Mankind has been processing materials since the dawn of time. Using a laser to aid in the manufacturing of products, however, is a relatively new development. Today’s high-power lasers are used for cutting, welding, drilling, marking, ablating and several other processes. And laser OEMs are answering the call with new developments in new laser technologies to increase the efficiency and quality of the lasers.
But there are two concepts that don’t change with these changing technologies. First, the power density must be kept consistent for the laser to process the material. And second, controlling these processes is paramount to ensuring a successfully manufactured product.
Power density (or energy density when discussing pulsed lasers) can be defined as the amount of laser power (or energy) that is concentrated down to what is commonly known as the “focused spot.” It is defined by two parameters: the laser power and the size of that focused spot.
Different materials require different ranges of power density applied from the laser to achieve the desired result. For instance, welding plastic requires much lower power density than cutting 1-in.-thick steel because of how the laser interacts with these two materials. Any deviation from this power density value and the hole is not drilled through, the two pieces are not joined and the shape is not consistently cut – which means parts are scrapped and time and money are lost.
Diagram of the operation of Ophir-Spiricon’s NanoScan scanning slit beam profiler.
Focused spot position
It is no secret that demand for higher average laser powers and higher pulse energies is increasing to achieve better throughput. This obviously presents a problem for the laser OEMs trying to push more light through physical components. Whether they be transmissive components, such as focusing lenses or cover glasses, or reflective components, such as directional or focusing mirrors, the greater the thermal effect, the more likely the system will experience a phenomenon known as “focus shift.” And when the location of the focused spot becomes unpredictable, so does the laser process.
Controlling the laser process through the measurement of the focus shift becomes more relevant the higher the laser power or energy goes. Most people working with these high-power lasers would agree that measuring the laser power and spot size (in order to determine the power density at the workpiece) is crucial to the success of the process.
But some laser users may not take into consideration that as the laser system ages, the greater effect thermal factors have on that system and the more important it is to measure that system’s performance for consistency. The laser systems will eventually deliver less power and an inconsistent focused spot size to the part being produced. Only through a rigid process control routine can the laser user better predict when to take maintenance actions on the laser system to keep its process optimized for peak performance.
Laser power measurement
Power measurement systems provide good indications of overall laser system performance. Users rely on embedded laser power meters to provide real-time feedback on the laser system and use these measurements as an indicator of how consistently the laser is behaving.
Technicians typically use a portable laser power meter to check laser powers at the workpiece to get a reading on how the overall system is behaving at that point. Both sets of meter measurements are valuable in determining if actions should be taken to correct any problem with the system.
The environments that these systems are used in are often times very harsh on the components given the amount of debris that is generated from the process. Particulates can settle on the laser system components, especially if the system is not maintained correctly, and can quickly degrade components, such as protective cover glasses and directional mirrors.
When this happens, thermal effects on the system have a greater effect on the way the laser interacts with the material. Technicians will see more steady reductions in laser power over time. But what will not be seen with the power measurement is the previously discussed shift in location of the focused spot with respect to the material being processed. Both behaviors can have a drastic effect on the outcome of the parts being manufactured.
At-focus beam measurement
The location of the focused spot is critical in most laser applications. One tool that is very useful in finding that location is a scanning slit beam profiler, such as a product made by Photon.
With a scanning slit profiler, a spinning drum rotates two orthogonal slits around a single-element detector to incrementally expose the laser. The position of the drum interfaces with an encoder to tell the position of the drum and accurately reconstruct the size of the laser and even mathematically represent 2-D and 3-D profiles of the laser simultaneously.
The advantage of this method is that because the laser is incrementally exposed, the relative power density of the laser being profiled without any attenuation is high. Scanning slit profilers can be put directly into the beam path to image the focused spot of the laser usually up to about 1,000 W of laser power, depending on the size of the focused spot. Using this method, the laser user can place the focused spot at the point of measurement without sampling or absorbing the beam to ensure that it is the correct size and that it is not changing over time. The laser user can also quickly and easily determine the best distance between the laser system and the part being processed.
One situation where this beam measurement technique is useful is with the processing of medical devices. Measurement of the focused spot – in addition to at-process laser power – is crucial and sometimes even required during application development, system runoff and continuous process validation.
Because access to the laser’s beam path after the laser is integrated into a system and put into production is often limited due to mechanical interferences, sampling the beam for purposes of attenuation is usually challenging. Since scanning slit profilers have a small footprint and require no attenuation, using them to measure the focused spot on continuous wave or quasi-CW beams has been successful.
BeamWatch GUI showing the beam caustic and beam waist profile, size, and location.
Non-contact beam measurement
As the power of the laser climbs, it becomes more impractical to place objects such as sampling mirrors or rotating drums into the beam path due to the relatively high power density. There is now a solution.
Recently, Ophir-Spiricon released a camera-based, non-contact beam profiling system called BeamWatch. It provides multi-kilowatt 1-micron wavelength laser users with unique data never before seen with any beam profiling system.
The signal supplied to and analyzed by the product is based on a physical property of light known as Rayleigh Scattering, where the high-concentrated light around the laser’s beam waist is scattered off of air molecules in its vicinity and captured by the camera. This system, with no moving parts, allows for a dynamic analysis of the laser’s waist without coming in contact with the beam, without the need for cooling.
For most industrial applications using multi-kilowatt lasers, the size and location of the beam waist must be held at consistent values to ensure correct power densities are being applied to the part. It wasn’t until the release of BeamWatch that the laser user had a way to measure these laser parameters.
For example, during some automotive welding applications, processing happens not at focus, but at some distance past focus because 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, which 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 so that each hole is consistently drilled, both from the top to bottom of each hole and from hole to hole. Dynamic, non-contact beam measurement provides a solution to this problem.
Ophir-Spiricon’s BeamWatch non-contact beam profiler.
Transmissive and reflective optics
Recent advances in beam delivery systems have led to new ways to deal with the problem of focus shift. Laser applications specialists have used Ophir-Spiricon’s non-contact beam analyzer to help develop these advanced products.
One design characteristic that helps with this problem is the application of reflective optics instead of transmissive optics, which are more susceptible to thermal effects from the laser. As laser powers continue to climb, they are applied to industrial processes.
Recently, measurements were made with the BeamWatch system on a 100-kW fiber laser being focused with reflective optics. Due to the relatively high laser power, this processing head exhibited an approximate 8-mm focus shift from beam on-time to approximately 20 seconds later. The fact remains that lasers will continue to have thermal effects on the system components with which they are integrated.
Spiricon’s BeamWatch in the forefront and Ophir’s 100-kW power measurement system in the background measuring a 100-kW fiber laser.
Even with advancements in laser technologies and the relatively higher efficiency that the 1-micron laser operates at, manufacturers are still faced with minimizing operating costs while maximizing throughput. And ensuring that their laser systems are operating as efficiently as possible is paramount to maximizing part production while minimizing downtime and part scrap.
With advancements in laser measurement technologies, the laser user and the laser technician have more tools than ever before to maximize the efficiency of their lasers and to keep their laser processes consistently high-quality. In addition to utilizing the latest technologies in laser systems to accomplish these goals, they can be used to better characterize and optimize the performance of these laser systems.
