Analysis of the hottest high power fiber laser

Analysis of high-power fiber laser

the so-called high-power fiber laser is a high-intensity light source positioned in the industries of machining, laser medical treatment, automobile manufacturing and military, compared with the low-power fiber laser as the carrier in fiber communication (the power is MW level). High power fiber laser skillfully integrates fiber technology and laser principle, and has cast the most advanced and sharp laser in the 21st century. Even in countries with advanced laser technology, fiber lasers are synonymous with cutting-edge, mysterious and seductive

I. fiber technology

the biggest feature of fiber lasers is that one fiber goes through to the end, and the whole machine highly realizes fiber integration. However, those lasers that only use fiber transmission in the external light guide part or LD pump source adopts PVC pipes with a diameter of 1m and are coupled with pigtails are not fiber lasers in the real sense

optical fiber is a solid glass fiber made of SiO2 as the matrix material. It is mainly used in optical fiber communication. Its light guiding principle is the total reflection mechanism of light. Ordinary bare optical fibers generally consist of a central high refractive index glass core (core diameter is generally.5 μ m) Middle low refractive index silicon glass cladding (core diameter is generally 125 μ m) And the outermost reinforced resin coating. Fiber can be divided into single-mode fiber and multimode fiber. Single mode fiber: the center glass core is thin (diameter 9 μ m+0.5 μ m) , it can only transmit one mode of light, and its inter mode dispersion is very small. It has the function of self selecting mode and limiting mode. Multimode fiber: the central glass core is thick (50 μ m+1 μ m) , it can transmit various modes of light, but its model gold public welfare fund began to enter Sichuan University, Beijing University of chemical technology, Fujian Normal University, Fuzhou University, Peking University, Xiamen University and many other colleges and universities, with large dispersion and impure light transmission

Figure 1

the fiber used in high-power fiber lasers is not an ordinary communication fiber, but a special fiber doped with a variety of rare ions, with a more complex structure and high radiation resistance --- double clad fiber

figure 2

double clad fiber has an inner cladding outside the core than ordinary fiber. For pump light, it is multimode, with large diameter and light receiving angle. It can absorb high brightness multimode pump light wantonly and gather a large number of photons in the fiber. Practice has proved that double clad fiber with D-shaped and rectangular cross-section has 95% coupling efficiency, so it has been widely used. For pulsed fiber lasers, a major issue is how to improve the radiation resistance of fiber. At present, the single pulse capacity of fiber lasers in the world can reach 20000w. How can a hair size fiber withstand such high laser radiation? Therefore, we must consider doping some special ions in the optical fiber to prevent the optical fiber from being burned out. For example, in the case of nuclear radiation, the fiber doped with cerium ion will not lose its light transmission ability due to dyeing, nor will it be deformed by heating

second, the traditional solid-state laser

laser is simply a wavelength converter --- short wavelength pump light excites doped ions to convert long wavelength optical radiation. It is generally composed of three parts: working material, resonator and pump system. Since fiber lasers are essentially solid-state lasers, here we only compare the characteristics of traditional nd:yag lasers

working substance:

working substance is the heart of solid-state laser. Its physical properties are determined by the matrix material, and its spectral properties are determined by the energy level structure in the activated ion. The widely used YAG laser crystal is formed by doping trivalent nd3+ into YAG single crystal. It mainly has the following obvious characteristics:

1. The growth rate of YAG rod is very slow, generally less than 1mm/h. At present, the largest crystal rod has a diameter of 40mm and a length of 180mm, so the laser gain is fundamentally limited and ultra-high power laser output cannot be achieved

2. As long as the working material is a crystal, it cannot avoid the thermal lens effect, thermal strain and thermally induced birefringence of the laser rod, and in serious cases, "laser quenching" and laser rod fracture occur; Therefore, the efficiency of YAG laser is very low

3. The main absorption spectral line of nd:yag rod is near 810nm, and its bandwidth is about 2nm. Therefore, the linewidth of the pump source should be strictly controlled, otherwise the ineffective absorption will cause heat loss. Therefore, YAG lasers generally need to add a huge cooling system

4. Because the radius of nd3+ is not completely consistent with that of y3+, there are inherent defects in the structure of nd3+ ion doped into YAG crystal, resulting in optical defects. It is not possible to mix high concentration of nd3+ into YAG crystal to achieve high gain, which is also the fundamental factor affecting the optical performance of laser

5. The average lifetime of nd3+ ion in metastable energy level is 300US, and its optimal Q-switch repetition frequency can only be 1/300us, that is, 3.3khz. Therefore, the Q-switch of YAG laser is generally set to kHz and cannot work at high frequency

optical resonator:

traditional optical resonator is mainly composed of two lenses coated on both ends of the working material, which plays the role of positive feedback, mode selection and output coupling. Compared with the unique cavity structure of fiber laser, the traditional optical resonator mainly has the following characteristics:

1. Because it is composed of two lenses, the resonator is affected by mechanical vibration, thermal lens effect and thermal focal length disturbance of crystal rod, which makes the laser unable to light normally, and requires extremely cumbersome dimming and monitoring

2. The cavity mirror is very sensitive to dust, moisture and debris, and it needs to be wiped regularly, otherwise the laser power will be affected

3. The length of the cavity is in contradiction with the output power, and the beam quality is in contradiction with the laser energy; High quality and high power output can be achieved only by using expensive mode selection/locking cavity

