January 6, 2012
The most powerful laser in the world can’t work without an effective, finely tuned beam delivery system. Its design hinges on the application, but a few basic elements lay the groundwork. Together they provide myriad options to find the most effective way to carry and shape the beam on its way from the laser source to the workpiece.
This article has been adapted and updated from Laser Mechanisms’ presentation, “Advanced Solutions for Laser Beam Delivery,” presented by Fiber Systems Manager Tom Kugler at FABTECH®, Nov. 15-18, 2009, Chicago, ©2009 by the Fabricators & Manufacturers Association and Society of Manufacturing Engineers.
These days lasers have permeated a wide cross section of metal fabrication applications: cutting, welding, brazing, surfacing, heat treating, drilling, and more. Laser types vary. Those performing conventional flat-sheet cutting have more options today, be it a solid-state fiber or disk laser or the CO2 gas variety. Welding, brazing, and surfacing applications can call for different laser types, including direct-diode, diode-pumped, flash lamp-pumped, and other iterations of YAG lasers. Still, that’s only half of the picture. The most powerful laser in the world can’t work without an effective, finely tuned beam delivery system.
Its design hinges on the application, but a few basic elements lay the groundwork. Together they provide myriad options to find the most effective way to carry and shape the beam on its way from the laser source to the workpiece.
Beams can be delivered and focused using a combination of transmissive (lens) or reflective (mirror) optics. Transmissive optical devices, the most common, are cooled indirectly, with water cooling close to where the optics are mounted. Reflective optics can handle higher heat levels, which is why many use them for high-powered applications. They can have copper substrates with machined channels just a few millimeters beneath the surface, allowing for direct water cooling (see Figure 1).
Transmissive optics work best for lasers up to 4 to 6 kW. Beyond this, reflective optics with copper substrates are much more capable because they result in much less thermal focus shift. Such shifts occur from heat buildup. Imagine a simple convex lens, thicker in the center than at the edges. When a laser passes through, the center of the lens will be warmer than the edges, because the center is where most of the laser energy is concentrated; in indirectly cooled systems, it also happens to be farthest from the cooling zone. This heating expands the optics slightly and so can actually move the focus point an appreciable amount. This thermal shift occurs over a few minutes.
Direct-cooled optics, on the other hand, have only a tenth of the thermal shift, and it occurs in mill-iseconds instead of minutes. The direct cooling, which minimizes the change in temperature at the optics, makes this possible.
Between the laser source and focusing head, industrial laser beams may be delivered via reflective optics or a fiber optic. The delivery method depends on the laser type and power involved. CO2 lasers generally require a series of reflective optics, while many solid-state lasers—including disk, fiber, as well as diode-pumped and flash lamp-pumped YAG varieties—may travel through a delivery fiber optic to the focusing head.
Just because a laser is solid-state doesn’t automatically make it deliverable through a fiber for all applications. For instance, certain high-peak-power YAG lasers used for drilling applications in the aerospace market employ standard mirrors to deliver the beam, like gas lasers. The beam energy and peak power are just too high for conventional fiber-optic delivery (see Figure 2).
For lasers using reflective optics, several elements help deliver the beam to the focusing head (or heads). A beam bender uses cooled mirrors to direct the laser beam 90 degrees from its original path. It also adjusts slightly to align the beam just right for the next stage of travel.
A beam shuttle essentially is a moving mirror. When introduced to the beam, it sends the laser beam down one of several paths. In a two-path system, a beam shuttle is placed above the beam path. To change the path, the beam shuttle moves in to intercept the beam, sending it down an alternative path. When the shuttle retracts again, the beam returns to its previous path. A system could have many beam shuttles, though only up to three is typical. Beams may be sent to various workspaces or, in some cases, a diagnostic device.
Shuttles and benders can’t send a beam to several places simultaneously. In other words, they don’t split the beam, only change its direction. To send a gas laser beam to several areas at once, beam splitters play a role. These entail an optic that’s only 50 percent reflective, roughly similar to a two-way mirror or mirrored sunglasses. Such an optic has special coatings that allow half the laser beam power to pass through, while reflecting the remaining beam power in a new direction. This means 50 percent of the beam power gets reflected down one beam path, and 50 percent gets reflected down another.
Many use beam splitters to process components generally half a meter apart or less. The splitter allows one laser power source to send energy to two focusing heads, which often process identical parts. The setup is cost-effective because a laser with twice the power is not twice as expensive. In other words, splitting one laser power source can be less expensive than buying two complete lasers. Such twin setups essentially can share elements of the same motion system, which also is less expensive than two separate systems.
In fiber optic-delivered lasers, most beam splitting occurs at the power source, when the beam energy enters the delivery fibers. This greatly simplifies sending beam energy to multiple work areas.
Fiber-delivered lasers require collimating. This takes the beam energy from the delivery fiber and changes it into a beam size—anywhere from 5 to 45 mm in diameter—that can be directed and focused with lenses and mirrors (see Figure 3).
A beam from a delivery fiber emerges as a cone of light that will continue to expand until it hits something that will collimate it to a parallel beam. The farther the collimator’s optic is from the delivery fiber, the wider the resulting parallel beam will be. The collimating optic is located at its focal point. If the optic has a focal length of 100 mm, it must be positioned 100 mm away from where the beam emerges from the delivery fiber.
Typical beam collimator focal lengths are 25 to 200 mm. High-powered lasers usually benefit from collimators with long focal lengths, because this allows for the power density to spread, putting less concentrated heat through the optics and extending their life.
