Signal-to-noise ratio improvements in microwave-assisted laser-induced breakdown spectroscopy

Abstract

A microwave-assisted laser-induced breakdown spectroscopy (MA-LIBS) was developed to enhance the signal-to-noise ratio (SNR) measurements of alumina oxide plates. A semiconductor-based 2.45 GHz microwave device was coupled to a semiconductor 1064 nm laser source using a helical antenna, which replaced the larger-size capacitor-like antenna. This work was motivated by the applications of laser-induced breakdown spectroscopy (LIBS) in decommissioning of Fukushima Daiichi Nuclear Power Station (FDNPS). When using a 50-meter optical fiber from the main source into the nuclear vessel, the LIBS laser energy was expected to decrease significantly. Using the low laser energy of equal to or less than 2.0 mJ, the MA-LIBS significantly enhanced the excitation of the neutrals of Aluminum (Al) and the background signals contained no continuum emissions. Significant increase of the emission intensities of Al I with the addition of microwave energy was observed which also increased the full-width-half-maximum (FWHM). The correlation of the laser and microwave parameters and the SNR were then analyzed. Results show the logarithmic correlation of the microwave pulse width and microwave energy to the intensity enhancement and SNR which is attributed to the absorption of the microwave energy. This correlation is consistent with variations of laser energy density, laser pulse number, and laser spot diameter. Using a delayed acquisition time, the emission intensities of Al I were only observed with the addition of microwaves. At the experimental threshold of laser breakdown energy using 0.3 mJ, the emission intensity of Al I was only observed with the addition of microwaves. Attempts to duplicate the same SNR measurements using the stainless steel and lead targets were also done. The microwave successfully enhanced the intensity emission and SNR of the Al I, Cr I, and Pb I excited species.