Pulsed Regime Research Articles

Ultra-short stripe microlasers fabricated with a focused ion beam etching technique.

Fabry-Perot (FP) lasers with a cavity length shorten down to 50 µm were investigated. One or two laser mirrors were formed by focused ion beam etching. InGaAs quantum dots of high density were used as the laser active region. The lasers operate in a pulsed regime up to at least 105°С. A comparison with lasers formed by the facet cleaving method (minimum length is 100 µm) reveals the similarity of the emission wavelength and the threshold current depending on the cavity length. This allows us to conclude that the etched facets do not significantly affect the non-radiative recombination rate and provide a reflection coefficient close to that of the cleaved facet. The maximum modal gain was estimated to be not less than 240 cm-1. No evidence of the excited-state lasing was found.

Enhancing He 23S1 state production in the burst regime of a nanosecond discharge

Abstract Atmospheric repetitive He discharge with 10\,ns current peak width working in the burst pulse regime was studied. The production of He $^3$S$_1$ and $^1$S$_0$ metastable states was examined using laser- collisions- induced fluorescence (LCIF) and atomic absorption spectroscopy (AAS). Both methods independently reveal a significant increase of $^3$S$_1$ population in the burst regime, depending on the plasma pulse separation time in the burst. Up to a seven-fold increase in the $^3$S$_1$ population was detected after the third pulse in a 3-pulse burst sequence (as compared to the value obtained after the first pulse or in a single pulse regime), not requiring additional applied power. The observed effect is explained by recombination between the electrons and the ground state ions (He$^+$, He$_2^+$) produced in the previous plasma pulse and refilling the volume of interest in few hundred $\mu$s.

Linking the Flow Regime of Papyrus‐Dominated Wetlands to Biologically Relevant Hydrologic Attributes

ABSTRACTThe dominant plant species in many African wetlands is Cyperus papyrus. Its adaptation to saturated and low‐oxygen conditions and its dense structure and height provide breeding and feeding grounds for unique flora and fauna. As a keystone species adapted to local hydrology, the flooding regime of papyrus offers the full range of hydrologic conditions and events essential to ecosystem health. However, no study has attempted to link papyrus wetlands' flow regimes to their biologically relevant hydrologic attributes. This study assesses hydrologic alterations of a papyrus wetland's flow regime due to rice irrigation. We develop a conceptual ecological model linking papyrus to hydrologic attributes to determine the consequences of changed environmental flow components (EFCs) on papyrus as a habitat. We find that agricultural water management considerably alters the magnitude, duration, timing and rate of change of EFCs, which could affect productivity (seed dispersal, germination and establishment; rhizome spreading; papyrus distribution across transects; and dispersal of floating mats) in papyrus wetlands. However, the effect on the papyrus wetlands' natural pulsed regime is negligible when the ratio of irrigated area to catchment area is no greater than 1:150. Overall, a better understanding of the threats of water diversion for agriculture is made by linking papyrus' flow regimes to biologically relevant hydrologic attributes. Knowledge of the roles of the various EFCs could provide opportunities for conserving and protecting papyrus wetlands, especially for systems at risk of altered flows.

Ultrahigh Resolving Power Ion Mobility Spectrometry with a Simple Pulser Circuitry.

The pulsing circuitry for high resolving power drift-tube ion-mobility spectrometry is based on three avalanche photodiodes. These are switched on by illumination through optical fibers, which provide electrical insulation of the driver circuitry from the high voltage. The setup was tested with a series of quaternary ammonium ions introduced with an electrospray ion source. Two instruments with drift tubes of 10 and 30 cm length were employed and a separation voltage of up to 23.7 kV. Resolving powers above 200 could be achieved for the longer tube, which are comparable to those obtained with a previously employed more elaborate electrically floating pulser. The new pulser allows the creation of common two-state injection pulses as well as tristate pulses known to reduce the discrimination of low mobility ions. A comparison between the two pulsing regimes showed that, as predicted by theory, for the longer tube, the discrimination of low-mobility ions in the two-state shutter mode was less significant than for the shorter tube.

