Increasing Pump Power Research Articles

Experimental Testing of Hydrophobic Microchannels, with and without Nanofluids, for Solar PV/T Collectors

Solar energy can be converted into useful energy via photovoltaic cells or with a photothermal absorber. While these technologies are well-developed and commercially viable, significant benefits can be realised by pulling these two technologies together in photovoltaic/thermal (PV/T) systems which can provide both heat and electricity from a single collector. Emerging configurations in the PV/T field aim to incorporate micro and/or nanotechnology to boost total solar utilisation even further. One example of this is the nanofluid-based PV/T collector. This type of solar collector utilises nanofluids—suspensions of nanoparticles in traditional heat transfer fluids—as both an optical filter and as a thermal absorber. This concept seeks to harvest the whole solar spectrum at its highest thermodynamic potential through specially engineered nanofluids which transmit the portion of solar spectrum corresponding to the PV response curve while absorbing the rest as heat. Depending on the nanoparticle concentration, employing nanofluids in a flowing system may come with a price—an efficiency penalty in the form of increased pumping power (due to increased viscosity). Similarly, microchannel-based heat exchangers have been shown to increase heat transfer, but they may also pay the price of high pumping power due to additional wall-shear-related pressure drop (i.e., more no-slip boundary area). To develop a novel PV/T configuration which pulls together the advantages of these micro and nanotechnologies with minimal pumping power requirements, the present study experimentally investigated the use of nanofluids in patterned hydrophobic microchannels. It was found that slip with the walls reduced the impact of the increased viscosity of nanofluids by reducing the pressure drop on average 17% relative to a smooth channel. In addition, flowing a selective Ag/SiO2 core–shell nanofluid over a silicon surface (simulating a PV cell underneath the fluid) provided a 20% increase in solar thermal conversion efficiency and ~3% higher stagnation temperature than using pure water. This demonstrates the potential of this proposed system for extracting more useful energy from the same incident flux. Although no electrical energy was extracted from the underlying patterned silicon, this study highlights potential a new development path for micro and nanotechnology to be integrated into next-generation PV/T solar collectors.

Numerical investigation of thermal performance of single‐walled carbon nanotube nanofluid under turbulent flow conditions

In this paper, a numerical study is performed to investigate the effect of Reynolds number and nanoparticle concentration on the thermal performance of single‐walled carbon nanotubes (SWCNTs) nanofluids flowing through a straight pipe with constant heat flux in a turbulent flow regime. The governing equations (continuity, momentum, energy, rate of turbulent production, and rate of turbulent dissipation) are solved using a computational fluid dynamic approach (ie, finite volume method). In the sensitivity analysis, the Reynolds number is varied between (10 000‐200 000) and nanoparticle concentration between (0%‐0.25%). The simulation results show that convective heat transfer (average Nusselt number) increases by 7.48% while the pressure drop and pumping power increase by 119% and 199%, respectively. In addition, at low Reynolds number (104), increase in nanoparticle concentration (0%‐0.25%) decreases entropy production rate by 16.95%. However, at higher Reynolds number (2*105), the entropy production rate increases by 149.77%. Furthermore, other results such as entropy generation number, Bejan number, second thermodynamic law efficiency, and thermal performance factor, which combine the first and second law of thermodynamics, are presented as functions of Reynolds number and nanoparticle's concentration. The overall result shows that SWCNT nanofluid will only be beneficial at low Reynolds number of (10 000‐50 000).

AlGaAs/AlGaInP VECSELs With Direct Emission at 740–770 nm

An optically-pumped vertical-external-cavity surface-emitting laser (OP-VECSEL) with 3.25-W output power emitting around 750 nm is demonstrated. The gain structure incorporates AlGaAs quantum wells (QWs) and barriers, and AlGaInP claddings. The emission wavelength could be tuned from 740 to 770 nm. The development addresses the need for high brightness lasers at a wavelength range that has proven difficult to reach. The demonstrated structure exhibits polarization-related peculiarities, which cause polarization switching under increased pump power due to mode competition. The presence of birefringence inside the active region is attributed to known long-range ordering within the AlGaInP claddings which causes distorted beam profiles. This influence on laser features has not been reported in VECSELs so far.

