Variable Pulse Repetition Rate Research Articles

Femtosecond tunable light source with variable repetition rate between 640 kHz and 41 MHz with a 130 dB temporal pulse contrast ratio.

We demonstrate a femtosecond tunable light source with a variable pulse repetition rate based on a synchronously pumped fiber-feedback optical parametric oscillator (FFOPO) that incorporates an extended-cavity design. The repetition rate can be reduced by an acousto-optical modulator in the FFOPO pump beam. The extended FFOPO cavity supports signal oscillation down to the 64th subharmonic. The high nonlinearity of the FFOPO threshold suppresses signal output for residual pump pulses that are transmitted by the pulse picker. We characterize the temporal pulse contrast ratio of the FFOPO signal output with a second-order cross-correlation measurement. This FFOPO system enables pulse picking with extraordinarily high values up to 111 dB suppression of adjacent pulses and exhibits a temporal contrast ratio that exceeds 130 dB. It generates fs-pulses with tunable wavelength from 1415-1750 nm and 2.5-3.8 µm and variable repetition rates ranging from 640 kHz to 41 MHz.

Adaptive Nonlinear Phase Compensation in a Femtosecond Hybrid Laser with Varying Pulse Repetition Rate

In this manuscript, an implementation of a tunable nonlinear phase compensation method is demonstrated on a typical femtosecond hybrid laser consisting of a fiber pre-amplifier and an additional solid-state amplifier. This enables one to achieve constant laser pulse parameters over a wide range of pulse repetition rates in such a laser. As the gain in the solid-state amplifier is inversely proportional to the input power, the shortfall in the solid-state gain at higher repetition rates must be compensated for with fiber pre-amplifier to ensure constant pulse energy. This increases the accumulated nonlinear phase and consequently alters the laser pulse parameters such as pulse duration and Strehl ratio. To overcome this issue, the nonlinear phase must be compensated for, and what is more it should be compensated for to a different extent at different pulse repetition rates. This is achieved with a tunable CFBG, used also as a pulse stretcher. Using this concept, we demonstrate that constant laser pulse parameters such as pulse energy, pulse duration and Strehl ratio can be achieved in a hybrid laser regardless of the pulse repetition rate.

Highly-Efficient Pulsed Mid-Infrared Generation Based on Intracavity Difference Frequency Mixing

We demonstrated highly-efficient pulsed mid-infrared generation based on intracavity difference frequency mixing inside a continuous-wave (CW) PPLN optical parametric oscillator (OPO). The near-infrared pulsed fiber source was located at 1120 nm with variable pulse width and repetition rate. The pump source of OPO was a CW linearly-polarized high-power 1060 nm fiber laser. The 1120 nm and 1060 nm pump beams were simultaneously incident into the OPO cavity. The 1060 nm pump beam built parametric oscillation firstly and generated high-power resonant signal beam inside the cavity. By controlling the PPLN temperature properly, the phase-matched difference frequency generation (DFG) occurred between the signal beam and 1120 nm pulsed pump beam in the PPLN crystal. Finally, the pulsed 1120 nm pump beam was successfully transformed to 3593 nm mid-infrared radiation at every power level and temporal characteristic. Both the slope efficiency and pump-to-idler conversion efficiency of the intracavity DFG process reached over 20%. In addition, the pulse shape of newly generated 3593 nm idler beam was basically the same as 1120 nm pump beam. The experimental results exhibited huge potential in frequency down conversion of low-power laser sources with special temporal characteristics.

Anomalously low ionization energy of phosphorus atoms in the electric field of silicon p-n junctions in the temperature range 10-20 K

The work extends the study of the ionization properties of phosphorus atoms as trapping/emission centres in the electric field of silicon p-n junctions operating at low temperatures. The goal is describing the ionization of phosphorus atoms by a single effective parameter, the ionization energy of phosphorus energy levels Eion . An approach to the study is based on manipulating the space charge concentration Neff in a nonirradiated silicon p+/n/n+ structure via filling phosphorus donors with electrons supplied by a laser pulse. Extracting the Neff from the experimental current pulse response shapes recorded at variable temperature and pulse repetition rate allowed building Arrhenius plot for evaluating Eion in the electric field. This value is shown to be 6.4±1.1 meV that is paradoxically low, being about 7 times less than the referred data.

