This work presents the development of a Z-depth system for Confocal Raman Spectroscopy (CRS), which allows for the acquisition of Raman spectra both at the surface and at depth profile in heterogeneous samples. The proposed CRS system consists of the coupling of a commercial 785 nm Raman Probe Bifurcated (RPB) with a 20x/0.40 infinity plan achromatic polarizing microscope objective, a Long Working Distance (LWD) of 1.2 cm, and a 50 μm core-multimode optical fiber used as a pinhole filter. With this implementation, it is possible to achieve both a high spatial resolution of approximately 16.2 μm and a spectral resolution of ∼14 cm−1, which is determined by the FWHM of the thin 1004 cm−1 Raman profile band. The system is configured to operate within 400–1800 cm−1 spectral windows. The implementation of a system of this nature offers a favorable cost–benefit ratio, as commercial CRS is typically found in high-cost environments such as cosmetics, pharmaceutical, and biological laboratories. The proposed system is low-cost and employs a minimal set of optical components to achieve functionality comparable to that of a confocal Raman microscope. High signal-to-noise ratio (SNR) Raman spectra (∼660.05 at 1447 cm−1) can be obtained with short integration times (∼25 s) and low laser power (30–35 mW) when analyzing biological samples such as in vivo human fingernails and fingertips. This power level is significantly lower than the exposure limits established by the American National Standards Institute (ANSI) for human laser experiments. Raman spectra were recorded from the surface of both the nails and fingertips of three volunteers, in order to characterize their biological samples at different depths. The measurements were performed in 50 μm steps to obtain molecular structural information from both surface and subsurface tissue layers. The proposed CRS enables the identification of differences between two closely spaced, centered, and narrow Raman bands. Additionally, broad Raman bands observed at the skin surface can be deconvolved into at least three sub-bands, which can be quantitatively characterized in terms of intensity, peak position, and bandwidth, as the confocal plane advances in depth. Moreover, the CRS system enables the detection of subtle, low-intensity features that appear at the surface but disappear beyond specific depth layers.

We report the improved performance of the power conversion efficiency of Stokes lines generated in an optical fiber (Corning LEAF or Corning MetroCor) by splicing a fiber segment with a lower MFD (Nufern 1060-XP or Nufern 980- HP). The experimental configuration 4-Km LEAF+1-m +1-m 1060-XP has better performance in the 1st Stokes of (1-0.39/0.67) 4.17 %, and in the 2nd Stokes of (1-0.55/0.64) 14 % compared to the configuration that uses 4-Km LEAF fiber without 1060-XP fiber segment. On the other side, the experimental configuration 4Km MetroCor + 0.5-m 980-HP, had an improved performance in the 1st Stokes of (1-0.64/0.5) 28 %, and in the second Stokes of (1-0.69/0.5) 38 % compared to MetroCor optical fiber without 980-HP fiber segment.

The majority of wind power is currently produced on high wind speed sites by large wind turbine, whereas small wind turbines often operate in light wind conditions. Small capacity wind turbines have not received the same engineering attention as their large counterparts. This is partially due to a number of unique problems that small wind turbines experience. The most relevant are: low operating Reynolds number (Re<500,000) and high angles of attack. Several studies have suggested that flow control devices such as the spherical tubercle could be used to increase lift before stall and generate more power in such situations. The aim of this study is to determine the effect of tubercle amplitude on aerodynamic performance of an airfoil at low-Re numbers (Re=300,000 & Re=400,000). Three amplitudes were considered in this study: A1=0.005c, A2=0.01c, and A3=0.03c. A detailed 2D simulation study is carried out using a calibrated Transition SST k-ω turbulence model to obtain aerodynamic coefficients and flow characteristics. Results indicate that small tubercles perform better overall than larger tubercles. The airfoil with the smallest tubercle outperforms the unmodified airfoil at both studied Reynolds numbers at angles of attack 0° – 4°. The analysis of the aerodynamic coefficients indicates that the improvement of the aerodynamic performance of the airfoils with tubercles is due to the reduction of the drag coefficient. Pressure, intermittency and wall shear stress contours suggest that the overall drag reduction is achieved through the decrease of friction drag.

