Pulsed Regime Research Articles

Pulse Power Generation Chronoamperometry as an Advanced Readout for (Bio)sensors: Application for Noninvasive Diabetes Monitoring.

We propose pulse power generation (PPG) amperometry as an advanced readout realized for Prussian blue (PB)-based (bio)sensors. In contrast to the conventional power generation mode, when the current response is generated upon continuous short-circuiting, the suggested pulse regime is fulfilled by periodic opening and shorting of the circuit. Despite PB being electroactive, the pulse readout is advantageous over conventional steady-state power generation, providing up to a 15-fold increased signal-to-background ratio as well as dramatically improved sensitivity exceeding 10 A·M-1·cm-2 for H2O2 sensors and 3.9 A·M-1·cm-2 for glucose biosensors. Such analytical performance characteristics are, most probably, achieved due to the enrichment of the diffusion layer by analyte mass transfer from the bulk upon opening of the circuit. Due to an improved sensitivity-to-background ratio, reduced flow-rate dependence, and enhanced operational stability, the regime allows reliable monitoring of blood glucose variations through sweat analysis with the on-skin device.

Transition to unstable oscillatory flames in porous media combustion

The flames inside porous burners with low filtration velocity are prone to oscillatory behavior similar to unstable combustion in narrow channels but accompanied by complicated dynamics due to the irregularity of porous structure, flow field, and thermal gradient formed as a result of heat recuperation. This work is a numerical study within the framework of the pore-scale simulation approach devoted to investigating the flame oscillation mechanisms and characteristics in a two-dimensional pseudo-irregular packed bed of particles generated using the Voronoi tessellation algorithm with constant pore size. The results demonstrate that unstable flame oscillations like flames with repetitive extinction and ignition (FREI) lead to the velocity pulsation in adjacent pore channels, where the stable flame fragments are exposed to the hydrodynamically-driven fluctuations. In the case of the flow rate close to the stability threshold, the flow pulses can reach a magnitude enough to shift the stable flame in the upstream quenching zone, promoting its oscillations with repetitive extinction and ignition. Results of parametric simulation with a variation of the particle diameter and flow velocity were summarized in the regime diagram revealing the transition between stable normal flame, FREI, and stable weak flame with the velocity reduction. The pulsating transitional regime was detected between the stable normal flame and the FREI. The amplitude and intensity of such pulsations increase monotonically with the flow rate reduction in a narrow range. Oscillations of the unstable flame and hydrodynamically-driven flame pulsations are similar in dynamics but different in their physical mechanisms. However, in irregularly packed beds of particles, both these phenomena unavoidably coexist and interfere significantly with each other due to the connectivity of the pore channels.

Multipass optical cavity for low density plasma polarimetry: numerical analysis and detection scheme

Measurements of internal magnetic fields are of primary importance in laboratory plasma physics; the most common diagnostic method is Faraday rotation measurement by means of polarimetry. Faraday rotation measurements are integrated along a line of sight: the overall rotation is proportional to the plasma density, magnetic field and probing wavelength, as well as to the integration segment length and hence to the plasma dimensions.As a results, measurements of small magnetic fields in small, low density plasmas becomes non-trivial; on the other hand, this is typically the situation of small, self organized laboratory experiments.Being the probing wavelength practically limited to the THz region by diffraction phenomena, the use of a multipass scheme is actually the only way to improve the diagnostic sensitivity. This work discuss an application study of a multipass cavity to the PROTO-SPHERA experiment, but the results can be easily generalized to others of similar characteristics.In particular, different layouts are analyzed (straight, folded, annular cavity) discussing the overall stability region, the impact of changes in plasma refraction and the overall performances in terms of photon lifetime. The performance analysis features a discussion on continuous vs pulsed regime as well as on some basic detection schemes (direct, closed loop feedback).

