International Journal of

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In this paper, design and fabrication of an internal resonant enhanced frequency doubling of the continuous-wave ytterbium-doped fiber laser at 1064nm using a Fabry-Perot bow tie cavity inside the fiber laser cavity is presented. The 3.5W power coupled into the enhancement cavity is amplified to 163W by the intracavity passive locking technique. By placing an LBO crystal within this resonant enhancement cavity, conversion efficiency of the second harmonic generation of the laser in continuous regime is increased from 0.023% to 51.42% (i.e. about 2200 times) which results to generation of 1.8W light at 532nm.

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In this paper, the interaction of Gaussian laser beam with under-dense plasma by taking the weakly relativistic ponderomotive nonlinearity has been investigated. For this purpose, the effect of the linear plasma electron temperature and upward exponential electron density profile on the laser propagation has been studied. The nonlinear second-order differential equation of the dimensionless beam-width parameter, f, on the distance of propagation, h, is derived by following WKB and paraxial approximation and solved numerically for the several initial electron temperatures. It is found that, the electron temperature ramp combined with upward ramp density profile would be caused stronger self-focusing where the beam width oscilates with less amplitude and smaller spot size. This could lead to further penetration of the laser beam inside the plasma by reducing the effects of diffraction.

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The random laser (RL) emission characteristics can be improved by many different routes including either the material processing or optimizing the concentration of the relevant constituents. These routes can be very hard and even not practical in many cases, leaving us with the search of new schemes for the externally improvement of the random laser performance. In this paper, we suggest a simple approach for the externally enhancement of the random laser emission properties that can be applied in any designed transparent random lasing structures with single mode or multi-mode emission. This approach is based on using an adhesive tape in order to introduce an external scattering medium to the lasing structure and also return back the amplified leaking photons. For our investigated sample with nonresonant feedback, it is demonstrated that the emission intensity can be increased by a factor of 4.2 and the random laser threshold can be decreased by a factor of 1.8.

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In this paper, an analytical model is presented to compare the monolithic end-pumped and distributed side-pumped arrangements in the master oscillator power amplifier (MOPA) of Q-switched (QSW) double-clad (DC) ytterbium (Yb)-doped fiber system. First, the time-dependent rate equations are solved numerically by the finite difference method and the output pulse characteristics are obtained. For more amplifying, the laser pulse is injected into the amplifier and the gain and saturation coefficients are obtained by using the best fitting between the outcoming data from solving rate equations and the transient amplification relation, based on the least squares method (LSM). Finally, the dependence of pump power and dopant concentration on the cavity amplifying parameters are investigated.

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In this work, for the first time, the improved lasing performance of a blue GaN-based laser diode is demonstrated by the introduction and vertical optimization of a new quadruple asymmetric waveguide structure. In the new proposed waveguide structure, in the first step, p-waveguide and electron blocking layers have been omitted. Then a triple asymmetry was considered for the design of an AlGaNp-cladding layer inside the waveguide structure. The performances of the conventional and proposed laser structures were theoretically studied using the photonic integrated circuit simulator in 3D simulation software. The 3deminsional simulations of carrier transport, optical wave- guiding and self-heating were combined self-consistently in the software. A good agreement was achieved between simulations and experiments by careful choice of different material parameters in the physical models. The effects of the AlGaN p-cladding layer properties on the performance of the new quadruple asymmetric waveguide GaN-based laser were theoretically studied. Threshold current, output power, and operation voltage were compared for different composition of Al, doping, and thickness of the AlGaN p-cladding layer. According to the simulation results, the optimized values of Al composition, doping, and thickness of the AlGaN p-cladding layer obtained for high-power performance.

