Tracey Ryba and David Havrilla
Improving high power, solid state laser processing
This article will discuss three types of solid state laser (SSL) beam delivery, processing optics, showing how to reduce thermal focus shift by using reflective optics; how the advances in beam quality have provided a creative way to make the laser more flexible by using a dual concentric core fiber to deliver two different beam qualities through the same fiber; and how higher beam qualities have allowed greater use of galvo scanner optics to perform welding remotely.
Controlling thermal focal shift
Keeping optical surfaces clean is critical when working with a 1 micron wavelength laser beam. Since the wavelength is much shorter than the CO2 laser, even a small particle in the optical path can cause significant damage or power losses. One can easily see the effects of thermal focus shift when comparing a 0 degree (straight) processing head with a camera and a 90 degree (right angle) processing head with a camera. In the 0 degree configuration, a transmissive optical plate is required between the lens and collimator to reflect visible light to the camera, while in a 90 degree configuration, the right angle bending optic is coated to reflect the laser wavelength and transmit the visible light to the camera.
In a transmissive optic, the laser beam passes through the optical material, and as much as 2% of the energy can be absorbed in the bulk material and face of the optic, even on a clean, well-designed optic. As one can imagine, 2% of 1000 watts is pretty manageable, but 2% of 16,000 watts can be extremely difficult to remove from a material that is not very conductive in its heat removing properties. On the other hand, a reflective optic will reflect nearly all the energy with very little losses in the coating and surface, typically less than 0.3%, so the heat needing to be removed from the optical material is very low.
A reflective optical processing head is ideally suited for high power, high duty cycle laser welding applications. The collimating lens and focusing lens have been replaced with reflective optics. Additionally, the reflective focus optic is coated to allow visible light to transmit to a camera located behind it. This leaves the protective cover slide as the only transmissive optical component in this configuration. FIGURE 1 shows the results of this configuration in high power applications as compared to transmissive optical configurations. As can be seen, the thermal focus shift was dramatically reduced, especially for high power levels where about a 75% reduction is realized, as compared to 0 degree (straight) transmissive configuration.
|FIGURE 1. Focus shift vs. time comparing reflective and transmissive.|
Dual core processing fiber
The desired beam quality for many laser welding applications is a flat-top beam profile with a moderate beam quality delivered in a relatively large spot size. On the other hand, laser cutting applications require a very good beam quality, typically a Gaussian profile that can be focused to a small spot. Using the appropriate beam quality for each allows for processing at the optimal focus point, which will allow some variations in part location, height, and focus shift at high power levels, while still achieving high quality, fast and very repeatable welds and cuts. In order to achieve these results one must typically use two separate fibers.
Fortunately, a new 2-in-1 fiber technology that contains two concentric fiber core diameters in one cable is now possible. In the cutting process, the laser beam is directed through the center 100 μm core, utilizing the high beam quality, and during the welding process, the beam is directed into the outer core (currently available in 400 μm or 600 μm diameters), thus delivering the optimal beam quality to the work piece.
The 2-in-1 fiber is ideal for use in laser processing systems; a single fiber can be routed through the axes to the processing head. This allows for either a quick change of the entire processing head or replacement of the cutting nozzle with a welding nozzle. When cutting, the cutting assembly is attached, and the laser beam goes into the center core. Then, when welding, the processing head is configured properly and an optical switch within the laser diverts the beam into the outer core of the 2-in-1 fiber.
FIGURE 2 shows how the fiber switching works, and an example of the measured intensity distributions in a sample part when coupling into the inner core (diameter 100 μm) and into the ring core (diameter 400 μm).
|FIGURE 2. Intensity distributions directly after the 2-in-1 fiber when coupling into the core (top) (diameter 100 μm) and into the ring core (bottom) (diameter 400 μm).|
At TRUMPF, comparative testing of the new 2-in-1 fiber vs. standard single core step index fibers has been done. In cutting trials, the core in the 2-in-1 fiber achieves the same cutting speeds and edge qualities as with standard step index fibers with the same diameter. The same applies to deep welding via the ring core, the quality of the weld seam and root, as well as the seam shape, which are comparable to the output from a conventional step index fiber with the same diameter. Whether cutting or welding, the 2-in-1 fiber safely and reliably reproduces the processing results achieved with conventional step index fibers
Remote scanner welding
Galvo based scanners have been around for many years now in laser material processing, primarily in laser marking based systems and fixed head systems in CO2 lasers. With advancement of high beam quality disk and fiber lasers, galvo based scanners are quickly expanding into industrial applications with the promise of reduced cycle times, cost reduction through simpler system design, large work envelopes and flexibility.
The laser light is delivered via a fiber optic cable to the programmable focusing optic (PFO) and - as in other types of processing optics - it goes through a collimator to produce a collimated beam at the optimized diameter to achieve high quality processing characteristics at the work piece, and to minimize energy densities on the steering mirrors and focusing lens.
The basic operation of a 2D PFO goes like this: After the beam is collimated, it is deflected off a mirror, which also allows visible light to pass back to an optional camera system. The deflected laser beam then is steered at the work piece by the combination of two mirrors mounted to precision galvo motors, one for each axis (X-axis & Y-axis) producing the motion, while the part being processed remains stationary. The deflected beam then passes through a series of lenses, called a flat-field lens assembly. By using multiple types of focal lenses in a stack, you can create an area where the laser beam focal plane remains constant. Add a motorized lens between the collimator and first galvo mirror, and you can then create adjustment of the focus in the Z-axis giving you a 3D working area.
Why use a PFO? On small parts it may be possible to have a fixed part and mount the PFO in a fixed position, giving a simple overall system design with no moving motion system other than the PFO. For parts larger than the work envelope, it can be possible to mount the part in a fixture and then provide a simple stage, allowing the welding of half or a quadrant at a time and then shift the part, or else the PFO can be mounted to a robot that can cover large areas and different angles. The weld speeds are the same as a traditional fixed optic processing head so the efficiency comes from removing unproductive re-positioning time (FIGURE 3). The most common applications of the PFO are found in the automotive industry today.
|FIGURE 3: Productivity of PFO vs. traditional laser fixed optic.|
This article has shown the limitations of traditional optics and how the designs have been pushed to the limits and the advancement that new high power and high beam quality lasers allow. As the technology continues to evolve, systems suppliers are looking forward to how it will challenge them to create the next generation of optics that will power the future of laser processing.
Tracey Ryba(Tracey.firstname.lastname@example.org) and David Havrilla are with TRUMPF Inc., Plymouth, MI.