Pump Absorption Research Articles

Influence of pump-beam parameters and crystal optical properties on propagation of thermally affected Bessel–Gaussian beams generated by a solid-state laser

In diode-pumped solid-state laser system, the generation and propagation of Helmholtz–Gaussian beam families such as Bessel–Gaussian (BG) beam can dramatically suffer from the effects of heat load. In this study, based on the ABCD matrix for BG beams passing through a graded-index medium, expressions for the thermal lens focal length and the output beam are obtained. Furthermore, to evaluate the impact of the pump-beam parameters and crystal optical properties, the intensity distribution of the zeroth- and first-order BG beams against radial distance together with its corresponding transverse intensity pattern on X–Y plane under different pump powers, pump beam waists, absorption coefficients, and thermal conductivities are simulated and compared. The simulation results indicate that for powerful narrow pump beam, the thermal-induced effects are so dramatic that the intensity profiles are altered, as the discernment of BG beams is not easily feasible; however, taking into account the large pump waist, high thermal conductivity and small absorption coefficient can partly cause the laser output similar to non-thermal one despite high pumping power is received. Therefore, the precise selection of laser parameters can considerably reduce the effects of induced heat on the generation and propagation of BG beams. The obtained results are a valuable hint to experimenters for beam identification and beam quality monitoring.

An improved cycle for heat pump application in hybrid-lift absorption/compression system integrated a wind energy and using organic fluid mixtures

For most parts of the world, the availability of cold sources is a considerable basis for progress. To subsist the demand for cold production, many refrigeration systems have been highlighted. Among these systems, a particular interest has been given to the compression-absorption hybrid refrigeration machine for which we integrated new organic mixtures (1,1,1,3,3,3-Hexafluoropropane / Dimethylacetamide (R236fa / DMAC), R245fa / DMAC (1,1,1,3,3-entafluoropropane / N, N’dimethylacetamide). This refrigeration system is usually adequate to meet most refrigeration requirements. The use of wind energy as a primary source for feeding this kind of system is very interesting because it is inexhaustible and is abundant in areas where the need for cold is enormous. The main objective of this work is the energy and exergetic optimization of a hybrid heat pump absorption / compression powered by a source of wind energy in order to improve its performance on the one hand and its adaptation to a wind source on the other. A simulation was also done to consolidate the results that were developed according to the calculus. The results of such a system show that the energy demand of absorption / compression refrigeration machines can be reduced to about 42% by switching to efficient energy systems. To summarize, such a study will allow making an economic optimization and a sensitivity analysis in order to reduce the investment cost; Furthermore, this novel configuration reduces the energy consumed by the compressor and thus approves improving of the efficiency of the heat pump for which COP can reach 0.75 contrary to the usual machine that has a maximum efficiency equal to 0.6.

紧凑型准连续泵浦调Q Nd∶YAG激光器

A compact end-pumped Electro-optical Q-switched Nd:YAG laser was designed in this paper. In order to achieve compact construction, laser diode arrays(LDAs) with collimated fast axes were used as pumping source. A spherical lens with a focal length of 40 mm and a cylinder lens with focal length of 25 mm were used as a group for the coupling lens. The LDAs' beam profiles on the incident and exit faces were calculated using Tracepro software. The simulation results indicate that when taking a Φ 6 mm×30 mm Nd:YAG with a doping concentration of 1.0at.% as the gain medium, the spots on the incident and exit faces were 5 mm(slow-axis)×4.5 mm(fast-axis) and 3 mm(slow-axis)×6 mm(fast-axis), respectively, and the absorption pumping power was about 83%. Dynamic temperature field distributions of Nd:YAG larer in 360 s at 22℃ and 60℃ were simulated by Ansys software. Output pulsed energy of 30 mJ and 25 mJ were achieved while the repetition frequencies were 30 Hz and 50 Hz, corresponding to a slope efficiency of 11.6% and 14.71%. The pulse durations were 18 ns and 16 ns, respectively. The experimental results show that the designed compact laser in this paper can achieve stable pulse laser output.