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Laser diode beam divergence is the main parameter for beam shaping and fiber optic coupling. Increasing the waveguide layer thickness is the conventional method to decrease the beam divergence. In this paper, the broadened asymmetric waveguide is introduced to decrease the divergence without increasing the optical power. The asymmetric waveguide was used to shift the vertical optical field to n-section, which has lower free carrier loss. The main target in this research is to minimize the internal loss to avoid the disadvantage of the broadened waveguide. Finally the beam divergence was decreased to 35 degrees that is very suitable for the conventional multi-mode optical fiber coupling and the optical power was increased to 2400mW in the laser diode with 100μm stripe width and 1mm cavity length. In addition to the fiber coupling, this improvement can be used for other direct applications that need beam shaping. 
 

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This paper presents the design procedure of folded-resonators for high-average power thin-disk lasers (TDLs). Because of the oblique angle of incidence in the laser path inside the resonator, folded resonators introduce astigmatism. Additionally, the dependency of the dioptric power of the active medium on the pump power made the resonator design more complicated. In the first section of this work, the disk thermal lensing was measured using a wavefront sensor, and the measurement procedure was presented and thoroughly discussed. The disk radius of curvature varied between 4.3 m to 6.4 m depending on the pump power. The disk was considered a variable lens inside the resonator based on the measurement results. V-shaped and L-shaped configurations' stability and M² factor were predicted, optimized, and compared. Astigmatism in the resonator parameters was considered and discussed. While the V-shaped cavity has better beam quality, the L-shaped cavity has less sensitivity to cavity misalignment. The primary approach of this paper was the resonator design of a cavity-dumped disk laser. However, the designed resonator configurations could be utilized in many laser resonators, such as industrial TDLs (to reduce the overall length of the system) and second harmonic-generation in TDLs.
 

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In this study, the self-guiding of an ultrashort laser pulse through air is investigated. Therefore, the terms of self-focusing, plasma defocusing and the pulse energy depletion due to the ionization, are considered in the wave equation. Then the laser pulse spot size equation is obtained using the source-dependent expansion method. Our results show that the laser pulse self-guiding occurs for the first twenty Rayleigh lengths. However, the laser pulse undergoes diffraction as it propagates further along the z axis. Moreover, it is seen that the back of the laser pulse is diffracted the most owing to the fact that the plasma is formed as the laser pulse propagates through air. It is also shown that the spot size variations affect the temporal and spatial profiles of the laser intensity, the laser pulse power and the ionization process.

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ta charset="UTF-8" >We investigate the Cu₂FeSnS₄ (CFTS) thin film. The raw materials of this thin film are copper, iron and tin, which are mixed in the form of tablets and then are deposited on a glass substrate through the process of pulsed laser deposition (PLD). The produced metallic thin films are then sulfurized to carry out the process of merging the element sulfur in the thin films and forming CFTS structure. We investigate the influence of sulfurization temperature and the laser pulse energy in the PLD process on the deposited CFTS thin films. The X-ray diffraction (XRD), Raman and UV-Vis analyses’ results show that by decrease in sulfurization temperature from 600 °C to 550 °C the crystal quality of the thin films is improved, which is realized by increase in volume and quality of the CFTS phase. On the other hand, the results confirm that the laser fluence is a decisive factor which should be taken into account to achieve an optimized structure.

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In this study, we investigated the production of silver nanoparticles by pulsed Nd: YAG laser ablation with λ=532nm in distilled water. The sodium citrate used to control the size of nanoparticles (Nps). The sample containing Ag NPs was characterized by linear absorption spectroscopy (UV-Visible spectroscopy) and transmission electron microscopy (TEM observation). The behavior of nonlinear optical properties of silver nanoparticles was studied using the Z-Scan method at two optimum numbers of the laser pulses and four optimum laser energy densities. In the Z-Scan method, the nonlinear thermal properties of Ag NPs were investigated under exposure to nanosecond laser pulse at λ=532nm. They were gained by fitting theoretical and experimental data. The values of the nonlinear refractive index (n₂) and the nonlinear absorption coefficient (β ) were compared concerning to two optimum numbers and four optimum energy densities. The results of the nonlinear refractive index showed a negative value for each sample, this means that samples act as a divergent lens, and the thermal self-defocusing effect can be the main factor of nonlinear behavior. Following the comparison of two quantities, n₂ and β , we found that the nonlinear refractive index increased when the number of laser pulses light increased. In addition, the nonlinear absorption coefficient decreased when the number of laser pulses light increased. As a result, the application of these Ag NPs for optical switching devices was investigated, which demonstrated that the large Ag NPs are applicable tools for optical switching devices.

