Baomin Bian

Initial formation process of laser-induced plasma shock wave in air

Abstract The initial formation process of laser-induced plasma shock wave in air during laser damage process has been studied experimentally with a well-designed optical fiber sensor based on detecting beam deflection. The experimental results indicate that the formation of the laser-induced plasma shock wave is the result of the overlap of vapor- and explosion-shock waves. The plasma explosion shock wave, which generates behind the vapor shock wave but has a higher speed, catches up with the vapor shock wave and overlaps it at a certain distance, giving the result of a strong shock wave in air. The distance is about 3 mm when a 170 mJ laser at 1.06 μm with 30 ns duration irradiates an aluminum target in air.

Monte Carlo simulation of Stokes vectors of polarized light scattering from two-dimensional random rough surfaces

A Monte Carlo model was established to simulate polarized scattering fields of two-dimensional rough surfaces based on the Kirchhoff approximation. Based on this model, numerical studies of the hemispherical distribution of Stokes vectors of scattered light from dielectric and metal rough surfaces were carried out. These surfaces have Gaussian distributions with correlation length of 3.1 µm and standard deviation varying between 0.1 and 0.6 µm. The results reveal that the V component of metal surfaces has peaks antisymmetric with the incident plane, whereas the V component of dielectric surfaces is almost zero. We consider that this property of the V component would provide a new method which could be used to distinguish the target material.

Instantaneous measurements of laser-induced plasma shock waves with an optical fiber sensor

High power laser-induced plasma shock waves in air were measured instantaneously with an optical fiber sensor based on detection beam deflection. It was proved that the time-of-flight deflection angle directly imaged the changing rate of the refractive index at the measurement position while a shock wave transmitted the detection beam. This method is suitable for measurements of shock waves in transparent flow fields with axisymmetric refractive index distributions.

Dynamics of particles removal in laser shock cleaning

Laser shock cleaning (LSC) has been proved an effective method to clean sub-micron and micron particles from solid surface during last five years. In this report, dynamics of the interaction between plasma shock wave and adhered spherical particles is analyzed in theory, considering the change of particle contact radius induced by the load of the shock wave. Analysis of the rolling mechanism at the initial contact of the shock wave with particles shows working gap has a serious influence to the cleaning and smaller diameter particles are more difficult to be removed with smaller cleaned area. Moreover, particle energy obtained from the shock wave is analyzed through which particle removal trace and cleaned area are studied combined reflection shock wave and irregular turnover of the particle into account. Removal of micron copper particles on a silica surface in air is experimented at different working gap. Results show that particles can be effectively removed within the suitable working gap, i.e., 0.8 mm for 150 mJ explosion energy, and higher working gap represents poorer cleaning efficiency. Moreover, the cleaning situation of the heavy contamination shows out an interesting phenomenon of the cleaned area (0.4cm2) profile that is an ellipse caused by the non-uniform pressure distribution of plasma shock wave.

Study on the testing technology of the strong bending losses from the optical fiber cables released from the bobbin

In the process of the high speed pay-off of the optical fiber cables, there are some problems unsolved such as testing the broken fibers or cables, monitoring the working states of the pay-off, and so on. In this paper, in order to solve these problems, a kind of dynamic testing technology to test the bending losses and the bending parameters of optical fiber pay-off cables based on the dual-wavelength, at the high pay-off speed, is proposed. Using the dual-wavelength method, the distribution of bending radius and bending length near the stripping points of the fibers on the optical fiber cable steel can be inversed by the difference of the bending loss index. On the other hand, a kind of dynamic dual-wavelength testing device is designed and developed, which is of broad band and high sensitivity and can obtain the periodic signals of the cable loss index with the changing of strong bending in real time. Besides, the static and dynamic calibration devices are designed, and the relationship between the two calibration methods is also established. Furthermore, the relationship between the curvature radius and the strong bending losses when the curvature radius is less than 6mm and the relationship between the bending length and the loss is studied experimentally. The results show that it has fast time response, strong anti-interference ability and high sensibility. There is obvious difference of the bending states between the scanning interval and the retrace interval, and the loss in the retrace interval increases greatly more than in the scanning interval. It is important to note that the minimum curvature radius of the strong bending cable is about 0.5mm at the speed of 200m/s. Our work will be beneficial to analysis the process and the mechanism of the fault of the fiber broking, as well as the fiber communication, fiber sensor, and so on.

