Per-pulse Energy Research Articles

Full-view LED-based optoacoustic tomography

Optoacoustic tomography is commonly performed with bulky and expensive short-pulsed solid-state lasers providing high per-pulse energies in the millijoule range. Light emitting diodes (LEDs) represent a cost-effective and portable alternative for optoacoustic signal excitation that can additionally provide excellent pulse-to-pulse stability. Herein, we introduce a full-view LED-based optoacoustic tomography (FLOAT) system for deep tissue in vivo imaging. It is based on a custom-made electronic unit driving a stacked array of LEDs, which attains 100 ns pulse width and highly stable (0.62 % standard deviation) total per-pulse energy of 0.48 mJ. The illumination source is integrated into a circular array of cylindrically-focused ultrasound detection elements to result in a full-view tomographic configuration, which plays a critical role in circumventing limited-view effects, enhancing the effective field-of-view and image quality for cross-sectional (2D) imaging. We characterized the FLOAT performance in terms of pulse width, power stability, excitation light distribution, signal-to-noise and penetration depth. FLOAT of the human finger revealed a comparable imaging performance to that achieved with the standard pulsed Nd:YAG laser. It is anticipated that this compact, affordable and versatile illumination technology will facilitate optoacoustic imaging developments in resource-limited settings for biological and clinical applications.

Nanosecond pulsed discharge dynamics during passage of a transient laminar flame

This work presents an experimental study of a nanosecond repetitively pulsed dielectric barrier discharge interacting with a transient laminar flame propagating in a channel of height near the quenching distance of the flame. The discharge and the flame are of comparable size, and the discharge is favoured at a location where it is coupled with the reaction zone and burnt gas. The primary goal is to determine how the discharge evolves on the time scale of the flame passage, with the evolution driven by the changing gas state produced by the moving flame front. This work complements the large body of work investigating the effect of plasma to modify flame dynamics, by considering the other side of the interaction (how the discharge is modified by the flame). The hot gas produced by the combustion had a strong effect on the discharge, with the discharge preferentially forming in the region of hot combustion products. The per-pulse energy deposited by the discharge was measured and found to increase with the size of the discharge region and applied voltage. The pulse repetition frequency did not have a direct impact on the per-pulse energy, but did have an effect on the morphology and size of the discharge region. Two distinct discharge regimes were observed: uniform and filamentary (microdischarges). Higher pulse repetition frequencies and faster-cooling combustion products were more likely to transition to the filamentary regime, while lower frequencies and slower-cooling combustion products maintained a uniform regime for the entirety of the time the discharge was active. This regime transition was influenced by the ratio of the time scale of fluid motion to the pulse repetition rate (with no noticeable impact caused by the reduced electric field), with the filamentary regime preferentially observed in situations where this ratio was small. This work demonstrates the importance of considering how the discharge properties will change due to combustion processes in applications utilizing plasma assistance for transient combustion systems.

Suppression of fibrillatory dynamics consisting of stable rotors by periodic pacing

Recent experimental studies have shown that a sequence of low-energy electrical far-field pulses is able to terminate fibrillation with substantially lower per-pulse energy than a single high-energy electric shock (see S Luther et al Nature 475 235–39). During this low-energy antifibrillation pacing (LEAP) procedure only tissue near sufficiently large conduction heterogeneities, such as large coronary arteries, is activated. In order to understand the mechanism behind LEAP, we have carried out a statistical study of resetting a medium filled by one or more stable spirals (‘rotors’) in a two-dimensional electrophysiological model of cardiac tissue perforated by blood vessels to the resting state (‘defibrillation’). We found the highest success probabilities for this defibrillation for underdrive pacing with periods 10–20 percent larger than the dominant period of the stable rotors in the unperturbed dynamics. If a sufficiently large number pulses is applied and an optimal pacing period chosen, the energy per pulse required for successful defibrillation is about 75–80 percent lower than the energy needed for single-shock defibrillation. Optimal conditions to control and suppress fibrillation based on stable rotors, hence, are similar to the ones in found for the case of an electrophysiological model displaying spatiotemporal chaos (‘electrical turbulence’) in an earlier study (see P Buran et al 2017 Chaos 27 113110). The optimal pacing period is found to increase with increasing strength of the electrical field strength used in the model. The success probability also increases strongly until the fourth or fifth pulse administered, which is strongly correlated to an observed increase of the fraction of re-excitable tissue with each subsequent pulse. Monitoring the fraction of excitable tissue in the model as key quantity of the excitable medium, moreover, enabled us to successfully predict the optimal pacing period for defibrillation.