Low-threshold absolute parametric decay instability in the monotonous density profile region in O1-mode ECRH experimental conditions

The possibility of the low-threshold absolute parametric decay instability excitation by the ordinary wave in the presence of a monotonous plasma density profile is analyzed. It is shown that in the cases of daughter upper hybrid and ion Bernstein waves the energy circulation between two nearby decay points can lead to an absolute instability at the pump power of less than half a megawatt. The instability saturation due to the ion Bernstein wave amplitude-dependent stochastic damping effect is considered and the rate of anomalous pump absorption is estimated. This scenario can explain the observations of anomalous scattering in the O1-mode electron cyclotron resonance heating (ECRH) experiments at the FTU (Frascati Tokamak Upgrade) tokamak in the presence of a monotonous density profile.

Generalized study on the pump light propagation in the distributed side-coupled cladding-pumped fiber with identical pump cores

Abstract In this paper, a generalized numerical model describing the pump light propagating in the distributed side-coupled cladding-pumped (DSCCP) fibers with identical pump cores is present, with which the pump light propagation in the DSCCP fiber is analytically investigated. It is found that the local pump absorption should be smaller than the inner-cladding pump absorption of signal fiber. It is also revealed that the DSCCP fiber length for a given total pump absorption should be inversely proportional to the up-limitation of local pump absorption which decreases monotonously with the pump cores number. Furthermore, it is found that the peak pump power coupled into the signal fiber increases monotonously, while its ratio to the total pump power (i.e., normalized peak pump power) decrease monotonously with the increment of pump cores number. The dependences of pump light propagation on the coupling coefficients and the inner-cladding pump absorption of signal fiber are also discussed. We believe that these results can provide significant guidance on understanding and designing of DSCCP fiber and its lasers and amplifiers.

Fabrication and laser performance of triple-clad Yb-doped aluminophosphosilicate fiber

Abstract We fabricated an Yb-doped triple-clad fiber consisting in adding to aluminophosphosilicate core and F300-pump cladding, low-refractive-index fluorine doped silica cladding, and demonstrated its laser performance based on an all-fiber oscillator. MCVD system, all-gas-phase chelate precursor doping technique, and a simple overcladding process were applied to make Yb-doped aluminophosphosilicate triple-clad fiber preform. Ultra-thin substrate tube and a two-stage deposition process were proposed to decrease and offset elements evaporation. The outer region of fiber core was tested with ∼1050 ppm Yb2O3, ∼10,800 ppm Al2O3, and ∼9950 ppm P2O5, while the central region was doped with ∼600 ppm Yb2O3, 8100 ppm Al2O3, and ∼7000 ppm P2O5, respectively (ppm is in molar here). Two-stage deposition process and triple-clad design results in pump absorption from Yb3+ ions about 0.67 dB/m at 915 nm. Tested in an all-fiber oscillator configuration forward-pumped by 915 nm laser diodes, 28 m-long triple-clad fiber presented 560 W laser output with optical-to-optical efficiency of 68.7%, and the corresponding beam quality M2 is ∼1.18. The results indicate all-gas-phase chelate precursor doping technique in combination with overcladding technique is potential to manufacture high power triple-clad fiber, and ultra-thin substrate tube as well as two-stage deposition process are very effective ways to decrease and offset element evaporation.

Thermal modeling, heat mitigation, and radiative cooling for double-clad fiber amplifiers

We report a detailed formalism aimed at the thermal modeling and heat mitigation in high-power double-clad fiber amplifiers. Closed form analytical formulas are developed that take into account the spatial profile of the amplified signal and pump in the double-clad geometry, the presence of the amplified spontaneous emission, and the possibility of radiative cooling due to anti-Stokes fluorescence emission. The formalism is applied to a high-power Yb-doped silica fiber amplifier. The contributions to the heat-load from the pump-signal quantum defect, as well as the pump and signal parasitic absorptions are compared to the radiative cooling. It is shown that for realistic cases, the local heat generation in kiloWatt-class fiber amplifiers is either dominated by the quantum defect or the parasitic absorption depending on the pump wavelength. In conventional designs, radiative cooling can be substantial only in properly designed amplifiers, when the pump power is tens of watts or lower, unless the parasitic absorption is reduced compared to the commonly reported values in the literature. We also explore the impact of the non-ideal quantum efficiency of the gain material. The developed formalism can be used to design fiber amplifiers and lasers for optimal heat mitigation, especially due to radiative cooling.