Thermal-hydraulic assessment of Once-Through Steam Generators for EU-DEMO WCLL Breeding Blanket primary cooling system application

Once-Through Steam Generators (OTSGs) were recently selected to be installed in the Primary Heat Transfer Systems (PHTS) related to EU-DEMO WCLL Breeding Blanket (BB). Referring to the Balance of Plant Direct Coupling Design (BoP-DCD) option, these components are used to deliver the thermal power removed from the two principal blanket subsystems, i.e. First Wall (FW) and Breeder Zone (BZ), to the Power Conversion System (PCS).OTSG design foresees a vertical component with primary water moving downward inside tube bundle and secondary fluid flowing throughout shell in counter current. The latter moves firstly downwards through the annular downcomer and then goes up in the central riser, where it boils up to super-heated steam conditions. Then, it is deflected by the upper tubesheet, flows again downwards along the steam downcomer and exits through the laterally connected steam nozzles.In the last years, as part of the research activities associated to the Work Package Balance of Plant (WPBoP), a detailed analysis of the steam generator, aimed at deeply understanding the main thermal-hydraulic aspects characterizing its performances during the pulsed regime of DEMO normal operations, was performed. To fulfil this scope, a complete model of the component was prepared by using the best-estimate system code RELAP5/MOD3.3, selected to be also the reference design verification tool.During operations, OTSG experiences full-load and low-load alternative phases with rapid transitions from one another. The steam generator capability to follow such load variations avoiding the occurrence of instabilities in the component was studied. The computational activity carried out was a preliminary thermal-hydraulic characterization of the component conceptual design during both pulse and dwell phases. The feedbacks of this analysis were also fundamental for the conceptualization of the steam generator mock-up to be installed in the planned STEAM facility that will be built at the ENEA Research center of Brasimone.

Recent advances in thulium-doped fiber lasers based on saturable absorber materials at 2000 nm

Thulium-doped fiber laser (TDFL) devices are a promising laser source technology for applications in the wavelength of 2000 nm, such as spectroscopy, sensing, medical, among many others. Several pulse regimes employing different approaches have been constantly demonstrated in the scientific literature. Particularly, the development of new saturable absorber (SA) materials with exceptional nonlinear optical properties has become an efficient methodology for generating short pulses in fiber laser cavities at 2000 nm. Looking for clarifying the current state of the art of TDFL technology at 2000 nm, we review in this paper the main achievements of TDFLs in the last five years (2022–2018) based on SA materials. Nonlinear optical properties of these materials, experimental setup arrangements, as well as new avenues are addressed in this review for the advancements in source laser technologies operating at 2000 nm.

Dynamic behaviors of special noise-like pulses and rogue waves in an L-band ultrafast fiber laser

We demonstrate a composite regime of the special noise-like pulses (NLPs) and rogue waves (RWs) in an L-band dispersion-managed fiber laser. Apart from stable ultrashort pulses, the fiber laser can easily generate unique NLPs with clear peak on broad spectrum at high pump power and proper polarization state. In this regime, these tiny pulses with irregular intervals experience random fluctuations, making the dynamic evolutions of these pulse envelopes look like the endless undulating mountains along the hundreds of intra-cavity roundtrips. In addition, the real-time observation based on the time-stretch technique shows that there are the unique lacking pieces and changeable pulse durations in this NLPs regime. Besides, the chaotic pulse packets with intensity fluctuations are observed at the proper condition, showing the typical statistical distribution with L shape, which can be called as the typical RWs. It is believed that these results will facilitate the further understanding of the nonlinear characteristics of the NLPs and RWs in ultrafast fiber lasers.

Generation of Longitudinal Ultrasound by Shear Strain Pulses in the Spin–Phonon Echo Regime

The possibility of generation of longitudinal ultrasonic pulses of gigahertz frequencies in the spin–phonon echo regime by acting on a magnetized isotropic paramagnet with nanosecond shear strain pulses is predicted. The frequency of generated longitudinal ultrasonic echo signals is twice as high as the frequency of pulses entered into the medium. This effect can be realized when the velocity of longitudinal ultrasound exceeds the velocity of shear strain waves by less than two times. This resonant mechanism of the pulsed regime of second harmonic generation, which is accompanied by a nonlinear transformation of transverse into longitudinal ultrasound, has no optical analog.