Lasing in III–V microdisk core–TiO2 shell lasers

Microdisk lasers having a III–V core coated with a TiO2 shell are experimentally studied under optical pumping. Initial core microdisk lasers with a 5–10 μm diameter comprising five layers of InAs/In0.15Ga0.85As quantum dots demonstrate room temperature lasing with excellent characteristics (threshold, quality factor) at the ground state optical transition in the 1.28–1.30 μm spectral range. Deposition of the TiO2 dielectric shell results in a decimation of the whispering gallery modes of the microdisk resonator and, for thicker (>100 nm) shells, in a blueshift of the lasing wavelength. Using numerical analysis, we demonstrate that coating a III–V core microdisk with a high-index shell causes strong penetration of the second and third radial modes into the shell, whereas the first radial mode remains nearly undisturbed. Though optical loss added by the TiO2 dielectric shell is low (it does not exceed 2 cm−1 even for a 250-nm-thick TiO2 layer), mode leakage to the TiO2 results in a decrease in the optical confinement factor and an increase in the threshold pump power. This effect is more pronounced in microlasers of the smallest diameter studied (5 μm). Thus, in addition to other applications, a TiO2 shell can be used for mode selection in microdisk lasers or for selective outcoupling of the emission to the waveguide structure, which requires proper adjustment of the TiO2 shell thickness and microdisk diameter.

Generation of high-energy and low-repetition rate hybrid pulses from a passively mode-locked Tm-Ho co-doped laser

This study presents a passively mode-locked Tm-Ho co-doped fiber laser at around 1840 nm with a low-repetition rate and high pulse energy. The laser delivers pulses at 93 kHz and produces a pulse energy of 18.5 nJ at 0.55 W pump power when the cavity length is 2.21 km. When the pump power is increased, high order of repetition rate outputs are observed. By a comparison of the spectra at various pump power levels indicates that the laser can be operated at hybrid multi-wavelength soliton mode-locking, depending on the pumping intensity. With the increase in pump power, single-wavelength soliton mode-locking and simultaneous of single- and dual-wavelength soliton mode locking are also observed. Besides, the system produces a laser at 1417 nm, which is induced by two-step absorption and the consequent stimulated emission. The low–repetition rate and high–pulse energy fiber laser at around 1840 nm has potential applications in sensing, processing, and surgery research.

Numerical investigation of dual‐pumped optical frequency combs based on silicon‐on‐insulator microring resonator

AbstractDual‐pumped optical frequency combs (OFCs) based on silicon‐on‐insulator (SOI) microring and their characteristics are numerically investigated. This indicates that a relative low pump power is required to generate the comb modes effectively due to the high Q factor and large nonlinearity of the SOI microring. The number of generated comb modes reaches to 28, and the spacing of comb modes is over 300 nm. As the pump power enhances to 12 dBm, the bandwidth of the OFCs (over 800 nm) and the number of the comb modes (over 64) ranged from the infrared to mid‐infrared wavelength region. Correspondingly, the field profile of the OFCs changes to irregular shape when the pump power increases. Meanwhile, it has been found that the intensity of the pedestal of the OFC is above −150 dBm when the Gaussian noise is introduced, which reduced the bandwidth and modes of the OFCs by affecting the pump powers' transferring efficiency. The OFCs based on SOI microring can be found applications in chip‐scale oscillations, waveform generation, and ultrafast spectroscopy.

All fibre Q-switched Thulium-doped fibre laser incorporating Thulium–Holmium co-doped fibre as a saturable absorber

A novel all fibre Q-switched Thulium-doped fibre laser (TDFL) is reported which includes a short length of a Thulium–Holmiumco-doped fibre (THDF) as a saturable absorber. A high repetition rate (27.26 kHz) coupled with a low pulse width (19.06μs) is obtained for single wavelength Q-switched pulse operation at an output wavelength of 1911.5 nm using a pump power of 200 mW. Increasing the pump power from 200 mW to 700 mW results in the repetition rate increasing from 27.26 kHz to 99.67 kHz and the pulse width decreasing from 19.06μs to 920 ns. The centre wavelength of the single Q-switched pulse was also red shifted from 1911.5 nm to 1932.5 nm with increasing the pump power. A 45 m length single-mode fibre (SMF-28) provided dispersion compensation, and effectively an SMF-THDF-SMF structure is inserted in the cavity which operates in a similar manner to an SMF-MMF-SMF structure, providing a strong multimode interference effect which supports dual-wavelength operation. A stable dual-wavelength Q-switched pulse was achieved at a threshold pump power of 213 mW. The dual-wavelength Q-switched pulse operation was generated at 1911.5 nm and 1914.5 nm with a repetition rate of 8.45 kHz and pulse width of 20.02μs. The dual-wavelength spacing of this pulse operation was 3 nm, which was in good agreement with calculations based on the multimode interference effect induced by the THDF. The repetition rate increased from 8.45 kHz to 70.65 kHz and the pulse width decreased from 20.02μs to 870 ns with increasing pump power. At the maximum pump power of 700 mW, the maximum output power was measured as 27.4 mW. The experimental results confirm that the THDF can be utilized as a SA to generate a stable and tunable single-wavelength Q-switched pulse output as well as dual-wavelength Q-switched pulse in the 2.0μm wavelength region.

