Design Of A Controlled Synchronous-measurement System Of Underwater LIBS Multi-dimensional Information And Its Application For Spectrum Normalization

The content of metal elements in inshore seawater is increasingly exceeding the standard,which is a threat to human living environment and health,so there is an urgent need for real-time monitoring of metal elements in coastal seawater.Laser induced breakdown spectroscopy(Laser-Induced Breakdown Spectroscopy,LIBS)has the advantages of real-time and simultaneous detection of multi-elements.however,due to the limitations of amorphous and compression resistance of water,underwater laser-induced plasma has the advantages of discrete and short lifetime,accompanied by the interference of laser energy,shock wave,bubbles and other factors,which leads to the complexity of underwater LIBS physical process and poor spectral stability.In order to solve the above problems,a synchronous measurement system of underwater LIBS multi-dimensional information is designed and built in this thesis,which includes the synchronous measurement module of LIBS spectrum and laser transmission probe(Laser-Beam-Transmission Probe,LBTP).The synchronous diagnosis of underwater LIBS spectrum,laser energy,shock wave and bubble evolution is realized and applied to spectral correction.Firstly,the thesis carries on the scheme design,device selection and parameter optimization of the underwater LIBS multi-dimensional information synchronous measurement and control system.The system is divided into three parts: LIBS excitation,LIBS spectrum detection and LBTP signal detection.The LIBS excitation module uses high energy and low repetition frequency pulse laser,the spectrum detection adopts plasma axial radiation collection mode,and the LBTP signal detection is illuminated by632 nm CW laser.A single point APD detector is used to quickly detect the time evolution of transmission-radiation signals.The work of this thesis takes the major metal elements(Na,Li,K)in seawater as an example to verify the method by configuring a standard aqueous solution with a certain concentration gradient.Based on the underwater LIBS multi-dimensional information synchronous measurement experimental system,the timing control and data acquisition host computer control software are designed and developed,and the synchronous acquisition of LIBS spectrum signal and LBTP reference signal is realized.The acquisition and control software of laser and oscilloscope is programmed by Qt,which realizes the basic control and state display of laser,and realizes the single trigger acquisition,time base and other related parameters control and waveform drawing of oscilloscope.The status parameters of host computer software and the actual monitoring results of the instrument are compared and analyzed.In order to evaluate the synchronous measurement and control system of underwater LIBS multi-dimensional information,the synchronous LIBS-LBTP data under different laser pulse energy and different ion concentration of aqueous solution were collected.By analyzing the correlation between underwater LIBS spectrum and LBTP signal,it is proved that there is a strong positive correlation between the peak area of Na,Li and K atoms excited by different laser pulse energy and the negative peak area of LBTP signal(R2 > 0.99).Taking the multi-dimensional information collected by LBTP as the reference signal,a spectral correction model based on PLSR(partial least square regression)is established,which effectively improves the stability of underwater LIBS spectral signal.At the end of the thesis,the full-text work is summarized,and the follow-up work is prospected from two aspects: "multi-dimensional data synchronous automatic acquisition and control software design" and "laser reflection measurement method".