KEYWORDS: Non line of sight propagation, Target detection, Image restoration, Object detection, Long wavelength infrared, Relays, Deep learning, Gallium nitride, Cameras, Sensors
The rapidly developing non-line-of-sight (NLOS) imaging technology in recent years is capable of intelligent visual perception of concealed targets, holding broad application prospects in security, emergency rescue, autonomous driving, etc.. Compared to active methods, passive NLOS imaging is promising to in real-world scenarios due to its low cost. This paper uses the long-wave infrared (LWIR) to detect multiple hidden targets. In contrast to the visible band, LWIR exhibits a higher proportion of specular reflection scattering on common relay surface but cannot represent details such as texture. Furthermore, passive NLOS imaging reconstruction is an ill-posed problem, leading to sparse and blurred features, which poses significant challenges for multi-target detection tasks. To address this, the paper proposes a deep learning method for collaborative multi-task image reconstruction and detection. The detection loss is backpropagated and fused with the imaging enhancement loss to guide the NLOS target reconstruction process towards high-quality detection results. Comparative experiments are conducted in multi-person target scenarios between the latest target detection methods and our method. The results indicate that our proposed method exhibits the best performance in terms of detection accuracy, recall rate, and the F1-score. Additionally, this paper demonstrates the generalization of the proposed method at different distances ranging from 10 to 20 meters. The related results provide data and methodological support for the advancement of NLOS imaging towards practical applications.
Experimental researches of the near-infrared laser with 1085nm wavelength and the mid-infrared laser with 3.8 microns wavelength irradiate on the transparent polyethylene film with a thickness of about 25 microns are carried out. Results show that the burn through time approximate exponentially decreases from 5.76s to 0.85s, for the average power density of the mid-infrared laser increases from 2.9W/cm2 to 37.2W/cm2, and the damage energy density is about 22.2J/cm2. The polyethylene film can be burned through by near-infrared laser irradiation with an average power density of 338.8W/cm2 for 18.6s, and the corresponding damage energy density is up to 6301.7J/cm2. The failure time exponentially decreases from 18.6s to 0.75s with the incident laser density increases from 338.8W/cm2 to 428.5W/cm2, and the corresponding damage energy density approximate exponentially decreases from 6301.7J/cm2 to 321.4J/cm2. The damage of near-infrared laser has obvious threshold effect. The polyethylene film would not be burned through until the laser power density reaches a certain high value, so the damage threshold of polyethylene film by near-infrared laser is 1 ~ 2 orders of magnitude higher than that by mid-wavelength infrared laser. The results are in good agreement with the absorption ratio of polyethylene film at wavelength of 3800 nm and 1085 nm measured under weak light.
In this work, the laser-induced periodical surface structure (LIPSS) on silicon is generated by 532nm nanosecond laser with pulse duration of 10ns and repetition frequency of 10Hz. The formation of LIPSS is observed under laser fluence of 260mJ/cm2 and pulse number of 600 shots with p- and s-polarized laser. For p- polarized laser, when the incident angle is 10°, 20°, 30°, 45°, and 60°, the LIPSS period is 400nm, 743nm, 902nm, 1148nm and 2525nm correspondingly. Besides, the orientation of LIPSS is always perpendicular to the polarization direction of the incident laser. For s- polarized laser, the LIPSS is perpendicular to the polarization direction and the period increases with the incident angle when it is less than 42°. The period is 457nm, 515nm, 549nm and 610nm at incident angle of 10°, 20°, 35°, and 41° correspondingly. when the incident angle exceeds 42°, a set of crossed LIPSSs emerge which are symmetric about the polarization direction. Meanwhile, the period reduces a lot which is around 400nm.
The thermodynamic properties of silicon plasma generated by picosecond laser in vacuum were studied by using spatiotemporally resolved emission spectroscopy technique. Temporal and spatial evolution behavior about plasma has been analyzed. Meanwhile, the temporal and spatial dependence of silicon ions with different charges were examined. Finally, the validity of assuming a local thermodynamic equilibrium (LTE) in the silicon plasma expansion was checked, and the deviation degree of LTE was measured. The results indicate that the assumption was valid only at longer (< 180 ns) delay times in the area z < 4 mm away from the target surface. At the initial stage of plasma expansion, the plasma deviates from LTE due to the sharp decrease of plasma temperature and electron density.
This study demonstrates manipulation of luminance from CaF2: Yb3+, Er3+ up-conversion (UC) micronano particles emissions and the fluorescence intensity improvement by using various surface plasmon resonance substrates which consist of SiO2, gold thin film, and nano cones from bottom to the top in the view of side structure. When they are applied, the optimum enhancement of the intensity of red up-conversion luminescence (UCL) peaked at 653 nm shows up to 189-fold. The distance dependent intensities of two UC micro-nano particles manipulated by holographic optical tweezers (HOT) are also illustrated in this article. The fluorescence intensities decrease when two UC micro-nano particles become near to the each other due to the inter-system transition at the much short distance.
