Pulsed Output Power Research Articles

Laser cleaning of Cu-based artefacts: laser/corrosion products interaction

<p class="Abstract">This study aims to develop a low invasive and selective laser cleaning procedure for the removal of reactive corrosion products on Cu-based artefacts without damage the substrate. In a preliminary step, laser cleaning was performed on two typologies of artificially corroded copper reference samples. The effect of the variation of laser parameters as pulse duration and output power, was thus evaluated on an oxide layer, simulating a protective patina, and a hydroxychloride layer, simulating a reactive corrosion products layer to be removed. The optimized cleaning procedure was validated on an archaeological artefact, a bronze coin. Morphological, microchemical and microstructural characterizations were performed by means of optical microscopy, confocal microscopy, field emission scanning electron microscopy, X-Ray diffraction and Raman spectroscopy, before and after laser cleaning. The experimental findings show that laser cleaning, in optimized conditions, can reduce the thickness of the hydroxychloride layers slightly affecting the oxide layers. The difference in the interaction with laser radiation of these two layers seems to be mainly related to the difference in grain size and porosity. Notwithstanding these encouraging results, in order to define the real feasibility of the laser cleaning procedure, a further validation on real artefacts is mandatory due to the variation in thickness and composition of the corrosion products formed during long-lasting uncontrolled degradation processes.</p>

Practical Model of Low-Volume Paddling-Based Sprint Interval Training Improves Aerobic and Anaerobic Performances in Professional Female Canoe Polo Athletes.

Sheykhlouvand, M, Khalili, E, Gharaat, M, Arazi, H, Khalafi, M, and Tarverdizadeh, B. Practical model of low-volume paddling-based sprint interval training improves aerobic and anaerobic performances in professional female canoe polo athletes. J Strength Cond Res 32(8): 2375-2382, 2018-Brief, intense exercise training using running and cycling as exercise interventions may induce aerobic and anaerobic adaptations in athletes from a wide range of sports. However, this has not been studied extensively for those sports in which the upper body is predominantly involved. Our purpose was to examine the effects of kayak paddling-based sprint interval training (SIT) on cardiorespiratory fitness and anaerobic performance. Sixteen professional female canoe polo athletes (age = 27.6 ± 1.9 years; height = 165.7 ± 5.2 cm; body mass = 62.6 ± 8.5 kg; body mass index = 22.8 kg·m; body fat = 23.8 ± 4.9%) were randomized to either an intense exercise training consisting of sets of 5 × 5-second maximum sprint efforts interspersed by a 10-second recovery between each sprint (3, 4, 5, and 6 sets/session from first to fourth week, respectively, with 3 minutes of rest between each set), performed 3 times per week for 4 weeks (n = 8), or a usual training control group (n = 8). Before and after the training period, aerobic and anaerobic measurements were assessed using a kayak specific test and Wingate protocol, respectively. Training increased V[Combining Dot Above]O2peak, O2 pulse, anaerobic threshold, peak, and mean power output in the SIT group compared with the control group (p ≤ 0.05) who showed no changes in these variables when tested 4 weeks apart without SIT. Paddling-based SIT was a potent stimulus and time-efficient strategy to induce rapid adaptations in aerobic and anaerobic performances in professional female canoe polo athletes who can use this training method to achieve fitness in a short period.

MoS2 saturable absorber prepared by chemical vapor deposition method for nonlinear control in Q-switching fiber laser**Project supported by the National Natural Science Foundation of China (Grant No. 11674036), the Beijing Youth Top-notch Talent Support Program, China (Grant No. 2017000026833ZK08), the Fund of State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, China (Grant Nos.