This study utilized solid-state lasers with a 50 ns pulse duration in a Q-switched mode of operation at wavelengths of 1.06 µm and 0.69 µm to investigate the hemispherical reflectivity of niobium. Our experimental results show that the reflectivity of niobium decreases notably as the laser fluence increases towards the plasma formation threshold for ablation at both studied wavelengths, which we attribute to changes in the absorptivity of the surface resulting from plasma formation. We also observed a significant effect of laser wavelength on the reflectivity values of the sample at low laser fluence. By determining the threshold fluence values for each wavelength, we estimated the surface temperature associated with the threshold fluence for plasma formation. Our calculations revealed discrepancies between published values for optically polished and mechanically polished niobium, which we suggest may be due to the presence of nano/micro defects, oxide films, and contaminants that amplify the wavelength-dependent effects on reflectivity. These findings have important implications for the design of optical components and laser processing techniques that use niobium, as well as for the development of accurate models of laser-material interactions. Further research is needed to fully understand the underlying mechanisms driving the observed effects and to explore potential applications of niobium in laser-based technologies.

In this report, we present our analysis of the relationship between critical power and stimulated Raman scattering in Raman fiber lasers. Through our research, we have established a connection between the R.G. Smith constant at critical power and the necessary pump power required to reach the maximum power delivered by the first Stokes just prior to the generation of the second Stokes. In our experiments, two setups were successful in reaching the second Stokes generation, one utilizing a glass–air interface as the output coupler without HR mirrors and the other using HR-FBGs for both Stokes in conjunction with a glass–air interface. We found that the 1 Km 1060-XP fiber has an R.G. Smith constant of ~4.94 at critical power, which when multiplied by 2 gives ~9.88, a value close to the R.G. Smith constant (9.75) for maximum Stokes corresponding to a pump power of 5.5 W, with an approximation of ~98.6%. Our results demonstrate the importance of knowing the R.G. Smith constant at critical power in estimating the necessary pump power to achieve maximum power delivery in any Stokes component.

Hoy en día los esfuerzos realizados en el desarrollo de nuevos materiales para ingeniería se han diversificado en diferentes ramas, enfatizando en este coloquio los aspectos ambientales, por lo que las investigaciones contenidas en los dos volúmenes de este handbook están enfocadas al desarrollo de materiales aplicados a la Ingeniería y al Medio Ambiente.

Se presenta una propuesta experimental para el desarrollo de un velocímetro láser Doppler que, por medio de interferometría de haces láser, permite detectar frecuencias de vibración para establecer el estado de vida útil de rodamientos. El diseño experimental consiste en un arreglo de siete fibras ópticas multimodo, seis de estas circundantes a una fibra central. Las seis fibras circundantes transportan luz de una misma fuente láser, pasan por una lente biconvexa, y en el punto focal de la lente, se genera la interferencia de los seis haces de luz que se manifiesta como patrón de franjas oscuras y brillantes.

This work presents a methodology to estimate the pumping power required for the first Stokes to reach its maximum stored energy level, before it generates the next Stokes. These estimates are achieved by experimentally measuring the critical power and the relationship between the pumping power (PP0) and the small signal of the stimulated Raman spread (PF0). For our study we used 1 km of 1060-XP fiber, experimentally obtaining Pcr = 6.693 W, PF0/PP0 = 6.759x10−6. With these experimental data, the pump power required for the first Stokes to reach its maximum stored energy level was 13.39 W, and the stored energy in the first Stokes was 9.88 W. It is important to note that the Raman threshold ln(PP0/PF0) = 11.9 is smaller than the initially reported ∼16.

