Dual-wavelength double-pulse laser machining

Bill Johnson, Andrew Forsman, and Steve Benda

Double-pulse technique can yield a significant advantage in speed and power when implemented using multiple wavelengths

M aking small holes and other structures in metals, plastics, and ceramics is a topic of wide-ranging interest.¹ Double-pulse laser machining (also known as SuperPulse) is a method that significantly improves speed and quality and has enabled the production of straight, clean holes as small as 5 µm in diameter and with an aspect ratio of 30:1 using nanosecond lasers and 532-nm laser light.^2,3

Double-pulse laser machining uses pairs of laser pulses that are timed so that some of the ablation products formed by the first laser pulse in each pair absorb the second laser pulse in each pair (see Fig. 1). The result is a modified laser-ablation process that is more efficient than direct laser ablation and that can reduce the deleterious effects of hole occlusion.⁴ The second laser pulse in each pair does not strike the workpiece directly but is absorbed by ablation products created by the first laser pulse in each pair.

This distinction is critical because the double-pulse format can be implemented using different wavelengths of laser light for the primary and secondary laser pulses. Furthermore, it is the properties of the ablation products that are targeted by the secondary laser pulses that determine the absorption efficiency of the secondary laser pulse. This can be combined to place another tool at the disposal of the laser process designer: using short-wavelength primary pulses and long-wavelength secondary pulses to process materials that are normally only susceptible to the short wavelength, but with less laser input power, greater speed, and better quality.

FIGURE 1. Schematic depicts the dual-wavelength double-pulse format.

Using double pulses
As a test, we combined second and third harmonics of two Nd:YAG lasers, whose firing is synchronized so that the 355-nm laser pulse strikes the workpiece 80 ns before the 532-nm laser pulse for each firing cycle of the lasers. Because each laser fires 10,000 shots/s, the 355/532-nm double-pulse combination strikes the workpiece 10,000 times each second.

The laser light is focused in a ~40-µm diameter spot on the workpiece by a 5-in. plano-convex lens. The time required to drill through the target material was measured using a photodiode to detect laser transmission through the drilled hole. This time is used to calculate the average material-removal rate and the energy required to make the hole and pierce the target material.

For targets opaque to the laser light, a photodiode placed behind the target detected the transmission of light through the target when it was pierced. On the other hand, when a target that was not normally completely opaque to the laser light was used, the photodiode was not reliable. When this was the case, the duration of the laser drilling process was varied until microscopic examination just barely revealed a hole on the back surface of the target.

Because the previously published work that established the enhancement in material-removal rate due to the double pulse technique used only 532-nm laser light, the use of a dual-wavelength double-pulse technique was first tested by drilling 0.5-mm-thick 304 stainless-steel targets.^2,3 For the 4-ns duration laser pulses used in the present work, the basic material-removal capability in a single laser pulse is increased slightly by using 355-nm light instead of 532-nm laser light (see Fig. 2).

FIGURE 2. Dual-wavelength double-pulse drilling of 0.5-mm-thick stainless steel. The solid lines show the energy required to pierce the target, for the double-pulse format and for each constituent wavelength separately. The dashed lines show the average material removal rates. It should be noted that, due to the 3W (300 mJ/pulse) constraint in the power of the 355-nm primary laser pulse, the ratio of the energies of the primary and secondary pulse in each double-pulse pair depended on the total power on the target. When drilling the stainless-steel target, the 355-nm primary pulse had the same energy as the 532-nm secondary laser pulse up to a total power of 6W.

However, using a dual-wavelength double-pulse technique dramatically increases the drilling speed and reduces the total energy required to drill a hole in the stainless-steel target. This verifies that the double-pulse technique can yield a significant advantage in speed and power requirements when it is implemented using multiple wavelengths.

Then, as an example of the use of dual-wavelength double-pulse machining in a plastic that is somewhat more opaque to 355-nm light than to 532-nm light, we chose a 1-mm-thick sample of polyethylene. This material was visibly more opaque to 355-nm laser light than to 532-nm laser light (see Fig. 3).

Clearly, the use of a dual-wavelength double-pulse format allows the drilling of the polyethylene with less energy and at higher rates, and this was done while not being forced to frequency-triple more than a minority of the laser energy to 532 nm. Furthermore, we were also able to use 532-nm laser light within the double-pulse format at levels that caused significant HAZ when the 532-nm laser light was applied by itself in the conventional single pulse format (see Figs. 3 and 4).

Other laser combinations

The double-pulse technique was implemented using a 532-nm wavelength primary pulse and a 1064-nm secondary pulse for drilling 50-µm-diameter holes through 1-mm-thick stainless steel. The result was that, by increasing the total power by 60%, the drilling speed more than tripled. In this test, the 532-nm laser light and the 1064-nm laser light were focused on the target by separate lens systems, and the beams were combined after their respective focusing lenses by a dichroic mirror.

This approach to simultaneously using multiple wavelengths to drill small, high-aspect-ratio holes also illustrates an important practical aspect to this technique-the focal spots produced by each beam that is combined to apply the double-pulse technique need good overlap. Our experience has been that optimal results are obtained when the spot overlap is at least 90%. It should also be noted that in each double-pulse pair, while the timing between the primary and secondary laser pulses is significant on the timescales of laser ablation and plasma dissipation, it is not significant on the timescales associated with the movements of galvo scanners and trepanning units.

There are other laser combinations that we have not tested but that we see no reason to avoid. For example, 266- or 213-nm light (used as the primary pulse) and 355-nm (used as the secondary pulse) light may be useful in processing certain plastics that are poor absorbers, even at 355 nm.

FIGURE 4. Entrance and exit holes for double-pulse (top) 8W and high-intensity (bottom) 10W 532-nm conventional technique.

The double-pulse method has been shown to work when applied using multiple wavelengths, enhancing material-removal rates in the percussion drilling of stainless steel by several times. Moreover, the dual-wavelength implementation of the double-pulse technique has shown a significant increase in drilling speed in plastic, as well as the ability to productively use a longer wavelength, which would ordinarily lead to immense HAZ, to even further increase the ultimate drilling speed.

Bill Johnson, Andrew Forsman, and Steve Benda are with General Atomics-Photonics Division (www.galasers.com), San Diego, CA, USA. Steve Benda can be contacted at [email protected].

References

1. H. Rohde in LIA Handbook of Laser Materials Processing, J. F. Ready, ed., 474 (2001); M. D. Perry et al., in LIA Handbook of Laser Materials Processing, J. F. Ready, ed., 499 (2001); LIA Handbook of Laser Materials Processing, J. F. Ready, ed., chapters 12 and 13 (2001).
2. A. Forsman et al., “Double-pulse laser machining as a technique for the enhancement of material removal rates in laser machining of metals,” J. Appl. Phys. 98, 033302-1 (2005).
3. A. Forsman et al., “Double-pulse format for improved laser drilling,” Photonics Spectra, 72 (September 2007).
4. D. Bäuerle, Laser Processing and Chemistry, Springer Berlin Heidelberg, 244 (2000).