Custom cooling

Matthew Dale discovers that laser cooling is an undefined art, as thermal management firms turn to adaptable modular systems to meet individual customer demands

Cooling solutions for industrial lasers can be complex and costly. With the development of smaller compressors over the past three years, cooling systems have been brought down in size, and even integrated within lasers themselves to help minimise the floor space of the machine. AMS Technologies, for instance, has developed a water-free cooling solution that reduces the footprint of its thermal management equipment.

With laser manufacturers each having their own ideas and philosophies about how a laser should be designed, it becomes increasingly challenging for cooling suppliers to offer a suitable thermal solution for each device using a defined range of cooling products.

‘In the future, especially for embedded assemblies, the mechanical constraints will always be different,’ commented Jan Meise, CEO of AMS Technologies. ‘There won’t be one chiller that covers everything; customisation will be needed in order to achieve that.’

The main focus, according to Erik Neumann, system manager of cooling at Glen Dimplex Thermal Solutions, is therefore not so much on having a series of products, but rather the right technology, the right processes and the right understanding of the customers’ requirements. ‘When you are able to understand the needs of the customer you can then look at your own technology portfolio and … design new components in a way that they can be combined to form an individualised cooling system suited to the new laser,’ he said.

Both Glen Dimplex and AMS Technologies therefore take a modular approach to cooling that enables them to adapt to the individual specifications of each customer. ‘Laser system manufacturers and integrators have a lot of varying needs,’ commented Neumann. ‘Laser manufacturers have new system generations and newly developed technologies, and so require multiple variations of cooling systems. System integrators work with entire laser networks, in addition to hydraulics and controller concepts, and so modular systems are an efficient way to address these complex situations.’

Using a modular approach, both firms are able to address individual customer demands, which cannot be addressed with standard defined products. ‘It’s more important to think of the system as a whole, and there you get away from the idea of a physical product and move towards a concept, an idea, a way of thinking that takes into account connectivity, integration, flexibility, predictive maintenance and Industry 4.0,’ Neumann explained. 

Going against the flow

For the past two years, AMS Technologies has been developing a water-free cooling system that could reduce floor space considerably for its laser customers. By removing the water loop, a number of components, including the pumps, water reservoirs, and one of the heat exchangers, are no longer needed in the system. Not only does this reduce the physical footprint of the cooling system, it also reduces the amount of servicing required to maintain it, and removes the risks of working with water.

A sealed compressor system for water cooling. Credit: AMS Technologies

‘If you ask any industrial laser system manufacturer, they would like to get rid of the water loop,’ Meise remarked. ‘Based on demos we have shown to our industrial laser customers, the removal of the water loop in cooling systems has generated lots of serious interest.’

The water-free products use the same inert refrigerant gas R134a that is in most of today’s chillers. However, in standard systems the gas is used to cool water that then goes on to cool the laser with a degree of stability. Now, with the water-free approach, AMS Technologies is using the gas to cool the laser directly.

‘Historically the gas would pass through a heat exchanger to cool the water; now it passes through an evaporator, which is commonly a cold plate with a laser diode mounted on it,’ Meise explained. ‘In this approach we are using the cold plate or micro channels of a diode laser as the evaporator for the vapour compression circuit.’

A number of demonstrators have been built using the waterless technique, which is starting to be embraced by AMS’s customers. However, there are still particular challenges that have to be overcome before it can be more widely accepted. ‘Beyond the demonstration of feasibility, a meaningful adoption will still require thoughts about serviceability of laser systems based on this cooling concept,’ Meise said.

Not only does removing the water loop and the corresponding components reduce the maintenance requirements of the overall system, it also increases the ease of its setup.

‘With the waterless approach, we deliver a closed cooling loop system to a customer, which is different to when we deliver a conventional chiller,’ explained Meise. ‘In the case of a conventional-type chiller the water loop – connected to the cold plate on which the laser is sitting – and the refrigerant loop can be serviced and exchanged separately. When initially installing this system you are filling the water after the laser is mounted. The water-free approach – where the evaporator sits close to the laser – is instead shipped as one closed loop format, meaning there will be no filling of the water circuit required out in the field.’ A lack of water also dramatically reduces the risk of potential catastrophic damage should a leak or fault occur in the cooling loop, as it would only be gas that sprays into the laser system.

The new approach provides a reliable, low maintenance option for today’s laser systems, according to Meise, and targets those developing new, smaller laser systems in the power domain of hundreds of watts. It is only through relatively recent developments, however, that the waterless technique has come about.