In order to study the scaling laws of optical components, we set up a model based on the heat conduction theory and thermodynamic theory. Then the similarity theory was used to the model analyzation. Finally, we demonstrate three conclusions which are related to the practical engineering application. The first one is that thermal damage behaviors of different scale optical components are similar when the linear power density of irradiated laser are the same. In other words, we should use the linear power density to represent the resistance of damage tolerance for optical components The second one is the judgement standard of scram time. We find the scram time of large-aperture system is certain times as much as the scram time of small-aperture system. The third one is about how to design the scaled experiment can we make two different scale laser systems obey the similar thermal damage behaviors. This study is of great help for the damage prevention of the optical components.
The thermal stress damage of optical elements always restrict the development of high power laser system. We studied the thermal damage mechanism of the optical elements with contaminants induced by high power continuous wave (CW) lasers. An experiment was carried out by a self-build optical element testing platform and a model based on the temperature field theory and thermodynamic theory was set up. We recorded the thermal stress damage process based on a 10 kW/cm2 level mid-infrared continuous wave laser. Then we calculated the thermal damage process of optical elements. The calculated results are in agreement with our experimental record. The results showed the success of modeling calculation in the thermal damage mechanism caused by contaminants.
Accurate measurement of the wall temperature is of great significance to investigate laser irradiation effects on a liquid tank. It was shown that the wall temperature couldn’t be measured accurately using the traditional installation method for the thermocouple. To overcome this problem, an effective installation technique was developed. First, a groove was carved on the rear surface of the metal casing of a liquid tank by laser irradiation. Then the thermocouple junction was welded to the measurement point and covered up by high-temperature heat conduction glue. The experimental results showed that the wall temperature could be measured correctly using this installation technique.
We present a versatile method to diagnose method to diagnose nanosecond laser induced plasma (LIP) plume with good temporal (10 ns here) and spatial (here sub-millimeter) resolution, without requiring the assumption of local thermodynamic equilibrium (LTE). The spatially resolved emission images from plasmas formed by 532 nm laser ablation of a silicon target in vacuum (10-7 mbar) with incident irradiance of 21 GW/cm2 were recorded at different time delays using a time-gated iCCD camera attached to a spectrograph and image optics. The spectroscopic emission lines associated with different charged species are assigned in the NIST Atomic Spectra Database. The further analysis of Stark broadened line shapes of those emission images allows tracking the plume dynamics and provides insight into the early time (i.e. within several tens of nanoseconds) mechanism of laser-target interaction and the subsequent laser-plasma coupling. The electron density (Ne) and temperature (Te) values and their variations with space and time are obtained from best-fitting model to the observed line shapes based on a non-LTE electron energy distribution function (EEDF) rather than a Maxwellian EEDF. The value of Ne and Te respectively declined from 1023 to 1021 m-3 and 10 to 0.1 eV since the plume expansion. The time-gated emission images and the spatial and temporal variation of the Ne and Te values both highlight the inhomogeneity of the LIP plume, and provide the future analysis and possible derivation of the electron emitting model from target surface after laser-lattice interaction within sub-nanosecond.
KEYWORDS: Silicon solar cells, High power lasers, Solar cells, Silicon, Laser irradiation, Pulsed laser operation, Electrodes, Lithium, Semiconductors, Beam splitters
Compared with traditional methods of energy supply, there is a great possibility to get a more remarkable enhancement of conversion efficiency for laser power (of proper wavelength and intensity) beaming to silicon solar cells. However, it should be noticed that cells may be damaged by high power laser. Based on the background, this essay explores high-power-laser's possible damage to silicon solar cells by analyzing IV curves (obtained by IV tester) and minority-carrier lifetime (measured by open-circuit-voltage-decay method). Research shows that, for 30s irradiation, minority-carrier lifetime decreases to some extent when irradiated by laser of over 5.5W/cm2 and the higher laser power density, the more degradation. Similarly, IV curves see a downward trend under laser of over 5.5W/cm2. In addition, there is a roughly linear relationship between lifetime and the decrease amount of short circuit current. Moreover, the degradation degree has a close relation with the maximum temperature. The prolonged illumination would not bring about more serious damage if one cell had already reached an equilibrium temperature.
