Laser Transmittance Research Articles

Real-time measurement of pump beam polarization through the transmitted light intensity in a polarized alkali metal vapor cell.

A laser beam with left-/right-handed circular polarization is generally used to create the oriented atomic spins for precision measurements in a spin-exchange relaxation-free (SERF) co-magnetometer. The fluctuation of laser polarization interferes with the spin polarization of alkali metal atoms, leading to the system performance degradation. Here, we report a method for real-time polarization state measurement by using the transmitted light intensity of the pump beam passing through the vapor cell. Based on the principle of circular dichroism, the optical absorption model of polarized alkali metal atoms is established. The simulation results of the transmittance of the pump laser with different polarization states through the alkali metal vapor cell are given and verified by experiments. The experimental results show that the circularly polarized beam has the weakest absorption, while the linearly polarized laser beam is absorbed the strongest. The achieved measurement accuracy stands at an impressive 98.83 %. This work provides a simple and easy-to-use way to measure the polarization state of the laser beam used in the vapor cell devices, particularly the microfabricated prototypes with limited space.

In situ syringe rotation system for heavy microparticle suspension stability in electrospinning technique.

Electrospinning allows the fabrication of polymeric nonwovens with a wide variety of inclusions in the micro-nanofibers. However, the electrospinning of microparticle-filled polymer solutions is still limited in particle size, density, and concentration, mainly due to suspension instability during the electrospinning process, so it is not commonly investigated despite the vast number of possible applications. In this study, a simple and effective novel rotation device was developed to prevent the settling of microparticles in the polymer solution during electrospinning. The stability of polyvinyl alcohol and polyvinylidene fluoride (PVDF) solutions with indium microparticles (IMPs) of (42 ± 7) μm diameter was evaluated using LASER transmittance inside a syringe, both static and rotating for 24h. While the static suspensions completely settled at 7minand 9h, respectively, depending on solution viscosity, the rotating suspensions remained stable throughout the experiment. The number and distribution of IMPs in PVDF electrospun mats were determined by optic microscopy and a novel x-ray imaging mapping method, showing 165% more IMPs in the mat obtained with the rotating syringe device. A simple analysis of the theoretical background of settling and rotating suspensions was included to understand the working mechanism of the device. Also, the electrospinning of solutions with high loadings of IMPs (up to 400% w/w PVDF) was accomplished. The simplicity and outstanding efficiency of the device shown in this work may serve as a solution to technical difficulties and as an encouragement to future research in microparticle-filled solution electrospinning.

The study of the Resultant of Attenuation in laser Radiance by Humidity Effecting on using two Types of water

This research sheds the light on the effects of one weather factors the humidity which are got from vaporing two rinds of water Tigress water and distilled water so they affect laser beams transmittance and change of humidity ranges. The laser of continuous CO2 with wave length 10.6 µm with power reads 0.8 mw and the laser of He-Ne laser with wave length of 632 nm with power reads 2.04 mw The beams of laser in the cell attenuation and measuring laser by using power sensor 's then using the effect factors after going on 2m. Then transmittance decrease with high humidity . The relation between transmittance ranges for beams of laser and humidity depend on spectrum of water vapor absorption, wave light of lasers, powers of. Therefore the effect of humidity that is got by vaporing water that this process becomes more effective on, the transmittance of laser than using other kinds of water. The reason is that, rivers water contains many melted elements which may vapor through increasing temperature to boiling and eventually it affects transmittance.

Measurement of microwave electric field based on electromagnetically induced transparency by using cold Rydberg atoms

