Michael Laha Michael Laha
Senior Product Line Manager

Ultra-High-Precision Laser Cutting—Pulse Length Matters

March 4, 2020


Traditional long-pulse and continuous-wave fiber lasers have come to dominate most metal-cutting applications over the past decade because they offer a great combination of cost characteristics, reliability and ease of use. But, for high-precision cutting applications, such as found in medical-product manufacturing, microelectronics fabrication and, increasingly, automotive production, they can’t always deliver the required finesse. In these applications, pulsed, diode-pumped, solid-state (DPSS) lasers frequently are the best choice. They come in a variety of pulse lengths, and at wavelengths in the infrared, visible and ultraviolet. Here we’ll provide some guidelines that explain when the cost of a DPSS laser is justified, and how to choose the best source for a particular job.  

For Feature Production on the Nanometer Scale

laser-cutting-Coherent-pulse-length-peak-powerFor the purposes of this discussion, precision cutting or microstructuring refers to the production of features in a material on the scale of 25 µm or less. Depending upon substrate thickness, a common aspect ratio is 10:1. These scale features are produced on metal substrates from 1 mm thick to thin foils of less than 10 µm thick.

Such precise applications typically require three main considerations:

  1. The ability to produce the desired feature size to begin with. 
  2. Cut-surface quality, i.e. the surface roughness and the production of debris or recast material. The ideal cut has straight (no taper), smooth sidewalls. Cut-surface quality can affect part functionality and also impacts cost should post-processing or rework be needed to smooth and edge, or remove debris.
  3. Controlling the heat input into the part, specifically to minimize the heat-affected zone (HAZ). The cutting process should not affect the bulk material properties, or surrounding features, in an undesirable way.

Numerous factors impact the considerations just mentioned. Some are external to the laser, such as the effects of an assist gas, the characteristics of the focusing optics, or the scan speed of the laser beam relative to the workpiece. In terms of the laser itself, three key laser parameters directly impact cut precision, quality and the HAZ: laser wavelength, power and pulse length.

Wavelength

While every material has its own unique absorption spectrum, in general, all metals reflect relatively well in the infrared, and absorb more strongly at shorter wavelengths. As a reminder, infrared (IR) light has a longer wavelength than visible light, which is, in turn, longer in wavelength than ultraviolet (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. 

laser-cutting-Coherent-photothermal-HAZ-photoablationAnother important factor in wavelength: 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 special 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 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 constant 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 energy × 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 the pulse energy be too high, not all of the laser energy is used efficiently for material removal, and bulk heating and the resulting HAZ again will occur. There’s typically a sweet-spot maximum material-removal rate for most metals that occurs when laser peak power reaches seven to ten times the material-ablation threshold (Fig. 1). 

Once the optimum pulse energy is determined, throughput (material-removal rate) may be increased by increasing the pulse-repetition rate. However, this action impacts other aspects of system operation, in particular the scanning speed or table speed. It’s usually necessary to have the laser beam move rapidly enough across the work surface to avoid a significant amount of pulse overlap. However, when operating at the optimum material-removal rate doesn’t produce a cut at the necessary depth,  multiple passes or some pulse overlap may be necessary. 

Pulse Length

laser-cutting-Coherent-stainless-steel-IR-UV-wavelengthLonger-pulse commercial DPSS lasers commonly employ a pulsing technology called “q-switching,” which delivers pulse lengths in the nanosecond range (10 to 9 seconds or a billionth of a second). But, these lasers can be designed to produce pulse lengths in the picosecond (10 to 12 seconds) or even femtosecond (10 to 15 seconds) range. 

Generally, shorter pulse lengths deliver a smaller HAZ. Furthermore, the impact of decreased pulsewidth generally is much more dramatic than that of decreased wavelength for a couple of reasons.

First, nanosecond pulse lengths tend to remove material by thermal meansheating the material until it boils off. But, at shorter pulse lengths (especially below 1 picosecond), another mechanism called photoablation starts to occur. 

In photoablation, the use of very short pulse lengths produces very high peak fluences that can directly break the molecular or atomic bonds holding the material together, rather than simply heating it. Plus, with the material exposed to the laser energy for such a short time, the energy can’t be carried beyond the area of impact, thus the surrounding area stays cold. Energy remaining after the bond-breaking process is carried away with the expelled particles. Together, these effects result in an inherently colder process with a significantly reduced HAZ (Figs. 2 and 3). Plus, as a very clean process, photoablation leaves no recast material, thereby eliminating the need for elaborate post-processing.

But, making DPSS lasers operate at shorter pulse lengths (called ultrashort pulse, or USP) requires more system complexity, and, therefore, greater cost. Thus, cost/part versus feature necessity should be weighed. With this in mind, USP lasers generally are reserved for the most demanding tasks where precision is absolutely critical. USP lasers also enable machining of traditionally difficult materials such as glass, ceramics, sapphire and diamond.

Help With Particular Applications

Most manufacturers seek to achieve the most cost-effective process. Besides laser purchase and operating costs, many other factors play into this, including throughput, scrap and rework rates; and consumables costs (e.g. assist gas). It’s a relatively large parameter space to explore, making assistance from the laser supplier or systems integrator a practical matter. Specifically, it can be helpful to partner with a supplier for some applications development, particularly on the workpiece material. Ideally, the supplier should have a variety of different laser sources available in order to determine a best choice for a given application. In the best case scenario, the supplier may even develop an optimum process recipe for the specific application need. MF

Author’s Note: An excellent treatment of this topic can be found in Fundamentals of Laser MicroMachining, a book by Ronald Schaeffer, Ph.D. The author wishes to thank Dr. Schaeffer for his assistance in the preparation of this article.  

Industry-Related Terms: Edge, Case, Continuous Wave, Functionality, LASER, Scale, Scrap, Spectrum, Substrate, Surface, Thickness
View Glossary of Metalforming Terms

 

See also: Coherent, Inc.

Technologies: Cutting

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