Experimental research on the energy coupling characteristic of 45# steel and 304 stainless steel under mid-infrared CW laser irradiation is carried out. Based on the classical electromagnetic theory, the theoretical formula of the energy coupling coefficient is derived under ideal condition. In order to obtain the energy coupling coefficient, an experimental system for reflectance measurement is set up by an integrating sphere. The curves about energy coupling coefficient and the temperature variation are measured respectively. The mechanism about the variation of energy coupling coefficient of sample under mid-infrared CW laser irradiation is also discussed. The experimental results show that the energy coupling coefficient of sample increased with temperature rising, but the curve in the heating stage is not consistent with the curve in the cooling stage, which means the change of the energy coupling coefficient is not a reversible process. Combined with the experimental phenomena and the energy dispersive spectrometry, the qualitative analysis about the differences between the 45# steel and 304 stainless steel is presented after irradiation. It indicates that the oxidation reaction has a significant effect on the laser interaction with sample. Accordingly, the variation of coupling coefficient of 304 stainless steel is not as obvious as that of 45# steel.
In order to measure the change of laser energy coupling coefficient with temperature in mid-infrared wave band, reflectance integrating sphere experiment system was designed and set up. 915nm CW laser was used to heat samples and the wavelength of probe laser is 3.8μm. Chopper and phase-locked amplifier were adopted in the system. Thermal imager was used to measure and record the temperature of samples during laser irradiation. The reflectance of steel and aluminum plates to 3.8μm was measured during 915nm laser irradiation. EDS analysis was done to investigate the change of elemental composition in the samples respectively. The experimental results show that, the results of reflectance and radiation temperature measured by this system are relatively accurate during laser irradiation. In the process of temperature rising from 300K to 785K, the color of 45# steel plates turns blue and black, while the color of aluminum alloy plates is basically unchanged. When temperature reaches about 700K, reflectance of 45# steel decreases obviously with the increase of temperature, while reflectance of aluminum is almost constant. The reflectance is probably determined by the oxide in the surface of samples which is consistent with the results of EDS analysis. Reflectance decreases with the increase of the content of oxygen in the surface. The reason of why the reflectance of aluminum is almost constant is that aluminum oxide is not generate massively under 750K.
The irradiation effects of LD laser on thin aluminum alloy plates are studied in experiments characterized by relatively large laser spot and the presence of 0.3Ma surface airflow. A high speed profilometer is used to record the profile change along a vertical line in the rear surface of the target, and the history of the displacement along the direction of thickness of the central point at the rear surface is obtained. The results are compared with those without airflow and those by C. D. Boley. We think that it is the temperature rise difference along the direction of thickness instead of the pressure difference caused by the airflow that makes the thin target bulge into the incoming beam, no matter whether the airflow is blown or not, and that only when the thin aluminum target is heated thus softened enough by the laser irradiation, can the aerodynamic force by the surface airflow cause non-ignorable localized plastic deformation and result a burn-through without melting in the target. However, though the target isn’t softened enough in terms of the pressure difference, it might have experienced notable deformation as it is heated from room temperature to several hundred degree centigrade.
Two kinds of anti-laser coating made of reflective / ablative resin, called reinforcement schemes of A and B, are applied to the glass fiber reinforced resin matrix composite plate. The anti-laser performance of these samples to the laser operated at the wavelength of 976nm is tested, under the case of a 0.3 Mach tangential airflow pass over the surface of the sample. The experimental results show that the laser damage threshold of the coating reinforced samples have increased more than 50% compared to the original sample, the reinforcement scheme B is better than A. The laser power density damage threshold of the coating reinforced samples to the near infrared laser is higher than 100W/cm2, under the irradiation time is 60 seconds. For the resin reinforced fiber samples, the removal process of the ablation residues has important effects on the perforation time of samples, when there is a strong airflow pass over the surface. The larger laser spot corresponding to the removal of the ablation residues is easier.
KEYWORDS: Resistance, Silicon solar cells, Solar cells, Laser applications, Laser energy, Silicon, Temperature metrology, Power supplies, Electrodes, High power lasers
The properties of 915nm laser power beaming to monocrystal silicon solar cells are investigated by measuring IV curves, temperature and etc. With the illumination intensity increased from 0.04W/cm2 to 0.58W/cm2, short-circuit current increases almost linearly from 0.14A to a maximum value of 3.07A. While the maximum power output peaks at a lower irradiation intensity of 0.46W/cm2, which can be also regarded as a turning point where IV curves begin to deteriorate from normal ones to oblique lines. During the period, the fill factor decreases continuously from around 74% to a stable value of 25%. To understand the experiment more clearly, theoretical analyses are conducted by virtue of Lambert W function. Based on the analyses, it can be concluded that the primary culprits influencing the cell’s output performance are the temperature and series resistance.