Due to the remarkable carrier mobility and nonlinear characteristic, MoS2 is considered to be a powerful competitor as an effective optical modulated material in fiber lasers. In this paper, the MoS2 films are prepared by the chemical vapor deposition method to guarantee the high quality of the crystal lattice and uniform thickness. The transfer of the films to microfiber and the operation of gold plated films ensure there is no heat-resistant damage and anti-oxidation. The modulation depth of the prepared integrated microfiber-MoS2 saturable absorber is 11.07%. When the microfiber-MoS2 saturable absorber is used as a light modulator in the Q-switching fiber laser, the stable pulse train with a pulse duration of 888 ns at 1530.9 nm is obtained. The ultimate output power and pulse energy of output pulses are 18.8 mW and 88 nJ, respectively. The signal-to-noise ratio up to 60 dB indicates the good stability of the laser. This work demonstrates that the MoS2 saturable absorber prepared by the chemical vapor deposition method can serve as an effective nonlinear control device for the Q-switching fiber laser.

Ultrashort pulse generation with an erbium-doped fiber laser ring cavity based on a copper oxide saturable absorber.

A soliton mode-locked erbium-doped fiber (EDF) laser has been experimentally demonstrated using copper oxide (CuO) thin film as a saturable absorber (SA). The dispersion of the EDF cavity including the CuO-SA was balanced by a suitable length of single-mode fiber (SMF). The fabricated CuO-SA has 3.5% modulation depth and 3.3 MW/cm2 saturation intensity. The mode-locked train pulses have 1.7ps pulse width and 983kHz repetition rate, while the pulse energy and output power are 1.29nJ and 1.27mW, respectively, at maximum pump power of 159mW. These results indicate that the CuO thin film is a good SA candidate for a fiber laser operating at a low pump power. To the best of our knowledge, this is the first demonstration of a CuO-SA-based mode-locked fiber laser.

Passive Q-Switching of a Yb:LuVO4 Laser with Cr4+:YAG: Approaching the Intrinsic Upper Limit of Repetition Rate**Supported by the National Natural Science Foundation of China under Grant No 11574170.

We demonstrate a diode pumped Yb:LuVO4 laser that can be passively Q-switched by a Cr4+:YAG saturable absorber having an initial transmission as high as 99.3%. A maximum pulsed output power of 2.35 W is generated at a repetition rate of 285.7 kHz, approaching or very near the intrinsic upper limit imposed by the recovery time of the Cr4+:YAG saturable absorber, and the resulting pulse energy, duration and peak power are, respectively, 8.2 μJ, 39.2 ns and 0.209 kW.

Design and Experiment of a High Power and Broadband Ku-Band TE11 Mode Gyro-TWT

The design and experiment of a high power and broadband Ku-band gyrotron traveling-wave tube (gyro-TWT) amplifier operating in the TE11 mode are presented in this paper. A new-type broadband rotating TE11 mode input coupler is applied to excite operating mode and a broadband three-disk beryllium oxide ceramic output window is used in the designed gyro-TWT. In order to suppress the potential oscillations, expand working bandwidth, and improve the heat dissipation capability of high frequency system, a periodic beryllium oxide and silicon carbide composite lossy ceramic rings loaded high-frequency circuit with a nonlinear small change output taper is adopted in the gyro-TWT. When the beam voltage is 70 kV and the beam current is 12 A, the experiment of this Ku-band gyro-TWT developed shows that the measured maximum output average power is ~20 kW at the frequency 16 GHz, corresponding to a maximum output pulse power of ~200 kW (10% duty ratio), a gain of 43.0 dB and a maximum efficiency of 23.8%, −3-dB saturation gain bandwidth of 2.6 GHz (~16.3%), and −1-dB saturation gain bandwidth of 1.5 GHz (~9.4%). Under the condition of 10-W constant input power driven, the measured −3-dB constant driven bandwidth is 1.4 GHz (~8.8%).

Time resolved Fabry-Perot measurements of cavity temperature in pulsed QCLs.

