With the increasingly extensive application fields of fiber lasers, the reliability of fiber lasers has attracted more and more attention, including the reliability of laser output performance, the reliability of electronic components, the reliability of optical devices, the reliability of systems and so on.
Most of these are closely related to the thermal characteristics of the laser itself.
In addition, temperature has a great influence on the performance of the laser, especially on the output power and output stability of the laser.
The heat of fiber laser mainly comes from pump source and gain cavity.
For the pump source, its conversion efficiency is about 50%, which also means that energy equivalent to the output optical power will be generated in the form of heat.
If the heat cannot be dissipated in time, the temperature of the internal chip will rise rapidly, and the central wavelength of the laser will drift with the increase of temperature.
For the gain cavity, only part of the pump light is converted into laser output after entering the active gain fiber, and the rest of the energy is converted into heat energy.
The thermal energy will increase the temperature of the gain medium, resulting in the broadening of the fluorescence spectrum and the short lifetime of spontaneous emission, which will reduce the energy conversion efficiency.
Therefore, heat management is of great significance to fiber lasers.
At present, the commonly used heat management technologies mainly include air cooling and water cooling.
Among them, air cooling heat dissipation technology is mainly used in low-power pulse lasers and low-power CW lasers, and water cooling heat dissipation is mostly used as the main heat dissipation measure in medium and high-power fiber lasers.
Two main ways of heat dissipation
1. Water cooling and heat dissipation
As the name suggests, water cooling is to use water to take away heat through heat exchanger (such as water cooling plate).
Its working principle is also very simple, that is, the cold water in the chiller flows into the heat exchanger through the water pipe, then comes out of the other port of the heat exchanger, and then flows back to the chiller through the water pipe. Through such continuous circulation, the heat is taken away from the inside of the laser.
The way of water cooling and heat dissipation is simple in structure and easy to maintain;
With strong heat dissipation capacity and good temperature uniformity, the cooling performance of the laser can be improved by using a chiller with greater cooling capacity.
At present, there are more than 500 manufacturers integrating and selling handheld laser welding machines on the market, which generally use water cooling.
However, in addition to the laser itself, the hand-held laser welding machine with water cooling and heat dissipation also needs additional water cooler and water, which leads to a significant increase in the overall volume and weight of the equipment and limited use environment.
2. Air cooling and heat dissipation
In a broad sense, air cooling refers to the use of fans to strengthen air convection and complete the heat exchange inside the machine.
With the improvement of technology, major laser manufacturers have begun to set foot in the field of air cooling and heat dissipation.
Last June, the global fiber laser giant I company launched air-cooled lightWELD 1500W handheld laser welding products;
In August, the first air-cooled a1500W intelligent laser welder was launched in China;
Air cooled fiber laser: reci, IPG, GW
These three lasers are mainly for the market segment of handheld laser welding.
Air-cooled lasers can make the work more flexible and portable.
They all use air-cooled heat dissipation without additional water-cooling equipment, which not only reduces the cost, but also greatly reduces the volume and weight of the equipment.
Although they are all called air-cooled lasers, the air-cooled heat dissipation schemes adopted are different, including fan heat dissipation, heat pipe radiator heat dissipation and compressor refrigeration heat dissipation.
(1) Fan cooling
In the laser, a substrate with good thermal conductivity (such as copper, aluminum nitride, etc.) is used to export the heat generated in the pump source and gain cavity, and then dissipate the heat by convection. This method is called convective heat dissipation.
According to the different driving forces of fluid flow, convective heat transfer can be divided into natural convection and forced convection heat dissipation.
Under the action of no external force, the fluid can flow spontaneously and conduct heat transfer only by the temperature difference of the fluid, which is called natural convection;
When there is an external driving force, that is, the fluid flows rapidly driven by fans, fans and other components, so as to take away heat. We call it forced convection.
Due to the slow heat dissipation speed and poor effect of natural convection, it can not fully meet the heat dissipation requirements of the laser.
Therefore, it is necessary to add fans to the whole cooling system to speed up the flow of air and turn natural convection into forced convection.
Cooling principle of fan
(2) Heat pipe radiator
Heat dissipation of heat pipe radiator means that the heat pipe realizes heat transfer by relying on the phase change of its internal working liquid.
This liquid has low boiling point and is easy to volatilize.
One end of the heat pipe is the evaporation end, which is connected with the heat sink inside the laser;
The other end is the condensing end, which is connected with the external heat sink and fan.
The tube wall is provided with a liquid absorption core, which is composed of capillary porous materials.