Lévy spectral intensity statistics in a Raman random fiber laser

The statistical behavior of spectral intensity fluctuations in a Raman gain random fiber laser is investigated for the first time, to the best of our knowledge. Through a fiber Fabry–Perot tunable filter, the intensity of the filtered single wavelength from random laser (RL) spectra is measured at different pump powers. The probability density function (PDF) of spectral intensity values is analyzed. Well below the lasing threshold, the intensity of a single wavelength is stable in the time domain, and the PDF exhibits a typical Gaussian behavior. However, around the threshold, strong fluctuations in intensity emerge. In this case, the statistical distribution is characterized by a power-law-tailed function consistent with a Levy α-stable distribution. We identify the statistical evolution as the injected pump power increases, from a Gaussian to a Levy and then back to a Gaussian distribution, which is similar to the case of RLs with a strong disorder due to the presence of photon scatterers and gain related to electronic transitions in the active medium, indicating the universal existence of such statistical properties in random lasing systems.

Lutetium (III) oxide film as passive mode locker device for erbium-doped fibre laser cavity

We reported a newly fabricated saturable absorber (SA) namely Lutetium oxide (Lu2O3) thin film for generating a stable and compact mode-locked Erbium-doped fibre laser (EDFL) operating at 1564 nm. The film SA was synthesized by embedding Lu2O3 into polyvinyl alcohol (PVA) host film. By incorporating the film SA along with 200 m long single mode fibre (SMF) into an EDFL cavity, the mode-locked laser with 0.97 MHz repetition rate stably emerged at an increasing pump power, ranging within 145-187mW. The pulse width was measured to be 2.12 ps. The highest pulse energy and peak power were measured as 7.64 nJ and 3.61 kW respectively and were obtained at 187 mW pump power. The fundamental frequency was examined to have a signal to noise ratio (SNR) of 61 dB. These laser performances suggest that the Lu2O3 film SA is feasible for becoming an alternative passive SA in the 1.55μm region.

Single-wavelength-tunable and dual-wavelength-switchable Tm-doped fiber soliton laser based on single-walled carbon nanotubes

We propose a single-wavelength-tunable and dual-wavelength-switchable Tm-doped fiber soliton laser with single-walled carbon nanotubes. The laser can deliver single-wavelength mode-locked pulse tunable from 1892 nm to 1924 nm. Dual-wavelength mode-locking operation can also be achieved by increasing pump power and rotating the polarization controller (PC), meanwhile the wavelength can be switched between 1883/1894 nm and 1905/1910 nm. Both the tunable and switchable operations are realized with great ease by solely adjusting the parameters of PC. The proposed Tm-doped fiber laser can operate in two mode-locking states, which is helpful for further understanding of the mode-locking mechanism and useful for practical applications.

A long single-span dispersion-decreasing-like fiber transmission system

We propose a novel unrepeated long single-span front-haul transmission system based on dispersion-decreasing-like fibers (DDLF). The modulation format is 160Gbps and PM-16QAM. The system transmission performance is effectively improved due to the balance of dispersion and nonlinear effects by using the DDLF, the balance of Raman amplification power and input optical power induced by equalization amplification between Raman amplifier and EDFA. The proposed system can effectively avoid the time delay of signal because of low dispersion of the DDLF system, and compensate the transmission attenuation impairment by employing the first-order Raman amplification at lower input power. The bit error rate (BER) of the system is better than 7% hard-decision forward error correction (FEC) threshold value of 3.8 × 10−3 in a wide range of input optical power. This system can be used in the long single-span front-haul transmission in special application scenarios. At higher input optical power, the nonlinear effect of the DDLF is the dominant factor affecting the performance of the transmission system, and nonlinear effect of the DDLF is increased with the increase of input optical power and Raman pump power, resulting in the system performance deterioration.

Manipulation of a stable dark soliton train in polariton condensate.

With a potential trap to mimic the finite-size effect, we predict the generation of dark soliton train in a nonresonant polariton condensate excited with the spatially homogeneous continuous wave (cw) field by exploiting the quantum interference in spatial domain. The number of solitons in the train is demonstrated to be controllable on demand in the domain of material parameters. We show that, similar to dark solitons in conservative systems, these nonlinear excitations have infinite lifetime and remain spatially localized even for the periodically oscillating and colliding state of dark soliton train, which do not depend on the parameters of the condensate. Especially, a final state of anti-dark soliton is observed for all kinds of dark soliton trains with the increase of pump power.