The thermal response of a cylindrical simulative warhead consisting of the steel casing and the TNT explosive irradiated by laser is simulated, basing on the smoothed particle hydrodynamics method. Preliminary computational simulation results show that, when the power density of 500W/cm2 continuous laser irradiation on a sealed explosive device consisting of the type 304 steel casing with thickness of 5mm and TNT explosive, compared with no airflow, the speed of 200m/s tangential airflow can reduce the thermal initiation time of 0.6s. In the case of incident laser power density is high, the convection cooling effect of tangential airflow can be neglected. The oxidation of airflow can significantly shorten the thermal initiation time of internal explosive.
The concept of using lasers to drill through rock has been discussed in the oil and gas industries since the development of the high-power laser. To evaluate the possibility of fielding a laser drilling system, two laser-related problems have to be investigated. The first is the irradiation effects of laser upon rocks; the second is the effects in laser transmission from the source to the rock deep in the well. This transmission includes two stages: the first stage is the transmission inside a fiber, which is packaged in a cable and has about the same length with the well depth; the second stage refers to the transmission process when the laser leaves the fiber and some transforming optics and transmits to the rock surface, during which the well conditions may impose tough restrictions. In this paper, experiment results of laser irradiation upon siliceous sandstone and granite are reported, and the fiber transmission loss is simulated, considering the main absorbing or scattering mechanisms inside fiber. And the laser transmission from the fiber end to the rock surface, in my view, may impose great challenge on the laser drilling technology.
The irradiation effects are studied, of solid-state laser on four kinds of plates (three of them are made of metal, the other, of composite), in experiments characterized by relatively large laser spot and the presence of surface flow. The thick iron samples, thin aluminum samples and thin carbon fiber/epoxy resin samples are subjected to air or N2 surface flow, while the box-shaped samples, containing a thin aluminum plate irradiated by laser, are filled with water. It is found that, besides the common role in all four cases cooling the plate by convective heat transfer, the fluid plays other different roles in different case influencing the dynamic response of the plate. The roles of the fluid in each case are described either with analytical boundary conditions or with differential equations, which are then incorporated into computational models. Numerical simulations are carried out, with results compared with the experiment results to explain the irradiation effects.
Experiments are performed to investigate the laser irradiation effects on thin aluminum alloy sheets subjected to tangential airflow. The wind blower generated airflow with a speed of about 100 m/s along the surface. For comparison, experiments in the absence of airflow are also conducted. Moreover, in order to know whether the combustion reaction takes place during the irradiation, we vary the composition of flow from air to nitrogen. The displacements of the sheets center are measured to see whether the tangential flow has a mechanical effect. The maximum temperature of the sheet is lower than 550 ℃ after 2 seconds irradiation with the laser power density of 173W/cm2. Accordingly, the structural parameters of aluminum alloy do not have distinct change and so do the features of sheets. The temperature curves in the air flow and nitrogen flow keep the same and both lower than that in no flow case. Moreover, the displacements measured in three cases do not have obvious difference. These experiment results indicate that the combustion reaction can hardly happen and the tangential flow only has a cooling effect. The maximum temperature reaches 600 ℃ when the laser power density rises to 400 W/cm2. Such a high temperature makes that the elastic modulus of aluminum alloy drops rapidly, which greatly softens the alloy sheets. The plastic distort of irradiated sheets confirmed this process. When the power density rises to 450W/cm2 big melt-through phenomenon is observed and there is viscous dripping under gravity in the no-flow case. However, in air flow and nitrogen flow, we can see the removal of macroscopic unmelted pieces of aluminum alloy sheet. The results indicate that the tangential flow mainly has two effects including cooling the target and removing the unmelted metal when the material is fully softened.
The irradiation effects of 976nm continuous-wave laser on carbon fiber reinforced E-51 resin composite is studied experimentally, with a 0.4Ma tangential airflow or 0.4Ma tangential nitrogen gas flow on the target surface. In order to simulate the thermal response of fiber reinforced resin composite materials subjected to combined laser and tangential gas flow loading, a three-dimensional thermal response model of resin composite materials is developed. In the model, the thermal decomposition of resin is described by a multi-step model. The motion of the decomposition gas is assumed to be one-dimensional, for the case that the laser spot is significantly larger than the thickness of the sample. According the above assumption, the flow of the decomposition gas is considered in the three-dimensional model without introducing any mechanical quantities. The influences of the tangential gas flow, the outflow of the thermal decomposition gas and the ablation(including phase change ablation or oxidative ablation)of the surface material on the laser irradiation effects are included in the surface boundary conditions. The three-dimensional thermal response model is calculated numerically by use of the modified smooth particle hydrodynamics (MSPH) method which is coded with FORTRAN. The model is tested by experimentally measuring the temperature profiles during carbon fiber reinforced E-51 resin composite subjected to combined laser and tangential gas flow. The predicted temperature profiles are in good agreement with experimental temperatures obtained using thermocouples.
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