The time-resolved atomic, molecular and optical science instrument at the Linac Coherent Light Source.

The newly constructed time-resolved atomic, molecular and optical science instrument (TMO) is configured to take full advantage of both linear accelerators at SLAC National Accelerator Laboratory, the copper accelerator operating at a repetition rate of 120 Hz providing high per-pulse energy as well as the superconducting accelerator operating at a repetition rate of about 1 MHz providing high average intensity. Both accelerators power a soft X-ray free-electron laser with the new variable-gap undulator section. With this flexible light source, TMO supports many experimental techniques not previously available at LCLS and will have two X-ray beam focus spots in line. Thereby, TMO supports atomic, molecular and optical, strong-field and nonlinear science and will also host a designated new dynamic reaction microscope with a sub-micrometer X-ray focus spot. The flexible instrument design is optimized for studying ultrafast electronic and molecular phenomena and can take full advantage of the sub-femtosecond soft X-ray pulse generation program.

Stable and Efficient Pulsed Mid-Infrared Laser Generation from an Er3+-Doped ZBLAN Fiber Laser

We demonstrate a stable and efficient passively Q-switched 2.8 μm Er3+-doped ZBLAN fiber laser with a broadband semiconductor saturable absorber mirror. Enabled by the broadband optical modulator, the stable Q-switched fiber laser can deliver a maximum average power over 700 mW with corresponding per-pulse energy of 8.19 μJ and a pulse width of 1.3 μs at a repetition rate of 88.6 kHz under an incident pump power of 3.8 W. In addition, the slope efficiency can reach 22.5%. To the best of our knowledge, this is the highest reported slope efficiency for the passively Q-switched Er3+-doped ZBLAN fiber laser.

Graphene/MoS2 heterostructure: a robust mid-infrared optical modulator for Er3+-doped ZBLAN fiber laser

We have prepared the graphene/MoS 2 heterostructure by a hydrothermal method, and presented its nonlinear absorption parameters and application as a nonlinear optical modulator in the mid-infrared region. Using the nonlinear optical modulator, stable passively Q-switched operation of an Er 3+-doped ZrF 4-BaF 2-LaF 3-AlF 3-NaF (ZBLAN) fiber laser at ∼2.8 μ m can be obtained. The Q-switched Er 3+-doped ZBLAN fiber laser can yield per-pulse energy up to 2.2 μJ with the corresponding pulse width and pulse repetition rate of 1.9 μs and 45 kHz, respectively. Our results indicate that the graphene/MoS 2 heterostructure can be a robust optical modulator for pulsed lasers in the mid-infrared spectral range.

Stable Q-switched operation of a resonantly diode pumped Er:YAG laser at 1617 and 1645 nm by Cr2+:ZnSe crystal

Using Cr[Formula: see text]:ZnSe as the saturable absorber, stable passively [Formula: see text]-switched operation of the Er:YAG laser at 1617 and 1645[Formula: see text]nm with per-pulse energy higher than 200[Formula: see text][Formula: see text]J can be achieved by controlling the output coupling ratios. The result indicates that the Cr[Formula: see text]:ZnSe crystal could be a robust nonlinear optical modulator for [Formula: see text]m pulsed laser.

Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging velocimetry.

Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging (STARFLEET), a nonseeded ultrafast-laser-based velocimetry technique, is demonstrated in reactive and nonreactive flows. STARFLEET is pumped via a two-photon resonance in N2 using 202.25 nm 100 fs light. STARFLEET greatly reduces the per-pulse energy required (30 μJ/pulse) to generate the signature FLEET emission compared to the conventional FLEET technique (1.1 mJ/pulse). This reduction in laser energy results in less energy deposited in the flow, which allows for reduced flow perturbations (reactive and nonreactive), increased thermometric accuracy, and less severe damage to materials. Velocity measurements conducted in a free jet of N2 and in a premixed flame show good agreement with theoretical velocities, and further demonstrate the significantly less intrusive nature of STARFLEET.

High-repetition-rate laser ignition of fuel-air mixtures.

A laser-ignition (LI) method is presented that utilizes a high-repetition-rate (HRR) nanosecond laser to reduce minimal ignition energies of individual pulses by ∼10 times while maintaining comparable total energies. The most common LI employs a single nanosecond-laser pulse with energies on the order of tens of millijoules to ignite combustible gaseous mixtures. Because of the requirements of high energy per pulse, fiber coupling of traditional LI systems is difficult to implement in real-world systems with limited optical access. The HRR LI method demonstrated here has an order of magnitude lower per-pulse energy requirement than the traditional single-pulse LI technique, potentially allowing delivery through standard commercial optical fibers. Additionally, the HRR LI approach significantly increases the ignition probability of lean combustible mixtures in high-speed flows while maintaining low individual pulse energies.

High-Pulse-Energy All-Normal-Dispersion Yb-Doped Fiber Laser Based on Nonlinear Polarization Evolution**Supported by the National Key Scientific Instruments Development Program of China under Grant No 2012YQ120047.

We report an all-normal-dispersion ytterbium fiber laser mode locked by nonlinear polarization evolution. With a 347-m-long all-fiber ring cavity, a pulse energy of 263 nJ at a repetition rate of 613kHz is achieved, which is the highest per-pulse energy directly obtained from an all-fiber mode-locked laser doped by ytterbium ions. The compact and operation-robust laser yields a well-shaped spectrum centered at 1032 nm with a bandwidth (FWHM) of 4 nm, and the slope efficiency is as high as 27.5%. The proposed low-repetition-rate high-pulse-energy mode-locked fiber laser will be a promising seed for all-fiber chirped pulsed amplification systems.

Generation and evolution of mode-locked noise-like square-wave pulses in a large-anomalous-dispersion Er-doped ring fiber laser.

In a passively mode-locked Erbium-doped fiber laser with large anomalous-dispersion, we experimentally demonstrate the formation of noise-like square-wave pulse, which shows quite different features from conventional dissipative soliton resonance (DSR). The corresponding temporal and spectral characteristics of a variety of operation states, including Q-switched mode-locking, continuous-wave mode-locking and Raman-induced noise-like pulse near the lasing threshold, are also investigated. Stable noise-like square-wave mode-locked pulses can be obtained at a fundamental repetition frequency of 195 kHz, with pulse packet duration tunable from 15 ns to 306 ns and per-pulse energy up to 200 nJ. By reducing the linear cavity loss, stable higher-order harmonic mode-locking had also been observed, with pulse duration ranging from 37 ns at the 21st order harmonic wave to 320 ns at the fundamental order. After propagating along a piece of long telecom fiber, the generated square-wave pulses do not show any obvious change, indicating that the generated noise-like square-wave pulse can be considered as high-energy pulse packet for some promising applications. These experimental results should shed some light on the further understanding of the mechanism and characteristics of noise-like square-wave pulses.

Stable high-energy Q-switched resonantly diode-pumped Er:YAG laser at 1645 nm

We demonstrate the generation of a stable high-energy Q-switched resonantly diode-pumped Er:YAG laser at 1645nm in passive and active operation modes. For the passively Q-switched Er:YAG laser, the bi-layer graphene saturable absorber (SA), which was transferred onto the highly reflective cavity mirror by an improved method, was applied to deliver a stable pulse train with per-pulse energy of about 0.1mJ. While in the active operation mode, the voltage-ON-type rubidium titanyl phosphate (RTP) Pockels cell as an electro-optic (EO) Q-switcher allows for the generation of a stable Q-switched pulse with per-pulse energy of up to 7.5mJ. Our work may provide a basis for the development of a high-energy Er:YAG laser at 1645nm to fulfill specific applications.