Level of Power Increase on Admission Determines Successful Treatment with Enhanced Anticoagulation with HeartWare Ventricular Assist Device Thrombosis

Purpose Ventricular Assist Device (VAD) thromboses remains an ongoing life threatening complication of VAD therapy. The ability to decide early which patients are at high risk would help treatment decisions. Methods 43 thrombus events between April 2013 and June 2018 were reviewed. All patients were on a combination of aspirin and warfarin and all implanted with Heartware HVAD. Patients were split into three categories related to their increase in pump power on admission (0-0.5 watts (w), 0.6-1w, 1>w). All patients were treated with IV heparin/ bivalirudin. If the thrombus was refractory to intravenous anticoagulation, patients received Alteplase or underwent surgical device exchange Results Watts increase 0-0.5 0.6-1 >1 Age 52 (±12) 48 (±14) 42 (±12) Gender 65% male 70% male 88% Male Diagnosis 10 Dilated Cardiomyopathy 7 Ischaemic Cardiomyopathy 11 Dilated Cardiomyopathy 6 Ischaemic Cardiomyopathy 6 Ischaemic Cardiomyopathy 3 Ischaemic Cardiomyopathy Time to thrombus (months) 22 (±17) 27 (±15) 32 (±25) INR 2.9 (±0.8) 2.6 (±0.5) 2.6 (±0.8) Heparin/ Bilirubin 9/8 13/4 5/5 Alteplase 35% 53% 77% Days in hospital 14 (±9) 24 (±26) 23 (±11) Admission Creatinine 96 (±39) 127 (±116) 140 (±79) Admission Bilirubin 18 (±11) 19 (±12) 29 (±23) Admission LDH 525 (±270) 659 (±426) 617 (±367) Resolved with anticoagulation 88% 70% 33% Mortality 6% 12% 0% Conclusion Those patients with a smaller increase in power (0 - 0.5) on presentation have a greater chance of resolution of thrombus with a conservative intravenous anticoagulation strategy. Conversely, those with higher watts (>1 over baseline) at presentation are at high risk and need to be considered for urgent transplantation or device exchange

Lutetium oxide film as a passive saturable absorber for generating Q-switched fiber laser at 1570 nm wavelength

Abstract We report for the first time, the application of Lutetium oxide (Lu2O3) thin film as a saturable absorber (SA) in generating a stable and compact Q-switched Erbium-doped fiber laser (EDFL) operating at 1570 nm. The film SA was prepared by embedding the Lu2O3 particles into a polyvinyl alcohol (PVA) host film. By integrating the film SA in a laser cavity, a stable pulsed laser appeared when the input pump power hit the threshold at 26.3 mW. The frequency of the pulsed laser rose from 18.4 kHz to 30.4 kHz, matching the increase of pump power from 26.3 mW to 71.6 mW. The shortest pulse width of 8.47 µs and the highest peak power of 8.98 mW were obtained at the maximum available pump power of 71.6 mW. Meanwhile, the maximum output power and pulse energy were measured at approximately 2.32 mW and 76 nJ, respectively. In addition, the signal to noise ratio (SNR) was 40 dB. These indicators suggest that Lutetium oxide film is a good passive SA that can be used to generate pulsed laser operating at1.55 µm region.

Mega-Hertz repetition rate broadband nano-second pulses from an actively mode-locked Yb-fiber laser

This work demonstrates an actively mode-locked all-normal dispersion Yb-fiber ring laser, delivering pulses of few nano-seconds (ns) to sub-ns duration at a 4 MHz repetition rate. Employing a filterless cavity architecture and different, long-length intra-cavity nonlinear fibers; a broadband output spectrum in the 1060–1140 nm wavelength region was obtained. With an increasing pump power, the pulse width reduced along with the appearence of new pulses, resulting into a muti-pulsing output. For a fixed pump power, the number of pulses and the amount of spectral broadening varied for different non-linear fibers. In the multi-pulsing state, the pulses were found to be located seperately in both time and wavelength, where Raman stokes corresponding to individual pulses could also be observed. The maximum output pulse energy achieved in a single pulse state was around 8.7 nJ, which increased to 14.5 nJ in a multi-pulse state. To the best of the authors’ knowledge, this is the first demonstration of a filterless active mode-locked fiber ring laser producing high-repetition rate broadband nano-second pulses in single and multi-pulse states at 1 µm.

