David Robson investigates the different methods of keeping laser systems at a constant temperature
The temperature of a laser system can determine its lifetime, performance and safety. Many systems simply burn out if they are too hot for too long. Thermal lensing damages the beam shape of most solid-state lasers, and the output wavelength of laser diodes closely depends on their temperature.
A recent trend has even been to take advantage of the symbiotic relationship between wavelength and temperature to produce highly-accurate tuneable lasers. In modern applications, such as medical surgery, where precision and reliability are everything, cooling is undoubtedly one of the most important factors to consider when building and integrating a laser system.
Apart from the very simplest applications, where fan cooling is sufficient, there are two main methods of cooling lasers: thermoelectric and liquid cooling, the advantages of which depend on the laser type and power, and the size and cost of the application.
Thermoelectric cooling devices
Thermoelectric cooling (TEC) devices are most useful for small, low-power laser diodes, working comfortably to remove excess heat at rates of less than 100W. They are noiseless, with a very high reliability and lifetimes of more than 100,000 working hours. They can also react very quickly to changes in temperature, and have an accuracy of 0.1°C.
The compact devices, made of a doped semiconductor, work in a similar way to the liquid-based systems, except electrons, rather than water, carry the heat away from the laser to a heat sink to release the energy into the surrounding area.
With dimensions as small as 2 x 2mm, they lend themselves to very compact applications, and can be easily welded to laser diodes for direct cooling. And for larger, more powerful lasers, they can themselves cool liquids which flow around the larger systems. While they are attractive for certain applications, Martin Räpple, product manager of thermoelectric systems at Ferrotec, warns users that care has to be taken over their integration, with the heat sink in particular needing careful consideration. ‘Their design is not simple, and the customer needs expert help from either the manufacturer or systems integrator.’ Overall, however, the coolers’ efficiency is increasing all the time and they are proving increasingly popular for low-power applications.
Liquid cooling devices
At heat outputs of greater than 100W, thermoelectric coolers become expensive and bulky, so more traditional methods of cooling are necessary. All of these pump a liquid around the laser to transfer the excess heat and dissipate it into a larger reservoir.
The simplest liquid cooler would merely pump water around the laser, and transfer the heat to the surrounding air. While this may be a cheap alternative, it does not provide accurate control over the temperature, which cannot be cooled more than the surroundings. It is, however, a cost-effective method and for some lasers, like CO2 and other gas lasers it is usually enough.
For large, industrial factories, liquid-to-liquid cooling is a popular alternative. In these systems, which require less maintenance than other methods, the water pumped around the laser can transfer the heat energy through an interface to each factory’s own refrigerated water supply. The laser cannot be plumbed in directly to this water supply, as it will not typically be of the required purity for delicate laser equipment.
In his time as sales director of Thermal Exchange, Chris Walker has seen a variety of disasters when customers didn’t take this advice. ‘Impure water likes to eat soft metals like the copper fittings in a circuit. I’ve seen applications where the whole laser system has been coated in copper due to this.’ Thermal Exchange’s own systems use plastic and stainless steel surfaces to try to prevent this from happening.
Again, liquid-to-liquid cooling does not provide an accurate, instantaneous control, and the cooled temperature is limited by the temperature of the factory’s refrigerated system, but it does transfer the heat into the factory’s water supply rather than dumping a potential 4kW of heat energy into the work area.
For large, high-power lasers, refrigerated cooling using vapour compression (like you’d find in your home fridge) provides by far the most accurate and reliable temperature control. As with TECs, the temperature can be specified to within 0.1°C, independent of the surrounding air. They can even be connected to alarm systems, which alert users or automatically shut down the laser if the temperature and pressure stray outside specified safety values.
Lytron's Kodiak recirculating chiller, from the inside and outside
However, they are also the most expensive alternative, and require more maintenance, particularly when the refrigerator valves become dirty and worn out. Surprisingly, they are also the easiest to integrate; Lytron supplies standalone refrigerated systems that can be installed within 20-30 minutes.
The engineers’ perspective
Greg Ducharme, a systems engineering manager at Lytron, described the advantages to laser manufacturers and integrators of using a specialist cooling company rather than building their own temperature controllers. ‘We’re the thermal experts and cooling is our forte. We produce 3,000 laser coolers a year, whereas a laser manufacturer may produce just 100, so we can do things more economically.’
Daryl McCoy, a laser and applications engineer at Photonic Solutions, however, believes that many companies, particularly manufacturers, would rather take control once the systems are bought. ‘It depends on the scale, but most people prefer to integrate the cooling system themselves.’
‘The decision of which system to use is based on a number of factors – the cost, the type and power of the laser – but it is usually driven by the space of the application,’ he says. He believes that, despite their limitations, thermoelectric and even air cooling are becoming increasingly popular, with more stringent environmental regulation on the flow and disposal of liquids in industrial settings.
‘My biggest advice is to listen carefully to the laser manufacturer’s specifications on the type of water to use. The biggest risk factor is contamination and corrosion, and the ramifications can be quite dire.’
With the ceaseless development of higher power lasers and greater precision applications, it seems clear that the methods for successfully controlling and stabilising a laser’s temperature will have to match the other technological advances in terms of strength, accuracy, and reliability.