Large Energy, Wavelength Widely Tunable, Topological Insulator Q-Switched Erbium-Doped Fiber Laser

We report on the generation of large energy, widely wavelength tunable pulses in an erbium-doped fiber laser passively Q-switched by a topological insulator saturable absorber (TI-SA). The TI-SA is prepared through an optical deposition method. Its saturating intensity and modulation depth are measured to be about 57 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 22%, respectively. We show that the high modulation depth of TI-SA allows the generation of stable Q-switched pulses with per-pulse energy up to 1.5 μJ and its broadband saturable absorption favors the tunable Q-switching operation from 1510.9 nm to 1589.1 nm. Our study suggests that TI: Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> could be a promising saturable absorber for both the high energy and broadband optical applications.

Simultaneous high-speed planar imaging of mixture fraction and velocity using a burst-mode laser

Simultaneous high-speed quantitative imaging of mixture fraction and velocity is demonstrated using the fourth- and second-harmonic outputs, respectively, of a burst-mode Nd:YAG laser. A tenfold increase in the record length and 16-fold increase in per-pulse energy are achieved compared with previous measurements of mixture fraction using burst-mode and continuously pulsed diode-pumped solid-state lasers, respectively. The high output energy is used for quantitative, high-speed mixture-fraction imaging with acetone planar laser-induced fluorescence, which also enables simultaneous particle-based velocimetry without interference from particle scattering. A semiquantitative model is used to determine the limitations on fourth-harmonic output energy due to the effects of transient absorption and thermal phase mismatch over a range of repetition rates. Data are presented for mixing within a turbulent jet (Reynolds number of 15,000) and are validated by comparisons with known turbulent mixing laws and previously published data.

Topological Insulator: &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$\hbox{Bi}_{2}\hbox{Te}_{3}$ &lt;/tex&gt;&lt;/formula&gt; Saturable Absorber for the Passive Q-Switching Operation of an in-Band Pumped 1645-nm Er:YAG Ceramic Laser

An in-band pumped 1.645-μm Er:YAG ceramic laser passively Q-switched by a topological insulator: Bi2Te3 saturable absorber is reported. The average output power could reach up to 210 mW, corresponding to a pulsewidth, a pulse repetition rate, and a per-pulse energy of 6.3 μs, 40.7 kHz, and 5.3 μJ, respectively. This result indicates that the topological insulator: Bi2Te3 could be a promising saturable absorber such as graphene, with potential applications for solid-state lasers.

Waveguide-like structures written in transparent polycrystalline ceramics with an ultra-low fluence femtosecond laser

We present a laser processing study of optically transparent ytrria stabilized zirconia (YSZ) ceramics (ZrO2-8 mol. % Y2O3) using unamplified femtosecond (fs) laser pulses of a few nJ and high repetition rate (70 MHz). The ceramics were fabricated using current activated pressure assisted densification (CAPAD) and have fine grain size and minimal porosity, producing a transparent material. Irradiation using fs laser pulses caused permanent changes in the optical properties of the irradiated zone. These laser written structures were found to confine He-Ne laser light (632 nm) in effect functioning as waveguide like structures and were written into the YSZ ceramics using a remarkably low per-pulse energy (5nJ). The number of passes with the laser i.e total incident pulses per unit area was found to significantly affect the waveguide writing. We believe that waveguides are regions were the concentration of oxygen vacancies and/or their associated free electrons have been altered by laser irradiation. We are not aware of previous reports of low fluence fs laser pulses being used to influence vacancy related defects to produce waveguides in ceramics. This new mechanism opens the door for writing strictures in optical ceramics with lower power than previously thought feasible.

