Increasing Pump Power Research Articles

Enhancement of second-harmonic generation based on the cascaded second- and third-order nonlinear processes in a multimode optical microcavity

Optical microcavities are often used to realize enhanced nonlinear optical interactions for highly efficient second-harmonic generation. With increased pump power, the efficiency of nonlinear frequency conversion can be increased further, while some other unwanted nonlinear effects will also emerge, leading to complicated dynamics or instability. Here, we study the interplay between cascaded second- and third-order nonlinear processes and investigate their impact on the second-harmonic generation in microcavities. It is found that the non-degenerate optical parametric oscillation (OPO) appears and the presence of $\chi^{(3)}$ process can modify the OPO threshold significantly when the multimode cavity is strongly pumped at the fundamental optical mode. One can even break the efficiency limitation of the second-harmonic mode restricted by the OPO by utilizing the interference between the OPO and the four-wave mixing. The present coherent interplay between nonlinear optical processes in microcavities is conducive to exploring new physics in the cavity nonlinear photonics.

3.5 kW bidirectionally pumped narrow-linewidth fiber amplifier seeded by white-noise-source phase-modulated laser

We demonstrate an all-fiber narrow-linewidth amplifier employing a bidirectional pump scheme and cascaded white-noise-source phase-modulated seed laser. The stimulated Raman scattering effect in the amplifier is investigated by substituting different types of seed lasers. The influence of pump distributions and seed injection power on mode instability (MI) in the amplifier is also experimentally investigated. As a result, a 3 dB linewidth of 0.175 nm and a beam quality of M2 ≈ 1.5 are obtained at the output of ~3 kW, without observation of MI and stimulated Brillouin scattering effect. With the further increase of pump power, MI occurs as the output exceeds 3.17 kW, along with beam quality degradation. Optical efficiency decreases to 71.5% at the ultimate output of 3.5 kW. Therefore MI becomes the main limitation to further power scaling.

Design and Numerical Modeling of Broadband Mid-IR Rare-Earth-Doped Chalcogenide Fiber Amplifiers

The design of broadband chalcogenide double-clad fiber amplifiers doped with praseodymium, dysprosium, or terbium ions is reported. A theoretical study of ultrashort pulse amplification in the 4– $5~\mu \text{m}$ range with allowance for realistic fiber parameters: dispersion, nonlinearity, background loss (2 dB/m), and Se-H impurity loss (7 dB/m at $4.6~\mu \text{m}$ ) is presented. It is shown that a gain of more than 15 dB can be achieved for dysprosium amplifiers and more than 25 dB for praseodymium and terbium ones. The energy of pulses with a repetition rate of 5 MHz can be increased from 10 pJ up to ~1 nJ for dysprosium fibers and up to several nanojoules for praseodymium and terbium fibers. Further amplification by increasing pump power may be limited by a low damage threshold.

Unified theory for excited-state, fragmented, and equilibriumlike Bose condensation in pumped photonic many-body systems

We derive a theory for Bose condensation in nonequilibrium steady states of bosonic quantum gases that are coupled both to a thermal heat bath and to a pumped reservoir (or gain medium), while suffering from loss. Such a scenario describes photonic many-body systems such as exciton-polariton gases. Our analysis is based on a set of kinetic equations for a gas of noninteracting bosons. By identifying a dimensionless scaling parameter controlling the boson density, we derive a sharp criterion for which system states become selected to host a macroscopic occupation. We show that with increasing pump power, the system generically undergoes a sequence of nonequilibrum phase transitions. At each transition a state either becomes or ceases to be Bose selected (i.e. to host a condensate): The state which first acquires a condensate when the pumping exceeds a threshold is the one with the largest ratio of pumping to loss. This intuitive behavior resembles simple lasing. In the limit of strong pumping, the coupling to the heat bath becomes dominant so that eventually the ground state is selected, corresponding to equilibrium(-like) Bose condensation. For intermediate pumping strengths, several states become selected giving rise to fragmented nonequilibrium Bose condensation. We compare these predictions to experimental results obtained for excitons polaritons in a double-pillar structure [Phys. Rev. Lett. 108, 126403 (2012)] and find good agreement. Our theory, moreover, predicts that the reservoir occupation is clamped at a constant value whenever the system hosts an odd number of Bose condensates.

Coexistence of rectangular and Gaussian-shape noise-like pulses in a figure-eight fiber laser.

