Handling the Heat

Designing optics for high laser powers


Cutting lasers with high laser power output have some obvious benefits. They allow fabricators to use the high precision and cutting quality of the laser beam for processing a thicker range of materials. Additionally, having a lot more power available often means a lot more cutting speed capability, or in other words, many more finished parts per hour coming from the laser cutting machine.

Very high laser powers also come with some challenges for machine builders, though. When people talk about high-power lasers delivering light energy at densities millions of times higher than the sunlight hitting one’s face outside on a hot day, it’s not an exaggeration. Taking many thousands of watts of infrared energy and concentrating it into an area a couple of hair thicknesses wide means handling a tremendous amount of heat, and to build a quality laser cutting machine means handling that heat consistently and reliably over time.

Effective cooling starts at the source, the literal laser source that is. Generating massive amounts of useful energy means generating a lot of waste heat, as well. The ability to liquid cool the internal components of the laser source is incredibly important for stable operation at high power output levels.

Some laser designs handle this heat load a lot better than others; the disk laser design, in particular, allows for very efficient direct cooling of all the internal laser components with a closed-circuit liquid chiller, ensuring continuously stable temperatures regardless of the operating mode required by the machine user.

Even the base plate on which the optics of TRUMPF’s TruDisk laser are mounted is comprised of a large mass of high-quality aluminum that is directly liquid-cooled, doing double-duty as a precision alignment surface and a high-capacity heat sink that removes heat load from the optics. Excellent cooling characteristics mean the disk laser source can scale to high power outputs without suffering wear and tear over time due to heat loading.

Fast delivery

After generating a multi-kilowatt laser beam, it has to be delivered from the laser source to the cutting head mounted on the machine’s motion unit. The motion unit moves the cutting head through the working range of the machine, and the cutting head does the work of shaping and focusing the raw laser beam coming from the laser source into a useful cutting tool.

Ultra-high-power lasers can handle thick cuts, but it requires a machine that can handle the high heat consistently and reliably over time.

Traversing the span between the output of the laser source and the input on the cutting head is the beam delivery system. In the case of fiber-guided lasers, the beam delivery system consists of a glass fiber surrounded by a reflective cladding that can carry and direct near-infrared wavelength light along its length. From the outside, this beam delivery fiber takes the shape of a flexible cable with a coupling at each end, the points where it connects to the laser source and cutting head.

The TruDisk laser source utilizes liquid cooling directly at the coupling points. However, the core of the delivery fiber itself cannot be directly liquid-cooled along most of its length. The glass core of the laser delivery cable can be as small as 100 microns, allowing the equipment operator to deliver that laser power as an extremely fine cutting tool at the workpiece, yet also meaning that very high energy densities are being achieved at a point along the optical chain that cannot be directly liquid-cooled.

At Fabtech 2022 in Atlanta, TRUMPF demonstrated the high-quality thick-material cutting its new 24-kW laser can accomplish.

To address this and ensure that the heat load on the beam delivery cable does not turn it into a wear point, TRUMPF uses a unique tapering design for the glass core of the delivery cable. At the ends of the cable where the liquid-cooled coupling points are, the glass fiber carrying the laser light is the final diameter required for optimal cutting conditions, but along the central span the glass fiber gently tapers to a larger diameter to reduce the energy density and, therefore, the heat loading on the delivery fiber.

Bring to a head

Being able to safely deliver the high-power laser beam to the machine is the final stage of laser management before all the hard work pays off in the form of value-added work. The cutting head is the final piece in the optical chain before the laser beam actually reaches the material to be processed.

The optical components inside the cutting head take the raw laser beam being delivered from the laser source and shape the beam into an actual cutting tool that can vaporize metal. Excessive heat loading at this stage can cause a premature component failure, leading to increased maintenance costs for the machine. In addition, changing heat loads on these optical components can cause the beam-shaping function of the cutting head to deviate, meaning the proper beam characteristics for the cutting process are not produced and loss of cut quality results.

To prevent the high energies the cutting head is managing from contributing to wear, TRUMPF uses multiple stages of liquid cooling applied directly at each optic. Gas cooling is used at the very bottom edge of the cutting head to help dissipate the reflected radiation from melting metal in the cutting process itself.

Watch the video to see the TruDisk 24001, TRUMPF’s ultra-high-power cutting machine, in action.

Finally, an array of sensors monitoring each optic ensures that smaller temperature variations occurring during typical operation do not cause the laser beam characteristics to drift, allowing the machine to automatically maintain cut quality. Engineered to handle the heat, all of these measures combined mean that TRUMPF’s TruLaser machines are ideally suited to operate at very high power output.


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