Maximum Pulse Repetition Rate Research Articles

Molybdenum gallium carbide saturable absorber as Q-switcher in an erbium-doped fibre laser

In this research work, molybdenum gallium carbide (Mo2Ga2C)—fabricated as a thin film via solution casting technique—was first introduced as a saturable absorber (SA) to regulate the intracavity losses in a Q-switched fibre laser. The fabricated Mo2Ga2C SA had undergone both optical and physical interpretations prior to its employment as a Q-switcher in an erbium-doped fibre laser (EDFL) ring cavity configuration, sandwiched in between the end-facets of the optical fibre ferrules. The Q-switched wavelength was observed at 1532.07 nm and the Q-switching parameters were determined, accordingly. The temporal parameters of Q-switched EDFL, which include the maximum pulse repetition rate, minimum repetition period and minimum pulse duration were indicated as 107.60 kHz, 9.29 μs, and 5.52 μs, correspondingly. The maximum output power, peak power and pulse energy of the Mo2Ga2C-PVA SA as the Q-switcher were 7.3 mW, 11.47 mW and 67.38 nJ, in accordance with a slope efficiency of 7.39 % and signal-to-noise (SNR) ratio of 57.6 dB.

Discharge Characteristics of Needle Plate Electrode in Transformer Oil Under Low-frequency AC Voltage

Low-frequency transformer is the key equipment of flexible low-frequency AC transmission systems for offshore wind power. As the low-frequency voltage is applied, the transformer has significantly different insulation characteristics from those of the conventional transformer, it is necessary to study the typical characteristics of partial discharge of oil-paper insulation under such voltage. In this dissertation, the characteristics of discharge of needle plate electrodes in transformer oil under power frequency to low-frequency voltage are experimentally researched by adopting the pulse current method, and the discharge statistics, discharge atlas as well as feature parameters are analysed. With the decreasing of voltage frequency, the inception voltage (IV) value of partial discharge slightly increases, the extinction voltage (EV) phases of positive half-cycle (PHC) voltage waveform, as well as negative half-cycle (NHC) voltage waveform appear earlier, and the maximum discharge (MD), average discharge (AD), and pulse repetition rate (PRR) all show a downward trend.

Phased-Array Approach to Air-coupled Ultrasound with Resonant Defect Excitation

Wave propagation in air-coupled ultrasound (ACUT) is slower compared to ultrasound in immersion technique. Due to the lower velocity of sound the maximum pulse repetition rate is five times smaller than in water coupled ultrasound. This can be compensated when using parallel transducers and receivers. For example, with five transducers a similar scanning rate can be achieved like in water based immersion technique. Further, the use of multiple probes allows for additional ways of testing. With beamforming, for example, it is possible to focus on specific spots of the specimen. In this work, a set-up with a 72 channel acoustic camera with broadband microphones and a frequency maximum at 80 kHz was used. Detection of the loudest source is easily possible with an acoustic camera using beam forming algorithms. By exciting the whole specimen with a single piezoshaker over a broad frequency range, it was possible to excite the resonant frequency of the defects, which makes the detection with an acoustic camera even more faster. With this method it took less than 1 s to scan an area of 300 mm x 300 mm of a glass fiber reinforced plastic part and find a damaged area.

Accumulation effect of active species in atmospheric-pressure plasma jet driven by nanosecond high-voltage pulses with MHz pulse repetition rate

With respect to successful applications of pulsed power in gas discharges, the enhanced generation of desired active species and control of plasma parameters as required are always decisive issues. In this study, a bipolar nanosecond high-voltage pulse generator with a maximum pulse repetition rate (PRR) of up to 1 MHz (i.e. a minimum pulse interval of 1 µs) in burst mode is developed, based on the principle of full-bridge converter and pulse transformer. This pulse source is used to generate an atmospheric-pressure plasma jet in Ar + 1%CH4 gas flow, and the influence of pulse intervals (from 1–10 µs) is explored. It is found that the pulse interval can strongly modulate the active species, i.e. a short pulse interval enhances the generation of C2 radial and H atom due to the accumulation effect, when the pulse interval is comparable with their lifetime, while it slightly suppresses the generation of Ar excited states and the energy fraction into electronic excitation. Reduced pulse intervals also prominently increase the energy fraction of vibrational excitation. This study demonstrates how the PRR effectively modulates active species and energy branching and enhances the generation of certain active species in atmospheric-pressure plasma driven by pulsed power.