We experimentally report the coexistence of the rectangular noise-like pulse (NLP) and the Gaussian-shape NLP in a figure-eight fiber laser. Benefiting from the strengthened nonlinear effect of a segment of highly nonlinear fiber (HNLF) in the cavity, the coexistent NLPs with various patterns, i.e. one rectangular pulse with one Gaussian-shape pulse, one rectangular pulse with two Gaussian-shape pulses and two rectangular pulses with one Gaussian-shape pulse, are formed depending on the cavity parameters setting. In particular, the evolution of these coexistent NLPs properties with pump power is investigated. It is found that the duration of the rectangular pulse always increases, while the Gaussian-shape pulse has almost no changes with the increasing pump power. The achieved results demonstrated for the first time, to the best of our knowledge, the pulses with different shapes can coexist under the NLP regime, which contributes to further understanding the fundamental characteristics of the NLPs and multiple pulses.

Intrinsic and environmental effects on the interference properties of a high-performance quantum dot single-photon source

We report a joint experimental and theoretical study of the interference properties of a single-photon source based on a In(Ga)As quantum dot embedded in a quasiplanar GaAs microcavity. Using resonant laser excitation with a pulse separation of 2 ns, we find near-perfect interference of the emitted photons, and a corresponding indistinguishability of $\mathcal{I}=(99.6{\phantom{\rule{0.16em}{0ex}}}_{\ensuremath{-}\phantom{\rule{0.16em}{0ex}}1.4}^{+\phantom{\rule{0.16em}{0ex}}0.4})%$. For larger pulse separations, quasiresonant excitation conditions, increasing pump power, or with increasing temperature, the interference contrast is progressively and notably reduced. We present a systematic study of the relevant dephasing mechanisms and explain our results in the framework of a microscopic model of our system. For strictly resonant excitation, we show that photon indistinguishability is independent of pump power, but strongly influenced by virtual phonon-assisted processes which are not evident in excitonic Rabi oscillations.

Experimental and numerical CHT-investigations of cooling structures formed by lost cores in cast housings for optimal heat transfer

This paper investigates the cooling performance of six different lost core designs for automotive cast houses with regard to their cooling efficiency. For this purpose, the conjugate heat transfer (CHT) solver, chtMultiregion, of the freely available CFD-toolbox OpenFOAM in its implementation of version 2.3.1 is used. The turbulence contribution to the Navier-Stokes equations is accounted for by using the RANS Menter SST k − ω model. The results are validated for one of the geometries by comparing with experimental data. Of the six investigated cooling structures, the one that forces the fluid flow to change its direction the most produces the lowest temperatures on the surface of the cast housing. This good cooling performance comes at the price of the highest pressure loss in the cooling fluid and hence increased pump power. It is also found that the relationship between performance and pressure drop is by no means generally linear. Slight changes in the design can lead to a structure which cools almost as well, but at much decreased pressure loss. Regarding the absolute values, the simulations showed that the designed cooling structures are suitable for handling the cooling requirements in the particular applications and that the maximum temperature stays below the critical limits of the electronic components.

1.9 μm square-wave passively Q-witched mode-locked fiber laser.

We propose and demonstrate the operation of Q-switched mode-locked square-wave pulses in a thulium-holmium co-doped fiber laser. By using a nonlinear amplifying loop mirror, continuous square-wave dissipative soliton resonance pulse is obtained with 4.4 MHz repetition rate. With the increasing pump power, square-wave pulse duration can be broadened from 1.7 ns to 3.2 ns. On such basis Q-switched mode-locked operation is achieved by properly setting the pump power and the polarization controllers. The internal mode-locked pulses in Q-switched envelope still keep square-wave type. The Q-switched repetition rate can be varied from 41.6 kHz to 74 kHz by increasing pump power. The corresponding average single-pulse energy increases from 2.67 nJ to 5.2 nJ. The average peak power is also improved from 0.6 W to 1.1 W when continuous square-wave operation is changed into Q-switched mode-locked operation. It indicates that Q-switched mode-locked operation is an effective method to increase the square-wave pulse energy and peak power.

