5 minutes to understand the heat dissipation of high-power semiconductor lasers
introduction
Semiconductor lasers were first researched from abroad, the earliest technology originated in the United States, Japan, mainly used in the military, and then with the iterative development of technology, began to be applied to the civilian market, used in optoelectronics, communications and other industries. With the development of China's national defense industry and optoelectronic manufacturing industry, the industry has begun to increase the demand for high-power lasers, and people have also begun to research high-power semiconductor laser devices. During the research, it was found that the light output quality of traditional semiconductor lasers could no longer meet the needs of today's people, and in order to increase the output power of semiconductor lasers, people began to continuously improve and analyze. During the research, it was found that half of the electrical energy of the semiconductor laser will be converted into heat energy when it is used, and if the heat dissipation of the semiconductor laser itself is not good, it will directly affect the life and use of the semiconductor laser, so the heat dissipation problem is one of the problems that researchers urgently need to solve.
Diode laser concept
Semiconductor lasers are by far one of the most widely used optoelectronic devices. With the continuous advancement of technology and the improvement of the mass production ability of devices, it can now be applied to more fields. Semiconductor lasers are lasers that mainly use semiconductor materials as a working substance, because the laser light generated will be different due to the different structure of the matter. Semiconductor lasers are characterized by their small size and long lifespan, and can now be used not only in the field of communication, but also in radar, sound measurement, and medical applications.
Classification of laser heat dissipation methods
At present, the main heat dissipation methods of lasers are divided into traditional heat dissipation methods and new heat dissipation methods, the traditional heat dissipation methods include: air cooling heat dissipation, semiconductor refrigeration heat dissipation, natural convection heat dissipation, etc., and the new heat dissipation methods include: flip heat dissipation, microchannel heat dissipation.
The heat dissipation structure and heat transfer process of semiconductor lasers
Figure 1 shows a diagram of the heat dissipation structure of a semiconductor laser.
The heat dissipation mechanism of semiconductor laser packaging is mainly composed of laser chip, soldering layer, heat sink, metal layer, etc. The soldering layer in the heat dissipation structure of the semiconductor laser is mainly used to connect the chip and the heat sink together by soldering. In order to achieve the purpose of reducing thermal resistance, high-power semiconductor lasers often use some materials with high thermal conductivity, such as gold-tin solder, during soldering. There will be many layers in the whole packaging process, these layers mainly include: chip, solder layer, heat sink, metal layer, the heat transfer effect of heat sink and metal layer will be used to conduct the heat energy of the laser chip, and finally make the semiconductor laser form a good heat dissipation, so as to prolong the service life of the laser.
Precautions to be taken in the analysis of thermal performance
The heat dissipation performance of high-power diode lasers is mainly evaluated by thermal resistance and heat flux, and attention needs to be paid to the heat flux at a limited temperature when evaluating it. If the temperature difference between the two is relatively large during the heat dissipation analysis, condensation will appear on the surface of the laser chip, which will not only affect the optical output power, but also affect the locking of the wavelength, and even damage the photoelectric performance of the circuit due to the condensation problem, which will ultimately affect the reliability. At present, the common way to reduce thermal resistance is to use thermal conductivity materials, which provide more optimization space for laser temperature reduction.
Traditional heat dissipation methods
Natural convection heat sink cooling and heat dissipation method
Natural convection heat sink cooling and heat dissipation is to use some materials with high thermal conductivity to take away the heat generated, and then dissipate the heat through natural convection. Scientists have also found that fins can also help dissipate heat, and can maximize the heat transfer rate in the heat dissipation system when dissipating heat. When the temperature is the same, the fin spacing decreases as the fin height increases. When using the substrate to place the heat sink vertically, it is necessary to increase the height appropriately, and improve the heat dissipation effect by increasing the height, which will reduce the cost a lot when using it. In practice, copper or aluminum nitride are often used as heat sinks, but the heat sink method cannot fully meet the heat dissipation needs of high-power semiconductor lasers.
Semiconductor cooling and heat dissipation (electrical cooling heat dissipation) method
The main characteristics of semiconductor cooling and heat dissipation methods are small size and strong reliability. Semiconductor cooling and heat dissipation methods often appear in high-power semiconductor lasers, because of the addition of TEC cooling, the size of the package is increased accordingly, and the cost of packaging also rises accordingly.
By adjusting the internal parameters of the TEC, the cooling control effect of the TEC can be improved. During their research, the researchers found that having the best heat transfer area ratio can maximize the TEC characteristic coefficient. During the research, it was also found that the ratio of the heat transfer area has a great relationship with the characteristics of the TEC material and the exchange area.