Unusual Evolutions of Dissipative-Soliton-Resonance Pulses in an All-Normal Dispersion Fiber Laser

We experimentally report the novel evolutions of dissipative-soliton-resonance (DSR) pulses with respect to the pump power in an all-normal-dispersion fiber laser based on a nonlinear optical loop mirror. By solely increasing the pump power, pulse breaking of the DSR pulses was observed apart from the typical pulse broadening at a fixed pulse peak power. Instead of arbitrarily broadening with the increase in the pump power or directly losing the mode-locking state, the process of pulse breaking was accompanied by the multipulse state and harmonic mode-locking pulses evolving from the original DSR pulse. In addition, to the best of our knowledge, a narrowing of the DSR pulse with the increase in the pump power was observed for the very first time. Further results show that these unusual evolutions of DSR pulses could be attributed to the changes in several laser parameters resulting from the increasing pump power under specific operating conditions.

Rectangular Pulse Generation From a Mode Locked Raman Fiber Laser

Rectangular and step-like pulses are generated in a mode-locked Raman fiber laser with a nonlinear optical loop mirror. The Raman laser has a typical figure-8 configuration. Stable rectangular pulses with a fundamental repetition rate of 733 kHz are obtained. As the pump power increases, the Raman pulse evolves to a step-like rectangular pulse. The maximum pulse energy is 64.1 nJ with a pulsewidth of 25.1 ns. After spectrally resolved measurements, the step-like pulse is proved to be induced by the dual wavelength oscillation. It is the first report of rectangular pulsed output in the mode-locked Raman fiber laser to the best of our knowledge.

Power of all-fiber amplifier increasing from 1030 W to 2280 W through suppressing mode instability by increasing the seed power**Project supported by the National Natural Science Foundation of China (Grant Nos. 61735007 and 61505260).

In this work, we investigate suppressing mode instability in detail by varying the seed power in a large mode area all-fiber amplifier with a fiber core diameter of . The transverse mode instability (TMI) thresholds are systematically measured for different seed power. Our experimental results reveal that increasing the seed power has a positive influence on enhancing the output power before the TMI effect appears, and finally the TMI threshold is approximately doubled from 1030 W to 2280 W when the seed power is increased from 27 W to 875 W. Almost 84.7% slope efficiency is reached with different seed power before the TMI threshold power. During our operation, we also find that in this type of LMA fiber the beam quality of the amplifier is degraded gradually instead of a sudden change as the pump power increases.

Strengthening mass transfer of carbon dioxide microbubbles dissolver in a horizontal tubular photo-bioreactor for improving microalgae growth

A CO2 microbubbles dissolver (CMD) was developed to facilitate dissolving inorganic carbon and strengthening mass transfer in a horizontal tubular photo-bioreactor system (HTPBRS), which enhanced microalgae biomass productivity with flue gas containing 15% CO2. The influence of pump power on the bubble formation and mixing effect was found to be more obvious than that of gas flow rate. Ceramic shell aerator was more favorable for reducing bubble diameter and enhancing mass transfer than traditional rubber strip aerator. Bubble formation time decreased by 53.4% and mixing time decreased by 68.9% in response to the increased pump power. When the base area ratio of ceramic shell aerator to dissolver in the HTPBRS increased, bubble formation time decreased by 19.6% and mass transfer coefficient increased by 80.9%. The biomass yield of microalgae Chlorella PY-ZU1 with ceramic shell aerator was 30% higher than that with rubber strip aerator in the HTPBRS.

MoS2 Q-switched 2.8 µm Er:ZBLAN fiber laser

Since the invention of graphene, two-dimensional (2D) materials have attracted great interest in both fundamental science and potential applications. Here, we report the saturable absorption features of 2D (multilayered structure) molybdenum disulfide (MoS2) in the mid-infrared (mid-IR) wavelength region, and use it to passively Q-switch an Er3+-doped ZBLAN fiber laser at 2.8 µm. This MoS2 saturable absorber has a moderate modulation depth of 5% at 2.8 µm, making it an appropriate candidate for light modulating in the mid-IR spectral range. In stable Q-switching, the Er3+-doped ZBLAN fiber laser is widely tuned in both repetition rate and pulse width. Through increasing pump power, the repetition rate and pulse duration are monotonically tuned from 36 to 70 kHz and from 1.84 µs to 806 ns, respectively. The maximum average power is 140 mW (at 70 kHz), giving a pulse energy of 2 µJ and peak power of ~2.48 W. Successful Q-switching operation of this 2.8 µm fiber laser confirms that 2D MoS2 has great potentials as efficient light modulating elements in mid-IR regions.