The majority of wind power is currently produced on high wind speed sites by large wind turbine, whereas small wind turbines often operate in light wind conditions. Small capacity wind turbines have not received the same engineering attention as their larger counterparts. This is partially due to a number of unique problems that small wind turbines experience. The most relevant are: low operating Reynolds number (Re<500,000), and poor performance at high angles of attack. Low and medium wind speed sites (Class II–IV) are more common than high wind speed sites, meaning there is a large source of energy not being taken advantage of. Several studies have suggested that flow control devices such as the spherical tubercle could be used to increase lift before stall and generate more power in such situations. The aim of this study is to determine the effect of tubercle amplitude on aerodynamic performance of an airfoil at low-Re numbers (Re=300,000&Re=400,000). Three amplitudes were considered in this study: A1=0.005c, A2=0.01c, and A3=0.03c. A detailed 2D simulation study is carried out, using FLUENT (a commercial CFD software) and the TransitionSSTk−ω turbulence model, to obtain aerodynamic coefficients and flow characteristics. Results indicate that small tubercles perform better overall than larger tubercles. The airfoil with the smallest tubercle outperforms the unmodified airfoil at both studied Reynolds numbers at angles of attack 0° – 4 °. Moreover, the airfoil with the largest tubercle outperformed all of the airfoils at an angle of attack of 0° and Re=300,000. The analysis of the aerodynamic coefficients indicates that the improvement of the aerodynamic performance of airfoils with tubercles is due to the reduction of the drag coefficient. Pressure, intermittency and wall shear stress contours suggest that the overall drag reduction is achieved through the decrease of friction drag. The decrease in friction drag is attributed to the thickening of the laminar boundary layer, caused by a more favorable pressure distribution around the airfoils with the aerodynamic improvements. Moreover, the drastic deterioration in aerodynamic performance at higher angles of attack is attributed to the turbulence generated by the tubercles. This study suggests that spherical tubercles could have a potential application in small wind turbines.

In this paper, ferrite nanoparticles of formula CoxFe3-xO4 (where x = 0.0, 0.5 and 1) have been synthetized, through the chemical co-precipitation method, in a basic medium through the use of Fe (III), Fe (II) and Co (II) solutions. The powders were characterized by Raman spectroscopy and X-Ray Diffraction (XRD). The diffractograms indicated the Co (II) ions occupied the tetragonal region, modifying considerably the distribution of the rest of the ions present in the inverse spinel structure. Raman spectra shows variations in the intensity of the characterized bands located at 460, 624 and 672 cm-1 for each structure.

Reflection of nanosecond laser pulses with different wavelengths (1.06 and 0.69 mu m) in ablation of titanium in air is studied experimentally. The laser wavelength effect on reflection is essential at low laser fluence values. However, it becomes negligible for laser fluence values by about an order of magnitude higher than the plasma ignition threshold. We speculate that the disappearance of the wavelength effect is explained by counter-acting processes of the laser light absorption in plasma, which increases with laser wavelength, and absorption in the surface layer, which decreases with increasing laser wavelength.

We report an experimental study of the signals delivered by a Raman fibre laser under different output coupling conditions. Our main results allow us to conclude that feedback reflectivity variations influence Stokes energy-transfer mechanisms. We also confirmed that in low-feedback systems high conversion efficiencies imply high thresholds for Stokes generation, whereas the contrary occurs for high-reflective output cavities.

The total reflectivity of tin and magnesium in ablation by nanosecond Nd:YAG laser pulses in air is studied. It was found that the high initial reflectivity of the studied metals undergoes a significant drop to values of 0.11 for Sn and 0.16 for Mg within a laser fluence range between about 0.8 and 11J/cm2. These reduced reflectivity values remain virtually unchanged with further increasing laser fluence. This study shows that a significant reflectivity decrease of the studied metals is caused by plasma formation in front of the irradiated surface. Below the plasma formation threshold, the reflectivity of the studied metals is observed to be virtually independent of laser fluence, indicating a small role of Drude׳s temperature effect on the reflectivity of the studied samples.

This paper presents a theoretical and experimental study on the behavior of the Raman gain efficiency for any Stokes and excitation wavelength. Analyzing the Raman threshold for two adjacent Stokes we find a mathematical relationship to the Raman gain efficiency. This equation scales inversely with wavelength and directly with the fiber attenuation, and also provides approximate quantities to numbers already published. From experimental data for a LEAF fiber, we construct two curves, one based on our equation and the other based on the approach normally used, and the comparison of the two curves reveals significant errors. Additionally, we show that the spectrum of the Raman gain efficiency has the same shape as the attenuation spectrum.