Transient time in nonlinear interaction of counter-propagating waves with the effect of pump absorption and phase mismatching

The parametric interaction of three waves, one of which travel in direction opposite to other, is associated with a number of unusual phenomena that find their applications. One of such phenomena is an extremely long transient process of amplitude changing of the output signal. In this paper, the impact of pump-wave absorption and the phase mismatch magnitude on the course of the process is investigated. It is shown that the absorption lead to significant changes in the process time, and the phase causes changes in dynamics but not in duration of the process.

Gain dynamics in a highly ytterbium-doped potassium double tungstate epitaxial layer

Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of KGd0.43Yb0.57(WO4)2, representing an Yb3+ concentration of ∼3.8×1021 cm−3, grown on an undoped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects, are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, first, a fraction of Yb3+ ions not contributing to stimulated emission, second, a reduction of population inversion owing to a parasitic energy-transfer process and, third, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ∼800 dB/cm, which was achieved without thermal management, is promising for a waveguide amplifier operating without active cooling.

Theoretical modeling and analysis on the absorption cross section of the two-photon excitation in Rb.

The cross-section is crucial for quantitative characterization and analysis of the absorption process. A model on the absorption cross-section of the simultaneous two-photon excitation in Rb-vapor four-wave mixing process is established by using the coupled-wave equation. Taken into account of the hyperfine structures for 85Rb and 87Rb, the third-order susceptibility and hyperfine line strength are calculated respectively. Then, the influences of hyperfine transition on cross section are investigated and simulation results agree well with the experiment results. The calculated results suggest that high pumping power intensity is essential in Rb two-photon excitation, while narrow linewidth is the limiting factor of high absorption efficiency by comparing normalized absorption profile between pumping beam and two-photon excitation process. Additionally, two approaches to improving absorption efficiency, linewidth narrowness of the pumping beam and absorption linewidth broadening, are proposed.

Numerical investigation of pulsed gas amplifiers operating in hollow-core optical fibers.

Optically pumped molecular gas amplifiers having a gain medium contained in a hollow-core optical fiber are investigated with numerical modeling to understand the primary physical processes that affect amplifier output and efficiency. A comparison of computational results with experimental measurements of incident pump, absorbed pump, and emitted mid-IR from a pulsed, acetylene-filled, hollow-core fiber amplifier [ Opt. Exp.25, 13351 (2017)] is used to explore the effects of various physical processes on pulsed amplifier operation. Single frequency, one-dimensional, time-dependent models are shown to align with experimentally measured lasing thresholds and ratios of absorbed pump to emitted laser energy but significantly over predict the amount of incident pump energy that is absorbed. A two-dimensional, time-dependent model that assumes Gaussian spectral and radial intensity profiles for the pump is developed and shows an improved ability to capture pump absorption. Results indicate that 1D, time-dependent models have utility in guiding experiments but the significant influence of the pump and laser propagation modes and the pump spectral characteristics on efficiency, threshold, and signal output must be explicitly included in high-fidelity high-power modeling.

Erbium – ytterbium codoped phosphate core/double silica clad composite optical fibres for compact amplifiers

We report a study of composite fibres as gain media of all-fibre amplifiers. A distinctive feature of the fibres is an erbium – ytterbium doped phosphate core in a double silica cladding. The high Yb3+ concentration (above 1021 cm−3) ensures effective pump absorption from the cladding at fibre lengths under 40 cm and effective erbium ion sensitisation. The silica cladding facilitates fusion splicing to standard fibres. The composite fibres offer a large gain coefficient: about 0.4 – 0.5 dB cm−1 at a signal wavelength of 1550 nm. The maximum gain at a fibre length of just 36 cm and an input signal power of 0.37 mW at λ = 1550 nm is about 13.5 dB. Under such conditions, the amplified spontaneous emission level is lower than the input signal level by 10 dB.

