FEATURE COOLING TECHNOLOGY
A 10kW direct diode module from Coherent, where each 100W diode bar is cooled by flowing water into micro- channels extending inside the structure
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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 28 ELECTRO OPTICS l MARCH 2014
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.’
It is likely that laser
sources will keep getting ever smaller, and, with them, the components used to cool lasers
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.’ l
@electrooptics |
www.electrooptics.com
Monty Rakusen
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