Total reflectivity of silver and molybdenum samples irradiated by high-intensity nanosecond Nd:YAG laser pulses in air of atmospheric pressure is experimentally studied as a function of laser fluence in the range of 0.1–100 J/cm2. The study shows that at laser fluences below the plasma formation threshold the total reflectivity of both silver and molybdenum remains virtually equal to the table room-temperature reflectivity values. The total reflectivity of these metals begins to decrease at a laser fluence of the plasma formation threshold. As the laser fluence increases above the plasma formation threshold, the reflectivity sharply drops to a low value and then remains unchanged with further increasing laser fluence. Calculation of the surface temperature at the plasma formation threshold fluence shows that the surface temperature value is substantially below the melting point that indicates an important role of the surface nanostructural defects in the plasma formation on a real sample due to their enhanced heating caused by both plasmonic absorption and plasmonic nanofocusing.

We present a simple novel technique that determines with precision the Raman gain efficiency for telecom fibers. The method is supported by a mathematical relation, which is the solution for the coupled differential equations that govern the stimulated Raman scattering. This technique is valuable due to its simplicity and consists of measuring the residual pump and the Raman scattering signals at the stimulated Raman scattering threshold. Our technique was proven for different fibers; as a result, we found that the rate of pump power to stimulated Raman scattering at threshold is lower than 16, which is the historically used value.

A comparative study on reflection of nanosecond Nd-YAG laser pulses in ablation of aluminum in air and in vacuum under the same other experimental conditions is performed. We find that, hemispherical total reflectivity of aluminum undergoes a sharp drop at the plasma formation threshold both in the air and in vacuum. The initial large value (0.8) of aluminum reflectivity decreases to a level of about 0.14 and 0.24 for ablation in the air and in vacuum, respectively. These decreased reflectivity values remain virtually unchanged with further increasing laser fluence. The reflectivity drop in the air is observed to be sharper than in vacuum. Our study indicates that the reflectivity drop is predominantly caused by absorption of the laser light in plasma. Nano/micro-structural defects present on practical sample surfaces play the important role in the plasma formation, especially for the ablation in the air, where the plasma formation threshold is found to be by a factor of 3 smaller than in vacuum.

In this work, we study the total reflectivity of mechanically polished metals samples. In our experiment, a Q-switched Nd:YAG laser that generates 50-ns pulses at a wavelength of 1064 nm is used for ablation of a sample. To measure the total hemispherical reflectivity we use an ellipsoidal light reflector technique. The total hemispherical reflectivity is studied as a function of laser fluence in the range of 0.1-100 J/cm(2). The experiments are performed in air at the atmospheric pressure.

A simple experimental configuration for measurement of the Raman gain coefficient is demonstrated. The Raman threshold condition plays a role important to calculate the critical power and the Raman gain coefficient. Analysis of the Raman threshold for second Stokes shows that the Raman gain coefficient scales with the inverse of the pump wavelength and the fiber attenuation, the obtained values are approximate to several quantities previously reported. With those physical properties the single pass evolution of pump and Stokes beams equations are simulated for different fiber lengths and several coupled pump powers. The numerical simulations show that the fibers losses and the numerical aperture play a predominant role in the Stokes generation. These results allow designing optical fibers efficient and/or poor in the Stimulated Raman scattering generation.

Hemispherical total reflectivity of copper, nickel, and tungsten in ablation by nanosecond Nd:YAG laser pulses in air of atmospheric pressure is experimentally studied as a function of laser fluence in the range of 0.1-100 J/cm(2). Our experiment shows that at laser fluences below the plasma formation threshold the reflectivity of mechanically polished metals remains virtually equal to the table room-temperature reflectivity values. The hemispherical total reflectivity of the studied metals begins to drop at a laser fluence of the plasma formation threshold. With increasing laser fluence above the plasma formation threshold the reflectivity sharply decreases to a low value and then remains unchanged with further increasing laser fluence. Computation of the surface temperature at the plasma formation threshold fluence reveals that its value is substantially below the melting point that indicates an important role of the surface nanostructural defects in the plasma formation on a real sample due to their enhanced heating caused by both plasmonic absorption and plasmonic nanofocusing.