Low Etendue Yellow-Green Solid-State Light Generation by Laser-Pumped LuAG:Ce Ceramic

A low Etendue (6.1 mm2) yellow-green solid-state light source with optical conversion efficiency up to 101.3 lumen/Watt is demonstrated by through high-power laser diode pumping of a static LuAG:Ce transparent ceramic plate. A 443-nm blue laser diode array with a full-width-half-maximum (FWHM) of <1.5 nm and a maximum output power of 15.2 W is used to attain a high spectral absorption efficiency. The output from the high-power laser diode array is collimated, shaped, despeckled, and focused to display a Gaussian-like profile with a 1.5-mm beam waist (1/e2 radius) to stimulate a 1.2-mm thick LuAG:Ce transparent ceramic plate with an emission FWHM spectra at 505–585 nm. No thermal quenching, pump absorption saturation, and obvious luminous flux decrease is found during test.

0419 Improvement in Sleep Maintenance and Sleep Quality with Ion Powered Pump Continuous Release and Absorption Melatonin: Results from a Self-Reported Patient Outcomes Study

0419 Improvement in Sleep Maintenance and Sleep Quality with Ion Powered Pump Continuous Release and Absorption Melatonin: Results from a Self-Reported Patient Outcomes Study

First demonstration of green and amber LED-pumped Nd:YAG laser

For the first time, to the best of our knowledge, a green (520 nm) and amber (592 nm) light emitting diode-pumped Nd:YAG laser is reported. The laser oscillator is a stable semi-planar resonator with a total length of 140 mm. The green (amber) light emitting diode-pumped laser produced a 107 (52) µJ laser energy, at 2.6 (0.7) J electrical pump energy. The oscillator operated at a low repetition rate (about 0.1 Hz) in free-running mode, where the laser spikes were initiated about 210–280 µs after the leading edge of the pump pulse. Moreover, the transverse mode profiles of the resonator, pump absorption efficiency, and optical gain have been studied in some detail.

Study on the adiabaticity criterion of the thermally-guided very-large-mode-area fiber.

The adiabaticity criterion of the thermally-guided very-large-mode-area (TG VLMA) fiber is presented based on the mode-coupling theory firstly, to the best of our knowledge. The requirement for the adiabatic propagation of fundamental mode is discussed systematically. It is revealed that the pump absorption plays the most important role and the adiabaticity criterion can be met as long as it is small enough. Then, the effects of the configuration parameters of TG VLMA fiber on the up-limitation of pump absorption for the adiabaticity criterion are investigated. It is found that for the straight TG VLMA fiber, reducing the initial refraction index and inner-cladding diameter and utilizing the bi-directional pumping scheme are beneficial to the adiabatic propagation of fundamental mode. The bent TG VLMA fiber is also studied. It is found that the bent fiber is much more difficult to meet the adiabaticity criterion than the straight one. The results show that even with the 100-cm bend radius, the pump absorption should be smaller than 1 dB/m to meet the adiabaticity criterion. It is suggested that enlarging the core-to-cladding ratio can be helpful for loosening the adiabaticity criterion of bent TG VLMA fiber. These pertinent results can provide significant guidance for understanding and designing the TG VLMA fiber and pertinent lasers and amplifiers.

