FEATURE
Issue: 

Beam blockers

Greg Blackman on the safety equipment for protecting against higher power and ultrafast lasers

New laser technology needs new safety equipment, so as industrial lasers become more powerful and offer shorter, higher energy pulses, the safety systems have to adapt accordingly. The ultrashort pulsed laser is a relatively recent introduction as an industrial tool, and there are still unanswered questions surrounding the best method for fume extraction and how blocking filters react to these short pulses. Meanwhile, continuous wave (CW) lasers are becoming more powerful, leading to demand for active laser protection windows, according to Laservision’s head of sales and marketing, Frank Billhardt.

In terms of laser safety eyewear, the market is driven by medical applications, Billhardt commented, a statement borne out to a certain extent by the programme for the Laser Institute of America’s International Laser Safety Conference in March in Atlanta, where medical laser safety will feature heavily (for more on the conference, see LIA news on page 26).

Billhardt remarked that there has been a lot of development in the absorbing dyes used in plastic filters for goggles, which are replacing conventional mineral glass filters, to give lighter and more comfortable eyewear. ‘Customised absorption curves, created by a mixture of several carefully selected and qualified dyes, especially for multiline medical laser systems, are in strong demand from medical laser manufacturers,’ he said.

One of the new dyes developed in the last year or so is a light grey, almost neutral colour dye, for protection against Nd:YAG laser radiation at 1,064nm. Before this, eyewear filters for the Nd:YAG wavelength were green; the new dye ‘gives better visibility for medical professionals viewing tissue’, commented David Bothner, director of sales and marketing at NoIR Laser, although Billhardt added that ‘there is still a lack of clear, colourless broadband filters for the NIR wavelength range’.

There are also now alternative absorbing materials to organic dyes, such as specialised particles, that can be moulded into polymers. These new materials offer higher damage threshold values than organic dyes in some cases, and coverage in areas that couldn’t be achieved with standard dyes, according to Bothner. ‘There’s now plastic eyewear for the 2µm range that didn’t exist before,’ he said. ‘Standard organic dyes that absorb at 2µm are not soluble in plastics, but these alternative materials can be moulded into polymer filters and protect against that wavelength range.’

Move to higher powers – now much more common in industry – and mineral glass becomes necessary, both for safety goggles and for windows in laser systems. However, acrylic windows are now available that withstand higher laser powers. Laservision’s blue acrylic P1P10 6mm-thick window for instance, is certified to D LB7 (OD 8+), the same D-rating covered by conventional infrared-protection glass windows between 1,030nm and 1,400nm, but offering a maximum dimension of 2 x 3m, much larger than the glass alternative.

These types of window are ideal for automated production cells with integrated lasers, according to Billhardt, such as 3D metal printing machines that run for several hours. Here, unobserved operation requires active protection and monitoring. ‘As 3D printing and 3D metal laser sintering are more common in production processes, adequate protection for such long time unobserved processes can only be achieved by active laser protection windows,’ he said. Laservision introduced this technology in 2010 with its first CE certified active plug-and-play window for 800nm to 1,100nm wavelengths.

Pitfalls of ultrafast laser safety

Active guarding windows protect against high power CW lasers for material processing, but the rise of ultrafast lasers for micromachining brings different laser safety requirements. Eyewear can protect against ultrashort laser pulses, but Bothner at NoIR warned that there are additional questions that need to be asked when purchasing safety equipment for these lasers.

‘As shorter pulses become more common, a lot of users will think a femtosecond laser operating at 808nm needs 808nm eyewear, but what the end-user and the providers of safety equipment need to question is, how broad is the pulse? It might be centred at 808nm, but if the pulse width is 40nm you need protection at 770nm. The LSO needs to be asking those questions,’ he said. ‘There are dyes that one could misuse, those that are centred at 808nm but have virtually no protection at 770nm or 850nm.’

