Pulse Packet Research Articles

FEATURES OF MEASURING THE LAYER OF PERIODIC PULSE SIGNALS BY THE SPECTRA ANALYZER

By reducing the band of analysis, the sensitivity of the spectrum analyzer increases, which allows to detect the harmonics of the test signal. Parameters of the test signal are limited by the standard adopted by the data interface. For typical spectrum analyzers when measuring the harmonics of periodic signals, the results of measurements become dependent on the parameters of the analyzer's settings. Therefore, it is expedient to determine the corrections to convert the harmonic level into a packet of pulses. It is shown that the correction, on which it is necessary to increase the results of measurement to calculate the harmonic level for a pulse packet, can be determined by the results of two measured signals in the line to which this signal propagates and emits. Measurement in the line requires additional devices to connect the analyzer's input to the line wires. At the same time, at the frequencies above 100 MHz it is difficult to fulfill the conditions for small distortion of the pulses during such measurements. But provided the parameters of the connection device are unchanged, the effect of the device on the results of measurement of identical harmonics with a relatively wide and narrow band will be the same. Therefore, the requirements for calibrating such devices disappear.

MIXI: Mobile Intelligent X-Ray Inspection System

A novel, low-dose Mobile Intelligent X-ray Inspection (MIXI) concept is being developed at RadiaBeam Technologies. The MIXI concept relies on a linac-based, adaptive, ramped energy source of short X-ray packets of pulses, a new type of fast X-ray detector, rapid processing of detector signals for intelligent control of the linac, and advanced radiography image processing. The key parameters for this system include: better than 3 mm line pair resolution; penetration greater than 320 mm of steel equivalent; scan speed with 100% image sampling rate of up to 15 km/h; and material discrimination over a range of thicknesses up to 200 mm of steel equivalent. Its minimal radiation dose, size and weight allow MIXI to be placed on a lightweight truck chassis.

Investigation of plume dynamics during picosecond laser ablation of H13 steel using high-speed digital holography

Ablation of H13 tool steel using pulse packets with repetition rates of 400 and 1000 kHz and pulse energies of 75 and 44,upmu {hbox {J}}, respectively, is investigated. A drop in ablation efficiency (defined here as the depth per pulse or upmu {hbox {m}}{/}upmu {hbox {J}}) is shown to occur when using pulse energies of E_{{mathrm{pulse}}} > 44,upmu {hbox {J}}, accompanied by a marked difference in crater morphology. A pulsed digital holographic system is applied to image the resulting plumes, showing a persistent plume in both cases. Holographic data are used to calculate the plume absorption and subsequently the fraction of pulse energy arriving at the surface after traversing the plume for different pulse arrival times. A significant proportion of the pulse energy is shown to be absorbed in the plume for E_{{mathrm{pulse}}} > 44,upmu {hbox {J}} for pulse arrival times corresponding to {>}1 MHz pulse repetition rate, shifting the interaction to a vapour-dominated ablation regime, an energetically costlier ablation mechanism.

Electron Accelerators for Novel Cargo Inspection Methods

One of the main factors limiting the performance of conventional x-ray cargo inspection with material discrimination (MD) is the interlaced mode of system operation. Such systems use pulsed linac or betatron x-ray generators and produce alternate bremsstrahlung pulses with lower- and higher- end-point energies. Consequently, these systems provide about 50mm lower penetration than a system operated in a non-interlaced mode, have a limited range of cargo areal densities with valid MD, and cannot perform MD of objects smaller than the pulse separation. Also, the limited pulse repetition rate of x-ray generators in interlaced mode limits the radiographic image quality at nominal commercial speeds of vehicles or trains.Several new methods of cargo inspection with MD were recently introduced to address the above-mentioned limitations: dual-energy methods based on Scintillation-Cherenkov detectors [1]; multi-energy method based on intrapulse time-varying of spectral content of x-ray [1,2]; multi-energy method utilized ramping-up energy packet of short x-ray pulses [3,4]; and methods based on multi-energy betatron [5,6]. All of these methods have electron accelerators as a core element. However, the accelerator requirements and, thus, their designs, are different for each system. In this paper, we will discuss the requirements for the accelerators, provide some details about their designs, and present several novel solutions for current and future projects.

