Carbon dioxide lasers experience rapid uptake

04 May 2007

A growing number of applications are turning to carbon dioxide lasers. Cathy McBeth explains what the far-IR source has to offer and what selection criteria potential buyers should bear in mind.

It has been said that the laser is a solution in search of a problem. In the case of the sealed carbon dioxide (CO₂) laser, this statement has taken on a life of its own. The ubiquitous CO₂ laser continues to offer new solutions to the scientific, medical and industrial marketplaces as the number of applications for it grows at a rapid pace. These compact lasers, traditionally used for cutting and marking, can now be found in applications ranging from cosmetic skin resurfacing, to flavour enhancement of our favourite foods, to spectrum analysis.

It is the industrial application of these lasers, however, that has seen the most phenomenal growth in the past decade. Sealed CO₂ lasers are now found in systems cutting sheet metal, marking 2D barcodes on electronics, drilling holes as small as 50 µm in surgical catheters and marking logos on glass windows. They also cut plastic automotive parts, engrave wooden plaques and weld steel tools – and the list continues to grow. To accommodate the increasing number of applications, a variety of laser resonator designs, power levels and model configurations are now available from several manufacturers, offering users a broad choice of products.

Operation and construction
A sealed CO₂ laser is one in which the laser bore (the area in which the plasma is constrained and lasing occurs) and gas supply are contained within a sealed tube. This is in contrast to higher-power flowing gas systems that require external gas tanks, pumps and filters.

The lasing medium is often a mixture of CO₂, nitrogen, helium and xenon gases that are contained in a metal, glass or ceramic resonator, with the lasing process initiated by a DC or RF energy source. Total gas pressure varies from about 20–200 Torr, depending on the size of the discharge gap and the drive frequency of the laser. The resulting beam radiates in the far infrared, most commonly at 10.6 µm. This wavelength is effective in processing a wide range of materials including wood, paper, plastics, glass, textiles, rubber and metals.

The first sealed CO₂ lasers were based on a glass-tube design with DC excitation of the gas. While these models are still manufactured, they are used primarily in medical applications and ultra-low-cost systems where short laser lifetimes are acceptable. Rugged RF-excited metal and/or ceramic resonator designs have largely displaced glass tubes in the industrial arena, proving themselves to be more efficient at gener-ating CO₂ laser output, and providing much longer operating lifetimes.

With output powers ranging from 10 to 600 W, sealed CO&sub2; lasers perform many processes, including cutting, welding, drilling, heat treating, cut-and-weld operations, marking and engraving. They are the key components to systems found in virtually any manufacturing industry – automotive, aerospace, electronic, construction and packaging, to name a few.

The low cost and minimal maintenance requirements of low-power CO₂ lasers make them appealing to many manufacturers. Sealed CO₂ lasers offer users the benefits of small size and rugged construction, combined with operating lifetimes in the thousands of hours. They can be mounted on robotic arms or moving gantry systems, and are simple to integrate.

While they have long been associated with processing organic materials such as plastics, rubber and wood, sealed CO₂ lasers are increasingly being used in applications that were once accomplished by other, higher-cost lasers or different technologies, such as marking and welding metals, and glass marking. Their expanded usage is largely due to improved process developments, as well as new CO₂ laser designs that better address the needs of these applications.

Cutting applications
Because of their compact size, sealed CO₂ lasers can be integrated into a variety of motion systems including flatbed cutting systems, multi-axis systems and robotic arms, to cut a wide range of materials. Focused spot sizes as small as 0.002 inch mean that intricate cutting patterns as well as straight line cuts are possible. The laser’s high-power density allows for high cutting rates with minimal heat input.

The typical characteristics of laser cuts are narrow, small heat-affected zones, square edges, low roughness and high resolution. Compared to cutting with a blade, a laser offers increased precision and flexibility without tool wear, and there are no consumables to buy. A few of the many materials that are typically cut with CO₂ lasers are:

• Plastics: plastic cutting is one of the most common applications of CO₂ lasers due to the high absorptivity of most plastics at 10.6 µm. Depending on the thickness and specific thermal properties of the plastic, as well as the speed requirements of the process, cutting can be achieved with 20–200 W of power.