Conceptual Design of the Steam Generators for the EU DEMO WCLL Reactor

In the framework of the EUROfusion Horizon Europe Programme, ENEA and its linked third parties are in charge of the conceptual design of the steam generators belonging to EU DEMO WCLL Breeding Blanket Primary Heat Transfer Systems (BB PHTSs). In particular, in 2021, design activities and supporting numerical simulations were carried out in order to achieve a feasible and robust preliminary concept design of the Once Through Steam Generators (OTSGs), selected as reference technology for the DEMO Balance of Plant at the end of the Horizon 2020 Programme. The design of these components is very challenging. In fact, the steam generators have to deliver the thermal power removed from the two principal blanket subsystems, i.e., the First Wall (FW) and the Breeding Zone (BZ), to the Power Conversion System (PCS) for its conversion into electricity, operating under plasma pulsed regime and staying in dwell period at a very low power level (decay power). Consequently, the OTSG stability and control represent a key point for these systems’ operability and the success of a DEMO BoP configuration with direct coupling between the BB PHTS and the PCS. In this paper, the authors reported and critically discussed the FW and BZ steam generators’ thermal-hydraulic and mechanical design, the developed 3D CAD models, as well as the main results of the stability analyses and the control strategy to be adopted.

Kinematic Dynamo Modeling and Its Peculiarities in Polar Latitudes

The results of calculations of the magnetic field structure in the kinematic approximation are presented, testifying to the consistency of the constructed numerical model with the structure of the flows of a conducting liquid: the instability of the convective process in a rotating spherical shell is reflected in the evolution of the magnetic field, manifesting itself in the pulsation regime of the field. It is shown that the features of the field evolution in the calculations are most clearly pronounced at high latitudes and have analogs in the behavior of the real geomagnetic field.

Sepiolite reinforced PEO layer on AZ 31

The present work deals with the investigation of the incorporation of nanosized ceramic particles (sepiolite) into an oxide layer formed on magnesium alloy AZ31 by plasma electrolytic oxidation (PEO). The actual (true) surface area of the sepiolite particles was determined to be 300 m2 g−1. The particles were exclusively electrostatically stabilized in the used electrolyte without visible sedimentation, at least over 27 days. Using a current controlled pulse regime, the particles are incorporated into the oxide layer simultaneously to the PEO process. The study clearly reveals that the movement and deposition of the particles is based on electrophoresis. Considering the measured mobility of the particles of μ ≈ 6 μm cm V−1 s−1 and a very low field strength in the electrolyte (0.4 V cm−1) the particles velocity was calculated to υ ≈ 2.4 μm s−1. The mass of deposited sepiolite was estimated to approximately 1.2 mg cm−2 under the chosen conditions. The fixation of the deposited particles is caused by sintering due to the heat development of the plasma discharge events.

Compact Yb fiber few-cycle pulse source based on precision pulse compression and shaping with an adaptive fiber Bragg grating.

We generate bandwidth limited 10 µJ pulses of 92 fs pulse width using an adaptive fiber Bragg grating stretcher (FBG) in conjunction with a Lyot filter. The temperature controlled FBG is used to optimize the group delay, whereas the Lyot filter counteracts gain narrowing in the amplifier chain. Soliton compression in a hollow core fiber (HCF) allows for access to the few-cycle pulse regime. Adaptive control further enables the generation of nontrivial pulse shapes.

20 MHz resonant photodetector for the homodyne measurement of picosecond pulsed squeezed light

A high-performance resonant balanced homodyne detector is a key element for the measurement of picosecond pulsed squeezed light, which is an important quantum resource in quantum-enhanced microscopic imaging. In this paper, we design and demonstrate a resonant photodetector for homodyne measurement with a maximum signal-to-noise ratio of 22.42 dB at resonant frequency of 20 MHz. Using this detector to pulsed regime, a maximum signal-to-noise ratio at resonant frequency is 10.02 dB, where a 40 dB subtraction capability at 80 MHz repetition rate of pulsed laser is obtained. With this design, the quantum noise of picosecond pulsed squeezed light is measured and the best squeezing level −1.7 dB below the shot noise level is clearly observed at 20 MHz.

Merging and disconnecting resonance tongues in a pulsing excitable microlaser with delayed optical feedback.