Schrödinger electrons interacting with optical gratings: quantum mechanical study of the inverse Smith–Purcell effect

Slow swift electrons with low self-inertia interact differently with matter and light in comparison with their relativistic counterparts: they are easily recoiled, reflected, and also diffracted form optical gratings and nanostructures. As a consequence, they can be also better manipulated into the desired shape. For example, they get bunched quite fast in interaction with acceleration gratings in presence of an external electromagnetic radiation, a phenomenon which can be desirable in development of superradiant coherent light sources. Here, I examine the spatiotemporal behavior of pulsed electron wave packets at low energies interacting with pulsed light and optical gratings, using a quantum-mechanical self-consistent numerical toolbox which is introduced here. It will be shown that electron pulses are accelerated very fast in interaction with the near-field of the grating, demanding that a synchronicity condition is met. To prevent the electrons to be transversely deflected from the grating a symmetric double-grating configuration is necessary. It is found that even in this configuration, diffraction due to the interaction of the electron with the standing-wave light inside the gap between the gratings, is a source of defocusing. Moreover, the longitudinal broadening of the electron pulse directly affects the final shape of the electron wave packet due to the occurrence of multiple electron-photon scatterings. These investigations pave the way towards the design of more efficient electron-driven photon sources and accelerators.

A benchmark facility for guided elastic wave actuation by piezoelectric discs

Key aspects of piezoelectric actuation of circular-crested guided elastic waves (ccGEW) in homogeneous plates are investigated experimentally and studied with finite element analysis (FEA). The specific implementation of fiber-optic strain transduction presented offers virtually distortion-free conversion of radial GEW displacements with a flat transfer characteristic and negligible interference with the wave to be studied. Due to a concentric arrangement of actuator disc, fiber-optic transducer and circular plate, a shuttling GEW pulse packet can be retained over an exceptionally long period of time. Hence accurate long term measurements of the excited surface strain in response to piezoelectrical actuation becomes feasible for specific GEW modes. Presumably, traveling-wave-like behavior of compressional ccGEW pulses including the transition of converging into expanding wave packets around the symmetry axis is observed for the first time. Minor deviations between FEA predictions and measurement results can be traced back to imperfections of the experimental setup or inaccurate material data. The presented approach may be used for thorough characterization of piezoelectric disc actuators and studies of ccGEW propagation in various homogeneous materials.

Generation of optical pulse packet using a fiber stacker for time fiducial applications

We present the concept of multicolor optical pulse packet generation based on modified fiber stackers featured with reflection geometries. Infrared radiation, visible and ultraviolet time fiducials were obtained by the amplification and frequency-conversion of the pulse packet generated in this fiber stacker. Application for the inertial confinement fusion (ICF) experiment diagnosis with time fiducials was demonstrated. With featured reflection geometries, the shaped packet pulses with uniform sub-pulse polarization states in a temporal window of ns range were generated in the fiber stacker. The design greatly simplifies the packet pulse generation for time fiducial and could be used for laser-driven ICF experimental diagnosis.

Peculiarities of ultrasonic pulsed immersion testing of blanks of railway axles

Research has been conducted and methods have been developed for revealing small-sized flaws in blanks of railway axles by ultrasonic immersion testing technique. It has been established that the use of probing pulse packets significantly improves the sensitivity of testing.

Discrete Chromatic Aberrations Arising from Photoinduced Electron-Photon Interactions in Ultrafast Electron Microscopy.

In femtosecond ultrafast electron microscopy (UEM) experiments, the initial excitation period is composed of spatiotemporal overlap of the temporally commensurate pump photon pulse and probe photoelectron packet. Generation of evanescent near-fields at the nanostructure specimens produces a dispersion relation that enables coupling of the photons (ℏω = 2.4 eV, for example) and freely propagating electrons (200 keV, for example) in the near-field. Typically, this manifests as discrete peaks occurring at integer multiples (n) of the photon energy in the low-loss/gain region of electron-energy spectra (i.e., at 200 keV ± nℏω eV). Here, we examine the UEM imaging resolution implications of the strong inelastic near-field interactions between the photons employed in optical excitation and the probe photoelectrons. We find that the additional photoinduced energy dispersion occurring when swift electrons pass through intense evanescent near-fields results in a discrete chromatic aberration that limits the spatial resolving power to several angstroms during the excitation period.

Microelectrochemical Machining at the Ultrasmall Interelectrode Gaps with the Use of the Packets of Nanosecond Voltage Pulses

The accuracy of the electrochemical machining is determined by the value of the interelectrode gap (IEG). The stability of the process of the anodic dissolution at the ultrasmall interelectrode gaps is connected with the optimal dosage of the electric energy put into the electrolyser. The decrease of the IEG up to 10…20μm demands of the narrow pulses of the voltage and small packets of pulses.The work deals with the theoretical and experimental researches of the process of the electrochemical micromachining at the ultrasmall IEG with the use of the packets of pulses up to 100…200ns.The method of the envelope curves on the maximums of the current pulses in the packet of pulses is used to define the accuracy of the machining and the speed of the leveling of the initial accuracy of the machining.The modeling of the process of the anodic dissolution at the ultrasmall IEG with the taking into account of gas generation and heating of the electrolyte.The produced experimental plant was used to carry out the researches of the electrochemical micromachining with the use of the nanosecond pulses voltage.

