In the case of high installation density, only a small amount of water is needed to meet the heat dissipation requirements. Therefore, the volume of an all-fiber laser is smaller than that of gas and solid-state laser systems with the same output power. And lighter weight.
8) High output power
After the output power of fiber laser exceeded 100W, the output power level increased rapidly. It only took more than three years to reach an output power level that exceeded the output power level achieved by YAG solid-state lasers and CO2 gas lasers for more than 30 years. Currently, The laboratory level of fiber lasers has exceeded 100,000 watts, 30,000 watts of optical fibers have been commercialized, and fiber lasers have been sold with an output power of 17000W. It can be predicted that the fiber laser will become the laser with the largest continuous output power for a long time.
9) Water and electricity saving costs
The fiber laser has excellent thermal performance, high electro-optical efficiency, and water and power saving. It is particularly important that it can be used without maintenance for a long time, which can save a lot of maintenance costs and time, and improve work efficiency.
10) Costs keep falling
The optical path of an all-fiber laser consists of optical fibers and optical fiber components. Due to the availability of raw materials, costs can be greatly reduced after the technology, products and markets mature. In addition to the optical path, semiconductor pump lasers are the main part of the cost of fiber lasers. From the development history and experience of optical communications, with the development of technology and the continuous expansion of market capacity, the cost of semiconductor pump lasers has been greatly reduced Will become an inevitable trend.
5 key technologies and development trends
Special fiber technology
All-fiber lasers need to use a variety of special optical fibers such as double-clad active fibers, double-clad photosensitive fibers, and energy transmission fibers. With the continuous increase of output power, the technical requirements for special fibers are also increasing. Therefore, special The development of optical fiber will play an important role in the development of fiber lasers. The new generation of special optical fiber represented by photonic crystal fiber will be gradually applied in the development of fiber laser. The development of special optical fiber will make the gain of active fiber higher, the power density it withstands, and the absorption of pump light more effective; it will make the fabrication of gratings easier, the stability of gratings better, and the gratings in optical fibers. The laser is more widely used; it will enable the energy transmission fiber to transmit higher power, be able to transmit high-power lasers over longer distances, and the range of wavelengths that can be transmitted is constantly expanding; it will make pump coupling easier to achieve and tolerate Higher pump power, lower losses, etc.
Cladding pump coupling
The cladding pump coupling technology of all-fiber lasers has an inestimable effect on determining the performance and level of fiber lasers. The fiber pump coupling devices and fiber power combining devices used for high-power all-fiber lasers are used under very high power conditions. The coupling efficiency must be high, the losses must be small, and the power to withstand must be large. The number of input light paths needs to be as much as possible. With so many extreme conditions, it is very difficult to produce high-quality pump coupling devices and power synthesis devices. However, there are many ways and means to achieve this, which is a challenging technology. Looking at the development trend of high-power all-fiber lasers, it is also required that the pump coupling device couples the pump light to the inner cladding while not affecting and damaging the core of the double-clad fiber as much as possible. Cascade pumping is achieved in the case of signal laser generation and transmission, and ultra-high power output is achieved. This paper believes that the development of pump coupling technology with the least impact on the core is the development direction of pump coupling devices. For optical fiber power combining devices, the goal pursued is to continuously increase the combined optical power.
Fiber grating technology
Fiber gratings in all-fiber lasers currently serve as a reflection cavity for the signal laser in the fiber core. However, with the further development of fiber laser technology, fiber gratings will have new uses in fiber lasers. The production technology of the new technology poses new challenges, and one of the directions worthy of attention is to make high-quality fiber gratings on large core multimode fibers.
Semiconductor pumped laser technology
Semiconductor pump lasers are the key components of fiber lasers. They have a significant impact on the reliability, lifetime, and manufacturing cost of fiber lasers. The development of a single long-life semiconductor pump laser with a wide light-emitting area has become one of the semiconductor pump lasers for fiber lasers. This trend is to continuously increase the output power of a single laser, continuously reduce costs, and further improve reliability. Among them, improving and innovating the packaging structure should be the core work, because the current packaging cost accounts for a high proportion.
Fiber laser complete machine technology
The overall design and production of all-fiber lasers involves a lot of knowledge, content, technology, technology, experience and know how. It is the core and most critical technology for all-fiber laser design and production, especially in new high-power all-fiber lasers. Today's development history is quite short, and there is still a lot of groundbreaking work to be done. For the whole machine design and production of all-fiber lasers, not only needs to be rationally designed for the application, but also shoulders the responsibility of improving and innovating the structure and scheme of the whole machine, as well as the improvement and innovation of important components and key technologies. At present, manufacturers designing and manufacturing fiber lasers all over the world have invested a lot in innovation.
Laser Beam Technology
Laser beam combining is currently a research hotspot in the field of laser technology in the world. Its purpose is to combine multiple laser beams into one beam output, which is an effective means to greatly increase the laser output power and brightness. Using the beam combining technology to synthesize the output of the fiber laser array, the output power of tens to hundreds of kW can be obtained in the future. Using it as the light source of the airborne laser weapon system can greatly increase the compactness and flexibility of the weapon system. The lethality and deterrence of large weapon systems.
Laser beam combining technology can be divided into two categories: coherent beam combining and non-coherent beam combining. Coherent grouping requires that the wavelengths and polarization directions of the output beams of all array elements are consistent, and the phase of each beam must be strictly detected and controlled, which is difficult to achieve. Non-coherent beam combining has no restrictions on the polarization direction and phase of the output beam of each array element. It only requires that its wavelength is within the spectral range of the beam combining element to achieve a scalar superposition of power. This beam combining method has a simple structure, a stable system and easy control. It has gradually become a research focus in the field of fiber lasers in recent years.