MW Pulse Research Articles

CPA-ready femtosecond pulses at 1 MHz from a custom recycled output Mamyshev oscillator

A cost-effective fiber laser architecture is introduced in which the output seed pulse is stretched and then returned in the oscillator for an additional single-pass amplification without spectral broadening. It is implemented in an all-PM-fiber configuration based on a Mamyshev oscillator with a low repetition rate of 1 MHz. It features a linear oscillator bounded by two offset chirped fiber Bragg gratings accompanied by a third one acting as a pulse recycling filter. The latter tailors the pulse profile in amplitude and phase to seed femtosecond chirped-pulse amplification systems without additional pre-amplification nor pulse stretching. A single-pump prototype generating 200-nJ, 100-ps pulses compressible to 290 fs at 1030 nm and at 960 kHz is demonstrated. Furthermore, simulations show how this new oscillator architecture can provide tailored seed pulses with high enough spectral energy density and low enough nonlinear phase to generate sub-200 fs, 40 µJ, > 180 MW pulses from an all-fiber setup involving a single tapered-fiber power amplifier, without pulse picking.

Comparative Study of Q-Switched Erbium Doped Fibre Laser with SWCNT-PVA Saturable Absorber of Different Ratios

This paper intends to compare the performance parameters of passive Q-switched erbium-doped fibre laser (EDFL) ring cavity configuration by employing SWCNT-PVA as a saturable absorber (SA). The single-wall carbon nanotube-polyvinyl alcohol (SWCNT-PVA) SA thin film fabricated by drop-casting technique is physically characterised to understand its surface morphology and thickness. The laser diode (LD) characterisation is attained to understand the attributes of LD before inserting the SWCNT-PVA into the ring cavity. The SWCNT-PVA SA of ratios 1:1, 3:2 and 2:3 is inserted into the cavity for analysis and comparison with each other. The pulses obtained from the Q-switched laser show that SWCNT-PVA SA of ratio 2:3 is the best SA as it possesses excellent power related parameters of the highest output power of 3.45 mW and highest pulse energy of 36.12 nJ with its moderate repetition rate of 96.25 kHz and pulse width 4.76 ms. The envisaged laser is presumed to have representational implications throughout fibre optic sensing, biosensors, range finding, and other fields for better possible prospects.

Spin coherence and magnetization dynamics of TMA2[KCo1-xFex(CN)6] toward coordination-framework spin qubits.

Metal compounds with S = 1/2 coordination-frameworks have been emerging as new powerful qubit candidates. In this study, we have reported the CN-based coordination framework TMA2[KCo1-xFex(CN)6] to be a qubit. We explored the magnetization dynamics and spin coherence of the magnetic dilution of the S = 1/2 Fe(III) complex TMA2[KFe(CN)6] (TMA = tetramethylammonium) in its Co(III)-based diamagnetic analogue TMA2[KCo(CN)6]. Alternating-current (AC) susceptibility data illustrate a slow magnetic relaxation upon applying a field of 0.1 T, which follows the phonon-bottleneck relaxation mechanism along with the Raman process. A magnetic relaxation time (τ) of 0.3 s (2% Fe) was realized at 1.8 K. Moreover, pulsed EPR data reveal a coherence duration of 1 μs (0.1% Fe) at 4 K with successful observation of Rabi oscillation at 4 K and 13 K (2% Fe) using MW pulses with variable irradiation-field strengths. The overall results indicate that TMA2[KCo1-xFex(CN)6] represents a promising qubit candidate, as it is capable of being placed in any superposition of the two distinct Ms states (Ms = +1/2 and Ms = -1/2).

Pulsed dynamic nuclear polarization: a comprehensive Floquet description.

Dynamic nuclear polarization (DNP) experiments using microwave (mw) pulse sequences are one approach to transfer the larger polarization on the electron spin to nuclear spins of interest. How the result of such experiments depends on the external magnetic field and the excitation power is part of an ongoing debate and of paramount importance for applications that require high chemical-shift resolution. To date numerical simulations using operator-based Floquet theory have been used to predict and explain experimental data. However, such numerical simulations provide only limited insight into parameters relevant for efficient polarization transfer, such as transition amplitudes or resonance offsets. Here we present an alternative method to describe pulsed DNP experiments by using matrix-based Floquet theory. This approach leads to analytical expressions for the transition amplitudes and resonance offsets. We validate the method by comparing computations by these analytical expressions to their numerical counterparts and to experimental results for the XiX, TOP and TPPM DNP sequences. Our results explain the experimental data and are in very good agreement with the numerical simulations. The analytical expressions allow for the discussion of the scaling behaviour of pulsed DNP experiments with respect to the external magnetic field. We find that the transition amplitudes scale inversely with the external magnetic field.

