Industrial use of femtosecond (fs) lasers for micromachining is increasing significantly, and the true potential of fs lasers is becoming evident. Today, fs lasers are used for a wide range of applications, including cutting and drilling of flat panel display glasses and films, cutting of implantable medical devices, ablation and scribing of solar cells, and surface structuring of various materials. Along with the drive for extremely high-quality processing, high process throughput is increasingly necessary, and as a result, there is a need for high power, shorter duration pulse rate femtosecond lasers with state-of-the art features. Today, fs lasers with output powers of >100 W at repetition rates of up to 10 MHz are available in the market.
When extremely high-quality machining is required, high power fs laser ablation has become a useful alternative to conventional non-laser processes such as milling, grinding and electrical discharge machining (EDM). However, the work of Neuenschwander et. al. has shown that there exists an optimal fluence for efficiently removing material and thereby lowering the thermal damage to the surrounding material1,2. They have demonstrated that this optimal fluence is ~ e2 times the threshold fluence, which for most of the materials is ~ 1 J/cm2. Taking this into account means high energy pulses can be efficiently used only by increasing the spot size and/or splitting the pulses into many pulses of smaller energy (burst pulses) or by increasing the number of spots (for parallel processing).
The Spectra-Physics Spirit high-energy industrial femtosecond laser.
The MKS Spectra-Physics Spirit 1030–100 high-power femtosecond laser, which is shown in figure 1, provides a wavelength of 1030 nm ±5 nm, a power of >100 W, a pulse energy of >100 µJ and a <400 fs pulse duration, with burst mode operation capability. In burst mode operation, each pulse can be split into several pulses, and the burst envelope can be shaped, i.e. the amplitude of each pulse within the burst envelope can be varied. Figure 2(a) shows burst pulses from a Spirit 1030-100 laser where the amplitude of the 5th pulse within the burst envelope is set to 0 percent, and figure 2(b) shows an example of burst shaping.
To investigate the effect of burst mode operation, MKS characterised the ablation rates and ablation efficiency in polycrystalline diamond (PCD), an ultrahard material, for a range of burst outputs from the Spirit 1030-100 laser. The experiment consisted of pocket milling volumetric regions in PCD, measuring the depth of the milled pockets and determining volume ablation rates and efficiencies. Variables included the number of pulses in the burst envelope and the average power (average pulse energy). The repetition rate was fixed to 1 MHz, and the spot size and scanning speed were kept constant with a pulse-to-pulse overlap of 50 percent.
Figure 3 shows the resulting dependence of volumetric ablation rate on the average power for single pulse, five-pulse burst and nine-pulse burst operation. The plot shows that at increasing power levels, burst mode operation results in enhanced ablation rates as would be needed to maintain an optimal fluence. At an average power of 100 W, a two-fold increase in ablation rate with a nine-pulse burst over that for a single pulse is observed. Figure 4 shows the normalised ablation efficiency versus average power for single pulse, five-pulse burst and nine-pulse burst operation. It can be seen that optimal ablation rates can be obtained at high average powers by increasing the number of pulses within the burst envelope. Results of the tests demonstrate the advantage of burst machining for enhancing material removal rate.
In summary, the Spectra-Physics Spirit 1030-100 laser has the ability to tailor pulse intensity in the time domain, and this approach has been proven to enable two-fold enhancement of material removal rates in PCD.
MKS ǀ Spectra-Physics
References
1Kramer, T., Zhang, Y., Remund, S. Jaeggi, B., Michalowski, A., Grad, L. and Neuenschwander, B. (2017). Increasing the specific removal rate for ultra short pulsed laser-micromachining by using pulse bursts. Journal of Laser Micro/Nanoengineering, volume 12, issue 2, pp.107–114.Available at: https://bit.ly/3nBlF3c2Beat Neuenschwander, B., Jaeggi, B., Foerster, D.J., Kramer, T. and Remund, S. (2019). Influence of the burst mode onto the specific removal rate for metals and semiconductors. Journal of Laser Applications, volume 31, issue 2, article no. 022203.
