RESUMEN
We report a theoretical and experimental study on stimulated Brillouin scattering (SBS) suppression in a monolithic fiber amplifier with filtered and amplified pseudo-random binary sequence (PRBS) phase modulation. Theoretically, we use a time-dependent three-wave coupled nonlinear system considering both active fiber and passive fiber to describe the acoustic phonon, laser, and Stokes characteristics in a fiber amplifier. The SBS threshold power after filtered PRBS phase modulation is numerically evaluated to obtain the optimal parameters, and the time-averaged distributions of the counter-pump power, laser power, and Stokes power at different positions along the fiber length of the fiber system are simulated. Also, we established a four-stage fiber amplifier system to verify our theory. The configuration of the fiber amplifier system includes a filtered and amplified PRBS phase-modulated single-frequency fiber laser, a three-stage pre-amplifier, and a counter-pumping main stage, subsequently. 2.5â kW output power with an FWHM linewidth of 9.63â GHz is accomplished by a domestic ytterbium-doped double-clad fiber with core/cladding diameters of 20.2/400â â µm. The reflectivity of the main stage is 0.049 at the maximum output power, which indicates the proposed architecture is under the SBS threshold. The experiments verify the accuracy of the theoretical model, which provides a reliable reference for evaluating the SBS suppression capability of the high-power narrow-linewidth fiber amplifier phase modulated by the filtered and amplified PRBS signal.
RESUMEN
In this paper, we establish a multi-stage fiber amplifier with pseudo-random binary sequence (PRBS) phase modulation. The stimulated Brillouin gain spectra of the main amplifier with both the unmodulated and pseudo-random binary sequence phase modulated configuration are measured (with corresponding output power), and the stimulated Brillouin scattering (SBS) threshold is investigated experimentally and theoretically. The pseudo-random binary sequence phase modulation parameters are optimized by theoretical simulation. With a two-stage preamplifier chain and a counter-pumping main amplifier stage, a maximum 3.05 kW output power with a slope efficiency of 85.9% is obtained experimentally. The central wavelength of the fiber amplifier is 1050 nm, associated with a full-width at half-maximum linewidth of 13.7 GHz. The stimulated Brillouin scattering reflectivity is below 0.01% at 3.05 kW at 13.7 GHz, which indicates that stimulated Brillouin scattering can be suppressed efficiently at this power and linewidth level.
RESUMEN
Hydrophobic surface drag reduction techniques are effective in reducing the frictional resistance of fluids, the adsorption of liquid molecules on hydrophobic surfaces can reflect the resistance to fluid flow through such solid surfaces. Based on molecular simulation technology, we investigate the adsorption characteristics of water molecules on hydrophobic surfaces to achieve rapid screening of hydrophobic materials in fire-fighting water supply systems. The Monte Carlo method was used to simulate the adsorption process of polymers and to analyze the effects of temperature and fixed adsorption quantity. Contact angle tests were also done to verify polymer hydrophobicity. The isothermal adsorption heat, water molecule distribution, and energy distribution were studied by molecular mechanics and molecular dynamics methods. Then, adsorption localization simulations and electrostatic potential distributions were used to predict possible adsorption sites on hydrophobic surfaces and single-molecule chains. Finally, the interaction energy, diffusion coefficient, and free volume were investigated to explain the adsorption mechanism at the molecular level. Simulation results show that, overall, PTFE was more hydrophobic and PES was more hydrophilic and at 298 K, the number of adsorbed water molecules was ranked as follows: PTFE < PVDF < PVC < PMMA < PPS < CSM < BD-HDI < BD-MDI < BD-TDI < PES. Furthermore, PTFE, PVDF, PVC, PES, and PPS have more stable adsorption configurations on the (0 -1 0) surface. According to the findings, hydrogen bonding dominates the interaction between water molecules and hydrophilic polymers, whereas π-π interactions increase water molecules' diffusion resistance in polymers with benzene rings. In addition, PES contains many sulfone groups and ether bonds, which disorganize the chain arrangement to provide more free volume, whereas the water adsorption rate of PTFE is reduced because its molecular chains are less convoluted and more organized.
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We report a polarized, narrow-linewidth Er:YAG laser operating at 1645 nm, in-band pumped by a continuous-wave (CW), narrowband 1532 nm fiber-coupled laser diode (LD). A maximum polarized continuous wave output power of 8.45 W was obtained, resulting in an optical conversion efficiency of 56% with respect to the absorbed pump power. The central wavelength was measured to be 1645.45 nm, with a full width at half-maximum of 0.13 nm. For Q-switched operation, pulse energy of 12 mJ at 100 Hz pulse repetition frequency and 95 ns pulse duration was yielded. To the best of our knowledge, this polarized pulse energy is the highest energy reported for a Q-switched Er:YAG laser pumped by a CW LD.