Excitability, encountered in numerous fields from biology to neurosciences and optics, is a general phenomenon characterized by an all-or-none response of a system to an external perturbation of a given strength. When subject to delayed feedback, excitable systems can sustain multistable pulsing regimes, which are either regular or irregular time sequences of pulses reappearing every delay time. Here, we investigate an excitable microlaser subject to delayed optical feedback and study the emergence of complex pulsing dynamics, including periodic, quasiperiodic, and irregular pulsing regimes. This work is motivated by experimental observations showing these different types of pulsing dynamics. A suitable mathematical model, written as a system of delay differential equations, is investigated through an in-depth bifurcation analysis. We demonstrate that resonance tongues play a key role in the emergence of complex dynamics, including non-equidistant periodic pulsing solutions and chaotic pulsing. The structure of resonance tongues is shown to depend very sensitively on the pump parameter. Successive saddle transitions of bounding saddle-node bifurcations constitute a merging process that results in unexpectedly large regions of locked dynamics, which subsequently disconnect from the relevant torus bifurcation curve; the existence of such unconnected regions of periodic pulsing is in excellent agreement with experimental observations. As we show, the transition to unconnected resonance regions is due to a general mechanism: the interaction of resonance tongues locally at an extremum of the rotation number on a torus bifurcation curve. We present and illustrate the two generic cases of disconnecting and disappearing resonance tongues. Moreover, we show how a pair of a maximum and a minimum of the rotation number appears naturally when two curves of torus bifurcation undergo a saddle transition (where they connect differently).

Genetic algorithm optimization of broadband operation in a noise-like pulse fiber laser

The noise-like pulse regime of optical fiber lasers is highly complex, and associated with multiscale emission of random sub-picosecond pulses underneath a much longer envelope. With the addition of highly nonlinear fiber in the cavity, noise-like pulse lasers can also exhibit supercontinuum broadening and the generation of output spectra spanning 100’s of nm. Achieving these broadest bandwidths, however, requires careful optimization of the nonlinear polarization rotation based saturable absorber, which involves a very large potential parameter space. Here we study the spectral characteristics of a broadband noise-like pulse laser by scanning the laser operation over a random sample of 50,000 polarization settings, and we quantify that these broadest bandwidths are generated in only sim 0.5% of cases. We also show that a genetic algorithm can replace trial and error optimization to align the cavity for these broadband operating states.

Active Tunable Pulse Shaping Using MoS2‐Assisted All‐Dielectric Metasurface

While optical pulse shapers have important applications in classical and quantum communication regimes and laser resonant cavities, engineering of group delay dispersion (GDD) remains one of their greatest challenges. Herein, by taking advantage of the electrically tunable optical properties of 2D material and the low‐loss nature of dielectric material. This paper demonstrates how an active tunable all‐dielectric metasurface assisted by 2D material can be leveraged to shape the temporal profile of a pulse. The proposed metasurface consists of an array of nanobars covered by a 2D sheet and positioned on a distributed Bragg reflector (DBR) as a perfect mirror to design a phase‐only modulator. Upon introducing in‐plane asymmetries, the quasi‐bound state in the continuum (QBIC) resonance emerges under normal incidence, which subsequently leads to achieving both significant GDD and the two regimes of pulse stretching and compressing via boosting the effect of the permittivity variation of molybdenum disulfide (MoS2). The monolayer MoS2 proves to be an excellent substitute for other tunable materials with inherent dissipative loss in the visible frequency range. Following such an active tunable geometrically fixed configuration, various pulse‐shaping operations are achieved, including compression (peak intensity up to 350%), expansion (peak intensity from 60%), splitting, and higher‐order distortion.

Pulse distortion caused by waveguide inhomogeneity in nonlinear optical wavelength converters

Low-noise integrated all-optical wavelength converters that can be operated in short pulse regime are essential tools to overcome contention resolution in a modern communication network, based on wavelength division multiplexing. Any imperfect functionality in such devices causes non-ideal optical power transfer to the converted data pulses. All imperfections during the preparation and operation of the wavelength converters can be addressed to the waveguide inhomogeneity which distorts data pulses to be converted. This paper reports different waveguide inhomogeneity effects on the pulse distortion while using periodically poled lithium niobate waveguide as wavelength converters. Three types of chi ^{(2)}-based nonlinear optical processes, including second harmonic generation, difference frequency generation, and cascaded second harmonic generation/difference frequency generation are numerically studied to show that any constant, linear, and quadratic waveguide inhomogeneity causes short pulse (down to 1 ns) distortion in such wavelength converters. In addition, it is shown that the reconstruction of textrm{sech}^2-shaped generated pulses is possible, when suitable upside-down quadratic variations of obtained inhomogeneity are deliberately induced in the waveguide. Notably, for pulsed second harmonic generation, the generated pulse can be compressed using an upside-down quadratic phase mismatch.