Overcoming the Lead Fiber-Induced Limitation on Pulse Repetition Rate in Distributed Fiber Sensors

Distributed fiber sensor (DFS)-based dynamic sensing has attracted increasing attention thanks to the growing demand in areas such as structural health monitoring and geophysical science. The maximum detectable frequency of DFSs depends on the maximum pulse repetition rate (MPRR), which is limited by the total length of the fiber under test (FUT). In some real-world applications, there is some distance between the interrogator and the monitoring site. Therefore, only a small part of the FUT acts as a sensing fiber (SF), while the other major part just acts as a lead fiber (LF), and the MPRR is limited by the LF and SF. Overcoming the LF-induced extra limitation on the MPRR is a practical problem for many DFS applications. In this paper, to the best of our knowledge, we propose a simple approach for overcoming the LF-induced extra limitation on the MPRR by dividing the DFS interrogator into two parts, for the first time. The proposed approach can be easily implemented for the real-world applicationsof DFSs whose LF is much longer than SF. It has been experimentally validated by using conventional phase-sensitive optical time domain reflectometry and Brillouin optical time domain analysis.

Self-starting Q-switched pulse generation in EDFL ring cavity based on Ta2AlC MAX-phase saturable absorber

A passive Q-switched erbium-doped fiber (EDF) laser is demonstrated using an MAX phase Ta2AlC saturable absorber (SA). By casting method, the Ta2AlC –PVA thin film is fabricated, and inserted into the laser cavity, which includes a 0.85 m segment of erbium doped fiber as the active medium pumped by a diode laser of 976 nm as the linear gain. The modulation depth, saturation intensity and non-saturable transmission of the MAX phase Ta2AlCSA are 7%, 15.2%, and 16.5 kW/cm2 respectively. The maximum pulse repetition rate (PRR) increase from 23.54 kHz to 111.4 kHz meanwhile, the pulse width gradually decreases from 8.175 μs to 1.589 µs at a range of a pump power from 30 mW to 150 mW and the highest pulse energy achieved is 68.2 nJ. Moreover, by adjusting the intra-cavity polarization controller state a tunable single–dual-triple-wavelength can be achieved with a tunable range of 5 nm. The experimental results demonstrated that the Ta2AlC MAX phase has a top placement as an artificial SA in Q-Switching fiber laser generation.

Fabrication of Sb2O3 by an improved chemical reaction assisted vertical micro sublimation method and its saturable absorber performance

Two-dimensional molecular crystals (2DMCs) are emerging ideal materials for future high-performance optoelectronic devices. People are constantly exploring new methods to solve the problem of difficult growth. Here, we design an improved chemical reaction-assisted vertical micro sublimation method based on vertical micro sublimation for the growth of two-dimensional Sb2O3 inorganic molecular crystals for the first time. The saturation absorption characteristics of the self-made Sb2O3 2DMCs were systematically tested, and the Q-switched laser output characteristics of Sb2O3 2DMCs at 1 µm were verified by using Nd: GYAP mixed crystal. The maximum average output power of 231 mW was achieved at 1080.6 nm, corresponding shortest pulse with a duration of 472 ns and maximum pulse repetition rate of 376 kHz. The maximum single pulse energy and the maximum peak power of the laser output were 0.614 µJ and 1.3 W, respectively. The growth of other 2DMCs would be motivated and its potential applications in the field of ultrafast photonics would be expanded with our findings.

Laser switching characteristics of enriched (7,5) single-walled carbon nanotubes at 640 nm

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter or chirality distribution have great potential in promoting carbon nanophotonics and nanoelectronics. In this paper, SWCNTs with dominant (7,5) species, which are fabricated by chemical vapor deposition method from a magnesia-supported Co catalyst, are applied for the nonlinear optical measurements. The SWCNTs exhibit efficient saturable absorption properties at a wavelength of 640 nm in terms of a modulation depth of 27.7% and a saturable intensity of 5.7 GW/cm2. Such a 640 nm absorption is correlated with the S22 absorbance bands of the semiconducting SWCNTs. Consequently, by employing the SWCNTs as a saturable absorber (SA), stable Q-switched pulses with the shortest pulse width of 200 ns and maximum pulse repetition rate of 238 kHz are achieved at 640 nm. Our findings indicate that subnanometer semiconducting SWCNT could act as an excellent optical switcher device at 640 nm, which may help to explore potential applications in visible nonlinear optics.

