Red hot cooling

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Katia Moskvitch finds that, as lasers become more powerful, innovative methods are needed to cool them

Lasers use more electrical energy than they can emit as light, with some of the energy ending up as heat. They therefore have to be cooled and, if heat is not extracted, the quality of the laser beam degrades. Lasers can be cooled with air, water and thermoelectrically, but cutting-edge cooling systems are being developed, and the recent advances in cooling technology have greatly increased the lifespan of lasers, especially for high-power systems.

The most efficient lasers are solid-state diode-pumped lasers and laser diodes. They can have an overall efficiency as high as 50 per cent, which means that there may still be a few tens or even a few hundreds of watts of energy that need to dissipate. That’s why cooling them is essential, said Marco Arrigoni of Coherent, a laser manufacturer based in Santa Clara, CA. And the higher the output power of a laser, the more important the cooling is, ‘because in high-power lasers, stability and beam quality will strongly depend on effective cooling’.

A failure in the cooling equipment or insufficient cooling can have catastrophic effects for the laser. For starters, it can lead to a change in the optical properties of a medium, causing differential expansion that might trigger mechanical stress and failure, such as the cracking of a laser rod. Also, it can cause temperature gradients – which, combined with changes in optical properties such as the refractive index, can lead to thermal focusing or defocusing of a beam. ‘At best you get unwanted beam aberrations, at worst this leads to optical damage,’ said Roland Smith, head of the plasma physics department at Imperial College London.

Lack of cooling can also detune some optical elements, for example laser diodes or thin etalons, and it can also detune phase matching in non-linear crystals. The bottom line is, without cooling, the laser may eventually overheat, burn out and fail. How well a solid-state laser is cooled down can also affect its lifetime and reliability, and play a role in its safe operation. The diodes’ temperature directly affects their output wavelength. Cooling systems have always been engineered in sync with any laser equipment, but which cooling system to use is an important issue to think about when deploying a laser.

Heat sinks

In general, the amount of cooling required depends on the average heat load that needs extraction, and on the best operating temperature required.

Some lasers are passively cooled – the heat from the laser dissipates into the laser case. But most lasers are cooled by forced airflow, cold water, thermoelectric (TE or ‘Peltier’) cooling, or liquid nitrogen. These methods often involve actively cooling a base plate where the heat from the laser crystal or the laser diode flows conductively into a metal sub-mount, sometimes with a diamond layer as an electrical insulator.

To cool thermoelectrically, a cooler actively controls the temperature of the laser active element. For this to happen, there needs to be a good thermal contact between the laser material and the cooling plate. This technique is typically used for cooling laser diodes at lower average powers, and could also be used for fibre lasers, which are always pumped by diodes. A small monolithic electrical device acts as a tiny heat pump, removing heat from the gain medium and from a mounting sub-plate. For example, Power Technology, a laser manufacturer headquartered in Little Rock, Arkansas, controls the temperature of its diodes to within 0.2°C over 5 to 40°C, according to Walter Burgess, the firm’s vice president of engineering.

When cooling a solid-state laser crystal like Ti:S, the laser is clamped in a metal block which is cooled by a Peltier cooler. It’s effective, but limits the amount of heat that can be extracted.

Liquid cooling

Liquid cooling methods rely on cold liquids drawing away the heat. The liquids circulate in a closed loop systems with a heat exchanger, ‘a bit like a car engine’, said Patrick Baird, laser physicist at the University of Oxford. This relatively old method of liquid cooling is still extremely important and widely used in many different systems.

When the laser material is cooled in this way, it is generally mounted in a chamber that allows the cooling liquid to come into contact with the laser material; the cooling liquid then flows into a chiller where the heat is removed by exchange with the air or a secondary cooling loop. It is important to make sure that the liquid does not create scaling or other corrosion of the tubing that would limit the efficiency of the heat exchange or limit the flow, noted Arrigoni.

Where the cooling liquid, usually water, is in direct contact with electrical connections, the purity of the water is critical, he added, as impurities cause the water to conduct, and this can lead to serious electrical problems. Improvements in techniques for removing dissolved materials and ions from water have contributed to the increase in laser systems’ lifetime.

Going cutting-edge

Higher power lasers need greater cooling and advances in cooling technology have increased the lifespan of these lasers. ‘Very small patterned conduits called micro-channel plates are used to cool very high power laser diode arrays that can produce kilowatts of power,’ said Arrigoni. ‘Even though these laser diodes are very efficient, considerable amounts of heat need to be removed from a very small device, with a cooling area of a few centimetres squared or even less. The overall quality of the micro-channel plate is an enabling element.’

Also, he added, it is important to look for smarter ways to provide an efficient path from the laser medium, where the heat is first produced or deposited, to the cooling plate or fluids. ‘Liquid nitrogen or cryogenic cooling is the most extreme method but also the most complex and costly. It should only be used where absolutely necessary. Although some companies producing Ti:S lasers cool them cryogenically, at Coherent we use cryogenic cooling only with the most extreme systems, like ones operating at 20W level of output power.’ In the last few years, the company has been able to cool thermoelectrically a 15W Ti:S amplifier that previously required cryogenic cooling, he added.

