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.
Non-line-of-sight(NLOS) imaging through fog has been extensively researched in the fields of optics and computer vision. However, due to the influence of strong backscattering and diffuse reflection generated by the dense fog on the temporal-spatial correlations of photons returning from the target object, the reconstruction quality of most existing methods is significantly reduced under dense fog conditions. In this study, we define the optical imaging process in a foggy environment and propose a hybrid intelligent enhancement perception(HIEP) system based on Time-of-Flight(ToF) methods and physics-driven Swin transformer(ToFormer) to eliminate scattering effects and reconstruct targets under heterogeneous fog with varying optical thickness. Furthermore, we assembled a prototype of the HIEP system and established the Active Non-Line-of-Sight Imaging Through Dense Fog(NLOSTDF) dataset to train the reconstruction network. The experimental results demonstrate that even in dense fog short-distance scenarios with an optical thickness of up to 2.5 and imaging distances less than 6 meters, our approach achieves clear imaging of the target scene, surpassing existing optical and computer vision methods.
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.
A high-energy quasi-continuous-wave (QCW) laser diode-pumped regenerative amplifier was demonstrated for using as a radiation source of laser-induced plasma. The seed source was an all-fiber amplifier, provided pulse width of 454 ps and single pulse energy of 7.7 nJ at a repetition rate of 24.17 MHz and a central wavelength of 1063.9 nm. The solid-state regenerative amplifier used a Nd:YAG crystal was side-pumped by QCW diode bars. With this system, high stability and high energy was generated at wavelength of 1064.1 nm, with pulse width of 392.1 ps and output average power was 4.04 W. The single pulse energy and peak power was 8.08 mJ and 20.6 MW, respectively. The M2 factor was about 1.48. The laser system will use as a picosecond radiation source for the following laser ablation and laser-induced plasma spectrum analysis. In the previous work, the process of laser-induced plasma was simulated by fluid dynamics. The temporal and spatial distribution of electron density and temperature was successfully simulated, corresponding the process of laser irradiation on target were recorded for set of materials (Si, Al, Cu). Finally, the mechanism and evolution process of the picosecond laser irradiation target were obtained.
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.
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.
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