Effect of process parameters on the formation of single track in pulsed laser powder bed fusion

There has been increasing interest in Laser Powder Bed Fusion (L-PBF) of metallic materials as a promising manufacturing technology. Although most L-PBF systems utilize laser beams with continuous wave emission (L-PBF(CW)), the possibility of using pulsed lasers (L-PBF(P)) has become available in some industrial L-PBF machines over the past few years. Previous studies suggest that the use of pulsed lasers could enable larger control of heat input and melt pool formation during the process, and could thus enable improvement of spatial resolution and feature sizes in L-PBF. In this study, the experiments were implemented using a pulsed laser in combination with continuous scanning movement instead of the ‘point-and-shoot’ method typically used by industrial L-PBF(P) machines of today. The experiments were executed using a trial L-PBF system (IPG ytterbium fiber laser, wavelength 1075 nm) for gas-atomized stainless steel 316L powder on compositionally similar substrates. Single tracks were melted with three different pulse lengths (50, 100, and 200 µs) by using a constant layer thickness of 50 µm, while varying pulse repetition rate, scanning speed and laser power based on six preset values of volume energy density (VED) of 36-120 J/mm3. In order to allow a comparison to be made, additional samples were manufactured by using the CW emission of the same laser. It was observed that the L-PBF(P) samples yielded narrower tracks in comparison to the samples manufactured using CW emission. In addition, the results of the experiments show that, while maintaining constant VED values, decreasing the pulse length or scanning speed decreased the widths of the tracks and their penetration into the substrate. Consequently, it was noticed that shorter pulse lengths require more overlap between consecutive pulses in order to produce continuous tracks. Pulsed emission shows potential for improving the spatial resolution of the L-PBF process.

Volume added surface barrier discharge plasma excited by bipolar nanosecond pulse power in atmospheric air: optical emission spectra influenced by gap distance

In this paper, volume barrier discharge with different gap distances is added on the discharge border of high-voltage electrode of annular surface barrier discharge for generating volume added surface barrier discharge (V-SBD) excited by bipolar nanosecond high-voltage pulse power in atmospheric air. The excited V-SBDs consist of surface barrier discharge (d = 0 mm) and volume added surface barrier discharges (d = 2 mm and 3 mm). The optical emission spectra are recorded for calculating emission intensities of N2 (C3Πu → B3Πg) and (B2 → X2 ), and simulating rotational and vibrational temperatures. The influences of gap distance of V-SBD on emission intensity and plasma temperature are also investigated and analyzed. The results show that d = 0 mm structure can excite the largest emission intensity of N2 (C3Πu → B3Πg), while the existence of volume barrier discharge can delay the occurrence of the peak value of the emission intensity ratio of (B2 → X2 )/N2 (C3Πu → B3Πg) during the rising period of the applied voltage pulse and weaken it during the end period. The increasing factor of emission intensity is effected by the pulse repetition rate. The d = 3 mm structure has the highest threshold voltage while it can maintain more emission intensity of N2 (C3Πu → B3Πg) than that of d = 2 mm structure. The structure of d = 2 mm can maintain more increasing factor than that of the d = 3 mm structure with varying pulse repetition rate. Besides, the rotational temperatures of three V-SBD structures are slightly affected when the gap distance and pulse repetition rate vary. The vibrational temperatures have decaying tendencies of all three structures with the increasing pulse repetition rate.

Experiment and Prediction of Ablation Depth in Excimer Laser Micromachining of Optical Polymer Waveguides

Extending the data transfer rates through dense interconnections at inter‐ and intraboard levels is a well‐established technique especially in consumer electronics at the expense of more cross talk, electromagnetic interference (EMI), and power dissipation. Optical transmission using optical fibre is practically immune to the aforementioned factors. Among the manufacturing methods, UV laser ablation using an excimer laser has been repeatedly demonstrated as a suitable technique to fabricate multimode polymer waveguides. However, the main challenge is to precisely control and predict the topology of the waveguides without the need for extensive characterisation which is both time consuming and costly. In this paper, the authors present experimental results of investigation to relate the fluence, scanning speed, number of shots, and passes at varying pulse repetition rate with the depth of ablation of an acrylate‐based photopolymer. The depth of ablation essentially affects total internal reflection and insertion loss, and these must be kept at minimum for a successful optical interconnection on printed circuit boards. The results are then used to predict depth of ablation for this material by means of adaptive neurofuzzy inference system (ANFIS) modelling. The predicted results, with a correlation of 0.9993, show good agreement with the experimental values. This finding will be useful in better predictions along with resource optimisation and ultimately helps in reducing cost of polymer waveguide fabrication.

Diffraction response of photorefractive polymers over nine orders of magnitude of pulse duration.

