The performance of high-powered lasers depends on effective cooling. High-powered lasers generate a significant amount of heat that must be removed from the laser system to avoid overheating critical components. Carbon dioxide (CO2) lasers, excimer lasers, ion lasers, solid-state lasers, and dye lasers all use liquid cooling to remove excess heat. Laser liquid cooling can help accomplish three goals: maintaining a precise laser wavelength and higher output efficiency, achieving desired beam quality, and reducing thermal stress on a laser system. Recirculating chillers, liquid-to-liquid cooling systems, ambient cooling systems, cold plates, and heat exchangers are a few of the cooling technologies used in laser systems' liquid cooling loops.
Low-powered lasers, such as small helium-neon or argon-ion lasers, may not require cooling or may come with their own cooling fan, which is generally sufficient. Some smaller gas lasers and many solid-state lasers contain their own built-in cooling system, usually a closed-loop heat exchanger. Larger gas lasers or other high-powered lasers, such as industrial CO2, large-frame argon- and krypton-ion lasers, and excimer lasers, however, typically require an external source of water flowing through the light-generating section of the laser system.
According to Coherent, Inc,1 manufacturer of lasers and laser systems, their ion lasers produce between 5kW and 55kW of waste heat as a by-product of the laser action. In order to avoid overheating critical components, Coherent notes the importance of efficiently removing this heat from the laser system and recommends the use of cooling water. Other laser systems may have more or less heat to remove, but the need for cooling remains.
One reason it's important to remove excess heat from a laser system is that an increase in temperature will result in an increase in wavelength. This wavelength increase can compromise a laser system's performance. Since the diode laser wavelength increases with an increase in temperature, the temperature must be uniform throughout the diode arrays in order to have high overall optical conversion efficiency in a pumping application.2 For example, the wavelength of light emitted from GaAs diode laser bars shifts at a rate of approximately 0.3nm/°C due to temperature related changes in bandgap energy and refractive index. To have a high overall optical conversion efficiency of light from GaAs diode bars in pumping some solid-state lasers, it is critical for the wavelength of light energy from each emitter to be within a very narrow wavelength band or within 1-2°C of each other. Cooling can help to keep the beam aligned in front of the emitter (± 5 microns).
Beam quality is also important in some laser applications. For example, with laser material processing, printing, marking, cutting, and drilling, strong beam focus is required. In high power lasers, the heating of the gain medium, such as the laser crystal, can cause thermal lensing3. These thermal effects in the gain medium can affect the laser wavefronts and therefore the beam quality. With diode-pumped solid-state (DPSS) lasers, the crystal must be cooled and the temperature should be controlled to 0.5°C. (See Figures 1 and 2.)
Keeping a laser cool and maintaining tight temperature control helps to ensure that the laser system is operating at the optimal wavelength. With liquid cooling of laser systems, power fluctuations will diminish and pointing stability and beam quality will improve.
Lower operating temperatures can also increase the lifetime of a laser system's components and reduce maintenance. This is especially true for DPSS lasers, since the mean time between failures (MTBF) is affected significantly by excess heat. It therefore makes sense for laser designers and manufacturers to integrate a cooling system within their laser. This helps ensure that the MTBF is extended and downtime is reduced, saving on operation and maintenance. By integrating a cooling system, it also helps the laser manufacturer ensure optimal laser system performance.
Commercially available recirculating chillers provide convenient cooling for a laser system. A compressor-based recirculating chiller's coolant temperature can be set to between -5.0°C and 35.0°C and maintain ±0.1°C temperature stability, ensuring that the laser system is operating at the optimal wavelength and as efficiently as possible.
Recirculating chillers are also more environmentally friendly and cost-effective than using tap water. Chillers are closed-loop systems that use active refrigeration. They are used for cooling laser systems when there is a high heat flux, high ambient temperatures, or when a laser system requires a chilled environment, such as with excimer lasers. Typically they have cooling capacities ranging from 800W-6kW, a PID controller, large thermal mass tank, and advanced refrigeration control circuits to ensure that they maintain the tight temperature stability needed for laser systems' pointing stability and beam quality. In addition, recirculating chillers can provide consistent flow and pressure to the system while maintaining control of the quality of the coolant. It's important to ensure that appropriate cooling system pressure is maintained, since excessive water pressure can create vibrations in the laser head.
Another benefit of recirculating chillers is that most of them are compatible with a variety of fluids. For example, many recirculating chillers are compatible with ethylene glycol (EGW) or propylene glycol (PGW) solutions, which offer corrosion and freeze protection. A recirculating chiller can also be fitted for compatibility with deionized (DI) water, including a DI cartridge to maintain a system's required resistivity. (DI water is electrically inert.)
Like a recirculating chiller, a liquid-to-liquid cooling system (LCS) offers precise temperature control of process water (fluid temperature to within ±0.5°C). However, it transfers heat to facility water via a liquid-to-liquid heat exchanger. An LCS is a solution for high heat-load or high ambient temperature applications where chilled facility water is available, as an LCS often has cooling capacities up to 20kW. For laboratories with several large lasers, an economical option may be the installation of a cooling tower on the roof of the facility to provide a common source of cooled water to all of the systems within the building. The cooling tower will exhaust the heat outside of the building, helping to maintain a comfortable work environment. With an LCS, the facility water never comes in contact with the laser system's water. The heat is transferred from the cooling system fluid to the facility water via the liquid-to-liquid heat exchanger. This is important because the water that circulates from a facility's cooling tower is often treated with fungicides, algaecides, and/or antifreeze, which may be too harsh for some laser components.
Ambient cooling systems, which have cooling capacities up to 3.5 kW, are a reliable alternative to refrigerated chillers and LCS for laser applications where precise temperature control and cooling below ambient temperature are not required. An ambient cooling system consists of a high performance heat exchanger, a fan, a pump, and a reservoir. The heat is moved from water circulating through the laser system into the ambient air by the liquid-to-air heat exchanger and fan, hence the term "ambient cooling system". Ambient cooling systems do not provide temperature stability, but they are a cost-effective means for heat dissipation.
Cold plates and heat exchangers are key components in liquid cooling loops for laser cooling. Cold plates are often used in conjunction with a recirculating chiller. Many lasers use tubed cold plates. However, aluminum vacuum-brazed cold plates that have liquid flow through channels are one type of cold plate more and more laser manufacturers are selecting. Manufacturers and end-users also use flat tube cold plate technologies. A cold plate may be mounted to the component requiring cooling, such as a thermoelectric, and will receive cold fluid from the chiller and transfer hot fluid back to the chiller. (See Figure 3.) Cold plates can also be designed as the electrodes of the laser system.
Heat exchangers are often found within cooling systems, such as chillers, LCS, and ambient cooling systems. Some laser manufacturers prefer to purchase a heat exchanger separately and integrate it themselves, connecting it to their own pump and reservoir.
High-powered laser systems require cooling for optimal performance and longevity. If maintaining a precise laser wavelength, beam quality, high output efficiency, and up-time are important, liquid cooling may be the answer. Recirculating chillers, LCS, ambient cooling systems, cold plates, and heat exchangers are becoming critical components in laser systems.
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