Toward coherent anti‐Stokes Raman scattering‐Raman optical activity for reaction monitoring in organocatalysis: Chiral vibrational signatures of the two atropisomers (R)‐ and (S)‐MOM‐BINOL in solution

AbstractCoherent anti‐Stokes Raman scattering‐Raman optical activity (CARS‐ROA) is a coherent version of spontaneous ROA spectroscopy, the latter being a well‐established chirally sensitive vibrational spectroscopic technique. A central major advantage of heterodyne‐detected CARS‐ROA over ROA is the drastic reduction of acquisition times from several hours to approximately 1 min due to the high ratio of the chiral Raman signal relative to the achiral CARS background. Here, we present the first demonstration of heterodyne‐detected CARS‐ROA of chiral molecules dissolved in an achiral solvent by using an experimental setup built from a conventional broadband CARS spectrometer. Specifically, the two atropisomers (R/S) of 2,2′‐Bis(methoxymethoxy)‐1,1′‐binaphthyl, a methoxymethyl (MOM) bis‐protected derivative of 1,1′‐Bi‐2‐naphthol (BINOL), dissolved in the achiral organic solvent dichloromethane (DCM) were characterized by employing ~1 ps, 1 mW of pump pulses and ~60 fs, 10 mW of Stokes pulses within approximately 4‐min acquisition time. MOM‐BINOL is an important precursor for the synthesis of a great variety of many chiral auxiliaries including binaphthyl phosphoric acids, which are an important class of chiral organocatalysts used in asymmetric reactions. Overall, this work is a first step toward kinetic reaction monitoring in organocatalysis by coherent ROA providing the necessary short acquisition times.

Conducting a three-pulse DEER experiment without dead time: A review

Double electron-electron resonance spectroscopy (DEER, also known as PELDOR) is used to study spin-spin dipolar interactions between spin labels, at the nanoscale range of distances. The DEER effect is obtained as a signal generated by echo-forming microwave (mw) pulses with an additional mw pump pulse applied at a different frequency. It is important to carry out measurements without artefacts induced by overlap of the pulses in the time scale. Such an experiment without the dead-time effect is achieved using the 4-pulse (4p) DEER method. The analysis of the literature performed here shows however that the 3-pulse (3p) DEER can also be free of the dead time problem, for which there are two possibilities. The first occurs using a specially designed bimodal resonator, for which the two frequencies are completely decoupled. The second possibility, which can be implemented for any commercial spectrometer, involves the signal correction based on an additional “blank” measurement with the pump pulse applied outside the EPR resonance. A detailed comparison of the 3p and 4p DEER data obtained previously by Milov et al. [Appl. Magn. Reson. 41 (2011) 59–67] shows that 3p and 4p approaches give similar results. The advantages of the 3p DEER techniques are discussed.

MoxTa(1-x)Se2 as saturable absorber for ultrafast photonics

Two-dimension ternary transition metal dichalcogenides have drawn great interests from both the fields of nanotechnology and materials science, due to their extraordinary physical performance, and great potential applications in nanodevice. Here, the applications of MoxTa(1-x)Se2 nanosheets in ultrafast photonics are discussed. The MoxTa(1-x)Se2 nanosheets are prepared by liquid phase exfoliation method. A MoxTa(1-x)Se2-based saturable absorber is fabricated by depositing these nanosheets onto the side surface of d-shaped fiber, which is inserted into a Er-doped fiber laser cavity. By adjusting polarization state and pump power, both harmonic mode-locking and large energy mode-locking operations are realized, respectively. In the former state, a 56th harmonic mode-locking is obtained, which corresponds to a repetition rate of 312.5 MHz. For the later state, the maximum average output power of 12.24 mW and single pulse energy of 2.19 nJ are obtained when the pump power is 500mW. The results demonstrate that MoxTa(1-x)Se2 can be widely used in ultrafast photonics and nonlinear optics.

