Laser-Induced Breakdown Spectroscopy by nJ Pulses at GHz Repetition Rate

Fiber lasers operating at GHz pulse repetition rate have shown exceptional performance in high-speed high-efficient material processing [1]. In laser-induced breakdown spectroscopy (LIBS), fiber lasers have attracted significant interest because of their advantages, including compactness, energy efficiency, stability, reliability, and cost-effectiveness. To the best of our knowledge, we employed for the first time a burst mode with GHz intraburst repetition rate pulses in laser-induced breakdown spectroscopy, reducing the required pulse energy by several orders of magnitude to the nJ level. We employed a custom-built Yb-doped 1040 nm fiber laser operating in burst mode with an intraburst repetition rate of 2.8 GHz and pulse energies ranging from 9 nJ to 196 nJ [2]. The system generates 40 ps pulses before compression and 650 fs pulses after compression, with a peak average power 14 W. LIBS experiments were carried out on stainless steel (SS) at a burst repetition rate of 100 kHz, using three different burst durations: 83 ns, 120 ns and 240 ns. We investigate the influences of burst duration and pulse energy on the optical emission spectrum of SS. As depicted in Fig.(a) it was observed that the blackbody-like spectrum which appeared while using high burst duration 240 ns, due to significant thermal effects, as most of the pulse energy is absorbed as heat by the target, is reduced when using the shorter burst durations. At GHz pulse repetition rate, this reduction in the ablation threshold enables materials to be ablated at much lower energy levels, generating a highly ionized plasma with intense emissions. As a result the thermal effects are minimized, leading to a spectrum with sharp, well-defined peaks. Subsequently, the impact of the laser pulse energy on the spectrum of SS was examined as displayed in Fig(b), at lower energies (26–57 nJ) the emission lines were relatively weak, and the signal to noise ratio (SNR) is also low. This was attributed to insufficient material ablation, as energy is near to ablation threshold, with most of it being used for plasma production. As the laser energy increases (57–196 nJ) the intensity of the emission lines increases significantly due to more effective material ablation and enhanced plasma excitation. Moreover, the SNR improves and the continuum background intensifies as the plasma temperature as the plasma temperature increases at higher energies, causing bremsstrahlung radiations to become dominant. In the second part of this study we focused on calculating the electron temperature and density to investigate how they vary with different burst durations and laser pulse energies. The Boltzmann method was used to calculate the electron temperature <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$T_{\mathrm{e}}$</tex>, while the Stark broadened profile of the 537.14 Fe I line was analyzed to estimate the electron density <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$n_{\mathrm{e}}$</tex>. At a burst energy 45.5 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{J}$</tex> the electron temperature was measured as 1812 K, 1600 K and 1514 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{K}$</tex> for burst duration 83 ns, 120 ns and 240 ns respectively. Similarly, the corresponding electron densities were found to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.959 \times 10^{17}, 1.874 \times 10^{17}$</tex>, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.386\times 10^{17}\text{cm}^{-3}$</tex>. It was concluded that the electron temperature and density rise with a decrease in the burst durations as shown in Fig.(c)&(d).