Microwave electric fields are measured by using cold Rydberg atoms. We obtain spindle-shaped cold atomic clouds in a magneto-optical trap and then pump the cold atoms to quantum state 5S<sub>1/2</sub>, <i>F</i> = 2, <i>m</i><sub>F</sub> = 2 by using an optical-pump laser. We obtain the Rydberg electromagnetic induction transparency (EIT) spectrum peak with narrow linewidth by the low temperature and small residual Doppler broadening. The results show that the typical EIT linewidth with 16 μK cold atoms is about 460 kHz which is 15 times narrower than that of 7 MHz obtained in the thermal vapor cell. The microwave electric field amplitude is measured by Autler-Townes splitting (EIT-AT splitting) in the cold atoms at frequencies of 9.2, 14.2 and 22.1 GHz, receptively. The results show that there is a good linear relationship between the EIT-AT splitting interval and the microwave electric field amplitude. The lower limit of the microwave electric field amplitude that can be measured in the linear region can reach as low as 222 μV/cm, which is about 22 times larger than the lower limit in the traditional thermal vapor cell about of 5 mV/cm. The improvement of the lower limit by EIT-AT splitting method is roughly proportional to the narrowing EIT line width by cold atom samples. This demonstrates that benefiting from the smaller residual Doppler effect and the narrower EIT linewidth in cold atoms, the cold atom system is more advantageous in the experimental measuring of the weak microwave electric field amplitude by using the EIT-AT splitting method. This is of great benefit to the absolute calibration of very weak microwave electric fields. Furthermore, the lower limit of the microwave electric field amplitude that can be measured is smaller than 1 μV/cm by using the change of transmittance of the prober laser at the EIT resonance, and the corresponding sensitivity can reach 1 μV·cm<sup>–1</sup>·Hz<sup>–1/2</sup>. These results demonstrate the advantages of cold atomic sample in microwave electric field measurement and its absolute calibration.

Hot-carrier tunable abnormal nonlinear absorption conversion in quasi-2D perovskite

Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm−2. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized “Fermi Sea” is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications.

ESTIMATING 3D FOREST STRUCTURE USING LIDAR DATA FOR RISK ASSESSMENT OF SLOPE COLLAPSE CONSIDERING ROOT SYSTEM DEVELOPMENT

Abstract. In order to predict slope failure, it is useful to be able to grasp the state of root system development of trees by remote sensing techniques. Since there is a positive correlation between tree diameter and root system development, a field survey to verify how predictable the number of trees and breast height diameter can be from LiDAR data was conducted in the Abukuma Mountains Granite Area and the Yamizo Mountains Jurassic Sedimentary Rock Area. It was carried out for the logging site. As a result, it was found that the relative tree density can be determined and that the laser transmittance and the sum of cross-sectional areas of the trees are inversely correlated.

Light transmittance dynamics and spectral absorption characteristics during auxiliary cryogen spray cooling in laser dermatology.

Cryogen spray cooling (CSC) is widely applied in laser dermatologic surgery to minimize the risk of non-specific epidermal thermal damage caused by the competitive laser energy absorption of epidermal melanin. However, the light absorption and attenuation by cryogen film and subsequent frost formation on the skin surface during CSC are needed to be investigated by using R-134a, R-404A, and R-32. A spray system equipped with an integrating sphere-based light collection apparatus was constructed to evaluate the time-resolved laser transmittance and spectral absorption characteristics induced by R-134a, R-404A, and R-32 sprays, under the clinical-used 755-nm and 1064-nm laser irradiations. No obvious light absorption peaks exist in the wavelengths of 370-1400nm. R-404A produces the largest average light absorbance (0.089), as compared to those of R-134a (0.066) and R-32 (0.068) in the near-infrared range (780-1400nm). Given the lowest boiling point and highest latent heat of R-32, the evaporation of liquid film and melting of subsequent frost are promoted, leading to smallest light attenuation. R-32 spray shows great potential in clinics owing to its high light transmittance, small light absorption, and high cooling capability. For R-32 spray, the durations between spurt termination and laser irradiation are recommended to be 8-100ms and 13-100ms with average light transmittances of 86% and 95% under 755-nm and 1064-nm laser irradiations.

Subwavelength Structures on Fused Silica for High-Power Laser Antireflection Applications

In the field of high-power lasers, subwavelength structures with both high transmittance and high laser damage resistance are required. However, in most of the studies, structural design tends to focus on optical properties. In this study, we research transmittance and surface electric field together. The surface light intensity, as an important damage factor, is investigated and quantitatively calculated using finite element method. Our results show that the surface light intensity has asymmetry on the front and rear surfaces, and is greatly regulated by the structural parameters. Based on this study, the subwavelength structures are designed which have both high transmittance and high laser damage resistance. Our research provides a new perspective to design of subwavelength structures, which is important for high-power laser antireflection applications.