Polymer-based laser safety eyewear is lighter and more comfortable

Bothner added that there is a lot of work being done to understand what happens with ultrashort pulses and absorptive filters in general. ‘There are dyes in polymer lenses that are tested and certified for short pulse laser systems. The important thing to understand is that not every short pulse system is the same, and that a dye that performs well at 200fs or 1ps might not respond the same way at 50fs or 10fs,’ he said. ‘Again, this is where the questions need to be asked, both by the user and the provider of the safety equipment, and to look at whether the filters have been tested for the laser system being used. In the US, there is research being carried out on absorptive filters and absorption at short pulses, and NoIR has a lot of test data on its filters that we share with our customers as a way to ensure that when the customer puts on eyewear, it’s rated to be safe for the specific laser.’

It’s not just safety eyewear that has to be carefully considered for use with ultrashort pulsed lasers, but fume extraction technology as well. ULT has carried out a study into the types of particles thrown up during micromachining with ultrafast lasers and found that standard filtration techniques are often not suitable. The company provides extraction filters and air purification systems, so it is not an independent study, but the work does highlight differences in the size and shape of particles from micromachining compared to those from conventional laser processing, something that Dr Stefan Jakschik, CTO of ULT, feels is not thought about by all laser system manufacturers.

‘Pulsed lasers have been around for a number of years, but system makers are only now starting to apply them and they are using standard filter techniques,’ Jakschik said. ‘System makers are definitely one group to be addressed concerning filtration methods for ultrafast laser processing, as well as the end user.

‘I would be very happy if all system makers were aware of the type of filtration required for ultrafast laser processing. However, not all system makers are aware of it,’ he added.

Continuous wave laser machining is a thermal process that melts the material, forming molten droplets that are easy to extract with standard filtration equipment. Pulsed lasers, on the other hand, operating at picosecond or femtosecond durations, don’t heat the part in the traditional sense, but work through a process called cold ablation whereby the energy of each pulse is high enough to remove material without putting any thermal energy into the surrounding component.

The particles ablated with ultrashort pulsed lasers are not melted; as such, they are much smaller, in the range of 50nm to 200nm, according to the study by ULT. ‘These particles in my understanding are more significant to the health concerns associated with material processing, because they are so small and can be breathed in and pass through the lung-blood barrier,’ commented Jakschik. He went on to say that this has not been tested, but, in theory, because the particulates are so small, they could potentially make their way into the blood.

In addition, these particles are not droplets, but are more like sugar cube-type structures, the study showed from scanning electron microscope images. ‘These kinds of particles can block standard filters quite quickly, forming a cake of material on the filter surface,’ Jakschik explained.

‘ULT has definitely changed its filtration approach for ultrashort pulsed laser processing,’ Jakschik stated. ‘Standard filters have a very low lifespan when extracting particles from cold ablation. The way filter systems are operated today can lead to a situation where the filters are not extracting particulates sufficiently, because if the filter breaks it blows out all the dust. You definitely need filters that don’t break.

‘The poorest standard technique is mat filtration, but this is used in many cases because it is the least expensive initial investment,’ Jakschik continued. ‘The mats don’t filter well and the lifetime is low.’

ULT recommends its LAS260 system for these ultrafast lasers, which uses an F9 pleat pre-filtration followed by a HEPA class H14 post filtration – this is just one of ULT’s units based on this technique. The company suggests that this is the best purification method in terms of quality of filtration and cost over six or 12 months.

The study tested ceramics, metals, glass, and semiconductors, and found that it is less expensive to use the ULT’s specific filtration method compared to standard filter techniques – by several orders of magnitude, according to Jakschik.

Back to school

‘The first defence against harm from a laser is training,’ commented James Saxon at Laser Components, which stocks laser safety products. With all the advances in laser safety, education remains a priority for laser users and those selling laser safety equipment.

‘We see a huge need for laser safety education especially in the trading and reseller sector,’ commented Billhardt at Laservision. ‘More and more dealers of standard working protection equipment like gloves, shoes, textiles and eyewear start to resell laser safety products without having the knowledge or background.’ Laservision runs a premium partner programme to qualify sales personnel from selected key distributors of its safety equipment.