Actual problems of modeling thermal-hydraulic processes in fast neutron reactors

The results of studies on hydrodynamics and heat transfer processes in fast neutron reactors are presented. Data on turbulent momentum transfer in rod bundles are analyzed. It is shown that the intensification of turbulent momentum transfer in the rod bundle channels is due to large-scale turbulent momentum transfer (secondary currents). The intensification of interchannel turbulent exchange in close-packed lattices of rods is explained. A dependence is obtained for the dissimilarity coefficients of interchannel convective exchange forced by wire wrapping in bundles of rods. The methods and results of numerical modeling of thermal hydraulics using the Monte Carlo method, thermomechanical analysis of the temperature field in fuel rod assemblies in the lifetime process are presented. The results of modeling based on a water model of temperature fields and the structure of coolant movement in the primary circuit of the reactor in various regimes are presented. A stable temperature stratification of the coolant was revealed in the peripheral zone of the upper chamber of the reactor above the side screens. It is shown that the process of boiling liquid metals in fuel assemblies has a complex structure, characterized by stable and pulsating regimes and a heat transfer crisis. The agreement between the results of experimental and numerical modeling is shown. A cartogram of the flow regimes of a two-phase flow of liquid metals in fuel rod assemblies has been plotted. The influence of the surface roughness of fuel elements on the boiling process and heat transfer during boiling of liquid metals is analyzed. Long-term cooling of a fuel assembly with a “sodium cavity” above the reactor core in accident regimes with boiling of liquid metals is shown. The objectives of further research are formulated.

Cavity-enhanced Faraday rotation spectroscopy for interference-free measurement of OH radical at 2.8 μm

An instrument based on cavity-enhanced Faraday rotation spectroscopy (CE-FRS) operating at 2.8 μm has been developed for interference-free measurement of OH radicals in the laboratory. By off-axis coupling of a continuous-wave laser into a high finesse optical cavity, FRS signal is obtained from balanced detection of time-integrated light intensity leaking out of the cavity in the presence of magnetic field. Radio-frequency white noise (5–520 MHz) was injected into laser current which reduced intensity fluctuations in cavity transmission, thus improved the signal-to-noise ratio of the spectroscopic signal by a factor of 2. The setup provides a simple and robust spectroscopic instrument for in-situ and highly-selective detection of paramagnetic species. We demonstrated the instrument’s capabilities using OH radical with concentration in the range of 1012 molecule.cm−3, generated by microwave discharge of water vapor at low pressure. The CE-FRS instrument exhibited a limit of detection of ∼ 1010 molecule.cm−3 in an integration time of 20 s, which is enhanced by a factor of 2.5 compared to cavity-enhanced wavelength modulation spectroscopy involving an off-axis integrated cavity output spectroscopy approach. A time-resolved FRS signal was recorded in a pulsed microwave discharge regime, giving a millisecond time resolution for the measurement of OH concentration profile. The developed instrument provides a potential analytical tool for the measurement of OH concentration for chemical kinetic study in reactor cells.

Development of a Semi-Autonomous Pulse and Receive Concrete Inspection System

Concrete structures are being subjected to increasing loads and used beyond their intended lifespan. When combined with operators shrinking maintenance budgets it is imperative structures are monitored for damage. Acoustic Emission (AE) and AE Tomography offer a method for damage detection and characterisation. However, they are encumbered by their equipment and setup requirements, and best used on areas where damage is already known to have occurred. An autonomous pulse catch system could be used to identify these areas. To explore the potential of such a system, seven concrete beams differentiated by their aggregate size were tested using an automated pulse and receive system. The effect of aggregate size, and pulse frequency on attenuation and wavespeed are investigated. The pulse regime consisted of narrowband pulses ranging from 30 kHz to 500 kHz. The results show that high frequency pulses experienced greater attenuation than low frequency pulses. Whilst a link appears between the size of the aggregate and the rate of attenuation of high frequency signals. A strong negative correlation between amplitude loss and pulse frequency was observed. For the specimen containing coarse aggregate, pulse velocity was constant between 100 kHz and 500 kHz, whereas specimens containing coarse aggregate experienced frequency dependant attenuation. This testing shows the significant potential for an automated pulse and receive system, which could lead to the development of an autonomous health monitoring system. In addition, the approach can be instrumental in developing large data sets, that can be used in Machine Learning processes, to develop a deeper understanding of AE signal propagation in concrete materials.