Switchable Dual-Wavelength Synchronously Q-Switched Erbium-Doped Fiber Laser Based on Graphene Saturable Absorber

We demonstrate a switchable dual-wavelength synchronously pulsed fiber laser Q-switched by graphene saturable absorber. Wavelength-resolved studies on the output Q-switched pulses show that despite of large wavelength spacing up to 26 nm, the two Q-switched pulses at each individual wavelength can be temporally synchronized with per-pulse energy up to ~ 70 nJ. Further experiments show that by adjusting the intracavity birefringence, dual-wavelength emission can be switched to another dual-wavelength operation regime with wavelength separation of 5.7 nm. Our experimental results are also qualitatively supported by the cavity linear transmission characteristics of the ring cavity.

Laser nitriding of niobium for application to superconducting radio-frequency accelerator cavities

Particle accelerators are a key tool for scientific research ranging from fundamental studies of matter to analytical studies at light sources. Cost-for-performance is critical, both in terms of initial capital outlay and ongoing operating expense, especially for electricity. The major factor is the niobium superconducting radio frequency (SRF) accelerator cavities at the heart of many of these machines. Presently, niobium SRF cavities operate near 1.9 K, well below the 4.2 K atmospheric boiling point of liquid helium to obtain sufficient performance. The consequent electric power costs are the most significant limit to operate the SRF cavities at 1.9 K. Transforming the cavity interior surface from niobium to δ niobium nitride (δNbN) with a critical temperature (Tc) ≅ 17 K instead of 9.2 K, appears to be a promising approach to raising the operating temperature. The traditional furnace method has nitrided niobium, but apparently have not been able to obtain δNbN.1 Moreover, furnace nitriding requires exposing the complete SRF cavity to an aggressive time-temperature history, risking mechanical distortion. As an alternative, laser gas nitriding has been applied successfully to a number of metals.2 A very recent review is available.3 The beam dimensions and thermal diffusion length permit modeling in one dimension to predict the time course of the surface temperature for a range of per-pulse energy densities. As with the earlier work,2 we chose conditions just sufficient for boiling of the niobium surface as a reference point. The treated materials were examined by scanning electron microscopy (SEM), electron probe microanalysis and x ray diffraction (XRD). The SEM images show a sharp transition with fluence from a smooth, undulating topography to significant roughening, interpreted here as the onset of ablation. Electron probe microanalysis measurements found a constant value of the nitrogen/niobium atom ratio to depths greater than the SRF active layer thickness. Certain irradiation conditions resulted in atomic ratio values consistent with formation of δNbN, and XRD data indicated only δNbN on top of the niobium metal.

Passively Mode-Locked Fiber Laser with a Sub-Megahertz Repetition Rate

We demonstrate an ultra-long cavity by which an all-fiber erbium-doped fiber laser is passively mode-locked by nonlinear polarization rotation. The length of the resonant cavity amounts to 466m, which can be achieved by incorporating a 420m highly nonlinear fiber. The laser generates stable mode-locked pulses with a 444 kHz fundamental repetition rate. A near transform-limited subpicosecond pulse is obtained without any dispersion compensation. The maximum average power of the output pulses is 5.16 mW, which corresponds to a per-pulse energy of 11.62 nJ.

Tunable and switchable dual-wavelength passively mode-locked fiber ring laser with high-energy pulses at a sub-100 kHz repetition rate

We demonstrate a tunable and switchable dual-wavelength passively mode-locked fiber ring laser in the anomalous dispersion regime that generates high-energy pulses at a sub-100 kHz repetition rate. The nonlinear polarization rotation (NPR) technique is employed in this configuration to allow for passively mode-locked operation. By exploiting the intensity-dependent loss caused by NPR, the mode competition could be efficiently alleviated. Moreover, taking advantage of the intrinsic spectral filter induced by the intracavity birefringence, the dual-wavelength separations as well as the wavelength lasing locations could be flexibly tuned by properly rotating the polarization controller. With a repetition rate of 85.8 kHz, the laser delivers the single pulse energy of 198 nJ at a pump power of 250 mW. The combination of high per-pulse energy and flexible tunability in this fiber laser is useful for practical applications.