Influence of gaseous laser in continuous-wave and pulse regimes on biomaterial characteristics

Aromatic plants and laser beam wavelength in the red range of visible spectra (623.8 nm), most commonly used for treating plant species, were selected. As there is not much information in the references, it is necessary to set a scale with qualitative and partially quantitative evaluations of the results. Specimens of thyme seed (Thymus vulgaris) were selected as aromatic plants for studying the effect of low-power lasers on plant development, and continuous He-Ne laser, with irradiation times in the range of minutes.

A novel online dispersive liquid-liquid microextraction for the spectrophotometric determination of free glycerol in biodiesel.

Biodiesel is an important alternative fuel produced from animal fats or vegetable oils. According to several world regulatory agencies, the acceptable threshold content for free glycerol in biodiesel is 200 mg kg-1. Exceeding concentrations may yield high amounts of acrolein after combustion. The analytical methods for glycerol determination are usually preceded by conventional liquid-liquid extraction that may affect precision and accuracy and impair analytical frequency. In this work, a multi-pumping flow system was proposed for the online dispersive liquid-liquid extraction of free glycerol from biodiesel followed by spectrophotometric determination. The analyte was transferred to the aqueous phase by mixing the sample and water under a pulsed flow regime. The emulsion was directed towards a retention column to remove the organic phase before chemical derivatization. Glycerol was oxidized by NaIO4 to generate formaldehyde, which reacted with acetylacetone in an ammonium acetate medium to yield 3,5-diacetyl-1,4-dihydrolutidine (λMAX = 412 nm). The optimization of the main parameters of the system was carried out by multivariate methods. The screening of variables was performed by fractional factorial design 24-1. The models for free glycerol determination and extraction were refined by central composite and full factorial 23, respectively. In both cases, the validation was carried out by analysis of variance, which yielded a satisfactory F-test value. After optimization, a linear range from 3.0 to 50.0 mg L-1 glycerol was observed. The detection limit, coefficient of variation and determination frequency were estimated to be 2.0 mg L-1 (n = 20; 99.7% confidence level), 4.2-6.0% (n = 20) and 16 h-1, respectively. The efficiency of the process was estimated to be 66%. After each extraction, the retention column (filled with 185 mg of glass microfiber) was washed with 50% ethanol solution to avoid carry-over effects. The comparative analyses of samples by the proposed and reference methods demonstrated the accuracy of the developed procedure at a 95% confidence level. Recoveries between 86 and 101% also indicated that the proposed procedure is accurate, suitable and reliable for online extraction and determination of free glycerol in biodiesel.

Crystalline waveguides with carbon nanomaterials for miniaturized pulsed lasers - INVITED

This presentation discusses the recent results on miniaturized pulsed solid-state lasers by utilizing femtosecond-laser inscribed crystalline channel waveguides and carbon-nanomaterial-based saturable absorbers. Based on optical characterization and optimization of the optical materials, integrated compact waveguide lasers present diverse pulsed operation regimes from Q-switching to continuous-wave mode-locking. Pulsing mechanism and various parameters in waveguide lasers are investigated to provide a basis for achieving higher performance of novel on-chip ultrafast lasers.

Third-order optical nonlinear features of Er3+ and Pr3+ activated multicomponent borate glasses in nanosecond pulse regime: A comparative study

In the current work, the borate-based glass specimens bearing composition (mol %) 5Na 2 O–10ZnO–10Bi 2 O 3 ‒(75‒ x )B 2 O 3 – x RE (where RE = Er 2 O 3 and Pr 6 O 11 , and x ranges from 0 to 5 mol %) were synthesized by conventional melt-quench method. The effect of rare earth ions concentration on photonic nonlinearities has been explored by the Z-scan method under the pumping of 532 nm in a nanosecond pulse regime. The Z-signatures revealed the occurrence of nonlinear absorption and refraction in the studied glass specimens. The nonlinear optical features were improved with respect to the rare earth concentration. The nonlinear optical features were superior for Er 3+ doped glasses compared to Pr 3+ doped glasses. This greatness of nonlinear optical features in Er 3+ doped glasses was attributed to the existence of a large number of electrons (11 electrons) in the 4 f outer shell compared to those existing in the 4 f outer shell of Pr 3+ (2 electrons). The enhancement of nonlinear gap features was also attributed to the creation of non-bridging oxygens in the host network when rare earths are loaded into the composition. The nonlinear photonic outcomes endorse that heavily Er 3+ loaded glasses are superior over Pr 3+ doped glasses for nonlinear optical device fabrications to work in the visible region.