Development of Figure‐of‐Nine Laser Cavity for Mode‐Locked Fiber Lasers: A Review

AbstractArtificial saturable absorbers (SA) are nonlinear optical devices widely employed in mode‐locked lasers. Among the artificial SAs are nonlinear polarization evolution (NPE) and nonlinear amplifying loop mirrors that can work in either figure‐of‐eight (Fo8) or figure‐of‐nine (Fo9) laser cavities. The NPE technique is highly sensitive to environmental perturbations. Both Fo8 and Fo9 laser cavities exhibit a higher environmental stability than the NPE technique. Here the recent advances of the Fo9 laser cavity, with a focus on the pulse formation mechanisms and the role of different cavity parameters that can enable ultrafast mode‐locking analyses are discussed. Besides, the recent development of using the Fo9 laser cavity to generate high‐energy rectangular laser pulses through either dissipative soliton resonance or noise‐like pulse regimes is also reviewed. In conclusion, the current issues and challenges of pulse formation through the Fo9 laser cavity are highlighted and recommendations for future research directions are proposed. This review is expected to provide a deeper insight into the Fo9 laser cavity as the next generation of artificial SA with interesting cavity structures, laser features, and output performances.

Modeling nanogratings erasure at high repetition rate in commercial optical glasses

Volume nanogratings (NGs) imprinted by infrared femtosecond laser in commercial optical glasses take the form of orientable subwavelength birefringent nanostructures, being composed of an assembly of nanopores. The existence of NGs strongly depends on the laser parameters and glass composition. Therefore, in this work, we tentatively model the erasure threshold of NGs in a pulse energy - repetition rate processing window. For this purpose, we combine i) a heat diffusion model to simulate the thermal treatment experienced by the glass upon laser irradiation, and ii) the Rayleigh-Plesset equation to take into account the evolution of a nanopore size during laser processing. We first determine a criterion for nanopores erasure, falling within a typical characteristic time of few tens of ns, for which the cooling of the last laser pulse absorbed by the material is progressively cooled. Then, considering a multipulse regime, and the dependence of the deposited pulse number on the thermal treatment experienced by the glass, the modeled NGs erasure threshold follows the experimental trend. Finally, by using a steady state regime for various repetition rates and adjusting the energy deposition (absorption coefficient or beam waist) as a function of the pulse energy, the NGs existence window can perfectly match the experimental values.

An insight in to microwave induced defects and its impact on nonlinear process in NiO nanostructures under femtosecond and continuous wave laser excitation.

This work demonstrates the impact of microwave (MW) irradiation on third-order nonlinear optical (NLO) processes in chemically deposited NiO nanostructure films. The optical nonlinearity of the NiO nanostructure films was studied using third-harmonic generation (THG) measurements in the pulsed femtosecond laser regime and the Z-scan technique in the continuous wave laser regime. In the ultrafast pulsed regime, THG measurements revealed a significant increase in the THG signal of MW-irradiated NiO nanostructures due to photoexcitation and relaxation processes, resulting from an enhancement in defect concentration. This increase in defect density upon MW irradiation was quantified by PL and XPS studies. Under continuous wave laser irradiation, the Z-scan technique showed an enhanced absorption coefficient of ∼10-1 m W-1 and a nonlinear refractive index of ∼10-7 m2 W-1. The high NLO values in both pulsed and continuous laser regimes indicate that MW-irradiated NiO nanostructure films hold promise for optoelectronic device applications.

Generation of noise-like pulses in a linear-cavity Tm fiber mode-locked laser based on FMF saturable absorber

As a real saturable absorber (SA) based on the nonlinear multimode interference (NL-MMI) effect, the devices based on the few-mode fiber possess numerous exciting characteristics due to their all-fiber structure, straightforward manufacturing process. In this paper, by employing a SA with the SMF-FMF-SMF structure in a linear cavity, we demonstrate a stable mode-locked Tm-doped fiber laser. At the pump power of 1 W, a single noise-like pulse (NLP) with a pedestal pulse duration of 28 ps and coherent peak width of 0.77 ps is generated at a central wavelength of 1940 nm with a 3 dB bandwidth of 10.51 nm. The stable noise-like pulse can be sustained from the lasing pump threshold up to the maximum pump power of 2.2 W without experiencing pulse splitting. The experiment results in a peak average output power of 116 mW, along with a corresponding maximum pulse energy of 24.73 nJ at a repetition frequency of 4.69 MHz. The high stability of our fiber laser is confirmed by the signal-to-Noise Ratio (SNR) of 63 dB, and its enduring stability is additionally validated over an 8-hour period. The experimental findings suggest that the SMF-FMF-SMF structure has significant potential in the simple and robust all-fiber mode-locked lasers operating in the noise-like pulse regime.