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The creation of single zeptosecond pulses is important in various scientific fields, particularly for studying time-resolved nuclear processes. In this study, a clean single sub-attosecond (270 zeptosecond) pulse is obtained in the simulation of a femtosecond laser with over dense plasma. First, the desired laser and plasma parameters are utilized to achieve the nano-bunches in the plasma surface and the desired spectrum up to 2000th harmonics is obtained. Then, different filters such as filtering the harmonics and different intensity filters are applied where, the special exponential function is due to a very clean single zeptosecond pulse.
 

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Sub-Doppler dichroic atomic vapor laser lock (DAVLL) is a modulation-free laser stabilization method that combines DAVLL and saturated absorption spectroscopy (SAS). The performance of this highly sensitive stabilization technique strongly depends on the characteristics of the generated error signal. The slope of the error signal determines the lock sensitivity or how fast the frequency compensation could be made in the feedback loop, and the amplitude of the error signal determines the lock stability or how much noise the feedback loop can tolerate before laser unlocking. We have analytically modeled the error signal of the sub-Doppler DAVLL considering all possible transitions between Zeeman sublevels and compared it with the experimental results. Using the analytical and experimental results, it is shown that the values of the required magnetic fields for maximizing the slope and amplitude of the error signal are close to each other. Selecting the mentioned values of the magnetic field for optimization of the sub-Doppler DAVLL error signal is highly useful for sensitive and stable laser locking.
 

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In this article, the conditions of pulse production in two mutually coupled lasers are studied. Based on the obtained characteristic equation and its roots, the dynamical behavior of the system and the threshold of the instability are analyzed. For the stable operation of the system and with the use of the time series curves, it is possible to study the dynamical behavior and the stability ranges of the laser in the presence of the saturable absorber and the gain environment. This paper aims to achieve from quasi-periodic behavior in a solitary laser to the generation of a pulse train from two mutually coupled lasers in the presence of saturable absorbers. Also, the stability range for a solitary laser and then for two coupled lasers in the presence of saturable absorbers have been studied.

 

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The simulation results of a 10-kW heat capacity slab laser are presented. Two different schemes for optical pumping with high-power laser diodes are investigated. The simulation of optical pumping using ZEMAX software demonstrates a uniform pump distribution within the laser slabs. Additionally, the temperature distribution in the laser slab is examined using COMSOL. The findings for two distinct laser designs reveal that increasing the slab dimensions reduces the temperature distribution and thermal issues. Furthermore, cooling schemes indicate that the cooling phase of a 10-kW HCL falls within the range of 20-40 seconds. A comparison of water and air cooling of the optically pumped slabs during the cooling phase demonstrates that water cooling is more efficient than air cooling. The simulation results confirm that the proposed laser will be an efficient device for laser material processing. A focused 10-kW HCL laser will melt the steel sheet after less than 1 s at 1490 K.

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In this research, an attempt is made to approach a specific type of carbon, such as graphene or carbon nanotubes by using pulsed laser ablation technique in deionized water environment with changing the laser factors such as wavelength and fluence. Nd:YAG laser with two wavelengths of 1064 and 532 nm and three fluence of 0.8, 1 and 1.2 J/cm² were selected that number of pulses was 5000 with a frequency of 10 Hz to be irradiated on the graphite target at about 10 minutes. The medium was distilled water. Graphite was located in the 40 ml of distilled water.
The effects of wavelength and fluence of the laser have been experimentally investigated on types of carbon characteristics with different analysis such as Raman scattering spectrum, FE-SEM images, UV–Vis-NIR spectrum and X-ray diffraction (XRD). By using the mentioned analysis, the type of synthesized nano carbon is studied.
This study evaluates the effects of the pulse energy and laser wavelength on properties of synthesized carbon nanoparticle in laser ablation method in medium of distilled water.