When it comes to laser diodes, ‘cooling is important to prevent the individual laser emitters from failing or the laser changing wavelength’, said Ian Musgrave, the Vulcan group leader at Rutherford Appleton Laboratory (RAL), one of the UK’s national scientific research laboratories operated by the Science and Technology Facilities Council, based in Didcot. Vulcan is a high-power laser system capable of delivering up to 2.6kJ of laser energy in nanosecond pulses and up to 1PW peak power at 500fs at 1,054nm.

Consistency required

Wavelength stability is fundamental for such applications as Raman spectroscopy. ‘If you make two Raman measurements at two different points in time, you want consistency in your probing laser,’ said Burgess. ‘If you interrogate the sample with two different wavelengths, from an unstable laser, you can introduce error.’ It’s the same with the output power stability. If you are making highly sensitive measurements based on laser power, changes in the source power appear as errors in the measurement, he said.

For high-power lasers, cutting-edge cooling methods have enabled higher average powers because of the ability to extract a greater amount of heat. ‘The ytterbium system, for example, is a three-level laser at room temperature but when cooled to cryogenic temperatures it comes close to being a four-level laser,’ said Musgrave. ‘For Ti:S, when cooled to cryogenic temperature, the thermo-mechanical properties of the crystal improve and this enables a higher average power to be achieved.’

Arrigoni cited the Legend Elite Plus ultrafast amplifier from Coherent as an example of cutting-edge cooling methods: ‘This was the first Ti:S amplifier to deliver pulse energy greater than 12mJ without resorting to the complexity and cost of cryogenic cooling. We’ve accomplished this by using a novel crystal and housing geometry to maximise the surface area that is actively cooled, relative to the overall size of the crystal.’

For other high-power systems, efficient cooling is necessary for the system to operate reliably, prevent meltdowns, and enhance the performance, making a system able to produce a ‘better’ beam or to run at a higher repetition rate. Minimising thermal lensing in the gain medium is important too – pumping gain crystals such as Ti:S or Nd:YVO4 ‘creates a thermal gradient in the crystal because as much as tens or hundreds of watts of pump light are sent on these crystals’, said Arrigoni. ‘The resultant changes in the crystal cause it to act like a lens.’

Maintaining alignment

It is important to offset this unwanted lensing with cooling because it may change the alignment of the laser, degrading the performance, said Musgrave. ‘For laser diodes, the output wavelength can be tuned by changing their temperature so a stable temperature is required to maintain the correct wavelength.’

Greater stability in laser output has benefited from cooling systems too, as the efficiency of a poorly cooled laser will decline with operating lifetime.

‘For semiconductor lasers, the gain curve moves in frequency with temperature,’ said Baird. ‘Usually the frequency of such a laser is controlled by optical feedback from, say, a grating with the laser’s gain curve optimised by temperature.’

High-power or energy lasers that are not very efficient are cooled in specific ways, usually with closed loop systems using distilled or deionised water – such as in the case of Nd:YAG or glass lasers. Some flash lamp-pumped systems, however, need to be heated slightly for optimum efficiency – alexandrite systems, for example. ‘The flash lamp and laser rod are then usually placed in an ellipsoidal reflector with cooling water flowing through the chamber,’ said Baird.

Arrigoni said that many lasers are operated in a light-regulated feedback loop to maintain the output power constant and ‘hide’ the deterioration of the laser performance due to thermal instability or simply aging.

Diode efficiency

Wherever possible, ion lasers or flash lamp-pumped lasers have been replaced by more energy efficient (and less heat generating) lasers, like diode-pumped solid-state lasers. These lasers employ diode pumping that can couple energy into the upper laser level more efficiently than broadband flash lamp pumping. Also, ‘the diodes are more efficient anyway, so this, together with a better wavelength matched into the absorption band of the gain medium, both reduces the size and the need for cooling,’ said Baird.

However, not all solid-state lasers lend themselves to diode-pumping. For example, high energy-per-pulse lasers require flash lamp pumping. One group of researchers at the University of Austin in Texas, led by laser physicist Todd Ditmire, are pushing the boundaries in exactly this area. Ditmire is also part of a team of scientists that formed National Energetics, a US company constructing high-power chirped pulse amplification lasers.

The researchers at the University of Austin are working on a technology to cool large aperture flash lamp-pumped disk amplifiers, which involves flowing thin layers of liquid via laminar flow up the faces of the slabs as they are pumped with the lamps. ‘This technique has allowed us to develop a Nd:glass disk amplifier with an aperture of 18cm, which can fire at one shot every 10 seconds,’ said Ditmire. ‘By comparison, up until now, almost all laser glass disk amplifiers with this large aperture could usually only fire once every 20 minutes. So we have made huge progress in the repetition rate of such high-energy lasers with our technique.’

And the aim is to go even further, ‘scaling up in repetition rate, so a system that might run at one shot per 20 minutes can run at a shot per second,’ said Smith.

RAL uses such a cutting-edge method to cool multiple diode-pumped slab amplifiers with fast gas flow in its Diode-Pumped Optical Laser Experiment (DiPOLE). This project is to develop the foundations of next-generation high energy, high power laser systems based on diode-pumped solid-state laser technology.

And in future, it is likely that laser sources will keep getting ever smaller, and, with them, the components used to cool lasers. ‘We are seeing a new generation of thermoelectric coolers using thin-film technologies,’ said Burgess. ‘This technology will assist laser manufacturers to make smaller laser sources.’