‘Over the course of the last two to three years, new mini compressors have emerged in the market, allowing the size of cooling systems to be reduced for lasers,’ said Meise. ‘Not only have compressors shrunk in size, they now have variable speed control, meaning they don’t need to be turned on and off constantly [as with previous compressors] and are much more efficient. The development of these variable speed controlled compressors is what has enabled the development of a water-free solution.’

Next-generation speed controlled compressors, in contrast to traditional on-and-off compressors, offer very low vibration and noise levels, allowing them to be positioned much closer to the laser and cool it with a high level of stability without risk of disrupting its operation. The compressors used by AMS can even be run on a supply as low as 24V, which opens up the possibility of using batteries to power a portable laser system – a current ambition for one of AMS Technologies’ customers. 

Embedded cooling

In order to maximise floor space efficiency, cooling systems are starting to be integrated entirely within laser systems themselves, an approach currently gaining interest within industry, according to Meise.

‘We see customer needs for smaller, portable laser systems where integration into small real estate is important,’ he explained. ‘While multi-kilowatt lasers will continue to use larger cooling systems located in separate rooms, for small assemblies in the domain of hundreds of watts, integration is really starting to become key.’

Embedding a cooling system within a laser, while reducing its physical footprint, also increases the complexity of its design, making it difficult to access when being serviced.

The issues of maintenance increase even further when water is involved, according to Meise, as the water will need to be routinely changed because of the build-up of algae and bacteria.

A mini liquid cooling kit (mLC-Kit) for use in a modular cooling setup. Credit: AMS Technologies

‘If you ask anyone that uses a water system to cool lasers they will say that one of the problems is that after a while the water turns green,’ he said. ‘Embedding this in a small confined area can create [additional] difficulties as the equipment still needs to be drained, cleaned and serviced.’

Despite these challenges, embedded cooling systems are already being used in the medical domain, Meise continued, with a number of AMS Technologies’ customers having embedded chillers into their equipment assemblies. ‘They want everything to be contained in a single housing,’ he said.

Glen Dimplex Thermal Solutions believes that embedded cooling systems will play an important role in Industry 4.0, where connectivity and the use and transfer of data will be crucial.

‘The integration of a cooling system is done in both a physical way and by the way of exchanging data inside and with the surrounding system,’ Neumann explained. ‘So users can utilise the data being produced by the laser and cooling systems as a whole.’

Additionally, embedded systems have to be designed using clever construction solutions to enable ease of maintenance despite difficult access, Neumann remarked. Glen Dimplex’s latest built-in chiller solution has been equipped with a refrigerant circuit that can easily be replaced in five steps and without the need of a refrigeration specialist.

Meise expects embedded systems to become more common in the future. However, they are not set to replace standalone chillers anytime soon, as the conventional format still favours systems comprising multiple lasers and machine tools that all need cooling.

Diamonds are for lasers

Element Six, a manufacturer of synthetic diamond, has developed a series of chemical vapour deposition (CVD) diamond heat spreaders that can be found at the core of high power CO2 and solid-state lasers. Through exploiting the extreme thermal conductivity, wide transparency range and low absorption of diamond, these heat spreaders remove much of the thermal load from the lasers’ internal components – such as the resonator – to enable a stable and enduring high optical quality of the laser beam.

‘In most cases, diamond is used exclusively due to the fact that other materials fail under similar conditions of use,’ commented Henk de Wit, general manager and business manager of optical at Element Six. ‘In these cases, diamond is a truly enabling engineering material giving access to previously unreachable levels of performance. This means no hot spot formation, so no beam distortion through thermal lensing, and low component temperature, resulting in extremely long [equipment] lifetimes.’

Element Six's Diafilm ETC700 diamond heat spreader

When applied correctly, diamond solutions offer the lowest operational temperatures under the highest power conditions, according to de Wit, who expressed that by using diamond components, the highest power density levels in both optical and electrical systems can be reached.

Element Six was selected by the European Commission’s Seventh Framework Programme for Research and Technological Development back in 2014 to help develop a new ultrafast pulse disk laser using its CVD diamond. The work was carried out as part of the three year project ‘Ultrafast High-Average Power Ti:Sapphire Thin-Disk Oscillators and Amplifiers’, which ended in December 2016. Within the project Element Six further developed its CVD diamond material to be mounted to the titanium sapphire disk as a heat spreader, improving its thermo-optical effects and allowing it to be pumped at higher powers.

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