Gas lasers still have much to offer

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David Robson evaluates the gas laser and whether it still has a future in the face of stiff competition from fibre and diode alternatives.

A research group from the University of Plymouth has used 25W CO2 lasers to engrave tiny serial numbers into the shells of live beetles. The research group, in conjunction with the Game Conservancy Trust and Rothamsted Research, used the technique to release and recapture potentially beneficial, predatory beetles in order to investigate their behaviour.

The beetles, sedated by chilling at 4°C for 30 minutes, were held in a small well while the laser engraved a unique serial number using the Synrad Fenix Laser Marker at a rate of up to 180 characters per second. After a number of trial-and-error experiments, the scientists were satisfied that the technique was harmless to the P. melanarius.

An interesting application in itself, but what makes it even more remarkable is the fact that industry experts have been predicting the death of gas lasers in favour of fibre and diode lasers for the past 15 years. However, this application is not alone; their precision, control, low cost, robustness and flexibility have meant that new applications for CO2, nitrogen, ion and helium-neon lasers are opening up all the time.

‘This is the most unusual application we’ve supplied lasers for,’ says Gary Broadhead, sales director at Laser Lines, ‘but our biggest market is in engraving and cutting.’ A more standard example of these applications would be the employment of 10W CO2 gas lasers to mark medical instruments, avoiding the risk of hazardous inks and providing a permanent and sterile means of identification.

The laser was used to mark PVC medical IV bags with health and safety information. The laser energy caused a colour change from semi-opaque to bright red-orange, producing a high contrast between the mark and the surrounding background, while barely penetrating the plastic. Flexography is a similar application: in this case, the laser is used to engrave printing rolls used for newspaper and magazine printing, and in the ceramic tile industry.

‘Due to the competitive price pressure, these lasers are quite affordable,’ says Broadhead. ‘They have the best pounds-to-watts performance ratio around. They are even used in schools for design classes, where they are connected to a PC with CAD software.’ In addition, they have very low maintenance: ‘They are sold like a light bulb – the customer plugs them in, and forgets about it.’

‘CO2 lasers are a mature technology. There haven’t been any quantum leaps recently, as the technology has been around since the 80s.’ However, development is far from dead. The customisation of resonator designs for different power levels has optimised performance, with narrow gap resonators providing greater laser beam intensity and improved beam quality for higher-power lasers, and free space resonators further improving the efficiency and cost-effectiveness of low-power lasers. In addition, ceramics, which have excellent thermal conductivity, have been used to aid cooling of heat generated by the plasma, which had previously been a big disadvantage of CO2 lasers.

The range of powers available in gas lasers, from less than 1W to a possible 20kW, is another factor that sets them apart. This too has improved recently, allowing a greater range of applications. Gerald Jones, general sales manager for Trumpf, says: ‘Fifteen years ago, CO2 lasers were typically in the 1kW range. However, there has been a gradual increase, and now powers of up to 6kW are common. This, together with better design, has opened up both welding and cutting applications.’ High power is achieved by increased stimulation of the laser source, which can cause thermal and mechanical stress, so the lasers need to be much larger to provide stability. The higher processing speeds could lead to a greater call for automated loading systems.

Jones believes that cutting is by far the largest application for high-power CO2 lasers. He, too, stressed their reliability for these jobs, saying: ‘They are an efficient laser source and a tried, proven, and well-known workhorse.’ They are used to cut sheet metal up to 10mm in thickness, for white goods and the automotive industry, and also to shape steel tubes for furniture and gym equipment. Lasers are much quieter than mechanical means, and prove to be more economical with the materials being processed. Lasers of roughly 4kW are also used to cut and process 3D components for the aerospace industry.

Processes in the aerospace industry have to be very tightly monitored, as heat may change the metallurgy of the components, which could result in weaknesses and cracks. CO2 laser beams are highly controllable, allowing the components to meet required standards. This is achieved with a high degree of synchronisation between the axis movements and the laser power control.

In addition to a greater range of powers, CO2 laser beams are also higher quality than solid-state beams. This means they can be focused to very small spot sizes, giving the high power density necessary for these applications.

These factors allow the high-precision cutting of thin glass used for cell phones, PDP and LCD displays using lower-power CO2 lasers from Coherent. This application uses a thermal process: a laser beam heats up the surface, after which a liquid stream provides a ‘thermal shock’ that directly breaks the glass. The quality of the break is so good that you don’t need an additional grinding process, unlike mechanical methods that would introduce micro-scratches.

‘The mechanical-cut glass does not have the same bending strength,’ says Frank Gaebler, product marketing manager for Coherent. ‘You don’t want glass that could crack in these heavily-used items like cell phones or PDAs.’ This process also provides a higher yield than other processes. It is gradually seeing more use, and is even seeing an introduction in printed circuit board cutting, as it produces less dust and debris than mechanical methods.