Tunable High-Power Q-Switched Fiber Laser Based on BP-PVA Saturable Absorber

Black phosphors (BP) nano-materials have been extensively investigated in the past years. However, the character of easily oxidizable limits BP's stability. Here a long-term-stable tunable Q-switched fiber laser with BP polymer composite (polyvinyl alcohol, PVA) as saturable absorber is realized. The modulation depth and the saturation power intensity of the BP-PVA film are 8.3% and 7.9 MW/cm2, respectively. A high average output power of 12.03 mW is obtained under the pump power of 400 mW. The shortest pulse width and maximum single pulse energy are 1.365 μs and 148.63 nJ, respectively. Meanwhile, the output spectra have a 2.5-nm blue shift when the pump power increases. The results prove that as a Q-switcher, BP polymer composite has a good performance on high power and excellent stability pulsed laser.

Dual-pulses and harmonic patterns of a square-wave soliton in passively mode-locked fiber laser

We demonstrate a square-wave soliton pulse passively mode-locked fiber laser. The mode-locked pulses are achieved by using a nonlinear amplifying loop mirror. Single-pulse operation at a fundamental repetition rate of 3.2 MHz is obtained. The optical spectrum presents the soliton feature of several sidebands. The pulse duration expands with increasing pump power, but the amplitude hardly varies. Pulse breaking occurs and a stable dual-pulse is obtained with a fixed interval of 48 ns. Harmonic mode-locked states can be achieved when the total pump power is higher than 740 mW. The harmonic pulses can also operate in both single-pulse and dual-pulse states.

An 8 cm long holmium-doped fiber saturable absorber for Q-switched fiber laser generation at 2-µm region

Abstract We demonstrate a Q-switched all-fiber laser operating at 2-µm region by adding a piece of 8 cm long holmium doped fiber (HDF) as a fiber saturable absorber (SA) in Thulium doped fiber laser (TDFL) ring cavity. Doping of Ho ions into yttria-alumina silica glass was done through conventional Modified Chemical Vapor Deposition (MCVD) technique in conjunction with solution doping process. The fabricated HDF has a linear absorption of 3 dB with a core diameter and a numerical aperture of 10 µm and 0.18, respectively. A self-started Q-switching operation begins at 418 mW pump level and continually dominant until 564 mW pump level. As the pump power increases, stable pulse train presence from 30.61 kHz to 38.89 kHz while the pulse width reduces from 3.18 µs to 2.27 µs. Both maximum output power and maximum peak power are obtained at 5.05 mW and 57.2 mW, respectively, while the maximum pulse energy is calculated to be 129 nJ. The signal-to-noise ratio (SNR) of the fundamental frequency is 50 dB. Our work may contribute to the discovery of stable, robust, and economic SA for pulse fiber laser generation at 2-µm region.

Confined-doped fiber for effective mode control fabricated by MCVD process

A confined-doped fiber was fabricated by a modified chemical vapor deposition (MCVD) process based on refractive index matching technology. With theory and experiments, we compared the confined-doped fiber and normal-doped fiber. We found that the confined-doped fiber with a core of 35μm and 0.07 numerical aperture could achieve single-mode output and improve the beam quality from 2.8 to 1.5 in the fiber laser. Meanwhile, it still possesses high laser efficiency and has good stability of beam quality with the increase in pump power. It suggests that the confined-doped fiber with a MCVD process may be the key material for a high-power fiber laser with excellent beam quality.

Evaluation of metal foam based thermoelectric generators for automobile waste heat recovery

This paper focuses on harvesting heat emitted by exhaust systems efficiently using thermoelectric generators (TEGs) and converting it to electricity. In a TEG that employs gaseous working fluid, up to 80% of the thermal resistance is due to the gas side. Maximizing the energy transferred from the hot exhaust gas to the hot side of the thermoelectric modules by suitable enhancement techniques can result in an efficiency gain for the TEG. To this end, we have investigated the performance of metal foam-based heat exchangers for reducing thermal resistance of the hot side in TEGs. A computational model of the metal foam-enhanced TEG, solving for the coupled thermal and electrical energy transfer processes, was developed to investigate the enhancement in system performance for a range of metal foam porosities and pore densities, and mass flow rates of the exhaust gas. Skutterudites with multiple cofillers were selected as thermoelectric materials. The primary performance metrics that were analyzed include the electrical power output and the associated pressure drop for various inlet conditions of the exhaust gas. Based on the trade-off between the increased pumping power required to offset the increase in pressure drop, and the gain in heat transfer coefficient with increase in mass flow rate of the exhaust gas, an optimal mass flow rate that maximizes the net electric power produced by the metal foam-enhanced TEG was obtained. The results show a critical exhaust flow rate for different pore densities of metal foam beyond which the net electric power produced by the TEG is less than of the TEG with no metal foam. At this critical flow rate, the maximum net electric power produced from exhaust waste heat by metal foam enhanced TEG is 5.7 (20 PPI) to 7.8 (5 PPI) times higher than that generated by the configuration without metal foam.