The development of a single mode fiber-based pulsed laser with variable pulse duration, energy, and repetition rate has enabled the characterization of photorefractive polymer (PRP) in a previously inaccessible regime located between millisecond and microsecond single pulse illumination. With the addition of CW and nanosecond pulse lasers, four wave mixing measurements covering 9 orders of magnitudes in pulse duration are reported. Reciprocity failure of the diffraction efficiency according to the pulse duration for a constant energy density is observed and attributed to multiple excitation, transport and trapping events of the charge carriers. However, for pulses shorter than 30 μs, the efficiency reaches a plateau where an increase in energy density no longer affects the efficiency. This plateau is due to the saturation of the charge generation at high peak power given the limited number of sensitizer sites. The same behavior is observed in two different types of devices composed of the same material but with or without a buffer layer covering one electrode, which confirm the origin of these mechanisms. This new type of measurement is especially important to optimize PRP for applications using short pulse duration.

Sub ablative Er: YAG laser irradiation on surface roughness of eroded dental enamel.

This study evaluated the effects of Er:YAG laser irradiation applied at varying pulse repetition rate on the surface roughness of eroded enamel. Bovine enamel slabs (n = 10) were embedded in polyester resin, ground, and polished. To erosive challenges, specimens were immersed two times per day in 20mL of concentrated orange juice (pH = 3.84) under agitation, during a two-day period. Specimens were randomly assigned to irradiation with the Er:YAG laser (focused mode, pulse energy of 60 mJ and energy density of 3.79 J/cm(2) ) operating at 1, 2, 3, or 4 Hz. The control group was left nonirradiated. Surface roughness measurements were recorded post erosion-like formation and further erosive episodes by a profilometer and observed through atomic force microscopy (AFM). Analysis of variance revealed that the control group showed the lowest surface roughness, while laser-irradiated substrates did not differ from each other following post erosion-like lesion formation. According to analysis of covariance, at further erosive episodes, the control group demonstrated lower surface roughness (P > 0.05), than any of the irradiated groups (P < 0.05). The pulse repetition rate of the Er:YAG laser did not affect roughness of dental enamel eroded. The AFM images showed that the specimens irradiated by the Er:YAG laser at 1 Hz presented a less rough surface than those irradiated at 2, 3, and 4 Hz.

The Role of Pulse Repetition Rate in nsPEF-Induced Electroporation: A Biological and Numerical Investigation.

The impact of pulse repetition rate (PRR) in modulating electroporation (EP) induced by nanosecond pulsed electric fields (nsPEFs) in mammalian cells was approached here by performing both biological and numerical analysis. Plasma membrane permeabilization and viability of Jurkat cells were analyzed after exposure to 500, 1.3 MV/m, 40 ns PEFs with variable PRR (2-30 Hz). A finite-element model was used to investigate EP dynamics in a single cell under the same pulsing conditions, by looking at the time course of transmembrane voltage and pore density on the ns time scale. The biological observations showed an increased EP and reduced viability of the exposed cells at lower PRR in the considered range. The numerical analysis resulted in different dynamics of plasma membrane response when ns pulses were delivered with different PRR, consistently with a phenomenon of electrodesensitization recently hypothesized by another research group.

A Blumlein-type, nanosecond pulse generator with interchangeable transmission lines for bioelectrical applications

The design and realization of a nanosecond, high-voltage electric pulse generator for bioelectrical applications is reported in this paper. A Blumlein type architecture was adopted, with some modifications, and realized in a microstrip line configuration with meander-shaped conducting strips, and with ultra-fast, high voltage solid state switches. Three microstrip-lines have been realized in such a way that, being interchangeable within the structure, they allow to match different load impedances. A fiber optic-based system was also realized to separate the high voltage side of the system from the switch-control circuit. The system is suitable for applying high voltage nanosecond electric pulses, with variable pulse duration, amplitude, repetition rate and polarity, to liquid media with different electromagnetic characteristics, hosted in electroporation cuvettes with different gap dimensions.

Groups of bats improve sonar efficiency through mutual suppression of pulse emissions

