Double-pulse Induced Breakdown Spectroscopy Detection of Femtosecond Laser Dielectric Fabrication and Its Applications Verification

Author:Cao Zhi Tao

Supervisor:jiang lan


Degree Year:2017





Dielectric is some kind of material with extremely high ablation threshold,the excellent physical and chemical characters of which contribute a lot to its extensive application in the optics area.Benefiting from the super short pulse duration and super high instantaneous power,the femtosecond laser has become the top priority in the flexible machining of dielectric material.The nonlinear absorption of femtosecond laser makes it possible for the high-precision fabrication of the dielectic material.Therefore,the femtosecond laser is widely utilized in the dielectric micro-/nano-fabrication,especially in the field of the interferometric fused silica fiber optical sensor fabrication.The fiber optical sensor fabrication demonstrates the super advantage of femtosecond laser micromachining in transparent dielectric as one promising application type.However,there are some obstacles existing in material removal process of dielectric by femtosecond laser.These obstacles lead to serious impediments in the further application of femtosecond laser.Up to now,the fundermental research of femtosecond laser and fused silica interactions stays on the level of off-line characterization of ablated morphology and in-situ measurement of electron dynamics by pump-probe technology.The plasma information in the nanosecond scale is rarely studied for deeper understanding of fused silica.As a principal method in the research of plasma character,laser-induced breakdown spectroscopy is a general technology for the plasma temperature and electron density measurement for metal and semiconductor material.But the application is limited due to the signal intensity and stability problem.Therefore,the research about signal enhancement of laser-induced breakdown spectroscopy is significant for both the technology application and the material ablation mechanism,which is the research field of great concern.The important research contents with innovation are mainly divided into the following parts:(1)Establish a multiscale measurement system for the multi-physical processes to achieve the in-situ characterization of femtosecond laser fabrication process in the time scale across 15 orders of magnitude.As a case study,the femtosecond laser deep-hole drilling process is measured including the ionization process in femtosecond timescale,initial plasma generation and following shockwave evolution in picosecond and nanosecond timescale,and plasma expansion in nanosecond timescale excited by the single pulse.The plasma ejection process in millisecond scale and the deep-hole formation in second timescale excited by the 10 Hz pulse percussion were also imaged by the multiscale measurement system.(2)Study the plasma emission spectrum signal enhancement effect of fused silica induced by the collinear femtosecond laser double pulse.Through the concrol of electron dynamics,it is found that the plasma emission intensity induced by the double-pulse femtosecond laser with interpulse delay exceeding 10 ps could be a dozen times of that induced by the single-pulse case.The maximum enhancement factor arrives 35 times at the interpulse delay of 120 ps when the laser fluence was 11 J/cm2.On the basis of local thermal equilibrium state,the plasma temperature and electron density were calculated by the Boltzman plot method and the Stark broadening method,respectively.The electron dynamics change in the double-pulse irradiation on fused silica is estimated to be the cause of the laser-induced breakdown spectroscopy enhancement.(3)Explore the plasma emission spectrum signal enhancement mechanism of fused silica ablated by femtosecond laser double pulse.The component of femtosecond laser induced plasma shows a great difference according to the material variety.The main difference is illustrated by the ionization degree of different material.The ionization degree of copper is so high that no intensity difference can be distinguished between the fast and slow part of plasma plume.By contrast,the single-pulse induced plasma emission of fused silica is mainly contributed by the fast part.Stronger plasma emission is achieved by the double-pulse irradiation under the same laser fluence,in which situation the partially ionized slow part by the single pulse is reionized by the second pulse.The reionized slow part becomes the main resource of plasma emission.Considering the characterization of ablation volume of fused silica,the ablation volume shows an opposite evolution with the change of interpulse delay when the delay exceeds 10 ps.It is demonstrated that the reionization of the slow part left by the first pulse is the main reason for the plasma emission enhancement which absorbs most of the energy of the second pulse.(4)Introduce the water assisting femtosecond laser ablation technology and the performance testing of two kinds of optical fiber sensors.The water assisting method aims at solving the debris in the femtosecond laser ablation process and the influence of fiber cylindrical structure on the focusing of femtosecond laser.The Fabry-Perot thermo-optic coefficient optical fiber sensor is designed with a complete pure silica structure in order to reduce the intrinsic temperature sensitivity of the sensor(order of disturbance:10-6RIU/℃).In addition,the second fiber end is coated with gold film to increase the reflectivity in the liquid environment for the thermo-optic coefficient measurement of different liquids.On the other hand,the multibeam interferometric optical fiber sensor is designed with a trench-embedding on the fiber taper.The trench depth is controlled to excite the cladding mode in the fiber taper to match the intensity of core mode using the femtosecond laser micromachining.Making advantage of the environment refractive index sensitivity of higher order cladding mode,the high sensitivity gas refractive index measurement of669.502 nm/RIU is achived for the mixture of nitrogen and helium.