Passively Q-switched erbium doped fiber laser based on graphene and carbon nanotube saturable absorbers

A passively Q-switched erbium-doped fiber laser (EDFL) was experimented on by employing graphene, single walled carbon nanotubes (SWCNT) and multi-walled carbon nanotube (MWCNT) saturable absorbers (SA). The SA film was obtained by embedding the graphene, SWCNT and MWCNT into polyvinyl alcohol (PVA). The graphene SA was prepared by dipping a PVA thin film into the graphene solution while carbon nanotubes SAs were prepared using the casting method and placed in the ring cavity to produce a stable pulse laser. Graphene, SWCNT and MWCNT SAs were operating at wavelengths of 1558.92 nm, 1557.98 nm and 1558.51 nm, respectively, whereas the continuous wave was 1560.72 nm at the input pump power of 56 mW. The pulse energy, output power, repetition rate and pulse width were compared in graphene, SWCNT and MWCNT SAs. The shortest pulse width retrieved in graphene, SWCNT and MWCNT were 3.90 µs, 3.62 µs 4.43 µs and produced at the repetition rate of 115.00 kHz, 130.70 kHz, and 89.13 kHz, respectively. In comparison to graphene and SWCNT SAs, MWCNT SAs exhibit the best performance in terms of output power of 2.19 mW and high pulse energy of 24.57 nJ in passively Q-switched EDFL.

Designing broadband pulsed dynamic nuclear polarization sequences in static solids.

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons with large thermal polarization to NMR-active nuclei via microwave (mw) irradiation. The ability to generate arbitrarily shaped mw pulses using arbitrary waveform generators allows for remarkable improvement of the robustness and versatility of DNP. We present here novel design principles based on single-spin vector effective Hamiltonian theory to develop new broadband DNP pulse sequences, namely, an adiabatic version of XiX (X–inverse X)–DNP and a broadband excitation by amplitude modulation (BEAM)–DNP experiment. We demonstrate that the adiabatic BEAM-DNP pulse sequence may achieve a 1H enhancement factor of ∼360, which is better than ramped-amplitude NOVEL (nuclear spin orientation via electron spin locking) at ∼0.35 T and 80 K in static solids doped with trityl radicals. In addition, the bandwidth of the BEAM-DNP experiments (~50 MHz) is about three times the 1H Larmor frequency. The generality of our theoretical approach will be helpful in the development of new pulsed DNP sequences.

Conserved spacing multi-wavelength mode-locked erbium-doped fiber laser based on barium titanate saturable absorber

A compact multi-wavelength harmonic passive mode-locked (MWHML) erbium-doped fiber laser (EDFL) has been demonstrated by integrating a Barium Titanate (BaTiO3) in the ring cavity configuration. The BaTiO3 with 9.5% modulation depth serves as a saturable absorber (SA) and is optically deposited by the Top-Down method to form an end face fiber integrated saturable absorber. Both the saturable absorption property and the large third-order nonlinearity of BaTiO3 assisted in generating a second-order harmonic pulse beside the fundamental frequency. Moreover, in real-time, eight wavelengths around 1560 nm with 0.7 nm wavelength spacing were produced with a maximum output power of 2.7 mW pulse duration of 5.2 ps. These results further suggest that BaTiO3 could be a good candidate for the saturable absorber and high nonlinear photonic device for related applications.

Generation of 565 MW of X -band power using a metamaterial power extractor for structure-based wakefield acceleration

We report the generation of up to 565 MW, 2.7 ns (FWHM) pulses at 11.7 GHz from a metamaterial structure in test at the Argonne Wakefield Accelerator. The highest power was generated by a train of eight 65 MeV electron bunches spaced at 1.3 GHz with a total charge of 355 nC. The metamaterial structure consists of 100 copper unit cells each consisting of a wagon-wheel plate and a spacer plate with a total structure length of 0.2 m. The 565 MW pulse generates a wakefield with a peak on-axis gradient of $135\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ that could be used to accelerate a trailing main bunch. An estimated surface electric field of over $1\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ is generated on the metamaterial plates at the peak power level but no evidence of breakdown was observed during testing. Tests with single electron bunches and with trains of bunches of up to 100 nC produced output power levels in excellent agreement with simulations. At higher total bunch charge, offsets of the bunches from the axis resulted in beam interception and reduced output power. Simulations indicate that a perfectly aligned bunch train would generate more than 1 GW of power from the structure.