 

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Background: Multiple Sclerosis (MS) is a chronic immune-mediated disease affecting the central nervous system, leading to various disturbances, including visual impairment. Early and accurate diagnosis of MS is critical for effective treatment and management. Scanning Laser Ophthalmoscopy (SLO) is a non-invasive technique that provides high-quality retinal images, serving as a promising resource for the early detection of MS. This research investigates a vessel-based approach for MS detection in SLO images using Long Short-Term Memory (LSTM) networks.
Material and Methods: A total of 106 Healthy Controls (HCs) and 39 MS patients (78 eyes) were enrolled. After implementing quality control measures and removing poor-quality or damaged images, the research utilized a total of 265 photos (73 MS and 192 HC). An approach for the early detection of MS in SLO images using LSTM network is introduced. This approach involves two steps: 1.It involves pre-processing and extracting vessels and then pre-training a deep neural network using the source dataset, and 2. tuning the network on the target dataset of SLO images.
The significance of vessel segmentation in MS detection is examined, and the application of the proposed method in improving diagnostic models is explored. The proposed approach achieves an accuracy rate of 97.44% when evaluated on a test dataset consisting of SLO pictures.
Through experiments on SLO datasets and employing the proposed vessel-based approach with LSTM, empirical results demonstrate that this approach contributes to the early detection of MS with high accuracy. These models exhibit the capability to accurately detect the disease with high precision and appropriate sensitivity.
 

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In this research, palladium nanoparticles (Pd NPs) were first synthesized using laser ablation in the deionized (DI) water environment. Also, metal-organic framework (MOF) was produced using the solvothermal method at a temperature of 150°C. To accumulate Pd NPs on the synthesized MOF, ultrasonic and magnetic stirring methods were used. Different analytical methods were used to investigate the structure and morphology of the synthesized nanocomposite. Also, the sensitivity of the synthesized nanocomposite to ethanol and methanol organic vapors was investigated. The results showed an increase in the response of the MOF in the presence of nanoparticles.

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Laser pulse overlapping (LPO) is an important factor affect the behavior of the laser treated surfaces. For laser surface treatment especially at high fluences, the laser beam must be focused to reach the desired fluence. Then laser beam or sample scanning in 2 directions is done to treat a surface area. In this paper, effect of the distance between laser treated lines (scanned in x direction) on the surface properties including morphological changes and wettability modifications of 304 stainless steel is investigated. The results show that the morphology and chemistry of the surface are influenced effectively by changing the overlap between laser treated line on the surface. Then, it should be considered as an important parameter in laser modification of a large surface with focused laser beam.

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In recent years, global climate change and population growth have exacerbated freshwater shortages. To address this issue, harvesting water from atmospheric fog has emerged as a promising technique. Inspired by natural processes, the fabrication of hybrid hydrophilic (HI) and superhydrophobic (SHB) surfaces has gained significant attention for enhancing water harvesting efficiency. This study presents a simple, cost-effective laser ablation method for creating wettability contrast surfaces with triangular and parallel patterns on brass metal. Through X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM), we investigated the structural and morphological effects on the wettability behavior of irradiated and non-irradiated brass. Additionally, we examined the influence of pattern shapes on water harvesting efficiency. Our findings indicate that triangular patterns significantly enhance water harvesting performance compared to parallel patterns.

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The modification of cell surface structures has become a focal point in cell biology, with methods like drugs, chemicals, and non-destructive techniques such as laser light exposure being utilized. In particular, exposure to femtosecond laser pulses has been found to increase cell permeability to formulations without causing thermal damage. This study aimed to observe and document the changes in the structure of Staphylococcus aureus bacteria when they were optically trapped and subjected to femtosecond laser pulses, along with the application of a medicinal substance, over 20 minutes. The research successfully determined the optimal power and exposure time of the laser light on the bacterial surface and demonstrated the ability of femtosecond laser pulses to enhance the efficacy of the medicinal substance.