Modified superstructure fiber Bragg grating for a filter application

Performance of a modified superstructure fiber Bragg grating is carried out by; simulation using a Matlab and then by optisystem7. Results for these two simulations are compared with an experimental investigation for three special FBGs in addition to a standard one. These four FBGs have one, two, three, and four regions. For each region, Bragg wavelength is increased around 6nm, with constant spacing between every two regions. Laser reflectance (R) and transmittance (T) spectra and their corresponding number of peaks and their bandwidths are observed and analyzed in a constant temperature and strain. Results for this investigation indicate enhancement for the modified FBG to be an effective filter that can be employed for communication as well as sense. Filtering the transmitted signals could be satisfied in a simple and more efficiency than the traditional device.

Pressure-sensitive characteristics of high-pour-point live oil and quantitative analysis of composition change

Previous studies on high-pour-point (PPT) oil have focused on the effects of temperature. There has been no systematic investigation of the pressure-sensitivity of crude oil, the deposition composition change of solid phases, and the effects of asphaltene deposition on wax precipitation. In the development of a high-PPT oil field, experiments using dead oil instead of live oil will exaggerate the effect of wax precipitation. In this paper, the wax appearance temperature (WAT), PPT, and asphaltene deposition pressure were obtained by the laser transmittance method. Subsequently, a new set of experimental devices and methods were developed to analyze the solid-phase components in live oil under various conditions. Finally, the viscosity of crude oil at various shear rates was measured with a capillary viscometer, and the wax network structure was evaluated. The experimental results showed that the WAT and PPT decrease with decreased pressure, and their fast decreasing ranges were 12–18 MPa and 15–18 MPa, respectively. The oil pressure-sensitivity not only includes the influence of wax; the appearance of the platform stage and trough pattern phenomenon proves that there is also the influence of asphaltenes. The main component of the deposition is C27+, and the amount of wax precipitated can reach 2.72%. For crude oil with a pressure lower than 15.83 MPa, the amount of asphaltene deposition reaches 0.04% at temperatures above WAT. Asphaltene can inhibit the aggregation and growth of wax crystal molecules, leading to a decrease in WAT and an increase in the viscosity at temperatures above WAT; however, the inhibition may be advanced or delayed with decreased pressure. Asphaltene also enhanced the strength of the wax crystal network structure, leading to an increase in the viscosity of crude oil at the critical shear stress under low pressure. When wax precipitation is dominant, there is a positive correlation between asphaltene deposition and wax, and the growth rate of wax is higher than that of nucleation in the early stage. This study improves the understanding of the effect of pressure on the physical properties of high-PTT live oil, provides reliable parameters for numerical simulations and the adjustment of the injection scheme, and lays the foundation for future evaluation experiments on wax removal chemicals.

A Fast Analysis Method for Blue-Green Laser Transmission through the Sea Surface.

The fast estimation of blue-green laser transmission characteristics through the fluctuating sea surface, such as refraction angles and transmittance, is very important to correct operating parameters, detection depth and anti-detection warning in airborne Light Detection and Ranging (LiDAR) applications. However, the geometry of the sea surface is changed by complex environment factors, such as wind and wave, which significantly affect the rapid acquisition of the blue-green laser transmission characteristics. To address this problem, a fast analysis method is provided to rapidly compute the blue-green laser transmittance and refraction angles through the fluctuating sea surface driven by different wind directions and speeds. In the method, a three-dimensional wave model driven by the wind was built to describe the wave spatial distribution varying with time. Using the wave model, the propagation path of the scanning laser footprint was analyzed using the proposed meshing method, thus the transmittance and refraction angles of the optical path can be fast obtained by using parallel computing. The simulation results imply that the proposed method can reduce the time consumption by 70% compared with the traditional analytical method with sequential computing. This paper provides some statistical laws of refraction angles and transmittance through the fluctuating sea surface under different wind conditions, which may serve as a basic for fast computation of airborne LiDAR transmission characteristics in complex environments.

Contamination of Coupling Glass and Performance Evaluation of Protective System in Vacuum Laser Beam Welding

Vacuum laser beam welding enables deeper penetration depth and welding stability than atmospheric pressure laser welding. However, contaminated coupling glass caused by welding fumes in the vacuum space reduces laser transmittance, leading to inconsistent penetration depth. Therefore, a well-designed protective system is indispensable. Before designing the protective system, the contamination phenomenon was quantified and represented by a contamination index, based on the coupling glass transmittance. The contamination index and penetration depth behavior were determined to be inversely proportional. A cylindrical protective system with a shielding gas supply was proposed and tested. The shielding gas jet provides pressure-driven contaminant suppression and gas momentum-driven contaminant dispersion. The influence of the shielding gas flow rate and gas nozzle diameter on the performance of the protective system was evaluated. When the shielding gas flow was 2.0 L/min or higher, the pressure-driven contaminant suppression dominated for all nozzle diameters. When the shielding gas flow was 1.0 L/min or lower, gas momentum-driven contaminant dispersion was observed. A correlation between the gas nozzle diameter and the contamination index was determined. It was confirmed that contamination can be controlled by selecting the proper gas flow rate and supply nozzle diameter.

