Laser cutting and thermal focus shift

Thanks to steady increases in laser power, the precision of laser cutting now benefits a wide range of applications, from fusion cutting of structural steel to faster and more efficient cuts of thinner materials. At Messer Cutting Systems, this is reflected in increasing reliance on high-performance fiber lasers for its cutting systems.

Messer Cutting Systems (Rödermark, Germany) is a global supplier of products and services for the metal processing industry. The company employs more than 800 people at its five main production sites. Products include oxyacetylene-, plasma-, and laser-cutting systems, ranging from handheld devices to special machinery for shipbuilding, as well as systems for oxyacetylene welding, soldering, and warming. The company conducts development projects on individual assemblies, software, and sensor technologies at the main facility, and then custom and market-specific development is finalized at local facilities.

Minimizing focus shift

When it comes to the development of new cutting systems, Messer knows that measurement technology plays a decisive role (FIGURE 1). According to Thomas Dünzkofer, project manager in development at Messer Cutting Systems, “Determining the focus shift, in particular, had been very time-consuming for us and it posed some risk when the lasers were of higher power. At the same time, we really needed those measurement results.”

Messer’s research team developed a new software algorithm that could be used to minimize the focus shift in their laser-cutting systems (FIGURE 2). It is based on time-resolved measurements specific to each type of cutting head.

Noncontact beam measurement

The noncontact beam measurement system allows a laser beam to pass through an opening into the interior of the system and then exit the other side—without experiencing any impact along the way. It can do this because it measures Rayleigh radiation. This refers to the scattering of electromagnetic waves from particles smaller than the radiation's wavelength, such as oxygen or nitrogen molecules in the air. The electric field of the laser radiation induces an oscillation in the dipole molecule at the laser's frequency, leading to elastic scattering at that same frequency (see "Measuring a high-power laser with Rayleigh scatter").

Inside the instrument, the scattered laser light is imaged from the side using a telecentric lens assembly on a CCD or CMOS camera. Each individual pixel in a single line of the CCD camera detects the scattered light as a measuring point of intensity in the beam profile. When used with a standard CCD or CMOS camera with 1090 × 2048 pixels, the system will measure 2048 individual profiles simultaneously. From these measurements, the beam and beam-quality parameters can be calculated according to ISO-13694 and ISO-11146 standards.

For Messer, the speed of the system is critical. Measurements at video frame rates make it possible to see—for the first time—any shift in focus, which is especially liable to occur right after the laser is turned on. Dünzkofer stated, “When cutting with lasers of high power and short approach paths, a shift in focus can definitely affect the cutting process.” This is why the research team at Messer developed a software component for use with the company’s laser-cutting instruments.

Measurement technology with a future

Advantages of laser cutting include precise cuts, sophisticated component geometries, and limited material melt. To keep pace, Dünzkofer is convinced that measurement technology for lasers will increasingly gain in importance, especially for new types of cutting heads. More and more, cutting heads allow setting of the focus position and focus diameter independently. The difference in focal length changes both the cut width and the Rayleigh length. Especially at higher powers, it is necessary to determine the thermal focus shift in relation to the focal length.

With the BeamWatch system, the R&D team at Messer Cutting Systems now can measure these parameters quickly and easily. The optimal cutting head can be selected for each application. From Dünzkofer's point of view, nothing stands in the way of using the tool in production or even service.

According to Dünzkofer, “The beam profiler is lightweight, compact, easy to transport and easy to operate. Because the beam simply passes through the instrument without being touched, neither the beam itself nor the gauge's reliability is affected. And you don't have to worry about power limitations.”

CHRISTIAN DINI ([email protected]) is Director – Global Business Development for Ophir (MKS Instruments), North Logan, UT; www.ophiropt.com.

Measuring a high-power laser with Rayleigh scatter

As the power of the laser climbs, it becomes impractical to place objects such as sampling mirrors or rotating drums into the beam path due to the relatively high power density.

Real-time beam measurement for lasers that are typically too powerful for direct measurement can be obtained by relying on the Rayleigh-scattering properties of the most common air molecules: nitrogen and oxygen. In Rayleigh scattering, the high-concentrated light around the laser’s beam waist is scattered off air molecules in its vicinity and captured by the camera. This allows for an analysis of the laser’s waist without coming in contact with the beam. The result is a beam analyzer with no water-cooling required, no moving parts, and no upper limit in the power of the laser being analyzed. And, since it is a camera-based system, it provides data up to 15 times per second—this allows the user to see more time-based characteristics of their laser system.

For most industrial applications using multikilowatt lasers, the size and location of the beam waist must be held at consistent values to ensure correct power densities being applied to the part. With the release of Ophir BeamWatch, the laser user now has a way to measure these laser parameters. For example, during some automotive welding applications, processing happens not at focus, but some distance past focus since a relatively larger beam with a lower power density produces desired results. If the focused spot is shifting due to thermal effects on the laser system after the beam is turned on, the location of the focused spot is not constant and therefore the power density of the beam changes, resulting in an inconsistent result over the duration of the weld.

A high-power laser being used in drilling applications for aeronautical and aerospace part production is another example where maintaining the location of the focused spot is critical. During these processes, thousands of tiny holes are drilled into parts for purposes of air-cooling parts that would otherwise be destroyed or deformed during use. Lasers with extremely high peak power and relatively long focal length lenses are used to drill these holes. The location of the focused spot—where the laser’s power density is highest—must be determined and strategically placed onto the part being processed so that each hole is consistently drilled, both from top to bottom of each hole, and from hole to hole. Maintaining the location of the laser’s focused spot has historically been a challenge since there has not been a way to dynamically measure this laser characteristic. Noncontact beam measurement provides a solution to this problem.