Pulsed Regime Research Articles

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.

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.

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, in which the cooling of the last laser pulse absorbed by the material is progressively cooled. Then, considering a multiple pulse regime and the dependence of the deposited pulse numbers on the thermal treatment, the modeled NGs erasure threshold follows the experimental trend. Finally, considering 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.

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.

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.

Highly efficient solid-state vortex laser in a robust and simple configuration

Vortex beams, known as a typical form of structured light, possess numerous applications in various fields. Their widespread application prospects have then sparked an in-depth analysis of the generation and manipulation of vortex modes in an active cavity, as well as the development of high-performance vortex lasers. In this paper, we report on a new class of highly efficient and high-power Nd:YAG vortex lasers in a robust and compact configuration, which allows direct generation of vortex beams with an easily controllable topological charge both in the continuous-wave and pulsed operation regimes. The on-demand generation of intracavity vortex modes is realized based on a Q-plate by controlling the geometric phase inside the laser resonator. The maximum output power in the continuous-wave regime is 4.11 W with a slope efficiency of 37.9%. Besides, the vortex pulses are also achieved by including a Cr:YAG crystal in the cavity as a saturable absorber. The shortest pulse width is 142.8 ns at a pulse repetition rate of 232.6 kHz, with a maximum average output power of 1.05 W. Vortex modes with other topological charges can be obtained by simply changing the corresponding Q-plate without sacrificing the lasing efficiency. The experimental results can shed some light on the design and building of highly efficient and high-power vortex lasers together with a well-defined controllable topological charge, aiming at some specific applications.

Microwave assisted efficient non-degenerate four-wave mixing in pulsed regime

We theoretically investigate a N-type 87Rb atomic system for efficient generation and control of a non-degenerate four wave mixing (FWM) signal in pulsed regime. The susceptibility of the atomic medium is customized as a gain profile by a weak probe pulse and two strong continuous wave control fields which allow us to generate the pulsed FWM signal. We study the propagation dynamics of the generated FWM signal inside the nonlinear medium. The FWM signal obtains the exact shape of the probe pulse and travels without changing the shape whereas, the probe pulse is absorbed inside the nonlinear medium. The conversion efficiency of this scheme without a MW field is 5.36%. However, a MW field that couples two metastable ground states enhances the conversion efficiency to 20.6%. The generation and control of such FWM signal in pulsed regime has important applications in signal processing, optical communication and information science.

Buildup of different emission regimes in a nonlinear polarization rotation modelocked all-fiber laser

We investigated experimentally and theoretically the buildup of light pulses in an erbium-doped sub-MHz all-fiber laser modelocked by nonlinear polarization rotation. We were able to study the buildup of two different emission regimes: standard solitons and noise-like pulses. In each case, we were able to determine the round-trips required to achieve a stable emission state. Temporal traces and optical spectra of single pulses were measured along the start-up transient of the laser. The experimental results were also confirmed by numerical simulations. Under the specific conditions of this laser, the soliton regime takes about 400 round-trips to reach single-pulse emission. In the noise-like pulse regime, it takes only 20 round-trips for the characteristics of noise-like pulses to show up; although a more steady-state emission is reached also at about 400 round-trips.

Brillouin light storage for 100 pulse widths

Signal processing based on stimulated Brillouin scattering (SBS) is limited by the narrow linewidth of the optoacoustic response, which confines many Brillouin applications to continuous wave signals or optical pulses longer than several nanoseconds. In this work, we experimentally demonstrate Brillouin interactions at the 150 ps time scale and a delay for a record 15 ns which corresponds to a delay of 100 pulse widths. This breakthrough experimental result was enabled by the high local gain of the chalcogenide waveguides as the optoacoustic interaction length reduces with pulse width. We successfully transfer 150 ps-long pulses to traveling acoustic waves within a Brillouin-based memory setup. The information encoded in the optical pulses is stored for 15 ns in the acoustic field. We show the retrieval of eight amplitude levels, multiple consecutive pulses, and low distortion in pulse shape. The extension of Brillouin-based storage to the ultra-short pulse regime is an important step for the realization of practical Brillouin-based delay lines and other optical processing applications.

Improved power and temperature performance of half-disk diode microlasers.

The power and temperature characteristics of Ø200 µm half-disk microlasers with a half-ring metal contact and high-density InGaAs/GaAs quantum dots are studied. In a continuous wave (CW) mode, the maximal optical power at 20°C was 134 mW, and the maximal CW lasing temperature reached 113°C. In a pulsed regime the maximal optical power of 1.6 W, limited by catastrophic degradation, was achieved. By comparing the CW and pulsed current-voltage characteristics, the dependence of a microlaser temperature on CW pumping current was determined. At CW currents corresponding to the maximal wall-plug efficiency, the maximal optical power, and complete lasing quenching, the laser temperatures were 60, 99, and 149°C, respectively.

Nimble vs. torpid responders to hydration pulse duration among soil microbes

Environmental parameters vary in time, and variability is inherent in soils, where microbial activity follows precipitation pulses. The expanded pulse-reserve paradigm (EPRP) contends that arid soil microorganisms have adaptively diversified in response to pulse regimes differing in frequency and duration. To test this, we incubate Chihuahuan Desert soil microbiomes under separate treatments in which 60 h of hydration was reached with pulses of different pulse duration (PD), punctuated by intervening periods of desiccation. Using 16S rRNA gene amplicon data, we measure treatment effects on microbiome net growth, growth efficiency, diversity, and species composition, tracking the fate of 370 phylotypes (23% of those detected). Consistent with predictions, microbial diversity is a direct, saturating function of PD. Increasingly larger shifts in community composition are detected with decreasing PD, as specialist phylotypes become more prominent. One in five phylotypes whose fate was tracked responds consistently to PD, some preferring short pulses (nimble responders; NIRs) and some longer pulses (torpid responders; TORs). For pulses shorter than a day, microbiome growth efficiency is an inverse function of PD, as predicted. We conclude that PD in pulsed soil environments constitutes a major driver of microbial community assembly and function, largely consistent with the EPRP predictions.