Upconversion luminescence of Ba3La(PO4)3:Yb3+-Er3+/Tm3+ phosphors for optimal temperature sensing.

To explore new upconversion (UC) luminescent materials for optical temperature sensing, a series of Ba3La1-x(PO4)3:0.1Yb3+,xEr3+ (abbreviated as BLP:0.1Yb3+, xEr3+, 0.01≤x≤0.03) and Ba3La1-y(PO4)3:0.1Yb3+,yTm3+ (abbreviated as BLP:0.1Yb3+, yTm3+, 0.005≤y≤0.015) phosphors was designed. For Yb3+-Er3+ codoped BLP, the UC emission spectra exhibit a gradual enhancement with increasing pump power, and two-photon process for the green and red emissions of Er3+ has been confirmed by studying the dependence of UC emission intensities on excitation power. When the temperature rises, the fluorescence intensity ratio (FIR) for the H11/22-4I15/2 and S3/24-4I15/2 emissions of Er3+ increases gradually due to the thermalization of populations at the H11/22 and S3/24 levels. With the temperature increased, the absolute sensitivity (SA) shows a continuous increase and reaches about 4.38×10-3 K-1 at 498K, but the relative sensitivity (SR) decreases gradually. For Yb3+-Tm3+ codoped BLP, the emission intensity of Tm3+ shows a gradual decrease with increasing Tm3+ concentration. By studying the temperature dependence of FIR for the (F23,F33)-H63 and H43-3H6 emissions, it has been found the sample exhibits a higher SR value but a lower SA value relative to the Yb3+-Er3+ codoped BLP.

Spectral dynamics of square pulses in passively mode-locked fiber lasers

We investigate experimentally and numerically the spectral dynamics of square pulses generated in passively mode-locked fiber lasers under the dissipative soliton resonance. The features of the transition from the single-peak spectral profile to the doublet spectrum with increasing pump power are studied. The used master equation takes into account the gain saturation, the quadratic frequency dispersion of the gain and the refractive index, and the cubic-quintic nonlinearity of the losses and refractive index. Experimental data are obtained for an Er:Yb-doped fiber ring laser. The theoretical and experimental results are in good agreement with each other.

Estimation of daily flow rate of photovoltaic water pumping systems using solar radiation data

Abstract This paper presents a simple model which allows us to contribute in the studies of photovoltaic (PV) water pumping systems sizing. The nonlinear relation between water flow rate and solar power has been obtained experimentally in a first step and then used for performance prediction. The model proposed enables us to simulate the water flow rate using solar radiation data for different heads (50 m, 60 m, 70 m and 80 m) and for 8S × 3P PV array configuration. The experimental data are obtained with our pumping test facility located at Madinah site (Saudi Arabia). The performances are calculated using the measured solar radiation data of different locations in Saudi Arabia. Knowing the solar radiation data, we have estimated with a good precision the water flow rate Q in five locations (Al-Jouf, Solar Village, AL-Ahsa, Madinah and Gizan) in Saudi Arabia. The flow rate Q increases with the increase of pump power for different heads following the nonlinear model proposed.

Numerical simulation of effect of pump distribution on spherical aberration of end-pumped laser

In order to study the spherical aberration of thermal lens when the laser crystal is pumped with the pump light distributed differently and the pump light under the end-pumping condition, in this paper we establish a single-ended pump and constant temperature thermal model to analyze the working characteristics of the Nd:YVO4 crystal. The steady state heat conduction equation and Zernike polynomials are solved, and the relationship between thermal spherical aberration and distribution of pump laser is established. The model is used in simulation, and the simulation results are further analyzed theoretically, showing that under the same pump power, the spherical aberration is greatest when the pump beam is of 2-order super-Gaussian distribution. The spherical aberration decreases with the increase of pump distribution coefficient k (except the Gaussian distribution). With the increase of pump power, the influence of pump distribution coefficient k on spherical aberration is aggravated gradually, and the difference in spherical aberration caused by different values of distribution coefficient k increases gradually. The range of strongest laser power of the 2-order super-Gaussian distribution pump is analyzed and simulated. Under same pump power, the maximum range of the strongest laser power of 2-order super-Gaussian distribution pump is 0.3-0.63 times the Gauss radius. The research methods and conclusions obtained in this paper have universal applicability and can be used for quantitatively analyzing the temperature distributions, thermal deformations, optical path difference distributions, and spherical aberration distributions of other laser crystals. At the same time, this study also provides a theoretical reference for improving spherical aberration from the perspective of changing the distribution of pump light and the laser output characteristics.