Polarization purity and dispersion characteristics of nested antiresonant nodeless hollow-core optical fiber at near- and short-wave-IR wavelengths for quantum communications

Advancements in quantum communication and sensing require improved optical transmission that ensures excellent state purity and reduced losses. While free-space optical communication is often preferred, its use becomes challenging over long distances due to beam divergence, atmospheric absorption, scattering, and turbulence, among other factors. In the case of polarization encoding, traditional silica-core optical fibers, though commonly used, struggle with maintaining state purity due to stress-induced birefringence. Hollow core fibers, and in particular nested antiresonant nodeless fibers (NANF), have recently been shown to possess unparalleled polarization purity with minimal birefringence in the telecom wavelength range using continuous-wave (CW) laser light. Here, we investigate a 1-km NANF designed for wavelengths up to the 2-μm waveband. Our results show a polarization extinction ratio between ~−30 dB and ~−70 dB across the 1520 to 1620 nm range in CW operation, peaking at ~−60 dB at the 2-μm design wavelength. Our study also includes the pulsed regime, providing insights beyond previous CW studies, e.g., on the propagation of broadband quantum states of light in NANF at 2 μm, and corresponding extinction-ratio-limited quantum bit error rates (QBER) for prepare-measure and entanglement-based quantum key distribution (QKD) protocols. Our findings highlight the potential of these fibers in emerging applications such as QKD, pointing towards a new standard in optical quantum technologies.

Photobleaching Effect on the Sensitivity Calibration at 638 nm of a Phosphorus-Doped Single-Mode Optical Fiber Dosimeter.

We investigated the influence of the photobleaching (PB) effect on the dosimetry performances of a phosphosilicate single-mode optical fiber (core diameter of 6.6 µm) operated at 638 nm, within the framework of the LUMINA project. Different irradiation tests were performed under ~40 keV mean energy fluence X-rays at a 530 µ Gy(SiO2)/s dose rate to measure in situ the radiation-induced attenuation (RIA) growth and decay kinetics while injecting a 638 nm laser diode source with powers varying from 500 nW to 1 mW. For injected continuous power values under 1 µW, we did not measure any relevant influence of the photobleaching effect on the fiber radiation sensitivity coefficient of ~140 dB km-1 Gy-1 up to ~30 Gy. Above 1 µW, the fiber radiation sensitivity is significantly reduced due to the PB associated with the signal and can decrease to ~80 dB km-1 Gy-1 at 1 mW, strongly affecting the capability of this fiber to serve as a dosimeter-sensitive element. Higher power values up to 50 µW can still be used by properly choosing a pulsed regime with periodic injection cycles to reduce the PB efficiency and maintain the dosimetry properties. Basing on the acquired data, a simple model of the photobleaching effect on a coil of the investigated fiber is proposed in order to estimate its sensitivity coefficient evolution as a function of the cumulated dose and its fiber length when injecting a certain laser power. Additional studies need to investigate the influence of the temperature and the dose rate on the PB effects since these parameters were fixed during all the reported acquisitions.

Dynamics of pulsating solitons with chaotic behaviors from a 1.7 μm ultrafast fiber laser

We demonstrate observation and dynamics of unique soliton pulsation regime in an ultrafast fiber laser at 1.7 μm. It is found that with either single-soliton pulse or multiple-soliton pulses in cavity the laser emission could exhibit periodic intensity and shot-to-shot spectra variations as measured based on the dispersive Fourier transformation (DFT) technique. Further, detailed studies turned out that the soliton pulsation can comprise numerous small pulses with random intensities, exhibiting chaotic characteristics. Besides, all the pulses within the multiple solitons possess identical pulsation period. In addition, despite of the fact that the temporal intervals between the multiple solitons are on the order of tens of nanoseconds among them, their pulsations demonstrate excellent synchronicity. The experimental observations are further revealed by numerical simulations. These results can provide insights into ways to optimize designs of laser sources and open up new potential for generation of pulsating solitons in ultrafast fiber lasers at different wavelengths.

