Laser-induced breakdown spectroscopy (LIBS) technology has enormous potential for deep-sea in-situ detection due to its real-time, non-contact advantages. However, the high density and incompressibility of water result in underwater laser-induced plasma with high particle number density and low temperature, causing lower intensity and poor stability compared to gas environments. To improve the underwater detection capabilities of LIBS, using long-duration lasers can significantly enhance the emission line intensity, as proven by many studies. This improvement also positively influences the signal-to-noise ratio (SNR) and signal-to-background ratio (SBR) of LIBS signals. In this study, we analyze the LIBS signal quality generated on a submerged aluminum target, including line intensity, SNR, SBR, and relative standard deviation (RSD), under different laser durations (6?25 ns) and energies (8?25 mJ). This provides an experimental basis for optimizing laser parameters in underwater LIBS.

A Q-switched Nd∶YAG laser with a wavelength of 1064 nm is used as the ablation source. The laser operates at pulse durations of 6 ns to 25 ns by varying the laser flash-pump voltage, with a repetition rate of 0.2 Hz. The laser passes through a half-wave plate and a Glan prism to adjust the laser pulse energy from 8 mJ to 25 mJ, monitored by a photodiode connected to an oscilloscope. A dichroic mirror transmits the laser beam and reflects the plasma emission light. Then the laser beam is focused into a quartz cuvette filled with deionized water by a pair of quartz plano-convex lenses L1 (f =38.1 mm). The cuvette (80 mm×80 mm×80 mm) has a wall thickness of 2 mm. The target is mounted on an X-Y-Z stage and submerged in water, with the laser focal position set at 0.75 mm below the target surface. The diameter of the laser spot on the target surface is about 250 μm, based on the measured crater size. Pure metals of aluminum (99.9%) are used, polished, and cleaned with alcohol before experiments. A water pump refreshes the water in the cuvette to reduce the interference from metal nanoparticles sputtered during continuous laser ablations. Plasma emission is reflected by the dichroic mirror and collected backward by a plano-convex lens L2 (f =50.8 mm). The emission spectra are then recorded by a fiber spectrometer with a wavelength range from 300 nm to 800 nm and a spectral resolution of 0.3 nm. Timings between the laser and spectrometer are controlled by a digital delay generator.

The laser focal position is set as 0.75 mm below the target surface by analyzing the LIBS under different laser energies and durations (Fig. 2). The LIBS spectra are affected by the shielding effect of the plasma in water at high irradiance, and the target is hardly ionized to generate plasma at low irradiance. SNR is analyzed by calculating the ratio between the Al I line intensity and noise filtered from the LIBS signal (385?415 nm). According to the contour maps of SNR versus laser energy and duration [Fig. 3(a)], SNR increases with laser energy when the laser duration exceeds 12 ns. However, when the laser duration is less than 9 ns, high laser energy can lead to a decrease in SNR. Using the minimum filter and adaptive iteratively reweighted penalized least squares (airPLS), we accurately estimate the baseline of the LIBS signal, enabling the correct calculation of SBR. By analyzing the contour maps of SBR versus laser energy and duration [Fig. 3(b)], we find that laser duration is the main factor affecting SBR. An increase in laser duration significantly improves the SBR due to the reduced recombination radiation. After spectral preprocessing, the RSD of 40 LIBS spectra decreases, and the stability of LIBS spectra improves [Fig. 4(a)]. However, RSD remains high when the laser duration is less than 9 ns and laser energy exceeds 18 mJ, due to the randomness of multiple independent breakdown [Fig. 4(b)]. After spectral preprocessing, the quality of the LIBS signal improves. The best LIBS spectral quality is observed at an irradiance of around 3.18×10⁹ W/cm² and a laser pulse width greater than 14 ns. High irradiance near or above the breakdown threshold of the solution leads to multiple independent breakdowns, as evidenced by the shadowgraphs of cavitation bubbles and shock waves, which decrease the quality of the LIBS spectra (Fig. 5). Additionally, the ablation crater created by a 6 ns laser is deeper than that created by a 17 ns laser, indicating that longer duration lasers distribute more energy into plasma radiation (Fig. 6).

In this study, we analyze the effects of different laser durations (6?25 ns) and energies (8?25 mJ) on underwater LIBS spectra generated on submerged solid targets. The results show that both excessively high and low irradiances decrease the quality of LIBS spectra under different laser focus positions, fixed at 0.75 mm in our experiment. SNR increases by about 1.7 times with the increase of laser energy when the duration is over 12 ns. SBR mainly depends on laser duration and increases significantly with longer durations. The RSD of LIBS spectra decreases, and stability improves with longer durations. However, multiple independent breakdowns seriously degrade LIBS signal quality with durations less than 9 ns and energies over 18 mJ. Optimal LIBS signal quality is achieved at irradiance around 3.18×10⁹ W/cm² and pulse widths larger than 14 ns. High irradiance leads to multiple independent breakdowns, decreasing LIBS spectral intensity and stability. The 6 ns laser ablation crater is deeper, indicating less laser energy converts into spectral radiation. Therefore, for underwater LIBS detection, selecting a laser with a long duration and high energy improves detection capabilities and laser energy conversion efficiency.