Lens on materials processing

Edmund Optics is one of the biggest producers of optical components globally, but is better known for work in medical or research applications. Now though, the company is entering the materials processing market, reacting largely to customer demand.

The company launched new laser mirrors at Laser World of Photonics that offer high laser damage thresholds targeted at laser materials processing, which is a different market from Edmund’s traditional science base.

Edmund Optics provides custom optical solutions as well as offering a vast off-the-shelf catalogue of optical components. Dr Thomas Kessler, executive vice president for global sales and vice president of European operations, commented: ‘Everything we do here [at Edmund Optics] is moving light; we don’t do anything else. We don’t create the light, we don’t destroy the light, we typically don’t detect the light – we just move it around. The laser is a light source so the materials processing market is a natural fit for us.’

While Edmund Optics is well known in many fields, it’s not primarily for materials processing. Kessler’s colleague Anthony Artigliere, director of Edmund Optics’ optical business unit, commented: ‘Laser materials processing is a growing area of interest for us and is an important market area going forward – but so far it’s always been behind the scenes. The optical components aren’t going to be publicised on the “box” of the laser system.’

Artigliere continued: ‘Laser processing is a small market for Edmund right now, probably so small that we don’t call it out separately when we do internal analysis of our markets, but it’s something we want to expand.’ 

Compared to optics for life science applications, Artigliere said that moving to industrial laser optics is a step up in terms of the grade of substrate material, the quality of the optical surface, and the coatings required because of the powers the optics are required to handle. The purity of the material the optic is made from can have an effect on the light passing through it. If the beam is shaped, then the contour of the lens is important, while high-grade coatings are required to protect the optic from high laser powers. ‘In general, the higher the power, the more precise the optical surface needs to be in terms of scratches and imperfections. If there’s a defect it tends to have an effect,’ Artigliere explained.

The high powers used in these applications can damage the lens, so most optics have a protective coating. Kessler said that uncoated standard optical glass like N-BK7 will reflect around 7 per cent of the light interacting with the lens on each surface. This means that only 86 per cent is transmitted, with 14 per cent of the laser power within the glass, which is really damaging for the lens, Kessler said. To avoid this, an anti-reflective coating is applied, using materials specific to the laser wavelength. 

Kessler added that if the laser is high power, the optic has to be able to withstand a certain power density and have a high laser damage threshold to ensure a long lifetime. ‘This changes the requirements of the coating,’ he said. ‘The coating design and process will be slightly different, especially regarding cleanliness and purity which have much tighter requirements.

‘One thing is that it [the laser] will actually remove the coating,’ he continued. ‘There is a certain amount of absorption of the laser; it could create a dark spot due to oxidisation which then creates more absorption and eventually it would not let the light through.’

Once the coating is ruined, the user has the option to replace the component or recoat it. However, recoating involves cleaning, grinding, and polishing the optic before putting on a new coating. 

Recoating takes time, and in a lot of industrial processes time costs money. Kessler explained that most users will instead have a number of replacement components to hand in order to make a quick change. ‘If you think of laser welding in automotive applications, then you have a production line and the next car is coming whether the system is working or not; you are better off to take out the whole thing and put in a new one,’ he said.

However, the conditions industrial optics must endure expose them to things such as sputtering from the work piece, which is, according to Artigliere a more common danger. Debris from laser processing can ruin a component but Artigliere explained that simply putting a debris shield in front of these expensive components can extend their lifetime.

In providing optics for materials processing, Edmund Optics is reacting to demands from the market. When new customers approach the company with requests for optics for new applications, Edmund’s engineers consider the light coming in at the start – in materials processing this is typically from the laser source – and the end application’s requirements, be this welding, ablation or cutting. 

Typically the laser beam has to be shaped and focused onto the work piece according to the application. Kessler said that when considering the optics for an industrial laser system, Edmund engineers have to ask: ‘What is the point of the light you want at the work piece? Is it parallel, rectangular, a light spot?’ How the beam is shaped determines the optical components in the system, which could be a lens or a diffraction grating. Some customers want to split the beam with a beam splitter to create two light sources to double the productivity of the system. Or prisms or mirrors could be used to bend the light around corners.

Kessler continued: ‘When we talk about welding, you can have point welding, but you can also have slot welding where you would rather have a laser line. You widen the laser in a line and change the dimension of the laser. This produces a homogenous light source for the welding process, but you need different components. It would not only be a lens; it could be an anamorphic lens that is stretching [the light].’

The characteristics and robustness of the optical system are largely dependent on what type of processing is being done, the material that is being processed, and the capabilities of the rest of the system. So, as both Kessler and Artigliere agreed, when designing a laser system it is best to go to the laser source to find out what can be achieved.

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