High-Power Lasers

High-power lasers play an important role in modern science, industry, and medicine. A significant milestone was reached on 5 December 2022, when Inertial Confinement Nuclear Fusion (ICF) achieved scientific breakeven, releasing more energy than the input laser energy. Additionally, Extreme Ultraviolet Lithography (EUVL) has enabled the development of microchips with 3 nm process nodes, marking a leap in semiconductor technology. These examples, together with the recent achievement of 10 PW (1015 W) laser output, herald remarkable advancements in technology and science. Laser systems are broadly classified based on their operating regimes into two main categories: Continuous Wave (CW) operation, where the laser is continuously pumped and emits a steady beam of light, and the pulsed regime, in which the laser produces single or multiple pulses at various repetition rates. This review will primarily focus on pulsed laser systems, exploring their various types and recent technological advancements.

UV-random lasing of ZnO films decorated with Au nanoparticles and modification of lasing threshold by surface plasmon effect

Threshold energy values for random lasing action are determined from spectral and temporal coherence studies in systems with incoherent feedback. Zinc oxide films with and without their surface having been decorated with gold nanoparticles stand simultaneously for the active and the scattering medium, whereas pumping is carried out by a conventional laser system operating at the picosecond pulse regime. The influence of the gold surface on both the threshold and the intensity of the optical response is investigated. Whenever localized surface plasmon resonances are supported, electron mobility at the gold boundaries facilitates reinsertion of the trapped electrons into shallow defect levels to the conduction band. In consequence the characteristic visible emission from ZnO (green) due to transitions from such levels is suppressed, thus lowering the pumping threshold prerequisite for random lasing emission. The tunning of the random lasing threshold is achievable by means of control on the morphology and size effects of the gold nanoparticles. This contribution shows that specific post-thermal treatments could be exploited to adjust the lasing properties of novel metal decorated systems, which already feature random lasing before decoration. In principle, the extension of the decoration should be rather small to minimize response losses due to screening side effects of the metal phase, which obtrudes direct contact of the pumping photons with the semiconductor system.

5.3 W/265 μJ Mid-IR All-Fiber Er3+:ZBLAN Gain-Switched Laser Based on Dielectric Fiber Mirror and Fiber-Tip Protection

We report a 2.8 μm all-fiber high-power and high-energy gain-switched Er3+:ZBLAN laser based on dielectric fiber mirror and fiber-tip protection. The fiber pigtail mirror, specifically designed for dichroic operation (i.e., anti-reflection at 976 nm pump wavelength and high-reflection around 2.8 μm laser wavelength), shows high damage density of >10 MW/cm2. An anti-reflection protective film is coated on the input tip of Er3+:ZBLAN fiber and an AlF3 endcap is spliced to the output tip of Er3+:ZBLAN fiber for mitigating the fiber-tip photodegradation and high-power catastrophic failure at 2.8 μm. The compact all-fiber cavity is formed by efficiently connecting the Er3+:ZBLAN fiber with dielectric fiber mirror using the standard FC/PC fiber adaptor. When the 976 nm pump operates in pulsed regime, the all-fiber mid-infrared gain-switched laser can be attained with two states of single-pulse and pulse-burst output. The extracted maximum pulse energy is 4.8 μJ in the single-pulse state, and the shortest pulse width is 426 ns. The pulse-burst mode can generate a maximum average power of 5.291 W and burst energy of 264.55 μJ. This work may offer a promising way to realize the low-cost, all-fiber, high-power and high-energy gain-switched laser at MIR wavelengths.

