In the air, Mach-zehnder interference system was set up to study the plasma expansion process of single crystal silicon induced by millisecond pulsed laser. Electron density is the main parameter of laser plasma characteristics. Calculation electron density of silicon plasma based on the relationship between the FWHM of Stark broadening of spectral line and the electron density. Experimental results show that: The existence material splash phenomenon is existence in silicon plasma generated by millisecond laser, the long pulse laser interaction with material has the thermal effect. Silicon plasma emission spectrum is strong in the distribution of the continuous spectrum, the discrete series of atoms and ions are superimposed on it. With the increase of the laser energy density, the electron density of the plasma increases.
According to the heat conduction equation, thermoelastic equation and boundary conditions of finite, using the finite
element method(FEM), established the three-dimensional finite element calculation model of thermal elastic ,numerical
simulation the transient temperature field and stress field distribution of the single crystal silicon materials by the
pulsing laser irradiation, and analytic solution the temperature distribution and stress distribution of laser irradiation on
the silicon material , and analyzes the different parameters such as laser energy, pulse width, pulse number influence on
temperature and stress, and the intrinsic damage mechanism of pulsed laser irradiation on silicon were studied. The
results show that the silicon material is mainly in hot melt under the action of ablation damage.According to the
irradiation of different energy and different pulse laser ,we can obtain the center temperature distribution, then get the
law of the change of temperature with the variation of laser energy and pulse width in silicon material; according to the
principal stress and shear stress distribution in 110 direction with different energy and different pulse, we can get the
law of the change of stress distribution with the variation of laser energy and pulse width ;according to the principal
stress distribution of single pulse and pulse train in 110 direction, we can get the law of the change of stress with pulse
numbers in silicon.When power density of laser on optical material surface (or energy density) is the damage threshold,
the optical material surface will form a spontaneous, periodic, and permanent surface ripple, it is called periodic surface
structure laser induced (LIPSS).It is the condensed optical field of work to generate low dimensional quantum
structures by laser irradiation on Si samples. The pioneering work of research and development and application of low
dimensional quantum system has important academic value.The result of this paper provides theoretical foundation not
only for research of theories of Si and substrate thermal stress damage and its numerical simulation under laser
radiation but also for pulse laser technology and widening its application scope.
In this paper, we establish a physical model to simulate the melt ejection induced by millisecond pulsed laser on
aluminum alloy and use the finite element method to simulate the melting and vaporization process of aluminum alloy.
Compared with the conventional model, this model explicitly adds the source terms of gas dynamics in the
thermal-hydrodynamic equations, completes the trace of the gas-liquid interface and improves the traditional level-set
method. All possible effects which can impact the dynamic behavior of the keyhole are taken into account in this
two-dimensional model, containing gravity, recoil pressure of the metallic vapor, surface tension and Marangoni effect.
This simulation is based on the same experiment condition where single pulsed laser with 3ms pulse width, 57J energy
and 1mm spot radius is used. By comparing the theoretical simulation data and the actual test data, we discover that: the
relative error between the theoretical values and the actual values is about 9.8%, the melt ejection model is well
consistent with the actual experiment; from the theoretical model we can see the surrounding air of the aluminum alloy
surface exist the metallic vapor; an increment of the interaction time between millisecond pulsed laser and aluminum
alloy material, the temperature at the center of aluminum alloy surface increases and evaporation happens after the
surface temperature reaches boiling point and later the aluminum alloy material sustains in the status of equilibrium
vaporization; the keyhole depth is linearly increased with the increase of laser energy, respectively; the growth of the
keyhole radius is in the trend to be gentle. This research may provide the theoretical references to the understanding of
the interaction between millisecond pulsed laser and many kinds of materials, as well as be beneficial to the application
of the laser materials processing and military field.
In this article ,we use finite element method to simulate the process of multi-pulse laser irradiate on fused silica.
Simulation results show that:during laser pulse radiation, the temperature rise of the laser-radiating fused silica surface
center point is nearly linear. During the pulse interval without pulse acting, due to none of thermal source or energy
concentration and the fused silica being at cooling phase, the temperature of the laser-radiating fused silica surface center
point declines sharply. At the fused silica surface where r=0.6mm, hoop stress turns to be tensile stress. When the next
pulse starts to act, the stress increases rapidly once again the maximum value of it is higher than the former one. Thus,
we can judge that the laser accumulation acts a major role in increasing tensile stress effect.
Meanwhile, we can figure out that the stress increase rate generated by the pulse laser of duty ratio 1:10 is higher than
the one of duty ratio 1:20 during the whole procedure. Along with shorter intervals of every two neighbor laser pulses of
duty ratio 1:10, the regression time of the tensile stress is comparably shorter. Thus, during the whole laser radiation
procedure, the initial tensile stress of every pulse of duty ratio 1:10 is greater than the one of duty ratio 1:20. Hence we
obtain the conclusion that, it’s much easier to generate damage when the pulse numbers in unit time increase with other
parameters of the laser do not change.
In order to improve the capacity of beam collimation for laser beam expander, it is necessary to design a more reasonable
and feasible structure of beam expander system. Laser beam expander is used to compress the laser divergence angle, in
order to reduce the energy losing in long distance scanning acquisition system. This paper introduces the working
principle and design idea of the laser beam expander, the collimating multiplying power focal length and the collimated
magnification formula of expander main, secondary mirror. According to the third-order aberration theory, Considering
the spherical aberration, sine difference and divergence angle, the reasonable analysis of optical path, ZEMAX optical
design software was used to design large-diameter laser beam expander and analysis and optimize, And given the actual
design data and results. Display the maximum optical path difference is ±0.01λ of the main light ray and each light ray.
To combination the rear- group objective lens of Galileo and Kepler beam expander, a large-diameter(1.475m) laser
beam expander was designed with 0.2m in the diameter, 1/2m in the relative caliber. In the objective lens System, a
high-order aspherical was used to the aberration of extra-axial point. we can see that the image quality is close to the
diffraction limit from the curves of wavefront. In addition to improve image quality effectively, the system has the
characteristics of simple structure, less costly and less design difficulty to compare with the other beam expanding
system. And make the output beam's divergence angle smaller, energy density higher, and the beam quality has been
greatly improved. The results show that the beam expander is fully meet the design requirements, the use effect is good.
Design and research of laser beam expanding system not only improves the quality of the laser beam in the laser system,
but also enlarge the application field of laser technology in photoelectric system.