When the laser generates heat, the evaporation end is heated, the working liquid evaporates rapidly, the steam flows to the condensation end under the pressure difference, and releases heat, which is discharged through the fan;
At the same time, the steam recondenses into liquid, and the liquid flows back to the evaporation section through the suction core (if it is a gravity heat pipe, there is no suction core, and the liquid adheres to the pipe wall and flows back to the bottom evaporation section by gravity).
If this cycle continues, the heat is transferred from the inside of the laser to the outside.
Heat dissipation principle of heat pipe radiator
The lightWELD 1500 handheld laser welding system of IPG company adopts the heat pipe radiator heat dissipation scheme.
The design and manufacture of lightWELD is characterized by small volume and light weight, which leads a new generation of changes in the current handheld laser welding machine.
In addition to welding, it also realizes the two in one function of handheld laser welding and cleaning.
The lightweld handheld laser welding machine adopts the air-cooled mode, without the power consumption of additional cooling water equipment.
It removes the pipes, components, control and maintenance links of the chiller, reduces the cost, increases the portability and improves the overall reliability of the system.
LightWELD 1500 handheld laser welding system
(3) Refrigeration and heat dissipation of compressor
Refrigeration and heat dissipation principle of compressor: the compressor compresses the refrigerant to turn the refrigerant into high-temperature and high-pressure gas and flow to the external condenser.
High temperature and high pressure gas condenses into low temperature and high pressure liquid, and the heat generated by liquefaction is discharged out of the machine with the fan.
The liquid refrigerant with low temperature and high pressure changes into a state of low temperature and low pressure and easy evaporation after depressurization through the expansion valve, and flows to the internal evaporator.
The evaporator absorbs heat to reduce the internal temperature of the laser to achieve the effect of cooling, and then the refrigerant vaporizes into a gas with high temperature and low pressure.
The gas refrigerant evaporated by the evaporator is compressed by the compressor again and circulates back and forth to realize the heat dissipation inside the machine.
Refrigeration and heat dissipation principle of compressor
The A1500w intelligent air-cooled handheld welding machine uses the scheme of compressor refrigeration and heat dissipation.
1500w intelligent air-cooled handheld welder
Comparison of heat dissipation methods
The structure of fan cooling is relatively simple.
It only diffuses the heat in the heat sink to the heat sink, and then uses the temperature difference between the heat sink and the ambient air to dissipate heat through the forced convection of the fan.
When the ambient temperature is too high in summer, the temperature difference between the heat sink and the air is too small, and the heat dissipation capacity will be greatly reduced.
It can only passively dissipate heat, which is greatly affected by the environment and can not accurately control the temperature.
Its advantage is that the overall equipment and control system are simple.
The heat pipe radiator has more heat pipes than the simple fan, so its structure is relatively complex.
It relies on the evaporation and condensation of the working material to quickly transfer the heat from the heat sink to the heat sink, and then dissipate the heat into the air through the fan.
It also belongs to passive heat dissipation, which can not accurately control the temperature and is greatly disturbed by the surrounding temperature.
The refrigeration and heat dissipation scheme of the compressor belongs to active heat dissipation.
Due to the existence of the compressor and expansion valve, the accurate temperature control can be realized by adjusting the flow and pressure of the refrigerant.
At the same time, the temperature of the refrigerant in the condenser is higher than the heat sink, which is conducive to the rapid transfer of heat to the air.
Its control system should be more complex;
At the same time, because its structure is much more complex than the above two schemes, the volume and weight of the equipment are also increased accordingly.
Traditional fiber lasers mostly use water cooling to dissipate heat.
First, the water is cooled by compression mechanism, and then the laser is cooled by water.
The air-cooled heat dissipation scheme directly uses the compression mechanism to cool the laser, abandoning the existence of water and reducing the intermediate heat transfer link.
Therefore, the heat dissipation efficiency is higher and the volume and weight can be made smaller.
In the laboratory, we use the constant temperature and humidity test chamber to set 35 ℃ to simulate the high-temperature service environment in summer.
Under the condition of full power of 1500W, we test the change of internal gain fiber temperature of laser with different air cooling schemes with time.
From the experimental data, it is obvious that the optical fiber temperature increases exponentially in the first few minutes and tends to be stable in about 10 minutes.
Due to the refrigeration function of the compressor, the laser can be actively cooled, so the temperature can be controlled below 60 ℃, and the temperature change is relatively stable;
The other two can only rely on passive heat dissipation, so the internal temperature is slightly higher than the compressor refrigeration scheme;
Among them, because the heat pipe has high heat transfer efficiency and can well export heat from the inside of the laser, its internal temperature is a little lower than that of the pure fan, and the temperature rise tends to be more gentle.
Variation of temperature with time when laser with different air cooling schemes outputs 1.5KW laser
(laboratory data may deviate from actual field use)