MHz Burst Research Articles

Manufacturing of high quality 3D microstructures in stainless steel with ultrashort laser pulses using different burst modes

In this study, the generation of 3D microstructures in stainless steel with ultrashort laser pulses using different burst modes will be presented. Femtosecond laser pulses in the BiBurst mode and picosecond laser pulses in the MHz burst mode are used to generate a predefined 3D microstructure. To smooth the structure bottoms and to remove deposits, a novel subsequent processing technique using femtosecond laser pulses in the GHz burst mode is performed. Two different processing strategies of the ablation and the subsequent treatment are applied to generate predefined 3D microstructures with low surface roughness and without deposits. In this way, smooth structure bottoms with a minimum surface roughness of 0.13μm can be obtained at a structure depth of 300μm.

Optimization of the ablation process using ultrashort pulsed laser radiation in different burst modes

The authors report on a solid-state burst mode amplified laser source capable of varying the pulse duration in the range of 0.24–10 ps and generating pulse trains (bursts) with an intraburst pulse repetition rate of 65 MHz (MHz burst mode) and 2.5 GHz (GHz burst mode). Furthermore, every pulse of a burst in the MHz burst mode can be divided into another GHz burst, called BiBurst. In this study, a laser radiating at 1030 nm wavelength is used to generate ablation areas and investigate the efficiency, the surface quality, and the process stability on stainless steel with different total fluences per burst, numbers of pulses per burst, and pulse durations in the MHz, GHz, and BiBurst modes. In the investigated parameter range, the highest efficiencies were obtained in the MHz burst mode with femtosecond pulses, but in this burst regime, the burst mode induced smoothing effect is dependent of the number of scans. In contrast, the process stability and the quality in the GHz and BiBurst are granted over a wide range of parameters. Thus, the actual advantages of the GHz and BiBurst regimes in terms of a high process stability and quality could be established.

Surface treatment on cobalt and titanium alloys using picosecond laser pulses in burst mode

The authors report on the results of surface treatment experiments using a solid-state amplified laser source emitting laser pulses with a pulse duration of 10 ps. The laser source allows the generation of pulse trains (bursts) with an intra-burst pulse repetition rate of 80 MHz (pulse-to-pulse time interval about 12.5 ns) with up to eight pulses per burst. In this study a wavelength of 1064 nm was used to investigate both ablation of material and laser-induced surface modifications occuring in metallic implant alloys CoCrMo (cobalt-chromium-molybdenum) and TiAlV (titanium-aluminum-vanadium) in dependence of the number of pulses and fluences per pulse in the burst. By using the burst mode, a smoothing effect occurs in a certain parameter range, resulting in very low surface roughness of the generated microstructures. It is demonstrated that at fluences per pulse which are smaller than the material-specific ablation threshold, a self-organized pore formation takes place if a defined number of pulses per burst is used. Thus, the advantage of the MHz burst mode in terms of a possible surface modification is established.

Micromachining of Invar Foils with GHz, MHz and kHz Femtosecond Burst Modes.

In this work, a burst mode laser is used for micromachining of 20 µm–250 µm thick Invar (Fe64/Ni36) foils. Holes were drilled by firing multiple pulses transversely onto the sample without moving the beam (percussion drilling). The utilized laser system generates a burst of a controllable number of pulses (at 1030 nm) with tunable pulse-to-pulse time spacing ranging from 200 ps to 16 ns. The sub-pulses within the burst have equal amplitudes and a constant duration of 300 fs that do not change regardless of the spacing in time between them. In such a way, the laser generates GHz to MHz repetition rate pulse bursts with a burst repetition rate ranging from 100 kHz to a single shot. Drilling of the material is compared with the non-burst mode of kHz repetition rate. In addition, we analyze the drilling speed and the resulting dependence of the quality of the holes on the number of pulses per burst as well as the average laser power to find the optimal micromachining parameters for percussion drilling. We demonstrate that the micromachining throughput can be of an order of magnitude higher when using the burst mode as compared to the best results of the conventional kHz case; however, excess thermal damage was also evident in some cases.

An induction voltage adder with MHz repetition rates and nanosecond duration based on printed circuit board stacked Blumlein lines.