Enhancement of hydrogen radical density in atmospheric pressure plasma jet by a burst of nanosecond pulses at 1 MHz

The generation and enhancement of active species in non-thermal plasmas are always decisive issues with respect to their successful applications. In this work, an atmospheric pressure plasma jet (APPJ) is generated in Ar + 1% CH4 gas flow by a bipolar nanosecond high-voltage (HV) source with a maximum pulse repetition rate up to 1 MHz (i.e. minimum pulse interval ΔT = 1 μs) in burst mode. The absolute density of hydrogen atom at ground state is measured by the two-photon absorption laser-induced fluorescence method. It is observed that with ΔT = 1 μs, the H atom density keeps increasing during the first eight HV pulses and later on, the H atom density is maintained at a quasi-stable value while more HV pulses are applied. When decreasing ΔT from 10 to 1 μs, while keeping the total number of HV pulses the same (with similar coupled energy), the peak H atom density increases by a factor of more than four times, but the decay of H atom density after the pulse burst with ΔT = 1 μs is faster. Another effect of short ΔT is to extend the axial distribution of the H atom outside the APPJ’s nozzle, and the ΔT = 2 μs case has the highest averaged H atom density when taking its temporal evolution and axial distribution into consideration. In this work, we propose that the intensive nanosecond HV burst is an efficient approach to enhance the active species density in non-thermal plasmas when a rapid response is required.

High-peak-power structured beams by an Nd:YAG/Cr4+:YAG laser in a near-hemispherical cavity.

An Nd:YAG/Cr4+:YAG passively Q-switched (PQS) laser in a near-hemispherical cavity is exploited to generate high-order structured pulsed fields. Under tightly focused on-axis pumping, radial-order Laguerre-Gaussian (LG) modes with controllable mode orders by the input pump power are realized to exhibit quite stable temporal behavior. The pulse repetition rates of radial-LG modes can reach up to 78 kHz with an average output power of 0.57 W and peak power beyond 300 W under a 5-W pump level. Furthermore, by introducing 1D off-axis pumping into the PQS laser, various structured pulsed fields with transverse morphologies as high-order Ince-Gaussian (IG) modes are further created. With clean and well-defined beam structures, the IG pulsed fields can be nicely reconstructed by the resonant modes of the inhomogeneous Helmholtz equation for spherical cavities. More importantly, these high-order PQS IG modes reveal highly regular pulse trains with the maximum pulse repetition rate beyond 20 kHz and overall peak power higher than 1.5 kW.

MOSFET Gate Driver Circuit Design for High Repetitive (200 kHz) High Voltage (10 kV) Solid-State Pulsed-Power Modulator

Solid-state pulsed-power modulators can effectively generate high repetition rate pulses, which are required in applications where the process rate depends on the repetition rate, e.g., plasma source ion implantation and deposition, plasma immersion ion milling, and diamond-like carbon coating. In this article, a solid-state pulsed-power modulator for generating high repetitive pulses is developed. Active pull-down circuits and designed metal-oxide-semiconductor field-effect transistor (MOSFET) gate drivers are applied to the modulator without increasing its size. The drivers enable a fast rise, fall time, and minimize pulsewidth, which are effective for high-repetition-rate pulse applications. The developed modulator exhibits a maximum output pulse voltage of ±10 kV, maximum output pulse current of 50 A, pulsewidth between 200 ns and 10 us, maximum pulse repetition rate of 200 kHz, and average output power of 10 kW. The operation principle of the designed MOSFET gate driver is analyzed in detail. Experimental results show that the modulator operates stably at a high repetition rate of 10 kV and 200 kHz and the feasibility of this proposed circuit for high repetition rate operation are verified.