Experimental demonstration of a quantum key distribution without signal disturbance monitoring

Quantum key distribution (QKD) enables two distant users, Alice and Bob, to share secret keys. In existing QKD protocols, an eavesdropper's intervention will inevitably disturb the quantum signals; thus, Alice and Bob must monitor the signal disturbance to place a bound on the potential information leakage. However, T. Sasaki et al. proposed a quite different protocol, named round-robin differential phase shift (RRDPS), in which the amount of eavesdropped information is bounded without monitoring the signal disturbance. Here, we present the first active implementation of the RRDPS protocol. In our experiment, Alice prepares packets of pulses, with each packet being a train with 65 pulses, and the global phase of each packet is randomized. Bob uses a 1-GHz, 1-64-bit actively controlled variable-delay interferometer to realize random switching of the different delays. Benefiting from the large pulse number of each packet, the high stability and low insertion loss of the interferometer, the system can distribute secret key over a distance of 90 km. Our results confirm the feasibility of performing practical QKD with this novel protocol.

Seeing the world through a dynamic biomimetic sonar

The outer baffle surfaces surrounding the sonar pulse emission and reception apertures of the biosonar system of horseshoe bats (family Rhinolophidae) have been shown to dynamically deform while actively sensing the environment. It is hypothesized that this dynamic sensing strategy enables the animal, in part, to cope with dense unstructured sonar environments. In the present work, a biomimetic dynamic sonar system inspired by the biosonar system of horseshoe bats has been assembled and tested. The sonar head features dynamic deforming baffles for emission (mimicking the bats’ noseleaf) and reception (pinnae). The dynamic baffles were actuated to change their geometries concurrently with the diffraction of the emitted ultrasonic pulses and returning echoes. The time-variant signatures induced by the dynamic baffle motions were systematically characterized in a controlled anechoic setting and the interaction between emission and reception dynamic signatures was investigated. The sonar was further tested in the context of natural environments with a specific focus on the interaction of dynamic ultrasonic pulse packets with natural targets. For both experimental approaches, a sonar with static baffle shape configuration was used as a reference to establish the impact of dynamic features.

A strategy for low electrode wear in meso–micro-EDM

Implementation of die-sinking EDM for precision machining of meso–micro-scale features with surface area smaller than 10mm2 down to 0.1mm2 is mainly restricted by electrode machining and electrode wear. In this work, micro-scale graphite electrodes with a projection area as small as 0.002mm2 and 1mm length have been machined. Process parameter analysis is carried out to analyse the wear behaviour of micro-scale graphite electrodes during erosion. Pulse duration, pause duration and rising current slopes have been found to be the primary parameters affecting the electrode wear. A low electrode wear strategy consisting of the wear neutral pulse packets is developed for erosion of micro-scale cavities and slots using graphite electrodes. Resource efficiency achieved through low electrode wear during roughing enables die-sinking EDM as a potential economic and energy efficient micromachining process.

Nanosecond Microwave Semiconductor Switches for 258…266 GHz

Optically controlled microwave switches open the way to commutate radiation frequencies up to terahertz. The switches are based on induced photoconductivity effect in semiconductors changing properties of resonant systems they are built in. The prospective applications—plasma heating, radars, particle accelerators, and spectroscopy—often require switching rapidness up to nanoseconds and coherence of output pulse packets between each other. Our waveguide semiconductor switches seem to be the most promising to satisfy these requirements. Several working switches have been built and tested for frequency range from 258 to 266 GHz at microwave power of 25 mW. The maximum possible microwave power to be switched is expected up to several watts or even higher with special heat dissipation means. The switches demonstrate nanosecond level of performance when controlled by a 10-ns green 527-nm laser with pulse energy of about 100 nJ.

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.

Oscillation-induced signal transmission and gating in neural circuits.

Reliable signal transmission constitutes a key requirement for neural circuit function. The propagation of synchronous pulse packets through recurrent circuits is hypothesized to be one robust form of signal transmission and has been extensively studied in computational and theoretical works. Yet, although external or internally generated oscillations are ubiquitous across neural systems, their influence on such signal propagation is unclear. Here we systematically investigate the impact of oscillations on propagating synchrony. We find that for standard, additive couplings and a net excitatory effect of oscillations, robust propagation of synchrony is enabled in less prominent feed-forward structures than in systems without oscillations. In the presence of non-additive coupling (as mediated by fast dendritic spikes), even balanced oscillatory inputs may enable robust propagation. Here, emerging resonances create complex locking patterns between oscillations and spike synchrony. Interestingly, these resonances make the circuits capable of selecting specific pathways for signal transmission. Oscillations may thus promote reliable transmission and, in co-action with dendritic nonlinearities, provide a mechanism for information processing by selectively gating and routing of signals. Our results are of particular interest for the interpretation of sharp wave/ripple complexes in the hippocampus, where previously learned spike patterns are replayed in conjunction with global high-frequency oscillations. We suggest that the oscillations may serve to stabilize the replay.