Temperature rise during operation is a central concern of semiconductor lasers and especially difficult to measure during a pulsed operation. We present a technique for in situ time-resolved temperature measurement of quantum cascade lasers operating in a pulsed mode at ~9.25 μm emission wavelength. Using a step-scan approach with 5 ns resolution, we measure the temporal evolution of the spectral density, observing longitudinal Fabry-Perot modes that correspond to different transverse modes. Considering the multiple thin layers that make up the active layer and the associated Kapitza resistance, thermal properties of QCLs are significantly different than bulk-like laser diodes where this approach was successfully used. Compounded by the lattice expansion and refractive index changes due to time-dependent temperature rise, Fabry-Perot modes were observed and analyzed from the time-resolved emission spectra of quantum cascade lasers to deduce the cavity temperature. Temperature rise of a QCL in a pulsed mode operation between -160 °C to -80 °C was measured as a function of time. Using the temporal temperature variations, a thermal model was constructed that led to the extraction of cavity thermal conductivity in agreement with previous results. Critical in maximizing pulsed output power, the effect of the duty cycle on the evolution of laser heating was studied in situ, leading to a heat map to guide the operation of pulsed lasers.

Terahertz master-oscillator power-amplifier quantum cascade laser with a grating coupler of extremely low reflectivity.

A terahertz master-oscillation power-amplifier quantum cascade laser (THz-MOPA-QCL) is demonstrated where a grating coupler is employed to efficiently extract the THz radiation. By maximizing the group velocity and eliminating the scattering of THz wave in the grating coupler, the residue reflectivity is reduced down to the order of 10-3. A buried DFB grating and a tapered preamplifier are proposed to improve the seed power and to reduce the gain saturation, respectively. The THz-MOPA-QCL exhibits single-mode emission, a single-lobed beam with a narrow divergence angle of 18° × 16°, and a pulsed output power of 136 mW at 20 K, which is 36 times that of a second-order DFB laser from the same material.

All-fiber high-power monolithic femtosecond laser at 1.59 µm with 63-fs pulse width

In this research, by adopting an alternative novel approach to ultra-short giant pulse generation which basically originated from difficulties with traditional employed methods, an optimized Er/Yb co-doped double-clad fiber amplifier is applied to boost output average power of single-mode output pulses to a high level of 2-W at 1.59-µm central wavelength. Output pulses of approximately 63-fs pulse width at 52-MHz repetition rate are obtained in an all-fiber monolithic laser configuration. The idea of employing parabolic pulse amplification for stretching output pulses together with high-power pulse amplification using Er/Yb co-doped active fibers for compressing and boosting output average power plays crucial role in obtaining desired results. The proposed configuration enjoys massive advantages over previously reported literature which make it well-suited for high-power precision applications such as medical surgery. Detailed dynamics of pulse stretching and compressing in active fibers with different GVD parameters are numerically and experimentally investigated.

Microwave switch based on mixed waveguide-coaxial tee for the cavity pulse shaper

The possibilities of microwave tee switches for the resonator pulse former are estimated. It is shown that the values of the pulse output power can reach units of MW with conversion factors above 102. The features of using a switch based on a waveguide-coaxial tee are studied to control the modes of accumulation and emission of energy of a high-Q resonator storage. The operability of the structure considered for obtaining nanosecond and subnanosecond pulses is shown.

Promoting the Range and Range Resolution of a LIDAR (DIAL) System Using a Suitable Pinhole Plasma Shutter

Pulsed transversely exited atmospheric (TEA) CO2 lasers, employed extensively in various applications such as light detection and ranging (LIDAR), have a pulse duration of about a microsecond due totheir nitrogen tail. In order to promote the measurement accuracy and the mean power of the laser pulse, the pulse duration should be shortened. In this research, we present the details of making a passive pinhole plasma shutter for a LIDAR (DIAL) system, which shortens the pulse duration of CO2 lasers from 1.5 μs to 25 ns in air at atmospheric pressure. This instrument increases the range resolution of the LIDAR system from 225 to 3.75 m. Also we show the results of investigation of the clipped pulse duration of the microsecond CO2 laser pulse using aluminum and copper pinhole metal targets with different pinhole diameters (1.5 and 1.8 mm) and at various laser output energies (338 and 309 mJ). Our experimental results show that the aluminum pinhole is more suitable than the copper pinhole for shortening the nitrogen tail of the CO2 laser pulse with a larger output average power. Thus, the range of the LIDAR system, which is proportional to the logarithm of the output pulse power, is increased.