Large-channel water cooling and heat dissipation method
In order to reduce the temperature of the heat sink, it is necessary to build a channel in the heat sink, and in order to achieve the effect of cooling, a certain water source needs to be added to this channel, so that the work of the laser will not be delayed. In view of this, researchers have found that the heat dissipation effect of the turbulent structure will be better than that of the traditional cavity structure, but the pressure in the channel will also increase. It is found that although large channels are widely used, due to the continuous increase of laser output power, the current large channel water cooling heat dissipation can no longer meet the heat dissipation needs of high-power semiconductor lasers.
New heat dissipation methods
With the increasing requirements for lasers in various fields, the traditional heat dissipation methods can no longer meet the current requirements, and more new heat dissipation methods need to be studied. At present, there are several new heat dissipation methods.
Flip chip placement method
Figure 3 shows the flip patch diagram. The traditional method of attaching the laser chip and the heat sink is to take the front side of the chip up, and the back cooling surface and heat sink are connected by solder, but the heat generation in the active area of the chip is mainly concentrated in the upper surface of a few microns of heat, and the upper surface and the lower surface are generally hundreds of microns apart, and the heat is conducted to the heat sink through such a long distance, and then to TEC refrigeration, and the heat dissipation effect is limited.
By improving the internal structure of the chip, adjusting the surface structure of the chip and the heating layer in the active area, and using the chip flip placement technology, the main heating surface of the chip is directly connected to the heat sink after passing through the soldering layer, and the heat dissipation efficiency of the laser can be improved by 20% or higher. Because the performance of the optical chip is strongly correlated with temperature, the higher the temperature, the more severe the wavelength drift, and the optical output power will also decrease or saturate, the heat dissipation effect can be greatly improved through flip mounting, the photoelectric output of the chip is more stable, and the performance of the entire laser has also been greatly improved, and the final performance needs to meet the performance requirements of the national military standard GR-468-CORE, some indicators are shown in Table 1.
Microchannel heat dissipation method
There are two main ways to dissipate heat from microchannels: (1) microchannels defined according to channel size; (2) Microchannels defined according to the effect of surface tension. Figure 4 is a typical diagram of a microchannel heat sink cooling structure.
Researchers did an experiment with a microchannel as a cooling device during the study, and found the heat dissipation characteristics of the microchannel through experiments, and the reason why the microchannel heat sink can dissipate heat is that there is a certain high heat flux. At the same time, the study also found that the microchannel will have a better heat dissipation effect. In addition, some people have found that the different groove shapes of the microchannel heat sink will also affect the heat dissipation effect. Numerous studies have found that the heat dissipation characteristics of the cosine channel are the best of all shapes. In addition, the researchers also found that the cooling device combining microchannels and glass micropipes can meet the heat dissipation requirements of high-power semiconductor lasers.
Lasers will be applied to microchannels when they are used, because microchannels will have better heat dissipation effect than traditional heat dissipation methods, and can meet the heat dissipation requirements of current high-power lasers. However, there is a disadvantage in the use of microchannels, that is, the microchannels are often blocked due to thermal deformation of cooling medium particles, which affects the heat dissipation effect, so it is necessary to use nanofluids to improve the heat exchange performance of the whole process.
Spray cooling heat dissipation method
Spray cooling is to spray the coolant onto the surface of heat transfer by atomizing with the help of pressure to achieve the purpose of cooling. The main characteristics of spray cooling are high heat transfer coefficient and low coolant flow. Researchers have found that when using water as a medium and using a solid conical nozzle for experiments, the surface of the microstructure can increase the effect of heat exchange. During the study, it was found that the coolness of spray cooling was related to the spray flow rate. In addition, the researchers also found a spray phase change cooler, in which the nozzle height in the spray cooling device is also very closely related to the heat dissipation effect.
Conclusion
In summary, the two most critical factors for improving heat dissipation are reducing the thermal resistance of the heat dissipation system and increasing the heat flux. When reducing the thermal resistance, materials with high thermal conductivity can be used to reduce it; When increasing the heat flux, it can be helped by increasing the heat transfer coefficient of the heat dissipation terminal. With the increasing requirements for the performance indicators of high-power lasers, many methods can no longer meet the application requirements, and more researchers are needed to conduct research through continuous efforts, so as to find more heat dissipation methods suitable for high-power semiconductor lasers.
