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The direct diode approach

Between 23 May and 9 June, one of the world’s largest light displays, Vivid Sydney 2014 not only lit up the Australian city, it also illustrated the importance of highly efficient, low-power diode lasers to the entertainment industry. A vibrant mix of colour and art filled the city making use of a diverse range of lighting techniques but with the staple of diode technology underpinning it.

However, the importance of diode lasers is apparent across the power spectrum. At the higher-energy end, Dilas’ industrial laser systems department is due to launch a 2.5kW diode laser this summer – aimed at brazing, surface treatment, and thin-metal welding. The new product will reconfirm the company’s position in the multi-kilowatt diode laser field. And other companies are also providing higher power diode laser technology – such as Trumpf, which provides a 6kW system.

Compared to other sources, diode lasers offer the end user advantages in terms of initial price, cost of ownership, and improved lifetime. They are a form of solid-state laser (SSL) and, over the past 10 years, there have been tremendous advances in SSL design and manufacture. Gerry Jones, laser sales manager at Trumpf UK, explained: ‘We are talking about a wall-plug efficiency of about 40 per cent; which is the most efficient of all industrial lasers. They have become more efficient, but also the beam quality has improved, and they are now available at higher power levels. At Trumpf we offer up to 6kW systems, but others go to greater powers than this. Those three things have made [diode lasers] much more attractive to customers.’

Dilas, which celebrates its 20th anniversary this September, specialises in diode components and systems and has seen a large increase in interest and sales of its industrial laser systems (ILS).

According to Steffen Reinl, sales and product manager of the ILS department: ‘In 2008 the company decided to start integrating more of its own fibre-coupled modules, which was the start of the ILS group. We will continue to expand this area to gain a larger market share. We already had the key technology in the components department, so it wasn’t that much of a step to move into the sale of the systems, and it offered more standardisation.’ He said: ‘This has proved very interesting to our industrial customers, because the diodes offer much larger efficiencies, require less cooling, have a smaller footprint, and this amounts to lower running costs. This is helping drive the technology. We are now able to replace older and established technologies.’

Jones agreed: ‘In several cases, we are looking to take out other laser types to be replaced by diode lasers, without any significant changes in overall system design or particularly high engineering effort.’ He also noted: ‘If you are changing from a totally different laser source such as a CO2 laser, you have to consider that it is operating with a different wavelength and therefore have to consider the health and safety. This means you have to ensure it is fully enclosed and that the windows of the enclosure are specified to the new laser, so if you change the laser type you may have to adapt the enclosure.’

However, Jones stated: ‘The multi-kilowatt laser sources are certainly a competitive source; it’s still a relative newcomer to the high-power laser market but it’s another choice.’

Jones explained that high-power sources have only been available for the past few years. He believes that end-users, suppliers and integrators will have to work together to establish the technology’s position. ‘Awareness needs to be raised, because often people don’t think of the diode laser immediately. Suppliers have to be sure that the customers are aware of the latest developments, and the customers have a duty to their own companies to be up to speed with what’s available. Certainly for multi-kilowatt brazing and welding applications, it’s a case of raising the profile.’

He continued: ‘As with a lot of emerging laser technologies, there may have been an earlier period when some people questioned the reliability [of diode laser systems], but not anymore. They’re here and they are here to stay.’

Today’s diode lasers have to be reliable. Jones pointed out that Trumpf’s main target market for these sources is the automotive industry, where machines are running 24 hours a day, seven days a week. He said: ‘That’s where they are being sold so, as far as we are concerned, there is no choice in the matter. They have to be reliable and there is no compromise on that for such industry sectors.’

These continuous manufacturing plants are where the lower operating costs will be appreciated most. The lower costs stem from the higher efficiency of the system. Reinl explained that part of the reason why these systems are more efficient is down to the semiconductor materials that are used to construct the diodes. ‘These have improved a lot over the past few years; they are now much more efficient.’ However it is not the whole story, Reinl elaborated: ‘Also, improvements made to the efficiency of both the coupling and the optical components increase the system’s ability to create a useful beam.’