Dual-wavelength harmonic mode-locked dissipative soliton resonance of Yb fiber laser

Dissipative soliton resonance (DSR) mode-locked fiber lasers have stimulated great interest recent years due to their unique pulse-breaking-free advantage for scaling soliton pulse energy. However, the mechanism accounting for pulse breaking to generate multipulses in DSR fiber lasers has not reached a consensus. Here, we show that harmonic mode-locked DSR 1-μm Yb-doped fiber laser in the dual-wavelength regime can be realized with a semiconductor saturable absorber mirror (SESAM). In the fundamental frequency (3.5 MHz) mode-locked state, the pulse width continuously broadens from 2 to 8.9 ns as the output power increases from 1.26 to 5.1 mW (pulse energy being from 0.36 to 1.46 nJ), while the pulse peak power is clamped at 0.17 W, highlighting the typical characteristics of DSR mode-locking operation. Through simply adjusting the SESAM’s feedback conditions, the DSR fiber laser can transits from the fundamental-frequency mode locking to harmonic mode-locked states. Harmonic states from the 2nd to 9th order have been observed, with pulse repetition rate enhanced from 7 to 31.5 MHz. Under fixed pump power, increasing harmonic orders leads to reduction of the pulse width and pulse energy but does not change the unique quantization feature of the peak power. The status of the SESAM also guarantees the generation of dual wavelengths centered at 1072.5 and 1079.5 nm with spectral widths of 2.7 and 1.9 nm, respectively. This dual-wavelength harmonic mode-locked DSR Yb fiber laser can find applications in various areas.

Abstract 14444: A Novel Role for the Hypothalamic Paraventricular Nucleus in Resuscitation From Opioid-Induced Respiratory Depression by Fentanyl in the Rat

Introduction: Opioid receptor antagonists, like naloxone, reverses opioid-induced respiratory depression (OIRD). We recently found that systemic oxytocin reverses fentanyl OIRD in rats. Naloxone triggers a surge in oxytocin release from the hypothalamic paraventricular nucleus (PVN) in anesthetized rats. Hypothesis: Given that naloxone induces PVN oxytocin release, we hypothesized that the PVN, an unrecognized brain region for opioid overdose reversal, is capable of respiratory resuscitation following fentanyl OIRD. Methods: Naloxone (5 mg/kg, i.v.) was given to conscious rats with indwelling venous cannulae. Blood samples were extracted at 5 min post-naloxone to measure plasma oxytocin by ELISA. Under anesthesia, phrenic nerve activity was recorded in vagotomized, paralyzed, and artificially ventilated rats to quantify inspiratory drive following fentanyl (60 nmol/kg, i.v.) and assess OIRD reversal potential following unilateral PVN nanoinjection of naloxone (3 nmol/kg, 50 nL) or vehicle (50 nL). In spontaneously breathing mice with channelrhodopsin expressed exclusively in oxytocin neurons, expired CO 2 was used to monitor rhythmic ventilation and to quantify bradypnea during OIRD. Following fentanyl, blue light (473 nm, 12 mW) pulses (10 ms, 20 Hz) or dummy pulses were delivered to the PVN using a 5 s On/1 s Off pattern for 3 min. Results: Systemic naloxone resulted in a surge in plasma oxytocin compared to vehicle in the conscious rat. Naloxone in PVN both reversed and prevented OIRD by systemic fentanyl in the rat. Photostimulation of PVN oxytocin neurons prevented lethal effects of fentanyl OIRD, whereas dummy pulses failed to prevent lethal respiratory arrest in the mouse. Conclusions: Here, we confirm that naloxone stimulates oxytocin neurons of the PVN and results in a surge of plasma oxytocin. PVN opioid receptor antagonism and photoactivation of PVN oxytocin neurons are individually capable of reversing OIRD. Findings also indicate that PVN oxytocin neurons, previously unrecognized in reversal of OIRD, are a novel target for resuscitation agents.

Double-pass pre-chirp managed amplification with high gain and high average power.