Full-field reconstruction of ultrashort waveforms by time to space conversion interferogram analysis.

Accurate amplitude and phase measurements of ultrashort optical waveforms are essential for their use in a wide range of scientific disciplines. Here we report the first demonstration of full-field optical reconstruction of ultrashort waveforms using a time-to-space converter, followed by a spatial recording of an interferogram. The algorithm-free technique is demonstrated by measuring ultrashort pulses that are widely frequency chirped from negative to positive, as well as phase modulated pulse packets. Amplitude and phase measurements were recorded for pulses ranging from 0.5 ps to 10 ps duration, with measured dimensionless chirp parameter values from -30 to 30. The inherently single-shot nature of time-to-space conversion enables full-field measurement of complex and non-repetitive waveforms.

Characterization of fast photoelectron packets in weak and strong laser fields in ultrafast electron microscopy.

The development of ultrafast electron microscopy (UEM) and variants thereof (e.g., photon-induced near-field electron microscopy, PINEM) has made it possible to image atomic-scale dynamics on the femtosecond timescale. Accessing the femtosecond regime with UEM currently relies on the generation of photoelectrons with an ultrafast laser pulse and operation in a stroboscopic pump-probe fashion. With this approach, temporal resolution is limited mainly by the durations of the pump laser pulse and probe electron packet. The ability to accurately determine the duration of the electron packets, and thus the instrument response function, is critically important for interpretation of dynamics occurring near the temporal resolution limit, in addition to quantifying the effects of the imaging mode. Here, we describe a technique for in situ characterization of ultrashort electron packets that makes use of coupling with photons in the evanescent near-field of the specimen. We show that within the weakly-interacting (i.e., low laser fluence) regime, the zero-loss peak temporal cross-section is precisely the convolution of electron packet and photon pulse profiles. Beyond this regime, we outline the effects of non-linear processes and show that temporal cross-sections of high-order peaks explicitly reveal the electron packet profile, while use of the zero-loss peak becomes increasingly unreliable.

Detachable fiber optic tips for use in thulium fiber laser lithotripsy

The thulium fiber laser (TFL) has recently been proposed as an alternative to the Holmium:YAG (Ho:YAG) laser for lithotripsy. The TFL's Gaussian spatial beam profile provides higher power transmission through smaller optical fibers with reduced proximal fiber tip damage, and improved saline irrigation and flexibility through the ureteroscope. However, distal fiber tip damage may still occur during stone fragmentation, resulting in disposal of the entire fiber after the procedure. A novel design for a short, detachable, distal fiber tip that can fit into an ureteroscope's working channel is proposed. A prototype, twist-lock, spring-loaded mechanism was constructed using micromachining methods, mating a 150-μm-core trunk fiber to 300-μm-core fiber tip. Optical transmission measuring 80% was observed using a 30-mJ pulse energy and 500-μs pulse duration. Ex vivo human calcium oxalate monohydrate urinary stones were vaporized at an average rate of 187 μg/s using 20-Hz modulated, 50% duty cycle 5 pulse packets. The highest stone ablation rates corresponded to the highest fiber tip degradation, thus providing motivation for use of detachable and disposable distal fiber tips during lithotripsy. The 1-mm outer-diameter prototype also functioned comparable to previously tested tapered fiber tips.

Enabling direct nanoscale observations of biological reactions with dynamic TEM

Biological processes occur on a wide range of spatial and temporal scales: from femtoseconds to hours and from angstroms to meters. Many new biological insights can be expected from a better understanding of the processes that occur on these very fast and very small scales. In this regard, new instruments that use fast X-ray or electron pulses are expected to reveal novel mechanistic details for macromolecular protein dynamics. To ensure that any observed conformational change is physiologically relevant and not constrained by 3D crystal packing, it would be preferable for experiments to utilize small protein samples such as single particles or 2D crystals that mimic the target protein's native environment. These samples are not typically amenable to X-ray analysis, but transmission electron microscopy has imaged such sample geometries for over 40 years using both direct imaging and diffraction modes. While conventional transmission electron microscopes (TEM) have visualized biological samples with atomic resolution in an arrested or frozen state, the recent development of the dynamic TEM (DTEM) extends electron microscopy into a dynamic regime using pump-probe imaging. A new second-generation DTEM, which is currently being constructed, has the potential to observe live biological processes with unprecedented spatiotemporal resolution by using pulsed electron packets to probe the sample on micro- and nanosecond timescales. This article reviews the experimental parameters necessary for coupling DTEM with in situ liquid microscopy to enable direct imaging of protein conformational dynamics in a fully hydrated environment and visualize reactions propagating in real time.