Highly efficient X-range AlGaN/GaN power amplifier

The development of microwave power amplifiers (PAs) based on transistors with an AlGaN/GaN heterojunction are discussed in terms of the possible enhancement of their efficiency. The main focus is on the synthesis of the transforming circuits, which ensure the reactive load at the second- and third-harmonic frequencies and complex impedance at the fundamental frequency. This makes it possible to optimize the complex operation mode of a PA; i.e., to reduce the scattering power and enhance the efficiency. A microwave PA based on the Schottky-barrier-gate field-effect transistor with 80 electrodes based on the GaN pHEMT transistor with a gate length of 0.25 nm and a gate width of 125 nm is experimentally investigated. The amplifier has a pulse output power of 35 W and a power-added efficiency of at least 50% at a working frequency of 9 GHz.

Reflection influence on the operation of a 0.42 THz second harmonic gyrotron with complex cavity

Since the average power of a 0.42 THz s harmonic gyrotron with complex cavity is small, the laser calorimeter AC2500H is adopted to measure the pulsed output power. However, the optimal spectral response of AC2500H is far from the operating frequency of the designed gyrotron. Thus the reflection on the laser calorimeter would be large, which would affect the mode competition in the designed gyrotron. In this paper, the influence of reflections from the laser calorimeter and output window is investigated by a multi-mode, time-dependent nonlinear code. From numerical simulation results, it is shown that the output power of the designed gyrotron is greatly decreased by the reflection from AC2500H although it doesn't cause excitations of competing modes. And it is found that when the reflection phase on the output window θr,n,1 is 5π/11, reflection magnitude on the laser calorimeter Γr,n,2 is 0.44 and the corresponding phase θr,n,2 is 3π/7, the variation of the calculated output power is very similar with the experimental result. Moreover, if the beam quality and Ohmic loss in the cavity wall are both considered, the calculated power can fit perfectly with the measured one.

Metalorganic vapour phase epitaxy of GaAs/AlGaAs nanoheterostructures for a quantum cascade laser

Short-period GaAs/AlGaAs superlattices, an active region, and a quantum cascade laser heterostructure have been grown by metalorganic vapor phase epitaxy, and their characteristics have been studied by high-resolution X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The heterostructures have been used to produce quantum cascade lasers emitting near 10 μm. Their output pulse power at 77 K is above 200 mW.

Myth of initial loading tritium for DEMO—Modelling of fuel system and operation scenario

Any fusion power reactors such as “DEMO” that achieves tritium self-sufficiency with breeding blankets can produce tritium by DD reaction followed by exponential breeding in the blanket within reasonable total operation period. The present study further suggests that realistic Power Ascension Tests (PAT) of DEMO can produce its tritium to be needed in the series of tests by its own program until reaching steady state full power operation, with no external supply or additional operation costs. Closed tritium fuel plant was described by a system dynamics model, and analyzed considering realistic PATs of DEMO, that will be mainly pulsed DD and low concentration DT. Typical PATs require years of operation from zero power criticality to full power, with pulsed power output and long dwell time between them. Output power is gradually increased in PATs to check the functions of reactor systems and components. In the case of fusion DEMO, zero power criticality corresponds to DD operation. While plasma may be fired in pulses, tritium plant is continuously operated to recover all the tritium produced by the DD and low DT burn. Depending on the different time constant of tritium retention in components, tritium is transferred by deuterium purge, and high concentration tritium is finally collected in the storage, to be available for the next tests at virtually no additional cost. Realistic “initial loading tritium” will be 100g for commissioning of tritium plant far prior to the plasma operation.