By utilizing the radio-frequency metal-oxide-semiconductor field-effect transistor switch, the multi-stage stacked printed circuit board Blumlein lines, and the induction voltage adder, a voltage generator with MHz repetition rates and nanosecond duration was designed and fabricated. The shunting current of the multi-stage stacked Blumlein lines, which consists of the coupling current at the load ends and the leakage current at the switch ends, is clarified. The circuit simulation and experiment of the three-stage stacked Blumlein lines are carried out; the result shows that the experiment and simulation agree well. The five-stage IVA prototype is designed for generating voltage pulses of ∼10 kV in amplitude and 8-11 ns in duration with MHz burst repetition rates. The voltage generator with MHz repetition rates and nanosecond duration demonstrated an important application prospect to reduce the electron beam initial emittance of the high energy accelerator.

Surface ablation efficiency and quality of fs lasers in single-pulse mode, fs lasers in burst mode, and ns lasers

In recent years, the burst-mode caught a lot of attention in the field of ultrashort-pulse laser micro machining. One of the major issues is the influence of the burst pulse number and frequency on ablation efficiency and quality. A recent publication reported of a significant increase in ablation efficiency when processing with ≥25 burst pulses at ≥100 MHz burst frequencies. This raises the question of whether processing with such high pulse densities can be attributed to non-thermal ablation, or whether a quasi-nanosecond laser ablation behavior is achieved. To answer this question, we determined ablation efficiencies as function of fluence for silicon, stainless steel, and copper and compared the ablation quality at the optimal fluence using the following laser systems: femtosecond laser operated in single-pulse mode, fs laser operated in 28-pulse-burst mode with a burst pulse frequency of 148 MHz, and a nanosecond laser with a pulse duration of 175 ns, which is identical with the temporal length of the burst pulse train. The comparison showed that the burst mode used produces similar surface morphologies and melt burrs as the nanosecond laser, but at about 2/3 of its efficiency.

Optical coherence elastography for evaluating Rayleigh wave propagation in soft matter-micellar fluids

Wormlike micellar (WM) fluids exhibit a self-assembly property and viscoelastic behavior which make them a promising material for representing biological fluids, ultrasoft tissue phantoms and drug delivery carriers. Although WM biochemistry has been investigated, the mechanical properties have been rarely discussed because of the complexity of soft matter systems. Here, we demonstrate that optical coherence elastography (OCE) is capable of measuring Rayleigh wave of micellar fluids. Three solutions with concentrations of 100, 200, 300 and 400 mM, were considered in the study. Ultrasound excitation to produce Rayleigh waves was accomplished using 7.5 MHz burst excitation signals of length 4.0 ms repeated at a rate of 19 Hz. The optical coherence tomography (OCT) acquisition was set to 50 lateral positions and 4000 A-scans for each lateral position at a 76 kHz scan rate for total acquisition time of 52.6 ms. Ultrasound-based shear wave elastography, implemented on a Verasonics system, was used to obtain group and phase velocity of the shear wave in all micellar fluids as reference. Our research results demonstrated that Rayleigh wave in micellar fluids can be evaluated by OCE and mechanical properties like Young’s modulus can be assessed. A wide of concentration ranges of WM fluids would provide a more realistic model to mimic biological fluids and tissues.

Excitation and electroporation by MHz bursts of nanosecond stimuli

Intense nanosecond pulsed electric field (nsPEF) is a novel modality for cell activation and nanoelectroporation. Applications of nsPEF in research and therapy are hindered by a high electric field requirement, typically from 1 to over 50 kV/cm to elicit any bioeffects. We show how this requirement can be overcome by engaging temporal summation when pulses are compressed into high-rate bursts (up to several MHz). This approach was tested for excitation of ventricular cardiomyocytes and peripheral nerve fibers; for membrane electroporation of cardiomyocytes, CHO, and HEK cells; and for killing EL-4 cells. MHz compression of nsPEF bursts (100–1000 pulses) enables excitation at only 0.01–0.15 kV/cm and electroporation already at 0.4–0.6 kV/cm. Clear separation of excitation and electroporation thresholds allows for multiple excitation cycles without membrane disruption. The efficiency of nsPEF bursts increases with the duty cycle (by increasing either pulse duration or repetition rate) and with increasing the total time “on” (by increasing either pulse duration or number). For some endpoints, the efficiency of nsPEF bursts matches a single “long” pulse whose amplitude and duration equal the time-average amplitude and duration of the bursts. For other endpoints this rule is not valid, presumably because of nsPEF-specific bioeffects and/or possible modification of targets already during the burst. MHz compression of nsPEF bursts is a universal and efficient way to lower excitation thresholds and facilitate electroporation.