Niobium carbide (Nb2C) MXene as a saturable absorber to assist in the generation of a wavelength tunable passively Q-switched fiber laser

Niobium carbide (Nb2C), a new member of the MXene family, is proposed as a potential saturable absorber (SA) material. Stable Q-switched pulses were established in the C-band wavelength region upon employing a Nb2C/PVA film-based SA in the erbium-doped all-fiber ring cavity. The generated EDF laser operated at a wavelength of 1558.73 nm with a corresponding 3 dB bandwidth and signal-to-noise ratio of 2.8 nm and 53.8 dB. A maximum pulse repetition rate and pulse energy of 75.19 kHz and 15.63 nJ were observed at a maximum pump power of 224.5 mW. Interestingly, the emission spectra can be continuously tuned across 44 nm ranging from 1522.32 nm to 1566.07 nm by controlling the tunable band pass filter that has been introduced in the laser cavity. Our findings reflect the feasibility of using Nb2C as an excellent broadband SA material that underpins new opportunities for photonic technology and other potential applications in nonlinear optics.

Passively Q-switched thulium fluoride fiber laser operating in S-band region using N-doped graphene saturable absorber

A passively Q-switched S-band fiber laser is proposed and demonstrated using a thulium-doped fluoride fiber as laser gain medium. The proposed design possesses significant advantages including broader tuning wavelength range as compared to previously reported Q-switched S-band lasers. The laser cavity is configured in a simple forward-pumped ring design with a 1400 nm single upconversion pumping scheme. Q-switching is achieved using a polyvinyl alcohol/nitrogen-doped graphene nanosheet (PVA–N-Gns)-based thin-film saturable absorber. Stable Q-switched pulses are obtained at a center wavelength of 1504.7 nm over a pump power range from 101.8 to 280.6 mW. The generated pulses have a maximum pulse repetition rate of 36.85 kHz and minimum pulse width of 4.8 μs at the pump power of 280.6 mW. The maximum pulse output power of 1.30 mW indicates a laser efficiency of approximately 0.63% while a signal-to-noise ratio of 57 dB shows pulse stability. The ability of the proposed cavity design to produce broad wavelength tunability range from 1450 to 1510 nm make it highly suitable for various application in the S and S + band regions.

Real-time laser energy meter of nanosecond and subnanosecond optical pulses

Abstract Energy meters have become one of the most important tools to determine the performance of pulsed lasers. In case of low power semiconductor lasers operating in a nanosecond and subnanosecond regime, the pulse energy measurement is difficult due to low sensitivity of the apparatus. Moreover, the energy measurement of pulses generated with high frequencies is still challenging for many commercially available meters. In this paper, an ultra-sensitive, high repetition rate, real-time and low-cost laser energy meter based on a high-speed photodiode is presented. The meter’s performance was optimized by using circuit simulation software to achieve maximum sensitivity and high-speed operation. In the experimental part, the minimum detectable energy and the maximum pulse repetition rate were determined to be 110 fJ and 2.28 MHz, respectively. To the best of author’s knowledge, the developed meter is the most sensitive and the fastest infrared pulse energy meter ever reported, including commercially-available devices.

Thermal loading on self-seeding monochromators in x-ray free electron lasers

One approach to achieve longitudinally coherent hard X-ray free electron laser (XFEL) radiation is to introduce a single-crystal monochromator in the beam path to generate a co-propagating, time-synchronized self-seeding radiation pulse. For modern high-repetition rate XFELs, however, thermal loading on the crystal can degrade the quality of the seed pulse. To assess limitations on the maximum XFEL pulse repetition rate imposed by the seeding crystal, a comprehensive study of thermomechanical, and X-ray diffraction effects within the crystal is presented for two consecutive self-amplified spontaneous emission pulses incident on both reflective and transmissive crystal monochromators.

188 ns pulsed Ho:Sc2SiO5 laser operating at 2107.2 m employing a few-layer graphene saturable absorber

A passively Q-switched Ho:Sc2SiO5laser operating at 2107.2 nm with a few layered graphene saturable absorber is obtained, corresponding to a beam quality factors ofMx2=1.19 andMy2 =1.21. A maximum average output power of 1.37 W and pulse energy of 16.08 μJ was achieved at an incident pump power of 13.2 W. The shortest pulse width of 188.3 ns and maximum pulse repetition rate of 85.26 kHz were observed, with a maximum peak power of 85.4 W.