• Wood: another classic CO₂ laser application is die-board cutting, which requires a large volume of highly accurate, repetitive cuts to be made. Laser cutting provides an extremely accurate and repeatable cut width in the die board.

• Steel: steel and stainless steel are the metals that are most commonly cut with CO₂ lasers. Steel can be cut with laser powers as low as 120 W for thin metals. Higher output powers are used to cut through thicker steels or to achieve faster speeds. Traditionally, these materials have been cut with high-powered, axial-flow lasers, but new models with higher output powers (240–500 W) and near-perfect beam quality have made this a viable application for sealed lasers.

Marking applications
Lasers offer users a flexible, inexpensive and environmentally friendly marking method. The implementation of lasers for marking is now becoming commonplace, in part because of their excellent mark quality, their lack of consumables and associated disposal problems.

As an increasing number of industries call for increased part traceability, CO₂ lasers are being utilized to mark Data Matrix and barcodes, serial numbers and date codes, as well as text and graphics. In addition to marking most plastics, wood and many composite materials, two increasingly popular marking applications for the technology include marking metal and glass.

• Metal: surprisingly, low-power CO₂ lasers can mark a number of metals, including mild and stainless steels, inconels and titanium. They are becoming an attractive alternative to the more expensive and maintenance-intensive Nd:YAG lasers, particularly as new, higher-power air-cooled laser models are introduced to the market. Metal-marking applications generally use CO₂ lasers from 50 to 200 W and benefit from fast rise/fall times and excellent beam quality.

• Glass: glass manufacturers and fabricators have long expressed an interest in CO₂ lasers for marking serial numbers, logos and codes on automotive glass, medical devices, windows and electronic parts. The problem, until recently, was that CO₂ lasers typically produced highly fractured marks on most glass types. New techniques have made it possible to produce crisp, high-quality marks directly onto glass with sealed CO₂ lasers, making them an attractive alternative to traditional glass-marking methods and expensive solid-state laser solutions. A 25 W CO₂ laser provides sufficient power for most glass-marking applications.

Selection criteria
It used to be that manufacturers offered a single laser design that was simply scaled up and down the power spectrum. Today, a range of different CO₂ technologies exist. For most applications, the composition of the material to be processed will be the primary factor in determining the laser required. That said, other factors that will affect an application include everything from price and size of the product to beam characteristics of the laser and the delivery optics that will be used in the end system.

The expansion and growing complexity of the many applications for sealed CO₂ lasers has driven some manufacturers to develop products that meet the specific requirements of their customers’ applications. Lasers can be designed from the start with power, mode quality and wavelength requirements that best match a given application.

For example, as a result of the advances in beam quality and the increasingly small size of the CO₂ laser, previous applications that would have required a very large CO₂ laser or a more expensive, shorter wavelength laser can be solved more economically. High-threshold applications such as cutting 1 mm thick titanium sheet, or even welding sheets of copper nickel alloy together have been demonstrated with less than 200 W. This is possible at lower power levels than before due to improvements in beam quality and optical pulse shape, which yield higher fluence levels at the focused spot. Faster rise/fall times have helped laser engravers progress from speeds of 40 to over 100 inch/s. Automotive customers can now choose an air-cooled laser model with enough power to mark serial numbers or 2D barcodes on steel parts.

An experienced sales engineer can be a valuable resource in determining the best fit for a customer’s application. Since every material will react differently to the CO₂ wavelength, it is important that potential customers discuss their application with a knowledgeable source. Some materials (copper and aluminium, for example) cannot be processed with a CO₂ laser, while others may char or discolour. Most manufacturers will provide processing trials for potential customers.

Today’s sealed CO₂ lasers are designed to accommodate the needs of the industrial marketplace. They are smaller, lightweight, and can be air-cooled at higher powers than ever before. Manufacturers not only offer multiple power levels, but multiple resonator designs to better address specific requirements. With an improved understanding of applications, more and more industries are learning of the benefits of low-power sealed CO₂ lasers.

• This article originally appeared in the April 2007 issue of Optics & Laser Europe magazine.