ANALYSIS: ULTRAFAST LASERS
“Provided the higher frequencies (MHz) can be utilised, a lower pulse energy system can outperform a higher pulse energy”
Figure 3: DeepCleave from Holo/Or enables thick cutting by giving a tight focus (less than 2µm) over a large depth of focus (over 1mm) – compared with a conventional axicon-generated Bessel beam
improving the overall efficiency of the system.
Pulse shape: Bessel beams & DOEs optimise energy delivery Beam shaping is particularly useful when processing glass. An axicon lens can be used to create a Bessel beam by refracting the incoming laser light to produce a series of overlapping annular beams that interfere constructively in the forward direction, resulting in a narrow beam over a longer focal distance.
One major consequence to using Bessel beams is that the peak intensity of the beam changes significantly through the focus, which results in an inefficient use of pulse energy and thinner processing widths. To overcome this limitation, a modified Bessel beam can be generated by using diffractive optical elements (DOEs), such as those made by Holo/Or, with a flatter energy intensity along the optical axis. This has been shown in Figure 3. This modified beam creates microcracks inside the glass, which are joined up in
alignment with the cut direction. The successive laser-induced cracks are close to each other, and an optimised process results in a polished surface instead of the striations seen when using a larger spot diameter or longer pulse duration (e.g. picoseconds). The optimised cut edge can withstand bending of the glass, unlike the rough edge of an unoptimised cut, which is much more susceptible to cracking under bending stresses. Other DOEs are used in
surface structuring where the pulse shape may be spatially more “top hat”, enabling smooth delivery of the energy to the surface without the excessive damage of the central peak in a gaussian beam profile. In this application too, there are advantages in having a small spot with a lower pulse energy and a higher repetition rate than using more energy and a lower frequency.
Burst mode: Ramping up process speed and quality Using bursts of pulses in a femtosecond laser allows for
a more efficient delivery of laser power to the material being processed. While each individual pulse removes less material, using three to five pulses in a burst (see Figure 4) can still deliver a greater volume of material removal per burst than a single pulse. Additionally, this technique provides more control over the material processing, resulting in less heat input and a smoother processing outcome. By implementing this method in glass processing, higher processing speeds and smoother edges have been achieved. The spacing of the pulses within the burst is also crucial, as spacing that is too narrow or too wide can lead to excessive energy input or more striations on the cut edge.
Wavelength: Size matters When processing with CW lasers or longer pulses (nanosecond to millisecond duration), it is evident that wavelength affects the laser material interaction more directly and that some materials can be processed much better
with shorter wavelengths. When moving to ultrashort laser pulses, there are other effects that come into play. A pulse of hundreds of femtoseconds will combine high peak power with short interaction times to process virtually all materials. The significance of wavelength is that a shorter wavelength (such as green or UV) can be focused to a smaller size. However, there are losses associated with the harmonic generation process, and when considering pure speed, infrared wavelengths can be more effective. Where ultra-small features are required (e.g. exceptionally fine holes), then such quality can be achieved effectively using green wavelengths (especially when processing copper) or UV wavelengths (e.g. when processing polymers).
Conclusions Investing in an ultrafast laser can pay dividends in throughput, either in ablation rate or in cutting speed. However, there are pitfalls to be avoided in the application realm. Too much pulse energy is not always helpful and provided the higher frequencies (MHz) can be utilised, a lower pulse energy system can outperform a higher pulse energy as long as the fluence is sufficient to process the material. Bursts and beam shaping can radically improve performance and are essential in some applications to achieve the optimum results. To avoid wasting energy, and to achieve the optimum throughput in an application, it is wise to consult with an experienced expert. When it comes to ultrafast laser processing, knowledge is power. EO
Figure 4: Improved ablation efficiency by lower peak energy delivered in short bursts at intervals of 25 to 50ns
www.electrooptics.com | @electrooptics
Dariusz Świerad is Sales and Marketing Director at Fluence, a Polish ultrafast laser manufacturer
May 2023 Electro Optics 13
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