Comparisons of laser-produced plasma in atmosphere between fiber-delivery and direct-focusing laser pulse

Highlights: • Fiber-delivery laser-produced plasma changes from a hemispherical to an oblate-cylinder structure in morphology. • Differences in plasma dynamics can be analyzed with different particle behavior obtained by spatial-resolved spectroscopy. • Comparison results indicate the peak irradiance of the focused laser also has a significant impact on plasma evolution. • The reason for differences in plasma evolution could be understood by energy deposition of laser absorption by plasma. -- Abstract: The evolution of the fiber-delivery laser-produced plasma induced by a single-lens laser probe and a double-lens laser probe were compared to the direct-focusing laser-produced plasma. It is found that the laser profile of the focused spot emitted by the fiber was uniform, showing a top-hat profile, with significant energy dispersion (standard deviation was increased to 1.2 from 0.76 and peak irradiance was decreased to 18.6 GW/cm² from 28.2 GW/cm²). Using the method of photography with an effective 3-ns exposure time, the fiber-delivery laser-produced plasma displayed obvious changes in plasma morphology from a hemispherical form to a flat hemispherical form, even an oblate-cylinder form. Via optical emission spectroscopy, it was obvious the duration of the N II emission was shortened by 50–120 ns, and the axial diffusion distance was also reduced by 0.2 mm. By combining shadow diagnostics and fast photography, it appeared clear that the shock wave was slowed down to 1.9–2.1 km/s, and was separated from the plasma plume with a longer delay of 500–600 ns. In addition, the fringe shift profiles recovered from Mach–Zehnder interferograms showed a lower electron density inside the plasma plume and a weaker degree of air disturbance at the shock wave position, consistent with the other diagnosis methods. These different evolutionary characteristics in the case of the fiber-delivery mode clearly indicated that the peak irradiance would also have a pronounced effect on the plasma process in addition to the average irradiance. This phenomenon could be related to the reduction in energy deposition of laser absorption by plasma for the fiber-delivery mode, and this reduction was caused by the decreased peak irradiance in the focused spot.