A Protocol for Transcranial Photobiomodulation Therapy in Mice.

Transcranial photobiomodulation is a potential innovative noninvasive therapeutic approach for improving brain bioenergetics, brain function in a wide range of neurological and psychiatric disorders, and memory enhancement in age-related cognitive decline and neurodegenerative diseases. We describe a laboratory protocol for transcranial photobiomodulation therapy (PBMT) in mice. Aged BALB/c mice (18 months old) are treated with a 660 nm laser transcranially, once daily for 2 weeks. Laser transmittance data shows that approximately 1% of the incident red light on the scalp reaches a 1 mm depth from the cortical surface, penetrating the dorsal hippocampus. Treatment outcomes are assessed by two methods: a Barnes maze test, which is a hippocampus-dependent spatial learning and memory task evaluation, and measuring hippocampal ATP levels, which is used as a bioenergetics index. The results from the Barnes task show an enhancement of the spatial memory in laser-treated aged mice when compared with age-matched controls. Biochemical analysis after laser treatment indicates increased hippocampal ATP levels. We postulate that the enhancement of memory performance is potentially due to an improvement in hippocampal energy metabolism induced by the red laser treatment. The observations in mice could be extended to other animal models since this protocol could potentially be adapted to other species frequently used in translational neuroscience, such as rabbit, cat, dog, or monkey. Transcranial photobiomodulation is a safe and cost-effective modality which may be a promising therapeutic approach in age-related cognitive impairment.

Effect of surface scallop tool marks generated in micro-milling repairing process on the optical performance of potassium dihydrogen phosphate crystal

Micro-milling is the prevailing method to repair laser-induced surface damage on potassium dihydrogen phosphate crystal components applied in high-power laser systems. Scallop tool marks are inevitably generated on repaired surfaces due to the interaction of neighboring tool paths involved in the micro-milling process. In this work, the effect of scallop tool marks on the optical performance of micro-milled crystal surfaces is theoretically and experimentally investigated. The results indicate that surface tool marks could be categorized into four bands having different period lengths (L) according to the levels of induced light intensification: high-frequency (0.1 μm ≤ L ≤ 0.7 μm), high-risk (0.7 μm ≤ L ≤ 1.2 μm), intermediate-frequency (1.2 μm ≤ L ≤ 100 μm), and low-frequency (100 μm ≤ L ≤ 1000 μm) bands. Tool marks in high-risk band play dominant role in degrading the repairing effectiveness that they should be strictly excluded. While for intermediate-frequency band marks, the induced light intensification is low, and the laser damage resistance and transmittance capacity are comparable to those of tool mark-free surface. The experimental results agree well with simulation results, which suggest that machining parameters corresponding to tool marks in intermediate-frequency band should be preferred in the actual micro-milling repairing process.

Bench-Scale Test Facility for Evaluating the Performance of Thermal Imagers for Fire Service Applications.

A bench-scale facility was developed for the evaluation of thermal imaging cameras. Smoke obscuration conditions in the optical smoke cell were characterized by measuring laser light transmittance through the cell. Measurements showed that the laser transmittance along the axial direction of the optical smoke cell was relatively uniform in the upper and lower halves of the cell for various smoke obscuration conditions. The thermal sensitivity of thermal imagers was investigated using the Michelson Contrast (CM) as a performance metric for a bar target viewed through the smoke-filled cell for different background thermal conditions. The results of the study indicate that the optical smoke cell can be utilized as a well-controlled and effective bench-scale test apparatus to evaluate aspects of the performance of thermal imagers for fire service applications.