We demonstrate, to the best of our knowledge, the first double-pass pre-chirp managed fiber amplifier. The double-pass fiber amplifier exhibits high gain allowing us to amplify chirped picosecond pulses from 20 mW to 113 W in a rod-type Yb-fiber corresponding to 38 dB gain. We study the dependence of static mode degradation (SMD) on the nonlinear phase shift (NPS) accumulated by the amplified pulse. Our results indicate that a larger nonlinear phase shift results in stronger nonlinear polarization evolution of the fundamental mode and leads to a lower threshold for SMD. After optimization, our pre-chirp managed amplifier seeded by 80 mW pulses delivers 102 W amplified power from the main output. The amplified pulses are compressed to 37 (55) fs with 90 (100) W average power by a grating pair (chirped mirrors). The double-pass configuration significantly simplifies the implementation of pre-chirp managed fiber amplifiers leading to an extremely compact system.

High-power laser testing of calcium-phosphate-based bioresorbable optical fibers

Silica optical fibers are employed in endoscopy and related minimally invasive medical methods thanks to their good transparency and flexibility. Although silicon oxide is a biocompatible material, its use involves a serious health risk due to its fragility and the fact that potential fiber fragments can freely move inside the body without the possibility of being detected by conventional methods such as X-ray imaging. A possible solution to this issue can be the development of optical fibers based on bioresorbable (i.e., biodegradable and biocompatible) materials, which exhibit the important benefit of not having to be explanted after their functionality has expired. The optical power transmission tests of recently developed single-mode (SM) and multi-mode (MM) bioresorbable optical fibers based on calcium-phosphate glasses (CPGs) are here reported. A continuous-wave (CW) fiber laser at 1080 nm with output power up to 13 W and picosecond laser sources at 515 and 1030 nm with MW pulse peak power were used to test the transmission capabilities of the CPG fibers. No degradation of the CPG fibers transmission under long-term illumination by CW laser was observed. A laser-induced damage threshold (LIDT) at a fluence higher than 0.17 J/cm2 was assessed with the picosecond laser sources.

Activation of Oxytocin Receptors and Oxytocin Neurons of the Hypothalamic Paraventricular Nucleus Are Individually Capable of Reversing Opioid Overdose

Opioid-induced respiratory depression (OIRD) can result in fatal respiratory arrest. The current gold standard for intervention is opioid displacement using high affinity opioid receptor antagonists, like naloxone (NLX), which reverse OIRD and, undesirably, opioid analgesia. We recently reported in rats that the most lethal clinically used opioid fentanyl (60 nmol/kg, i.v.) causes OIRD that is reversed by systemic NLX (2.5 µmol/kg, i.v.). Multiple reports indicate that systemic NLX rapidly triggers release of the analgesic neuropeptide oxytocin (OXTN) from the hypothalamic paraventricular nucleus (PVN) both in opioid-dependent and opioid-naïve rats. Given that OXTN microinjections into specific nodes of the respiratory network can stimulate breathing, we hypothesized that OXTN can reverse fentanyl OIRD. We recorded phrenic nerve activity (PNA) to index neural inspiration in anesthetized (urethane/chloralose), vagotomized, paralyzed and artificially ventilated rats and determined that systemic peptide and non-peptide activation of OXTN receptors robustly reversed PNA arrest by fentanyl (60 nmol/kg, i.v.) (P<0.0001, n=4-6). Because nearly all systemic NLX-induced OXTN release is from the PVN, we tested the possibility that PVN NLX could counteract OIRD.PVN NLX (3 nmol/kg, 50 nL) not only reversed PNA arrest by fentanyl (60 nmol/kg, i.v.) (P=0.0238, n=3), but prevented it altogether (P=0.0107, n=3-4). To determine whether direct stimulation of PVN OXTN neurons could reverse fentanyl OIRD, we used an optogenetic approach. Mice with channelrhodopsin selectively expressed in OXTN neurons were anesthetized (ketamine-acepromazine-xylazine) and instrumented with arterial and venous catheters along with an optical fiber cannula dorsal to PVN. While spontaneously breathing, expired CO2 was used to monitor rhythmic ventilation and to quantify bradypnea during OIRD. Following systemic fentanyl (60 nmol/kg, i.v.), blue light (473 nm, 12 mW) pulses (10 ms, 20 Hz) were delivered to the PVN using a 5 s On/1 s Off pattern for 3 min. Photo-stimulation, which did not stimulate basal respiration rate (P=0.9056), prevented lethal effects of fentanyl OIRD (P<0.0001, n=3). Whereas dummy pulses during fentanyl OIRD permitted apneas as long as 51.1±16.5 s and proved to be lethal, optogenetic stimulation significantly attenuated apneas to a maximum 2.9±1.6 s (P<0.0001) and resulted in respiratory rate progressively recovering to 60.7±0.4% of baseline by the end of photo-stimulation. Although the expired CO2 increase post-fentanyl was similar (P=0.0334-P<0.0001) prior to PVN photo-stimulation between cohorts receiving blue light and dummy pulses (P=0.2171), the latter failed to prevent lethal respiratory arrest (P<0.0001). Findings indicate that systemic treatment with OXTN receptor agonists and activation of PVN OXTN neurons, both previously demonstrated to be analgesic, can reverse fentanyl overdose in rodents without requiring analgesia-reversing opioid receptor antagonism.