Panoramic Measurements of the Output Pulse Power of Receiv-ing/Transmitting Devices of a Microwave Band

Panoramic Measurements of the Output Pulse Power of Receiv-ing/Transmitting Devices of a Microwave Band

Carrier envelope offset frequency detection and stabilization of a diode-pumped mode-locked Ti:sapphire laser.

We demonstrate the first diode-pumped Ti:sapphire laser frequency comb. It is pumped by two green laser diodes with a total pump power of 3 W. The Ti:sapphire laser generates 250 mW of average output power in 61-fs pulses at a repetition rate of 216 MHz. We generated an octave-spanning supercontinuum spectrum in a photonic-crystal fiber and detected the carrier envelope offset (CEO) frequency in a standard f-to-2f interferometer setup. We stabilized the CEO-frequency through direct current modulation of one of the green pump diodes with a feedback bandwidth of 55 kHz limited by the pump diode driver used in this experiment. We achieved a reduction of the CEO phase noise power spectral density by 140 dB at 1 Hz offset frequency. An advantage of diode pumping is the ability for high-bandwidth modulation of the pump power via direct current modulation. After this experiment, we studied the modulation capabilities and noise properties of green pump laser diodes with improved driver electronics. The current-to-output-power modulation transfer function shows a bandwidth larger than 1 MHz, which should be sufficient to fully exploit the modulation bandwidth of the Ti:sapphire gain for CEO stabilization in future experiments.

Repetitively Q-switched laser operation of a miniature Yb:LuPO_4 crystal rod

We report on efficient repetitively Q-switched laser operation of a miniature Yb:LuPO4 crystal rod with emission wavelengths ranging from 1000 to 1010 nm, at different repetition rates over a range of 1−50 kHz. An average output power of 6.55 W was produced at 50 kHz of repetition rate, with a slope efficiency of 40%; in a low-repetition-rate operation at 2 kHz, the maximum pulsed output power generated was 1.43 W, with pulse energy, duration, and peak power being, respectively, 0.715 mJ, 12.5 ns, and 57.2 kW.

The Nonlinear Designs and Experiments on a 0.42-THz Second Harmonic Gyrotron With Complex Cavity

Terahertz (THz) and sub-THz science and technology are very hot today. However, it is difficult to obtain high-power source of THz wave. Gyrotrons are well known as one of the most promising source for powerful THz radiation. In this paper, a 0.42-THz second harmonic gyrotron with complex cavity operating at TE17.4 is numerically simulated and designed by adopting self-consistent nonlinear theory with the first-order transmission line equations. Meanwhile, the mode competitions in the designed gyrotron have been investigated carefully by using time-dependent multimode nonlinear theory. It is proved that the operating mode can be first excited and other competing modes are well suppressed in the complex cavity. Based on these numerical simulations, the designed complex cavity gyrotron has been manufactured and measured in the Terahertz Science and Technology Research Center at the University of Electronic Science and Technology of China. The measured results show that the designed gyrotron can stably operate at TE17.4 with pulsed output power of 19.3 kW when the beam current ${I}_{{0}}$ is 4.4 A, beam voltage ${U}_{{0}}$ is 51 kV, and magnetic field ${B}_{{0}}$ is 8.03 T. The corresponding frequency and interaction efficiency are 421.645 GHz and 8.6%, respectively.

Challenges of Spinel Li4Ti5O12 for Lithium‐Ion Battery Industrial Applications

Rechargeable lithium‐ion batteries (LIBs) offer the advantages of having great electrical energy storage and increased continuous and pulsed power output capabilities, which enable their applications in grid energy storage and electric vehicles (EVs). For safety, high power and durability considerations, spinel Li4Ti5O12 is one of the most appealing potential candidate as an anode material for power LIBs due to its excellent cycling stability and thermal stability. However, there are still a number of challenges remaining for Li4Ti5O12 battery applications. Herein, an updated overview of the latest advances in Li4Ti5O12 research is provided and key challenges for its future development (i.e., fast‐charging, specific capacity, swelling, interface chemistry, matching cathode and electrolyte as well as batteries design and manufacturing) are highlighted.