How bats adapt their sonar behavior to accommodate the noisiness of a crowded day roost is a mystery. Some bats change their pulse acoustics to enhance the distinction between theirs and another bat's echoes, but additional mechanisms are needed to explain the bat sonar system's exceptional resilience to jamming by conspecifics. Variable pulse repetition rate strategies offer one potential solution to this dynamic problem, but precisely how changes in pulse rate could improve sonar performance in social settings is unclear. Here we show that bats decrease their emission rates as population density increases, following a pattern that reflects a cumulative mutual suppression of each other's pulse emissions. Playback of artificially-generated echolocation pulses similarly slowed emission rates, demonstrating that suppression was mediated by hearing the pulses of other bats. Slower emission rates did not support an antiphonal emission strategy but did reduce the relative proportion of emitted pulses that overlapped with another bat's emissions, reducing the relative rate of mutual interference. The prevalence of acoustic interferences occurring amongst bats was empirically determined to be a linear function of population density and mean emission rates. Consequently as group size increased, small reductions in emission rates spread across the group partially mitigated the increase in interference rate. Drawing on lessons learned from communications networking theory we show how modest decreases in pulse emission rates can significantly increase the net information throughput of the shared acoustic space, thereby improving sonar efficiency for all individuals in a group. We propose that an automated acoustic suppression of pulse emissions triggered by bats hearing each other's emissions dynamically optimizes sonar efficiency for the entire group.

High power and high energy monolithic single frequency 2 μm nanosecond pulsed fiber laser by using large core Tm-doped germanate fibers: experiment and modeling

We report a high power and high energy all-fiber-based single frequency nanosecond pulsed laser source at ~1918.4 nm in master oscillator-power amplifier (MOPA) configuration. The pre-shaped pulsed fiber laser seed with a variable pulse duration and repetition rate were achieved by directly modulating a continuous wave (CW) single frequency fiber laser using a fast electro-optical modulator (EOM) driven by a arbitrary waveform generator (AWG). One piece of single mode, large (30 μm) core, polarization-maintaining (PM) highly thulium-doped (Tm-doped) germanate glass fiber (LC-TGF) was used to boost the pulse power and pulse energy of these modulated pulses in the final power amplifier. To the best of our knowledge, the highest average power 16 W for single frequency transform-limited ~2.0 ns pulses at 500 kHz was achieved, and the highest peak power 78.1 kW was achieved at 100 kHz. Furthermore, mJ pulse energy was achieved for ~15 ns pulses at 1 kHz repetition rate. Theoretical modeling of the large-core highly Tm-doped germanate glass double-cladding fiber amplifier (LC-TG-DC-FA) is also present for 2 μm nanosecond pulse amplification. A good agreement between the theoretical and experimental results was achieved. The model was also utilized to investigate the dependence of the stored energy in the LC-TGF on the pump power, seed energy and repetition rate, which can be used to design and optimize the LC-TG-DC-FA to achieve higher pulse energy.

Probing background ionization: positive streamers with varying pulse repetition rate and with a radioactive admixture

Positive streamers need a source of free electrons ahead of them to propagate. A streamer can supply these electrons by itself through photo-ionization, or the electrons can be present due to external background ionization. Here we investigate the effects of background ionization on streamer propagation and morphology by changing the gas composition and the repetition rate of the voltage pulses, and by adding a small amount of radioactive 85Kr.We find that the general morphology of a positive streamer discharge in high-purity nitrogen depends on background ionization: at lower background ionization levels the streamers branch more and have a more feather-like appearance. This is observed both when varying the repetition rate and when adding 85Kr, though side branches are longer with the radioactive admixture. But velocities and minimal diameters of streamers are virtually independent of the background ionization level. In air, the inception cloud breaks up into streamers at a smaller radius when the repetition rate and therefore the background ionization level is higher. When measuring the effects of the pulse repetition rate and of the radioactive admixture on the discharge morphology, we found that our estimates of background ionization levels are consistent with these observations; this gives confidence in the estimates.Streamer channels generally do not follow the paths of previous discharge channels for repetition rates of up to 10 Hz. We estimate the effect of recombination and diffusion of ions and free electrons from the previous discharge and conclude that the old trail has largely disappeared at the moment of the next voltage pulse; therefore the next streamers indeed cannot follow the old trail.

Compact and high repetitive pulsed power modulator based on semiconductor switches

This paper describes the design of a compact and highly repetitive pulsed power modulator based on semiconductor switches. Output specifications of the proposed modulator are as follows: variable output pulse voltage, 1-10 kV; width, 1-10 μs; pulse repetition rate (PRR), 1-50,000 pps; and average output power, 10 kW. The proposed solid-state pulsed power modulator has two main parts: an IGBT stack that hasa gate drive circuit for the pulse output with a variable pulse width and repetition rate, and a capacitor charger that has a controllable output voltage. To connect all the storage capacitors in a series for high voltage pulses, a simple and reliable IGBT stack structure was proposed based on the Marx generator. In addition, the gate driver circuit, which supplies power and signals to all the IGBTs simultaneously, was introduced. This providesa superior protection function against the arc and the short. To charge the storage capacitors, a novel 10 kW (10 kV, 1 A) high voltage capacitor charger with the combined advantage of a series-loaded resonant converter and a ZVS (zero-voltage-switching) full-bridge pulse width modulation (PWM) converter was proposed and designed especially for the solid-state pulsed power modulator. Theexperiment results verified that the proposed scheme and structure can be used effectively for a high voltage pulsed power modulator that requires variable voltage, repetition rate, and pulse width, depending on the process of the applications.