RESUMEN
Four coals samples at different ranks were analyzed by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and solid-state 13C nuclear magnetic resonance (NMR). The calculated coal molecular model was constructed according to the experimental data. The mode of evolution of four coal molecules with different metamorphic degrees was explored. The results indicate that the nanostructures of these four coal molecules mainly consist of aromatic structures, aliphatic structures and oxygen-containing functional groups. The coal metamorphic degree is the most important factor affecting the evolution of the coal molecular nanostructure. By increasing the coal rank, the aromatic carbon content and aromatic system increase, while the aliphatic carbon content and aliphatic system decrease, and the species and content of oxygen containing functional groups are also reduced. During the evolution of the molecular microcrystalline structure, the degree of vertical order of the aromatic structural unit, the flatness of the aromatic structural unit (La), the average crystallite stacking height (Lc), and the average number of crystallites in a stack (n) increase, while the interlayer distance between aromatic sheets (d002) decreases; the short-range ordering of the coal structure is mainly caused by changes in the orientational arrangement from intramolecular aromatic layers to intermolecular aromatic layers when low-rank coal molecules evolve to high rank coal molecules. The structural evolution mechanism of coal molecules of different ranks has been revealed through the analysis of the mode of evolution of the molecular structure the coal. This study enables us to better understand the nanostructure evolution mechanism of coal molecules at different ranks.
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The continuous wave (CW) and passively Q-switched performances of Nd:CaYAlO(4) crystal with both a- and c-cut were demonstrated. The CW output powers of 1.15 W and 1.26 W were obtained under the pump power of 8.96 W with slope efficiencies of 15.2% and 16.8% for a- and c-cut samples, respectively. As a result, new dual-wavelength all-solid-state lasers at 1080 nm and 1081 nm were achieved with c-cut crystal. By using Cr(4+):YAG wafer as saturable absorber, we performed Q-switching experiments. The highest average output powers and shortest pulse widths were measured to be 0.798 W, 10.6 ns and 0.537 W, 9.6 ns for a- and c-cut samples, respectively.
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An Nd-doped Lu(1.5)Y(1.5)Al(5)O(12) (Nd:LuYAG) crystal was obtained by Czochralski method. Absorption and emission spectra were recorded at low and room temperature. Continuous wave (CW) and passively Q-switched laser operations of Nd:LuYAG crystal were, to our knowledge, demonstrated for the first time. A CW output power of 1.67 W with slope efficiency of 39.8% was obtained. In the passively Q-switched operation, the shortest pulse width, largest pulse energy, and highest peak power were achieved to be 9.6 ns, 61.7µJ, and 6.4 kW, respectively, with Cr(4+):YAG crystals as the saturable absorbers.
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The ability of coal to adsorb methane depends on the coal microstructure; however, the research on its exploration is still underway. In this paper, a new method was adopted to investigate the evolution characteristics of the crystallite structure of eight different rank coals and its influence on the methane adsorption capacity. The crystallite lattice parameters, including d 002, L c, L a, N ave, and f a, were determined by curve fitting analysis of X-ray diffraction (XRD) spectra. The methane adsorption experiments were carried out through a static capacity method, and the methane adsorption parameters (V L, P L) were measured. Correlations were established for the crystallite lattice parameters and the methane adsorption parameters. From the results obtained, there is a good negative linear relationship between V L and d 002 and a good exponential relationship between P L and d 002, indicating that the increasing d 002 can weaken the methane adsorption capacity. V L displays an exponential increase with increasing L c and N ave, whileP L presents a linear decrease, but reverse variations are emerged in the process of change for both, and the methane adsorption capacity is weaken temporarily. V L presents a lognormal distribution with increasing L a, and the minimum value appears at L a = 1.85-1.9 nm. V L and P L both obey lognormal distribution with increasing L a/L c, but their trends are completely opposite, and the methane adsorption capacity is the strongest at L a/L c = 0.85-0.9. As f a increases, V L and P L present an overall exponential increase and an overall exponential decrease, respectively, but reverse changes also emerge. The methane adsorption is related to the crystallite structure characteristics of coal. Finally, the influence mechanism of the crystallite structure evolution on the methane adsorption capacity was analyzed, which has great significance for prevention of gas disasters in underground coal mines.
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Outburst simulation experiments facilitate understanding coal and gas outburst in underground mining. With the help of the mid-scale simulation system, a model based on similitude principle, coal seam sandwiched by roof and floor, was constructed to conduct an outburst experiment. It had a three-dimensional size of 1500 mm × 600 mm × 1000 mm with 0.5 MPa gas pressure. The experimental procedures include specimen preparation, moulding, sealing, gas charging and adsorption, and completion. The outburst process was investigated by analyzing the gas pressure variation, temperature variation, outburst propagation velocity, particle size of outburst coal and energy transformation. During the experiment, each gas charging was accompanied with gas pressure or temperature fluctuation because of coal behavior of gas adsorption-desorption. The outburst propagation velocity was 17.2 m/s, obtained by a mass-weighted calculation of velocities of outburst coal. The small-size coal particles have a higher desorption rate and tend to participate in outburst process. According to energy conservation law, the energy forms of the outburst included elastic strain energy (Ee), gas expansion energy (Ep), internal energy of coal (ΔU), breakage work (W1), throwing out work (W2) and gas-flow loss energy (ΔE), and each was calculated respectively. Gas potential energy, including gas expansion energy and internal energy of coal, registered a larger percent and was far greater than the strain energy. And it can be the main factor influencing the occurrence of low-threshold outburst. The experimental system provides a feasible way to study the initiation and evolution of coal and gas outbursts.