Generation of 1.5 MW–140 GHz Pulses With the Modular Pre-Prototype Gyrotron for W7-X

In anticipation of an Electron Cyclotron Resonance Heating system upgrade for the stellarator Wendelstein 7-X, a 1.5 MW - 140 GHz continuous-wave gyrotron is under development. In order to provide a first experimental verification of the scientific RF and electron beam optics design of the gyrotron with ms pulses, the Karlsruhe Intitule of Technology has developed a short-pulse pre-prototype gyrotron. In this work, we present details regarding the construction of the pre-prototype as well as measurements from the first experimental campaign delivering up to 1.6 MW in short pulses.

2-GHz watt-level Kerr-lens mode-locked Yb:KGW laser.

We report on a 2-GHz high-power Kerr-lens mode-locked Yb:KGW laser pumped by a single-mode fiber laser. The output performance for two different output coupling rates was investigated. Stable bidirectional mode-locking operation at the repetition rate of 2.157 GHz was obtained with a 0.6% output coupler. The average output powers of bidirectional operation are 741 mW and 746 mW, with 123-fs and 126-fs pulse durations, respectively. By using a 1.6% output coupler, unidirectional mode-locking is achieved with 145-fs pulse duration and 1.7-W average output power, which, to the best of our knowledge, is the highest average power from Kerr-lens mode-locked GHz femtosecond oscillators.

Initializing 214 Pure 14-Qubit Entangled Nuclear Spin States in a Hyperpolarized Molecular Solid.

Quantum entanglement has been realized on a variety of physical platforms such as quantum dots, trapped atomic ions, and superconductors. Here we introduce specific molecular solids as promising alternative platforms. Our model system is triplet pentacene in a host single crystal at level anticrossing (LAC) conditions. First, a laser pulse generates the triplet state and initiates entanglement between an electron spin and 14 hyperfine coupled proton spins (quantum bits or qubits). This gives rise to large nuclear spin polarization. Subsequently, a resonant high-power microwave (mw) pulse disentangles the electron spin from the nuclear spins. Simultaneously, high-dimensional multiqubit entanglement is formed among the proton spins. We verified the initialization of 214 pure 14-qubit entangled nuclear spin states with an average degree of entanglement of Eav = 0.77 ± 0.03. These results pave the way for large-scale quantum information processing with more than 10 000 multiqubit entangled states corresponding to computational (Hilbert) space dimensions of dim >1053.

Feasibility of lasing in the GaAs Reststrahlen band with HgTe multiple quantum well laser diodes

Operation of semiconductor lasers in the 20–50 µm wavelength range is hindered by strong non-radiative recombination in the interband laser diodes, and strong lattice absorption in GaAs-based quantum cascade structures. Here, we propose an electrically pumped laser diode based on multiple HgTe quantum wells with band structure engineered for Auger recombination suppression. Using a comprehensive model accounting for carrier drift and diffusion, electron and hole capture in quantum wells, Auger recombination, and heating effects, we show the feasibility of lasing at λ = 26, …, 30 µm at temperatures up to 90 K. The output power in the pulse can reach up to 8 mW for microsecond-duration pulses.