![]() A schematic diagram of the experimental setup is shown in Fig. To generate a variable time delay between two pulses, a setup resembling a Michelson interferometer was used by employing a pair of ultrafast P-polarized beam splitters (680 – 1080 nm), which can split the P-polarized output beam in 50/50. Through an optical parametric amplifier (Light Conversion), the laser wavelength can be tuned from 290 to 2600 nm. The femtosecond laser (Spectra-Physics) used in the experiments was polarized linearly, with a central wavelength of 800 nm, pulse duration of 50 fs, and repetition rate of 1 kHz. As a consequence, the drilling efficiency has improved and the aspect ratio of the microholes in PMMA has increased. A visible femtosecond double-pulse microdrilling method is proposed to manipulate the photon – electron interaction process during the ablation and to achieve free-electron-density adjustments. In this paper, the wavelength of a femtosecond laser is tuned to the visible-light zone by a commercial- ized optical parametric amplification (OPA) system, combined with a self-built, double-pulse setup where the time delay between two pulses can also be tuned at the same time. It further changes the corresponding material properties and photon absorption process. In fact, except for the subpulse number and pulse delay, adjusting other parameters, such as the wavelength and pulse energy in a pulse train, can also control the free- electron density. By using a pair of pulses with controlled separation, the machining precision and quality were enhanced effectively. ![]() Recently, the feasibility of free- electron-density adjustments by manipulating the temporal or spatial distribution of femtosecond lasers has been proved by theoretical and experimental works. This offers new possibilities for controlling transient localized material properties and corresponding phase-change mechanisms. A femtosecond pulse duration is shorter than many physical characteristic time periods, which makes it possible to manipulate electron dynamics, such as excitations, ionizations, recombinations, densities, and the temperature of electrons. Hence, a smaller feature size, greater spatial resolution, and better aspect ratio can be achieved. As a result, the heat- affected zone, where melting and solidification occur, is reduced significantly. A femtosecond laser is capable of depositing energy into a material in a very short time period before thermal diffusion. This disadvantage makes the results of drilling unacceptable for some high-precision applications, for example, fuel injectors, spinnerets, in- jection nozzles, etc. By reducing beam divergence and improving beam homogeneity, nanosecond UV lasers can be used to fabricate deep holes, but the quality of the micro- structures appears to deteriorate with increasing aspect ratio. Pulsed lasers have an advantage in drilling microholes because of their con- trollable pulse durations and repetition rates. ![]() A variety of lasers have been used for microhole drilling studies, including both continuous-wave and pulsed lasers with pulse durations in nano- to femtosecond ranges. temperature, which make it suitable for a microhole drilling study.
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