Reliability and efficiency are also opening new markets for the sub-1kW diode market. Pangolin is a USA-based company whose areas of interest span the industrial biomedical, 3D printing, illumination, and laser-entertainment markets. It is predominantly concerned with the lower-power diodes. The company’s chief operating officer, Justin Perry, said: ‘The laser entertainment market is one where we are seeing a lot of activity. They are eating up these diode lasers for a couple of reasons. The convenience of the diode lasers is a large part of the reason. In the past, the ion laser projectors that people used were physically larger and required more electricity, cooling, and a larger housing. Regardless of the application, it was not as easy to integrate the laser. The new lasers are less expensive and much easier to work with. The reliability of diode lasers is much greater; the older gas lasers would be useable for a maximum of 8,000 hours, but with diode lasers you can typically get around 20,000 hours of lifetime before it requires replacing. It’s a substantially better product to work with.’

He continued: ‘You can also extract more colours from a diode laser than from traditional sources and this allows you to create much more vivid and beautiful shows. This means that a lot of [laser entertainment] companies are advertising that they use diode solutions, and this was certainly one area in which we have seen a surge in the use of the technology.

‘From what we have gathered from several trade shows, the market is growing across the board. Another new application that we have seen is in 3D printing. Many of the patents for a stereo lithography machine expired and now products are being developed that are at a price point that a typical household can afford, and we are seeing that market segment explode now. So because of diode lasers’ affordability and convenience, new areas and applications are becoming possible.’

At the higher-power end of the applications for diode lasers, companies are coming up against the issue that as the power increases, the beam quality deteriorates. Perry explained that, with poor beam quality: ‘The light tends not to be as tightly bound straight out of the diode, and users may have an issue shaping it. We have heard clients say they have issues with the shaping of the beam and so we are looking into it and we have experimented with a couple of projects.’ Without being able to shape the beam, applications such as beam-cutting become more difficult.

Reinl explained that one way around this was by stacking multiple diodes that produce a different wavelength and combining them using a waveplate – the process is known as multiplexing – and so the power can be improved while keeping the beam quality high.

Trumpf’s Jones said that for cutting applications the other SSL types, such as disk and fibre, possess a higher beam quality and are preferred over diode lasers as they produce better results.

However, for brazing and welding Jones thinks customers can and should seriously consider diode technology.

Preventing a nasty shock
It’s not just the higher power sources that have to overcome issues if they are to find new applications. Pangolin’s Perry stated: ‘Smaller product design, more efficient production of the laser, are allowing the diodes to be used in a greater number of consumer applications such as in cars for headlamps. As the technology becomes cheaper, it’s hard to say where the market is going to go, it has an almost limitless potential. I don’t see laser technology going down at all. From all accounts, every industry report shows that it will grow.’

But there are limiting factors for the diode. Perry warned that the lower-power sources are vulnerable to voltage fluctuations from common phenomena such as electrostatic discharge (ESD), saying: ‘The internal structure of the diode is what makes it so vulnerable to the discharge. The new green lasers that are coming to the market are susceptible to ESD. Also UV, VCSEL, higher power IR, and quantum cascade diodes are at risk.’

Perry pointed out: ‘In a manufacturing facility, there is dust, people walking around, and they may not be wearing the correct clothing. All this can cause a build up of electrostatic charge which could accidently be transferred to the diode, causing either degradation or complete failure of the diode. It can be generated just about anywhere, and is one of the main causes of diode failure.’

To combat this, Pangolin developed Lasorb, which is attached directly to the anode and cathode of the diode. Perry said: ‘The main thing that we look for is a change in voltage. Then Lasorb absorbs this damaging voltage spike before it enters the diode which eliminates the chance of it causing damage.’

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