Study on the mechanical analysis and the testing technology of the optical fiber cables released from the bobbin

Accurate measurements of forces applied to the optical cable reels with high spinning speeds, will render information on the breakdown of optical fibers, and thus improve the odds of success and un-winding efficiency. In this paper we analyze and deduce the cable wire stress at high pay-off speeds. A high-sensitive opti-mechanical testing sensory device is designed to measure both the axial tension of the cables and the radial pressure of the cable reels at varying stress points simultaneously. The time resolution of this new device is less than 0.015ms, the response time is up to 15μs, and its sensitivity is about 500pc/N, which satisfies the mechanical testing requirements at high spinning speeds. In addition, the spinning speed of 260m/s led to the break-down of the optical fibers, and the spinning speed of 250m/s tested finally led to a deceleration near the end of the broken fibers. It is obvious that this kit can meet the requirement to obtain the periodic signals of the varying forces for each layer and each turn of optical fiber cables. Moreover, we found that the pay-off fiber cable is safe with the unwinding speed of 250m/s and the break-down of optical cables happens during the deceleration process. However, it is under the unwinding speed of 260m/s that pay-off fiber cables broke during the experiment. The abnormal breakdown signals are captured at these unwinding speeds, respectively.

The model of random signals generated by optical particle counter and the instrument improvement

In order to study and improve atmospheric and air pollution monitoring sensor, a new mathematical model of random signals is established based on measuring process of light scattering signals analyzed by laser particle counter which combines the high speed data acquisition card PCI-9812 and optical particles counting sensor. The measured random signals can be divided into stability constant part and random variation part. The performance of the instrument is improved by both this model and analytical methods. Statistical distributions of the amplitude of the standard particles with different diameters are studied by the original experiment and improved one. The resolving power of particle size could attain more than 90%. The results reveal statistical distributions match well with lognormal distribution with a natural number as an independent variable. The lognormal distribution plays an important role in describing the random fluctuation characteristics of random process in both theories and experiments. Furthermore, both normal and lognormal distribution fitting are applied in analyzing the experimental results and testified by chi-square distribution fit test and correlation coefficient for comparison.

An equivalent method to analyze the electrical effect induced by laser plasma

Electrical effect induced by laser plasma in air is measured using a tiny probe placed in close to a metal target. Analysis shows that the resulting signal wave varies with detection distance. Based on the testing system, an equivalent circuit model was proposed to analyze the formation mechanism and evolution of the electrical signal and its dependence on the probe distance. The observed signal peak polarity overturn was also analyzed and explained. Finally, our method provides an explanation for the effects of the testing angle on the probe signals according to the time and space evolution of the laser plasma.

Research on generated mechanism of scattering characteristic of random rough dielectric surface

On the basis of electromagnetic theory, the scattering light intensity from a series of dielectric surfaces with different roughness is calculated by using monte-carlo method and the boundary condition of Kirchhoff approximation. The geometry profile of rough surfaces obeys Guass distribution and all of the surfaces have the same corelation length. With the increasing of rms height, the width of diffuse scattering intensity distribution, the attenuation of scattering peak value and its moving to the normal direction are observed. After theoretical analysis, it is obtained that the statistical distribution of the local slope varies with the rms height and reflecting index of these local slope are the main reasons for these phenomenon.

The invariant statistical rule of aerosol scattering pulse signal modulated by random noise

A model of the random background noise acting on particle signals is established to study the impact of the background noise of the photoelectric sensor in the laser airborne particle counter on the statistical character of the aerosol scattering pulse signals. The results show that the noises broaden the statistical distribution of the particles measurement. Further numerical research shows that the output of the signal amplitude still has the same distribution when the airborne particle with the lognormal distribution was modulated by random noise which has lognormal distribution. Namely it follows the statistics law of invariance. Based on this model, the background noise of photoelectric sensor and the counting distributions of random signal for aerosols scattering pulse are obtained and analyzed by using a high-speed data acquisition card PCI-9812. It is found that the experiment results and simulation results are well consistent.