FEATURE COOLING TECHNOLOGY
cooling L
Red hot
Katia Moskvitch finds that, as lasers become more powerful, innovative methods are needed to cool them
asers 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,
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
In high-power lasers, stability and beam quality will strongly depend on effective cooling
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
26 ELECTRO OPTICS l MARCH 2014
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
@electrooptics |
www.electrooptics.com
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