A High Voltage Diode-Transistor Generator of Nanosecond High Voltage Pulses

A small generator of high-power high voltage nanosecond pulses with a 12 V power source is considered. At a load of 75 Ω, it provides the generation of pulses with an amplitude of ~17 kV, a front of ~4 ns, and an energy of ~15 mJ. The maximum pulse repetition rate is 15 kHz. The generator contains an inductive energy storage device and a capacitive storage device with an operating voltage of 1 kV. The formation of high voltage pulses is carried out by a high voltage current chopper in the form of a block of drift diodes with a sharp recovery. The conditions for the effective operation of the diodes are provided by a block of parallel-connected IGBT transistors with an operating voltage of 1.2 kV. The possibility of using a generator for multispark ignition of the spark plug of an internal combustion engine is shown.

Optical shutters for a compact femtosecond Yb:KYW laser

A stable passive mode-locking regime for a Yb3+ :KY(WO4)2 laser with optical pumping by a continuous diode laser was obtained using two types of optical shutter with a saturable absorber based on coupled GaAs/In0.25Ga0.75As/GaAs quantum wells. The shutters differed in their reflecting part: the first type was a semiconductor mirror and the second type was a dielectric one. The typical parameters of the mode-locking regime were as follows: a pulse duration of about 200 fs for both types of shutter, an average power of 2.54 W for the all-semiconductor shutter (SESAM) and 0.92 W for the dielectric mirror with a saturable absorber (DSAM). The efficiency relative to the incident pump power was 57% for the SESAM and 19% for the DSAM. The maximum pulse repetition rate was 982 MHz. The measured recovery time was about 2 ps for both saturable absorbers. The mirror damage threshold estimate for the SESAM was at least 8.87 mJ cm−2.

High-Voltage Charging Power Supply Based on an LCC-Type Resonant Converter Operating at Continuous Conduction Mode

This article describes the design, implementation, and testing of a 2.0-kW high-voltage rep-rate charging power supply based on an non-isolated inductor-capacitor-capacitor (LCC)-type resonant converter operating at continuous conduction mode, which provides a constant charging current with high control accuracy/efficiency. The design of the proposed capacitor charging power supply (CCPS) has been carried out in such a way that to be a low cost, simpler, compact, and efficient driver for the miniature pulsed plasma sources (PF-3J: 10 kV, 84 nF, 48 nH, and PF-225: 15 kV, 2 μF, 50 nH), operating at repetitive discharges with the following specifications: 20 kV, 2.0 kW, and maximum pulse repetition rate up to 50 Hz. In order to understand the required conditions of such a power converter toward the functional stability, a detailed theoretical study of, and the design considerations along with the simplified design equations taken for, the proposed converter are discussed from a practical viewpoint. Due to the use of a fast complex programmable logic device (CPLD) controller, the synchronization of the start of the leg switching voltage with the moment of zero-current detection was observed at the switching frequency of 125 kHz. The functional stability of the CCPS at repetitive discharge mode was verified via operation at low and high frequencies. The results of diagnostics successfully confirmed the accurate performance of the control and sampling circuits for the moderate repetition rate of 0.5 Hz with the repeatability of about 0.1%. Finally, the developed rep-rate high-voltage charging power supply was shown to be very reliable, even under faulty operating conditions in the system.

Holmium oxide thin film as a saturable absorber for generating Q-switched and mode-locked erbium-doped fiber lasers

We demonstrated a new type of transition metal oxide (TMO) material, Holmium oxide (Ho2O3) polymer film SA for compact passively Q-switched and mode-locked erbium-doped fiber laser (EDFL) generations. The film SA has a thickness of ~40 µm and modulation depth of ~45. It was fabricated by embedding Ho2O3 nanoparticles into PVA polymer solution. The Q-switched laser operated stably at a center wavelength of 1563 nm within a pump power range of 45�97 mW. The maximum Q-switching pulse repetition rate and the narrowest attainable pulse width were 115.8 kHz and 0.64 µs, respectively; while, the maximum pulse energy was obtained as 129.36 nJ. On the other hand, the mode-locked EDFL emerged stably within a pump power range of 62�180 mW at a center wavelength of 1565.4 nm. The repetition rate was 17.1 MHz, while the pulse width was 650 fs. Additionally, the maximum output power was measured as 9.01 mW; while, the corresponding maximum pulse energy was obtained as 0.524 nJ. This Ho2O3 film SA offers simplicity and reliability in the design of compact and portable fiber pulsed laser generators particularly in the 1.55-µm regime. © 2019