Zhibo Ni

Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS) with Standard Reference Line for the Analysis of Stainless Steel:

In this work, calibration-free laser-induced breakdown spectroscopy (CF-LIBS) is used to analyze a certified stainless steel sample. Due to self-absorption of the spectral lines from the major element Fe and the sparse lines of trace elements, it is usually not easy to construct the Boltzmann plots of all species. A standard reference line method is proposed here to solve this difficulty under the assumption of local thermodynamic equilibrium so that the same temperature value for all elements present into the plasma can be considered. Based on the concentration and rich spectral lines of Fe, the Stark broadening of Fe(I) 381.584 nm and Saha–Boltzmann plots of this element are used to calculate the electron density and the plasma temperature, respectively. In order to determine the plasma temperature accurately, which is seriously affected by self-absorption, a pre-selection procedure for eliminating those spectral lines with strong self-absorption is employed. Then, one spectral line of each element is selected to calculate its corresponding concentration. The results from the standard reference lines with and without self-absorption of Fe are compared. This method allows us to measure trace element content and effectively avoid the adverse effects due to self-absorption.

The Influence of Acquisition Delay for Calibration-Free Laser-Induced Breakdown Spectroscopy

Time-resolved spectra of neutral and ionized atomic emissions from slag sample are measured by laser-induced breakdown spectroscopy (LIBS). Various factors affecting the calibration-free CF-LIBS method are carefully analyzed, and subsequently these factors are either avoided or corrected. Plasma temperature and electron density are calculated by Saha–Boltzmann plot and Stark broadening of Ca, respectively. At the same time, self-absorption and local thermodynamic equilibrium have been carefully studied. An automatic spectral lines elimination algorithm is applied to calculate plasma temperature and element concentration. The calculated element concentrations show marked changes with acquisition time increasing. Due to the influence of continuous spectrum at early times and self-absorption at late times, the large absolute errors sum is obtained in these two periods. The smallest absolute errors sum corresponds to the gate delay time 1.5 μs < td < 2 μs for our experimental setup.

Research on coherent Rayleigh backscattering and signal processing method in phase-OTDR

There have been a number of papers focusing on fiber distributed sensing with coherent Rayleigh backscattering published. However, up to now, very limited research articles on investigation of coherent Rayleigh backscattering signal waveform and its physical mechanism have been reported. This paper first proposes a theoretical derivation to illustrate coherent Rayleigh backscattering waveform. The theoretical model is then proved with numerical simulation and experimental measurement. In addition, signal processing method is an important factor on the performance of a phasesensitive OTDR system. An improvement of signal processing method, which is consisted of digital average, moving average and interval subtraction, with good effect on locating external perturbation is also introduced.

Investigation on Emission Spectra of Reheating and Pre-ablation Dual-Pulse Laser-Induced Breakdown Spectroscopy *

Two configurations of orthogonal double-pulse laser-induced breakdown spectroscopy (DP-LIBS) and their corresponding signal enhancement mechanisms were investigated in this paper. For pre-ablation DP-LIBS configuration, the first laser was input in parallel to the sample surface while the second laser was perpendicularly input onto the sample surface. For reheating DP-LIBS configuration, the first laser was perpendicularly input onto the sample surface and the second laser was input in parallel to the sample surface. The signal intensities were enhanced using both orthogonal DP-LIBS configurations compared to single laser pulse LIBS. The differences of physical mechanisms involved for both schemes were analyzed using time-resolved spectroscopic technique. Continuous spectrum intensity was reduced using pre-ablation DP-LIBS excitation while it was increased with reheating DP-LIBS excitation. Nine consecutive intensity measurements were performed. Fifteen spectral lines were investigated. The average relative standard deviation of fifteen lines for pre-ablation, reheating DP-LIBS and single pulse LIBS was 5.83%, 1.69% and 4.82%, respectively. It was demonstrated that signal repeatability of reheating configuration was better than that of pre-ablation configuration. A correlation between the enhancements of emission line intensities and their excitation energy levels was established for reheating and pre-ablation double-pulse approach. It was found that reheating configuration showed better improvement in sensitivity at high excitation energy levels compared with pre-ablation configuration.