Tips for optimizing laser processes
A nifty thing about working with lasers is that there are always new and better machines coming on the market. Fifteen years ago, most people hadn’t heard of disk, USP (ultrashort-pulse) or fiber lasers. Now, they can be found throughout mainstream manufacturing, and they have greatly expanded laser users’ ability to confront and overcome manufacturing challenges.
No matter how good a laser is, though, if used incorrectly bad parts will result. There are many considerations when optimizing a laser process. Some suggestions for doing that follow. (Due to the limitations of space, I’ve narrowed the discussion to lasers used in the focused-beam mode, which is more common than lasing in the imaging mode.)
New laser technologies are helping users overcome many manufacturing challenges. But even the most advanced equipment must be optimized in order to ensure the production of good parts. (Shown is a bioresorbent stent cut with a TRUMPF TruMicro Series 5000 ultrashort-pulse laser.) Image courtesy of TRUMPF.
My best advice is to start at the source—the laser—and work downstream. Most focused-beam lasers emit a beam with a Gaussian profile—a round beam with “wings” (Figure 1). The hottest part of the beam is its middle; the weakest energy within the pulse is represented by the wings. This type of beam is relatively easy to direct and control, because it is round, but the nonuniformity in the beam’s energy density can damage a workpiece by creating a heat-affected zone on its surface.
The best way to minimize HAZ is to make sure the beam doesn’t distort as it propagates through the optical system. The beam needs to be as uniform as possible as it exits the laser. This is easier said than done, and partly depends on the type of laser and its wavelength.
Some lasers incorporate technology that optimizes beam output. An example is a DPSS (diode-pumped solid-state) laser with harmonics. The harmonics are generated via nonlinear crystals. As crystals wear, their positions must be changed to maintain optimal performance. Most users have routines in place for repositioning crystals, as well as adjusting temperature settings and other factors that affect crystals. Another suggestion is to regularly change a laser’s dessicant. Water vapor inside the laser head can degrade laser performance.
These and other laser housekeeping chores should be performed before attempting to optimize the beam path.
Figure 1: The energy output of a laser beam with a Gaussian profile is strongest in the middle. Image courtesy of PhotoMachining.
Once the optimal laser output is reached, it is time to move down the optics stream. A rule of thumb is to use the smallest number of optics necessary to do the job correctly and safely. Optics, which include turning mirrors, telescopes, polarizers and focusing lenses, are either refractive (light is transmitted through the material) or reflective (light bounces off a surface).
Most optics are coated with a transmission enhancer or antireflection enhancer that improves performance and increases wear life. These dielectric coatings are almost invisible to the naked eye, making visual observation a poor way to assess their condition.
A better way is to measure the energy (or power) of the beam before it enters and after it exits the optic. This is done with an energy meter. The difference between the readings is the attenuation caused by the optic. Good laser systems have a beam-efficiency rating in the high-90s, meaning a beam’s power, after passing through all optics, is attenuated only a few percentage points. For transmissive optics, the substrate must be free of burns that can attenuate the beam.
Figure 2: A series of crosshairs cut with a laser in order to focus it. The outer samples show material removal is incomplete, indicating an out-of-focus laser. The center crosshairs were cut when the laser was in focus. Image courtesy of PhotoMachining.
To illustrate the importance of minimizing beam attenuation, consider a laser with 10 optics, all of which are 99 percent efficient. Upon hitting the target, the beam will have about 85 percent of its original power. If the optics were 95 percent efficient, the resulting beam-on-target power would be about 60 percent. Such a setup throws away a lot of good, usable photons and would probably create problems upstream in terms of excess heat and potential damage to the optics.
Once the beam is directed toward the processing area in the most efficient manner, it is time to find the best focus setting. A simple way to do this is to reduce the laser output and then laser-cut marks on a piece of material. Place the stage, with the material on it, well above the expected focus position. Then, move the stage incrementally in the Z-direction through the focus range, cutting a series of crosshairs. In Figure 2, the center set of crosshairs depicts the midpoint—the sweet spot—of a laser’s focus range.
The next step is to decide where to position the focus. It usually is placed at the surface of the material to be processed. But frequently, like when the material is very thick, it is better to place the focus at the side of the material opposite the beam-entry point or in the material’s bulk. The exact location depends on the lens’ focal length, which affects the depth of field, placement of the lens in the holder, and whether galvanometers or a fixed beam is used.
As mentioned, space doesn’t permit discussion of all the factors that influence optimization of a lasing process. But concentrating on the laser and beam delivery—especially the focus—will help ensure the success of any optimization effort.
— R. Schaeffer
Ronald D. Schaeffer, Ph.D., is CEO of PhotoMachining Inc., a high-precision laser job shop and systems integrator in Pelham, N.H.