New laser safety technologies are being investigated, such as responsive eyewear filters that are transparent but absorb upon laser energy hitting them, but as laser technology drops in price and becomes more available, proper safety training is as important as ever.  

Glass versus polymers

Mineral glass filters for safety eyewear give the greatest protection against laser radiation, but recent advances in organic dyes mean that polymer filters can now meet many more laser safety needs. James Saxon at Laser Components explained that ‘when polymer goggles can be used, these are recommended as they are lighter and more comfortable, and polymers are generally cheaper as well.’

Laser Components offers a support service for laser safety eyewear, evaluating all the laser parameters, giving a list of options of filters required.

The dyes for polymer filters are baked into the material, which makes them scratch resistant and means a single filter can contain multiple dyes to give it a wide protection range. But they melt if the power becomes too great.

Glass filters have reflective coatings and can therefore withstand a lot more power, but are generally heavier and the protection range might only be across two or three different wavelengths, Saxon said. Glass filters are also susceptible to scratching, which compromises laser safety.

Deciding on whether to use a mineral glass or a polymer lens depends on the intensity of the laser light per unit area. ‘A 10W beam focused down to a pinprick would give a very high intensity per unit area, whereas a more divergent beam would have a lower intensity per unit area,’ explained Saxon.

Extreme laser safety

Research centres like the Extreme Light Infrastructure (ELI) and the European XFEL being built in Hamburg, Germany contain some of the most powerful lasers in the world, all for probing fundamental physics. Ensuring safety at these sites presents its own unique set of challenges, something that laser safety specialist Lasermet is currently working on for the European XFEL facility.

Lasermet is installing an interlock safety system for the XFEL centre. ‘In these projects, there are multiple rooms with multiple sources delivered to multiple target areas – the laser light could go to any one of those areas,’ explained Paul Tozer, managing director of Lasermet. ‘From a safety interlocking point of view, it’s a bit of a nightmare. It’s a big departure from the normal laser safety interlock situation, where there is a laser contained in a laboratory.’

Lasermet’s Safety Logic Plus interlock system

Lasermet has done work in the past for the UK Atomic Weapons Establishment (AWE) where there were a number of target chambers to which the laser could be delivered.

‘With these more complicated setups, you get to a situation where you have to solve it with logic,’ Tozer said. Lasermet built its Safety Logic Plus interlock system for AWE because the organisation wouldn’t accept a software-based device.

Safety Logic Plus uses DIN rail mount logic gates, which are hardware based. The user develops a logic diagram for a safety system; there is then a physical logic gate on the DIN rail for each logic gate in the safety plan. The product conforms to the safety integrity level SIL3, and to the safety control system standard EN13849-1 at performance level E, as do all the company’s laser interlock systems.

‘The other thing that makes the design of safety systems for the ELI and European XFEL projects a safety interlocking nightmare is that you’ve got secondary radiation produced by these systems,’ commented Tozer. ‘Instead of the laser radiation being a danger of serious injury, there is a danger of death. Essentially, you’re walking into a room, the laser source isn’t in that room, but if the laser source is connected to it there is danger of death. You have to have a means of detecting in a foolproof way whether the beam is going into that room, and not allow the beam to go into there until you’ve gone through a series of safety procedures.’

In the ELI project, the beam is delivered through vacuum tubes with special gates and mirrors. Each of those has an interlock switch, so the safety system detects where the beam is being directed from the position of the mirrors.

‘You have to define the circumstances under which the laser is allowed to fire,’ Tozer said. ‘Because of the dangers involved you need to build up [the logic diagram] very carefully. You don’t want it to be flexible; the point is you mustn’t change it.

‘Once you get multiple rooms – say six rooms – where you could produce the laser radiation and another six experimental halls where you could deliver it, the logic is quite complex,’ he concluded.

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