Page 29 - MetalForming April 2020

unique absorption spectrum, in gen- eral, all metals reflect relatively well in the infrared, and absorb more strongly at shorter wavelengths. As a reminder, infrared (IR) light has a longer wave- length than visible light, which is, in turn, longer in wavelength than ultra- violet (UV ) light. So, cutting efficiency typically is worst in the IR spectrum, and increases at shorter wavelengths. Plus, the strong absorption of UV light means that it doesn’t penetrate far into the material, which minimizes the HAZ.
Another important factor in wave- length: Shorter-wavelength lasers can be focused to smaller spot sizes (the equivalent to using a smaller-diameter drill or narrower saw blade). This further enables the production of finer features.
But, guess what? Pulsed DPSS lasers all produce IR light, and getting shorter wavelengths out of them involves spe- cial optics that increase cost and reduce output power. So, in general use the longest wavelength that gets the job done because higher power is more

Photothermal Interaction
readily available, and the cost per watt is lower.
Power
DPSS lasers for materials processing virtually always are used in a pulsed mode, rather than in a continuous wave where laser output remains con- stant over time. The laser outputs a series of light bursts, and pulse length refers to how long each burst lasts.
For pulsed lasers, the term power really refers to the combination of pulse energy and repetition rate (pulse ener- gy × repetition rate = average power). For a given pulse energy, decreasing the pulse length, or pulse duration, increases the peak power.
Each material features an ablation threshold—a certain minimum pulse energy required to produce material removal, rather than just bulk heating. If the pulse energy isn’t higher than the ablation threshold for the material, the workpiece will be heated but no material removal will occur. Should
Picosecond- Pulse-width Laser Beam
No Signifcant Microcracks or Surface Debris
Recast Material
Atomized Vapor
Minimal Heat Affected Zone
Fig. 2—This graphic schematically sum- marizes the benefits of material removal via photoablation, possible with ultra- short-pulse (USP) lasers, versus photo- thermal material removal, which occurs using nanosecond-regime lasers.
Fabrication: Laser Cutting
Microcracks
Heat Affected Zone
Nanosecond- Pulse-width Laser Beam
Surface Debris
Photoablation

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