A Novel Approach for Investigation of Chain Transfer Events by Pulsed Laser Polymerization

AbstractPulsed laser polymerization (PLP) with subsequent analysis of molecular mass distribution (MMD) is used to determine the rate coefficient of chain transfer to an agent A, ktrA, by varying pulse repetition rate such that the contributions of PLP‐induced and chain‐transfer‐induced peaks to the MMD change to a significant extent. It is shown by simulation that the relative heights of these peaks may be used to estimate ktrA. The method is applied to evaluation of the rate coefficient of chain transfer to dodecyl mercaptan with butyl methacrylate polymerizations at −11, 0, 20 and 40 °C. The Arrhenius parameters for this coefficient are determined to be: A(ktrA) = (2.2 ± 0.6) × 106 L · mol−1 · s−1 and Ea(ktrA) = (22.1 ± 0.7) kJ · mol−1. magnified image

Hydrodynamic size distribution of gold nanoparticles controlled by repetition rate during pulsed laser ablation in water

Most investigations on the laser generation and fragmentation of nanoparticles focus on Feret particle size, although the hydrodynamic size of nanoparticles is of great importance, for example in biotechnology for diffusion in living cells, or in engineering, for a tuned rheology of suspensions. In this sense, the formation and fragmentation of gold colloidal nanoparticles using femtosecond laser ablation at variable pulse repetition rates (100–5000Hz) in deionized water were investigated through their plasmon resonance and hydrodynamic diameter, measured by Dynamic Light Scattering. The increment of the repetition rate does not influence the ablation efficiency, but produces a decrease of the hydrodynamic diameter and blue-shift of the plasmon resonance of the generated gold nanoparticles. Fragmentation, induced by inter-pulse irradiation of the colloids was measured online, showing to be more effective low repetition rates. The pulse repetition rate is shown to be an appropriate laser parameter for hydrodynamic size control of nanoparticles without further influence on the production efficiency.

UV laser-ablated surface textures as potential regulator of cellular response

Textured surfaces obtained by UV laser ablation of poly(ethylene terephthalate) films were used to study the effect of shape and spacing of surface features on cellular response. Two distinct patterns, cones and ripples with spacing from 2 to 25 μm, were produced. Surface features with different shapes and spacings were produced by varying pulse repetition rate, laser fluence, and exposure time. The effects of the surface texture parameters, i.e., shape and spacing, on cell attachment, proliferation, and morphology of neonatal human dermal fibroblasts and mouse fibroblasts were studied. Cell attachment was the highest in the regions with cones at ∼4 μm spacing. As feature spacing increased, cell spreading decreased, and the fibroblasts became more circular, indicating a stress-mediated cell shrinkage. This study shows that UV laser ablation is a useful alternative to lithographic techniques to produce surface patterns for controlling cell attachment and growth on biomaterial surfaces.

Breakdown behavior of electronics at variable pulse repetition rates

Abstract. The breakdown behavior of electronics exposed to single transient electromagnetic pulses is subject of investigations for several years. State-of-the-art pulse generators additionally provide the possibility to generate pulse sequences with variable pulse repetition rate. In this article the influence of this repetition rate variation on the breakdown behavior of electronic systems is described. For this purpose microcontroller systems are examined during line-led exposure to pulses with repetition rates between 1 KHz and 100 KHz. Special attention is given to breakdown thresholds and breakdown probabilities of the electronic devices.

A novel scheme for a highly selective laser ion source

A new type of resonance ionization laser ion source, which shall combine the advantages of a laser ion source with those of an ion trap, is proposed. The primary purpose of such a laser ion source trap, which is based on a gas-filled linear radio-frequency quadrupole ion trap system, is the decoupling of evaporation and ionization processes. Furthermore optimum temporal control on the generated ion bunch is obtained. Both effects will lead to a significantly increased isobaric selectivity and ion beams of low emittance. A large variety of operational modes, ranging from quasi-dc to microseconds-bunched radioactive ion beams with variable pulse width and repetition rate, can be chosen freely and beam energies can easily be altered. The principles and the layout of LIST will be discussed on the basis of atom and ion trajectory simulations and resulting performance parameters.