Optical property measurements of 235 mm large-scale Ti:sapphire crystal

A Ti:sapphire crystal with a diameter of 235 mm and thickness of 72 mm was grown by the heat exchange method (HEM). The absorption intensity of the crystal at 532 nm averaged at 91%. The figures of merit (FOMs) at different positions of the crystal were measured and the FOM value in the central region was found to reach 90. The transmittance laser beam was intact with no obvious distortions and had only a small deformation compared with the incident laser beam. A small-signal amplification experiment was performed on the Ti:sapphire crystal and a gain of more than 6 times was achieved with a pump energy density of 1.98 J/cm2. These tests indicate that the 235 mm Ti:sapphire crystal has excellent optical qualities and will further improve the energy output of a 10 PW laser system.

Numerical and experimental investigation on laser transmission welding of fiberglass-doped PP and ABS

Fiberglass-doped polypropylene (PP) and fiberglass-doped acrylonitrile butadiene styrene (ABS) are more difficult to be laser transmission welded than other pure plastic because of its low laser transmittance, and this paper aims to quantificationally investigate the relationships of welding parameters, molten pool area and shear strength in welding fiberglass-doped PP and fiberglass-doped ABS by simulation and experiment method. Firstly, comparing with previous researches, several technique difficulties in this welding experiment are pointed out. Secondly, a new hybrid heat source model is proposed to build three-dimensional finite element model for predicting temperature distribution and molten pool geometrical size by coupling Rotary-Gauss body heat source and Gauss plane heat source. Then, a group of taguchi experiment is completed to obtain the characteristics of welded joint, and to evaluate the precision of hybrid heat source model under various welding parameters. Furthermore, the mathematic relationships between molten pool area, laser energy per unit of area and shear strength are fitted out to search optimal welding parameter. Eventually, microstructure observation and fracture appearance analysis experiments are implemented to reveal the root cause of above relationships. This proposed hybrid heat source model and mathematical relationships exhibit commendable value for guiding practical production of laser transmission welding.

Oxygen effects on the performance of XeCl excimer lasers

We have investigated the degradation of window transmittance of XeCl excimer laser with oxygen, from which it was analyzed the laser performances such as stability of output energy, pre-ionization voltage, and spatial shift of laser beam. We found that oxygen suppressed the generation of by-products due to the chemical reactions between electrode material and chlorine. The degradation of transmittance ratio of laser window with oxygen improved from 10.4% to 1.4% after 20 million shots compared to without oxygen. Analyzing XPS spectrum for the contaminated window, we have confirmed that W and Cu on window surface were reduced in case of with oxygen, which means oxygen has a role to suppress the contamination on window surface.

The influence of laser ablation plume at different laser incidence angle on the impulse coupling coefficient with metal target

A calibrated pendulum measuring device and a dimensionless analysis method were used to measure the impulse coupling coefficient at different laser intensities with aluminum, steel, and iron targets. The experiment was performed with a pulsed laser with the wavelength of 1.06 μm and the pulse duration of 7 ns. The experimental measurements of the variation of the impulse coupling coefficient versus the laser energy density agree with the theoretical prediction, and the optimum laser energy density correlated with the maximum impulse coupling coefficient corresponding to the theoretical predictions. The impulse coupling coefficients with laser incidence angles of 0° and 45° are compared for understanding of the effects of the ablation plume on the impulse coupling effect, and the experimental result shows that the impulse coupling effect grows as the incidence angle changes from 0° to 45°. Furthermore, the transmittance of the incident laser through the ablation plume in front of the target is deduced from the impulse measurements, and the effect of the ablation plume on the impulse coupling at high laser intensity is discussed. In order to investigate the weak impulse coupling effect, which is difficult to obtain from the experiments, the impulse coupling coefficient at low laser energy density was calculated by the finite element simulation.

Attenuation characterization of 532 nm and 1064 nm laser propagating in rain

Investigation of the influence of rain attenuation on the laser propagating performance was performed. In this paper, the rain attenuation model was modified based on the engineering application. The attenuation coefficients of 532nm and 1064nm laser at different rainfall rates were calculated, and the factors that affect the energy attenuation were analyzed. It is revealed in the simulations and experiments, that the energy transmittance of the two lasers decreased with the increase of rainfall rate, and the further propagating distance was, the more obvious the decline. Moreover, the increased divergence angle of the beam would enlarge the disparity of the energy transmittance of the two lasers, and the energy transmittance of 1064nm laser reduced more than that of 532nm. The experimental data were well matched with the simulation results, and the maximal deviation was about 5%.