Analysis of the EU-DEMO fuel cycle elements: Intrinsic impact of technology choices

In the context of the EU-DEMO pre-conceptual design phase and the setup of a fuel cycle simulation tool, we analysed twelve prime technology candidates of the fusion fuel cycle to determine the impact of these choices and future R&D priorities. The main function and working principle of these technologies are shortly characterized and two evaluation parameters are introduced: the Technology Readiness Level and the DEMO Importance Level. The former is an approach adapted from literature to evaluate the maturity of a technology in the categories theory, experiments and integration. The latter grades the performance, reliability and impact on system robustness of each technology deriving its criticality. Ultimately, both parameters are combined to form a matrix of research priorities. Technologies of high importance and low maturity, namely the breeding blanket, metal foil pump and thermal cycling absorption process are ranked highest priority. Second priority is given to technologies of lesser criticality and maturity, such as vacuum pumps. Technologies of high maturity and importance are graded third priority. They require proper integration on a system level to counteract the boundary conditions they impose. Last priority is given to well established, reliable technologies such as cryotraps and depleted uranium getter beds.

Highly-Efficient Ho:SiSc2O5 Laser Pumped by a Narrow Line-Width Tm:YAP Laser at 1.94 μm

We present a highly-efficient continuous-wave Ho:SSO laser pumped by a diode-pumped Tm:YAP laser with a narrow linewidth (NL) of 0.3 nm. With the free-running (FR) Tm:YAP laser, we obtain a maximum output power of 2.23 W at an absorption pump power of 7.2 W, corresponding to an optical conversion efficiency of 31% and a slope efficiency of 42.6%. With the NL Tm:YAP laser, we obtain a maximum output power of 2.88 W at the same absorption pump power. The optical conversion efficiency increases to 40% when the slope efficiency increases to 55.5%. The output linewidth of the Ho:SSO laser is 0.8 nm when we use the Tm:YAP laser with a narrow linewidth of 1.8 nm pumped by a FR Tm:YAP laser. The beam quality also changes from 1.31 to 1.22.

Theoretical study of the effect of pump wavelength drift on mode instability in a high-power fiber amplifier

This paper presents an investigation of the effect of pump wavelength drift on the threshold of mode instability (MI) in high-power ytterbium-doped fiber lasers. By using a semi-analytical model, we study the effects of pump wavelength drift with a central pump wavelength around 976 nm and 915 nm, respectively. The influences of the pump absorption coefficient and total pump absorption are considered simultaneously. The results indicate that the effect of pump wavelength drift around 976 nm is stronger than that around 915 nm. For more efficient suppression of MI by shifting the pump wavelength, efficient absorption of pump power is required. The MI thresholds for fibers with different total pump absorptions and cladding diameters are compared. When the total pump absorption is increased, the gain saturation is enhanced, which results in the MI being mitigated more effectively and being more sensitive to pump wavelength drift. The MI threshold in gain fibers with larger inner cladding diameter is higher but more dependent upon pump wavelength. The results of this work can help in optimizing the pump wavelength and fiber parameters and suppressing MI in high-power fiber lasers.

Benefit of Nd3+ ions to the ~2.7 µm emission of Er3+: 4I11/2→4I13/2 transition in Nd,Er: CaLaGa3O7 laser crystal

Nd3+ was introduced into Er3+: CaLaGa3O7 crystal for an enhanced ~2.7 µm emission. The structure parameters of Er: CaLaGa3O7 and Nd,Er: CaLaGa3O7 crystals are determined by the X-ray Rietveld refinement method. The effects of Nd3+ on the spectroscopic properties and mutual energy transfer mechanisms were investigated. It is found that Nd3+ greatly improves the absorption pump efficiency and thus enhances the Er3+ ~ 2.7 µm emission by nearly 5 times, meanwhile, quenches the ~ 1.5 µm near-infrared emission. Therefore, the self-termination effect for ~ 2.7 µm erbium laser is suppressed successfully. Furthermore, the energy transfer efficiencies of Nd3+→Er3+ and Er3+→Nd3+ are estimated to be 92.3% and close to 100%, respectively. Besides, the radiative transfer microscopic parameters between Nd3+ and Er3+ were theoretically analyzed. The results indicate that the introduction of Nd3+ is very beneficial in achieving ~ 2.7 µm laser in Er3+: CaLaGa3O7 crystal, which could act as a potential gain media for LD pumped MIR laser.