Multishot laser damage of multilayer dielectric mirrors in the near-infrared subpicosecond regime

The laser damage resistance of dielectric components of high-power laser facilities to laser irradiation depends significantly on the irradiation sequence. In the short pulse (fs) regime, it is known that continuous irradiation of these components leads to a reduction in the damage threshold, reflecting a laser fatigue effect. Conversely, in the long pulse (ns) regime, progressive irradiation of these components leads to an increase in the damage threshold, reflecting a laser conditioning effect. In this article, we experimentally evaluate the competition between the effects of laser fatigue and laser conditioning for multilayer dielectric components irradiated in the subpicosecond pulse regime in the infrared (∼1µm) through different test sequences. For this purpose, we implemented an original test sequence derived from an S-on-1 type protocol, which consists of irradiating the component until damage. By repeating this sequence at different set points, it was possible to estimate the progressive reduction in damage threshold with the number of laser irradiations and to compare it with that observed during the fluence ramps. Particular attention was also paid to the precise knowledge of the test beam irradiating the component, as a dependence of the beam surface on the test set point was highlighted.

Transition to oscillatory instability of lean methane–air flames in microchannels

Micro-flow reactors and porous media burners are prone to instability in the form of flame with repetitive extinction and ignition (FREI) near the region of a temperature gradient. Although significant progress has been made in this field, there is still a lack of understanding regarding the nature of such stable-to-unstable transition. Some studies reported a smooth and continuous instability transition in microchannels, interpreting this phenomenon as a supercritical bifurcation. Meanwhile, others have observed that the transition is subcritical, with rapid, stepwise evolution of the stable flame to FREI. This study aims to rigorously determine the transition character theoretically and numerically using a two-dimensional model with a precise resolution of the bifurcation parameter (flow velocity) near the critical point and gradual wall temperature ramp. The results indicate that the transition is a subcritical bifurcation with strong hysteresis. However, the amplitude of the limit cycle born in the bifurcation point could be small compared to FREI-like oscillations. Moreover, further development of the instability depends significantly on the channel width. In relatively wide channels, the stable regime transforms immediately into the extinction/ignition one with rapid growth in amplitude. In moderate channels, complex transitional dynamics were observed with an evident pulsating regime, which evolved into FREI through mixed-mode oscillations. In narrow channels, the amplitude of transitional pulsating flame becomes comparable to the fully-developed extinction/ignition cycles, and the bifurcation diagram has a continuous character with no significant jumps or discontinuities. Such qualitative variation of transition character provides a possible explanation for contradictory interpretations presented in the literature.Novelty and Significance StatementCurrently, there is no consensus about the character of transition between the stable and extinction/ignition regimes in microchannels. Some studies have reported a continuous instability transition with an evident pulsating regime, interpreting it as a supercritical bifurcation, while others have observed subcritical bifurcation with a rapid transition. The novelty of this research lies in the rigorous determination of the bifurcation type and further instability development in microchannels of different widths based on an accurate simulation of flame dynamics near the critical point with very small steps of the flow velocity variation, along with the bifurcation theory. The study contributes to understanding the instability transition nature and may explain the different interpretations of the transitional phenomenon found in the literature. The results offer valuable insights for designing micro-scale and porous media combustion devices.

Enhanced nonlinear optical properties and optical limiting performance of graphene modified polyaniline composite under pulsed laser regime

Polyaniline (PANI) and its graphene-doped composite (PANI-GNP) were subjected to open-aperture (OA) z-scan studies to investigate their nonlinear optical (NLO) properties and optical limiting (OL) behavior. The z-scan experiments were carried out using a nanosecond pulsed Nd:YAG laser at a wavelength of 532 nm. The z-scan studies revealed that PANI and PANI-GNP composites exhibited a reverse saturable absorption (RSA), and graphene doping modified the NLO behaviour and improved the OL property. The nonlinear absorption coefficient (β), OL threshold, and imaginary part of nonlinear susceptibility ( χi(3) ) of PANI and PANI-GNP were calculated for three different laser input pulse energies and found to be in the order of 10−9 cm W−1, 106 W cm−2, and 10−10 esu, respectively. These results qualify the PANI-GNP composite as having potential applications in OL and optoelectronic devices. The successful incorporation of graphene into PANI in PANI-GNP composite was confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). Linear optical properties were analysed using UV–visible absorption spectroscopy. To the best of our knowledge, the present work is the first attempt to explore the intensity-dependent NLO properties, OL behaviour and the calculation of their related NLO parameters of PANI-GNP composites.