The second big application for high-power CO2 lasers is welding. As a rule of thumb, every 1.5mm of penetration into steel requires one kilowatt of power so, for deep penetration, 4kW to 15kW lasers are used for the production of automotive components such as gears, shafts, and the car body.

For another kind of gas lasers, Helium-Neon lasers, the range of wavelengths proves to be the biggest advantage. Like diode lasers, ‘HeNe’ lasers work primarily in the red part of the spectrum, but unlike diode lasers, they can also produce a range of green, yellow and orange laser light. The emission can be tuned using a prism in the laser cavity that is tilted using a motor to transmit different wavelengths. These lasers work at much lower powers, typically 35mW.

HeNe lasers are even cheaper than CO2 lasers. The narrower line-width and thermal stability of the beams make them ideal for use in holograms and interferometry. The beam is also naturally circular with low divergence, unlike the elliptical beam of diode lasers that must be corrected with complex optics. In the past they were used widely in barcode scanners, but laser diodes are now used for this application because they are cheaper and smaller.

Ion lasers too, with Argon and Krypton as their gain medium, produce a stable, controllable output of up to 12 different wavelengths. However, their disadvantages mean they are now often overlooked for many applications. ‘I am often asked who in their right mind would choose an ion laser over a solid-state laser,’ says Paul Ginouves, director of the ion laser division at Coherent. ‘It is a legitimate question: they are very large, and have an efficiency of one hundredth of a per cent, but their tuneability and extremely high beam quality open up a number of applications.

‘The biggest market is the semiconductor industry. Ion lasers are used to engrave optical photomasks that are used as a template when creating printed circuit boards. They are also employed in the inspection of semiconductor wafers.

‘Biological spectroscopy is another big market, because their range of wavelengths means they can be used for a number of different experiments. Even though solid-state systems could be held in the palm of your hand, you would need many of these to produce the wavelengths necessary for every experiment.’

The wavelengths ion lasers produce are ideal for the sorting of red blood cells, which absorb blue-green light, from white blood cells, which reflect it. The amount of light reflected could allow the user to determine the number, size and morphology of the cells. JDSU too produces HeNe lasers that are used in similar flow cytometry applications.

HeNe lasers are also used for fingerprint identification, as the grease on fingers fluoresces under certain wavelengths of light. Other possible applications include cartography, printing, DNA sequencing and biotechnology engineering.

Argon lasers are much more powerful than HeNe lasers, making them suitable as a source of illumination in wind tunnels to study the airflows over the surfaces of cars and planes.

They are similarly used in the quality control of aircraft and truck tyres to test for bubbles of air trapped between layers of remoulded material. The tyres are put in a vacuum, and as the air is evacuated, the bubbles expand and are highlighted on a hologram made with the laser. The high power of the lasers reduces the exposure time necessary in this kind of application.

Ion lasers have a large market in the entertainment industry, particularly in China and theme parks such as Disneyland, where they are used for light shows due to their high visibility and high output. However, this may not be for long: ‘We are on the cusp of seeing solid-state lasers invading this space,’ says Ginouves.

A laser services by Laser Support Services.

Although the production of gas lasers may have decreased in recent years, there is still plenty of call for repair and maintenance companies such as Laser Support Services. Grahame Rogers, managing director, says: ‘We formed 16 years ago to provide a repair service for ion lasers, and we are currently repairing more lasers now than in our first year.’ He attributes this success to tapping into the semiconductor manufacturing market, which uses a large number of gas lasers for wafer inspection.

‘We repair and refurbish ion lasers that can vary from being one to 20 years old. There is still a very large installation-base of ion lasers worldwide, and there is a constant need to repair them.’ Typically, optics and control components need replacing, as well as the plasma tube, which becomes eroded by plasma at the bore of the tube.

The controllability, flexibility, and stability of gas lasers, both in terms of wavelength and power, seems to have prolonged their place in the market for the time being. ‘The market is flat at the moment; these lasers will die out eventually, but it will not happen in the foreseeable future,’ says Steve Knight, director of Laser Lines, regarding HeNe lasers. CO2 lasers in particular seem safe from extinction: ‘Fibre lasers may take on some of the applications of CO2 lasers, but the market of CO2 lasers grew by 20 per cent last year,’ says Gary Broadhead, sales director of Laser Lines. ‘I can see it having a very long, healthy future. We are seeing more new applications all the time.’

Nitrogen lasers

After years of practically no activity, we may be about to witness a resurgence of the nitrogen laser. LTB Lasertechnik has been developing the lasers since 1990, believing they will provide a low-cost and ecologically friendly alternative to excimer and diode laser UV sources for analytical instrumentation.

The lasers have been considered old-fashioned and out of date since the 70s, with millions of dollars being spent on the development of excimer and diode lasers. However, nitrogen lasers are low-maintenance and allow pulses as short as 50ps, without producing any pollutants.

One of the biggest concerns had been the unreliability of these lasers. However, it is hoped that this had been eliminated in LTB’s latest model, the MNL 100, along with improved heat stability and increased longevity of the gas quality inside the laser channel.