4. The initial laser excited from the laser crystal is not a single-mode light, but a beam with a diameter of several millimeters. The single-mode output must be realized through the cavity mirror attenuation or mode selection mechanism, thus reducing the overall conversion efficiency

pumping system:

because the advantages and disadvantages of the lamp pumping system are well known, here we will only talk about some characteristics of the pumping mode of DPSSL:

1. Because dppssl is mainly slightly improved on the pumping system, it can only alleviate the thermal effect of the laser rod, but cannot fundamentally eradicate the disadvantage of the crystal laser

2. It is necessary to strictly control the wavelength of LD between nm, either add a cooling system or a wavelength lock, which is determined by the spectral characteristics of Nd: YAG crystal

3. If the pump light is focused on a few millimeters of crystal end face for end pumping, first, high power output cannot be achieved; Second, the pump light should not be too strong, otherwise the film may fall off, the crystal rod cannot dissipate heat in time, and even the rod distortion may occur

4. If the pump light pumps the crystal side, it is generally multimode output. If special measures are not taken, the beam quality cannot be improved

5. The laser directly emitted by LD is a highly Gaussian astigmatic beam. During end pumping, various optical elements need to be added to calibrate and focus the pump light on the crystal. These additional optical elements will be affected by mechanical vibration, dust and humidity, thus reducing the conversion efficiency

III. low power fiber lasers

the output power of fiber lasers for general communication is generally milliwatt level, and its typical structure is shown in the following figure:

it is significantly different from our traditional industrial lasers for processing:

1. Use ion doped fiber as working material

2. Use fiber Bragg grating to replace optical lenses to form an optical resonator

3 LD pump source can be seamlessly coupled with doped fiber through pigtail

4. The light guide part also uses fiber output directly

but the single-mode core diameter of this kind of laser is only 9um, and only end pump can be used, which can not bear too high power density; In addition, the single-mode fiber core puts forward strict requirements for the mode of LD. Only single-mode light can be coupled into the fiber core for effective pumping, but it is a pity that high-power single-mode LD has not been achieved so far; Finally, strong pump light coupling in a very thin core will have serious nonlinear effects, which will change the optical performance and reduce the conversion efficiency. Due to the influence of power, this kind of laser has been limited to the field of optical communication; At the same time, due to the huge industry gap, almost no one dared to associate it with laser processing. Therefore, high-power output is the biggest bottleneck in the development of fiber lasers, and almost all research work is carried out around this problem

although most people in China did not realize high-power fiber lasers until after 2002, IPG was established in Russia after at least 20 years of hard research, and SPI was also established in Britain after at least 30 years of research. They all shoulder important national defense missions in the Cold War era, have received the full support of the state, and have always been top secrets in the military field

IV. high power fiber laser

the following figure is the schematic diagram of high power fiber laser from IPG company of Russian technology. According to the three components of the laser, it is analyzed as follows:

working material ----- double clad special fiber:

1. The single-mode fiber core is composed of quartz material doped with ytterbium ions and other elements, which is used as the laser oscillation channel; The inner cladding is composed of pure quartz material whose transverse size and numerical aperture are much larger than the core and whose refractive index is smaller than the core. It is a multimode fiber that receives multimode LD pump light; It is precisely because the doped activated core is separated from the multimode inner cladding that receives multimode pump light that the possibility of multimode optical pumping and single-mode optical output can be realized, which virtually resolves the contradiction between laser power and beam quality

2. The whole double clad fiber adopts D-type and other structures, with small optical rotation effect, full absorption, and optical optical conversion of more than 80%

3. Countless branch fibers are generated on both sides of the optical fiber. Each branch can be seamlessly coupled with the LD with pigtail to form a point pump, which can greatly improve the output power and avoid a series of thermal effects caused by the traditional end pump

5. The optical fiber is made of quartz material with better performance than ordinary glass, and doped with high radiation resistant ions. The whole section of the optical fiber can withstand laser energy up to 10000w without thermal damage. People have higher and higher requirements

6. Yb3+ has no excited state absorption and can be highly doped. At the same time, the optical fiber can reach hundreds of meters, which can greatly improve the laser gain and increase the heat dissipation area; The optical fiber disk is on the heat sink and can work stably with simple air cooling

7. The absorption spectrum of yb3+ is 10 times wider than that of nd3+. It is very loose for the mode of LD light source and is hardly affected by wavelength temperature drift, which can greatly improve the conversion efficiency

8. Yb3+ energy level is a simple two-level, and its metastable life is three times that of nd3+. A small power pump source can accumulate and store a large amount of energy in the excited state, which is very suitable for forming a high-density ion number inversion in a very narrow fiber core, so as to output a stable strong laser

optical resonator ---- fiber grating:

1. Fiber grating uses the photosensitivity of fiber materials: that is, the interaction between external incident photons and the fiber core causes the permanent change of the refractive index of the latter. The essence of the spatial phase grating formed in the core of single-mode fiber by UV laser direct writing method is to form a narrow-band filter or reflector in the core

2. Fiber Bragg grating is engraved on both ends of the fiber core. When the activated ion emits a continuous broadband light and transmits it to the grating, it will selectively reflect back a corresponding narrow-band light (such as 1064nm) and return to vibration along the original transmission fiber; Other stray light is directly transmitted