Role of gas environment in the process of deep-hole drilling by ultrashort laser pulses

Abstract

Fax: (0711) 6856842The detailed study of the role of air pressure in deep hole drilling by femtosecond and picosecond intense laser pulses (Ti:A1203 and Nd:YAP lasers) was performed in the range 1÷1000 mBar. Steel sample plates were mostly tested, experimental data obtained for ceramic materials is also presented. The following ablation parameters were measured and analyzed: ablation rates and their dependence on the channel depth, ablated crater morphology, optical transmission in channels after through hole formation. Both percussion and helical drilling regimes were used.Special attention was paid to two strong gas assisted effects typical of sub-picosecond and subnanosecond material ablation, which are low threshold gas breakdown in deep channels and nonlinear interaction of ultra-short intense pulses with air resulting in conical emission. Unwanted aspects of both phenomena were shown to disappear in a moderate vacuum of lOO mBar. A new approach to formation of such a vacuum in drilled channels was also proposed and experimentally modeled using ultra-high repetition rate nanosecond laser pulses.Keywords: laser ablation in air and vacuum, ultra-short pulses, deep channels, air breakdown, white light continuum generation, ultra-high repetition rate laser exposure

Percussion Drilling Hole in Cu, Al, Ti and Ni Alloys Using Ultra-Short Pulsed Laser Ablation

“…Further, as the number of pulses increased at the same laser energy, molten material around the top surface of the hole decreased, and in some cases, disappeared. Between tens and thousands of pulses, for example 50 and 1000 pulses, shown in Figure 6c,d, resulted in irregularly shaped holes in Ti alloy and Ni alloy induced by conical emission (CE) [29]. The same structure was reported for other metals machined by ultrashort pulsed laser ablation, such as SS304 [30] and steel [31].…”

Peculiarity of metal drilling with a commercial femtosecond laser

“…In vacuum, plasma production is greatly reduced and shielding does not occur, so for the fundamental pulse with pedestal the drill rate in vacuum is larger than in air. The existence of plasma shielding for longer duration pulses in air and its absence in vacuum has been demonstrated by others [18]. Here it is worth mentioning that estimates using the Keldysh formula show that the multiphoton ionization produced by the femtosecond component of the laser pulse cannot create dense plasma.…”

Initiation of air ionization by ultrashort laser pulses: evidence for a role of metastable-state air molecules

“…The time-integrated photodiode signal was calibrated as a function of the incident pulse energy in the absence of any absorption effects under vacuum conditions. In the experiment we used either a narrow bandpass interference filter (800 nm central wavelength, 10 nm FWHM) or a longpass filter (>600 nm) and obtained identical results that indicated negligible spectral broadening of the incident radiation observed for higher laser intensities [12,14,27]. Note that the used experimental scheme with analysis of the transmission of the central beam fraction is more sensitive to plasma initiation than measuring the total transmitted pulse energy, especially near the air breakdown threshold, due to stronger attenuation of the most intense central part of the beam [6,8,28,29].…”

“…When a femtosecond laser pulse propagates in air, many nonlinear optical effects, such as multiphoton ionization (MPI), tunnel ionization (TI), optical Kerr effect, Raman scattering, white-light generation, and conical off-axis emission, come into play to induce air breakdown, thus affecting the pulse properties [11][12][13][14]. The formation of laser-induced filaments, enabling the beam to propagate over extended distances is mainly due to the competitive action of MPI and selffocusing governed by the Kerr effect [12].…”