1.7 μm band narrow-linewidth tunable Raman fiber lasers pumped by spectrum-sliced amplified spontaneous emission.

A 1.7μm band tunable narrow-linewidth Raman fiber laser based on spectrally sliced amplified spontaneous emission (SS-ASE) and multiple filter structures is proposed and experimentally demonstrated. In this scheme, an SS-ASE source is employed as a pump source in order to avoid stimulated Brillouin scattering. The ring configuration includes a 500m long high nonlinear optical fiber and a 10km long dispersion shifted fiber as the gain medium. A segment of un-pumped polarization-maintaining erbium-doped fiber is used to modify the shape of the spectrum. Furthermore, a nonlinear polarization rotation scheme is applied as the wavelength selector to generate lasers. A high-finesse ring filter and a ring filter are used to narrow the linewidth of the laser, respectively. We demonstrate tuning capabilities of a single laser over 28nm between 1652nm and 1680nm by adjusting the polarization controller (PC) and tunable filter. The tunable laser has a 0.023nm effective linewidth with the high-finesse ring filter. The stable multi-wavelength laser operation of up to four wavelengths can be obtained by adjusting the PC carefully when the pump power increases.

Plasmon-Induced Selective Enhancement of Green Emission in Lanthanide-Doped Nanoparticles.

By introducing an 18 nm thick Au nanofilm, selective enhancement of green emission from lanthanide-doped (β-NaYF4:Yb3+/Er3+) upconversion nanoparticles (UCNPs) is demonstrated. The Au nanofilm is deposited on a microfiber surface by the sputtering method and then covered with the UCNPs. The plasma on the surface of the Au nanofilm can be excited by launching a 980 nm wavelength laser beam into the microfiber, resulting in an enhancement of the local electric field and a strong thermal effect. A 36-fold luminescence intensity enhancement of the UCNPs at 523 nm is observed, with no obvious reduction in the photostability of the UCNPs. Further, the intensity ratios of the emissions at 523-545 nm and at 523-655 nm are enhanced with increasing pump power, which is attributed to the increasing plasmon-induced thermal effect. Therefore, the fabricated device is further demonstrated to exhibit an excellent ability in temperature sensing. By controlling the pump power and the UCNP concentration, a wide temperature range (325-811 K) and a high temperature resolution (0.035-0.046 K) are achieved in the fabricated device.

Effect of slot-jet position on the cooling performance of the hybrid trapezoid channel and impingement module

This paper investigated the effect of slot-jet position on the cooling performance of the hybrid trapezoid channel and impingement module. Cooling performances of trapezoid channels with different slot-jet positions were comparatively discussed using the realizable k-ε turbulent model. Apart from the channel with α = 0.25 (the dimensionless distance that the slot-jet deviates from the symmetry axis of trapezoid channel), both the temperature and the temperature difference (the difference between maximum temperature and minimum temperature) of the module bottom surface firstly decline and then rise with the increase of α. When the slot jet is at the position of α = 1.00, the module achieves the minimum bottom temperature. While the slot jet is at the position of α = 0.50, the module reaches the minimum bottom temperature difference. Considering the bottom temperature and bottom temperature difference, the slot-jet position α = 0.75 is recommended in the practical application. Besides, the difference among the cooling performances of channels with different slot-jet positions gradually decreases with the increase of pump power. Furthermore, the cooling performance of the channel with α = 0.25 is deeply affected by the pump power. At the low pump power, both bottom temperature and bottom temperature difference of the channel with α = 0.25 are larger than that of the channel with α = 0.00. While at the high pump power, not only is bottom temperature of the channel with α = 0.25 lower than that of the channel with α = 0.00, but also temperature difference of the former is smaller than that of the latter.