In fiber-delivered systems, a simple formula gives an idea of the ultimate spot size that ends up on the workpiece: Spot diameter = Delivery fiber diameter x (Focus optics focal length / Collimator optics focal length). This is why a longer-focal-length collimator not only means that the collimated beam diameter will be larger, but also that the final focus spot size on the workpiece will be smaller (see Figure 4).
Lasers with high beam quality and small fiber diameters require collimators with short focal lengths. This is because these beams have very low divergence; they spread only slightly when they emerge from the delivery fiber. It doesn’t take a long focal distance to collimate it to parallel, and then focus it back down to the desired focus spot. In a typical multimode fiber laser, as now used in various machines that cut flat sheets, the beam that emerges from the delivery fiber may have a diameter between 20 and 45 mm. Single-mode lasers—used most often for low-power applications like microwelding—might have a beam diameter of only 5 or 10 mm. (Note some laser basics: A “10-micron” beam produced by a CO2 laser and a “1-micron” beam from a fiber or disk laser refers to the laser light’s wavelength, not its beam diameter.)
Finding and maintaining the optimal focus spot size and shape depend on the application, but all lasers use some common tools to attain the spot and keep the focus as consistent as possible throughout the processing cycle.
Autofocusing Head. In this setup, the focusing head adjusts based on feedback from either a contact or noncontact workpiece sensor. In the noncontact arrangement, the gas nozzle acts as a capacitive sensor to measure the gap between it and the conductive workpiece below. This signal then directs a servo system that adjusts the head’s height to maintain optimal focus, even if the workpiece surface height varies unexpectedly. For nonconductive material, a traditional contact sensing system, including a boom with a rollerball on the end, may be used.
To perform these adjustments, a laser cutting machine may move the head in Z. Or a focusing head may have a self-contained Z-axis adjustment, which works well with certain applications, such as heads mounted on robots, which may not be able to react fast enough to offer in-process height adjustment.
Trepanners. These optics send the beam in a circle to cut precise holes very quickly. An optical trepanner has a pair of wedge-shaped optics separated by a small distance. Rotating one of those wedges creates an offset away from the beam centerline, moving the focus spot rapidly. By adjusting those wedge positions and rotating them together, the system creates a circular trepan motion of variable radii and speeds (see Figure 5).
Another trepan strategy uses servomotor-driven mirrors to change the length of the beam path so that its focal point moves in a circle. Think of the beam cross section as the wheel on an old-fashioned steam engine, and the focal point as the rod linkage point on the wheel’s outside diameter. The beam position itself doesn’t move left to right or up and down. Instead, shifting the beam-path length causes one axis of the beam to change rapidly, allowing the focal point to travel quickly in a circle.
Scanners and Galvo Mirrors. Placed just before or after a focus lens, these servo-driven mirrors direct the laser beam quickly from point to point. They are used especially in remote welding applications, in which the focus lens can be a meter or more away from the workpiece surface.
Nozzles. Literally hundreds of different kinds of nozzles are available (see Figure 6). To select the best one requires weighing many variables, including part features, the operation at hand (cutting, welding, cladding, heat treating, and so on), part and material features, as well as the laser’s wavelength. Nozzle selection really is a science unto itself.
For processing an object with mechanical obstructions and other restrictions to work area access, a long, skinny nozzle might suit. Flat-sheet cutting with capacitive height sensing may require a short, stubby nozzle. Still, the more long and slender, the more mechanically fragile nozzles can be. Orifice diameter also changes with the parameters. For many applications, machined nozzles suffice, but critical laser processing may benefit from cold-formed nozzles, which offer a smooth, mirrorlike finish.
Material type and thickness also play a role. For instance, in an application involving metal foil, a laser system may benefit from a shower nozzle, in which small gas-jet holes surround a central orifice. This distributes the assist gas around the circumference of the nozzle and prevents that thin sheet of foil from fluttering during laser operation.
Through-the-Lens Viewing. For years high-powered laser systems have used a visible infrared laser to help the operator focus the beam. Unfortunately, these visible lasers focus in a different plane than the actual processing laser. So for precision work, which may require precise focusing as well as general process monitoring, many have used through-the-lens viewing. Cameras mounted above the beam path can be set so their focal point is the same as the laser beam’s. With these systems, what you see is really what you get.
Lens Drawers. A focusing head can have cartridges—or lens drawers—that accept lenses of different focal lengths. A typical system for laser cutting might have a cartridge with a 5-in.-focal-length lens and, just above, another cartridge with a 7.5-in.-focal-length lens. Such two-lens systems allow for quick lens changeover. The nozzle will have the same standoff from the workpiece, even after a lens change. Because of the drawer positions, no further focusing head adjustments should be necessary (see Figure 7).
Crash Protection and Avoidance. Crash protection systems basically act as an interlock. When broken, the system shuts down, saving the machine from more serious damage. These can entail mechanical switches, proximity switches, pneumatics, and other tools. Crash protection simply prevents the destruction of the focusing head.
They also can save a large, expensive workpiece. Say a laser cuts a large piece of sheet and crashes halfway through the operation. Some crash protection devices allow the operator to reset the head and start again at a certain point, so that the entire workpiece isn’t lost to the scrap pile.
Certain systems also offer crash avoidance. Using the same sensing tools as the autofocusing function, crash avoidance technology continually sends signals back to the controller. If a workpiece surface isn’t where it should be, it sends an alert, attempting to stop a crash before it happens.
There’s more to the laser than its type. Whether you are using a CO2, fiber, disk, flash-lamp or diode-pumped YAG, or anything else, choosing the proper beam delivery and focusing system can be vital. If not delivered and focused properly for the application, no laser beam—no matter its quality—can be truly effective.
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