Photon diagnostics at the FLASH THz beamline.

The THz beamline at FLASH, DESY, provides both tunable (1-300 THz) narrow-bandwidth (∼10%) and broad-bandwidth intense (up to 150 uJ) THz pulses delivered in 1 MHz bursts and naturally synchronized with free-electron laser X-ray pulses. Combination of these pulses, along with the auxiliary NIR and VIS ultrashort lasers, supports a plethora of dynamic investigations in physics, material science and biology. The unique features of the FLASH THz pulses and the accelerator source, however, bring along a set of challenges in the diagnostics of their key parameters: pulse energy, spectral, temporal and spatial profiles. Here, these challenges are discussed and the pulse diagnostic tools developed at FLASH are presented. In particular, a radiometric power measurement is presented that enables the derivation of the average pulse energy within a pulse burst across the spectral range, jitter-corrected electro-optical sampling for the full spectro-temporal pulse characterization, spatial beam profiling along the beam transport line and at the sample, and a lamellar grating based Fourier transform infrared spectrometer for the on-line assessment of the average THz pulse spectra. Corresponding measurement results provide a comprehensive insight into the THz beamline capabilities.

High Frequency Power Supply with 3.3 kV SiC-MOSFETs for Accelerator Application

Pulse switching characteristics of newly developed 3.3 kV SiC-MOSFET were investigated and they were provided for a prototype switching power supply (SPS). With supply voltage of 2.5 kV and the load resistor of 100 Ω, rise time Tr and fall time Tf were 76 ns and 88 ns respectively. The SPS exhibited a successful 2.5 kV-20 A-1 MHz burst mode operation.

Experimental Demonstration and Circuitry for a Very Compact Coil-Only Pulse Echo EMAT.

This experimental study demonstrates for the first time a solid-state circuitry and design for a simple compact copper coil (without an additional bulky permanent magnet or bulky electromagnet) as a contactless electromagnetic acoustic transducer (EMAT) for pulse echo operation at MHz frequencies. A pulsed ultrasound emission into a metallic test object is electromagnetically excited by an intense MHz burst at up to 500 A through the 0.15 mm filaments of the transducer. Immediately thereafter, a smoother and quasi “DC-like” current of 100 A is applied for about 1 ms and allows an echo detection. The ultrasonic pulse echo operation for a simple, compact, non-contacting copper coil is new. Application scenarios for compact transducer techniques include very narrow and hostile environments, in which, e.g., quickly moving metal parts must be tested with only one, non-contacting ultrasound shot. The small transducer coil can be operated remotely with a cable connection, separate from the much bulkier supply circuitry. Several options for more technical and fundamental progress are discussed.

J0420303 100MHz域バースト波送信器を用いた高調波画像化([J042-03]超音波計測・解析法の新展開(3),機械材料・材料加工部門)

J0420303 100MHz域バースト波送信器を用いた高調波画像化([J042-03]超音波計測・解析法の新展開(3),機械材料・材料加工部門)

Multiple-pulse pumping for enhanced fluorescence detection and molecular imaging in tissue

Applications of fluorescence based imaging techniques for detection in cellular and tissue environments are severely limited by autofluorescence of endogenous components of cells, tissue, and the fixatives used in sample processing. To achieve sufficient signal-to-background ratio, a high concentration of the probe needs to be used which is not always feasible. Since typically autofluorescence is in the nanosecond range, long-lived fluorescence probes in combination with time-gated detection can be used for suppression of unwanted autofluorescence. Unfortunately, this requires the sacrifice of the large portion the probe signal in order to sufficiently filter the background.We report a simple and practical approach to achieve a many-fold increase in the intensity of a long-lived probe without increasing the background fluorescence. Using controllable, well separated bursts of closely spaced laser excitation pulses, we are able to highly increase the fluorescence signal of a long-lived marker over the endogenous fluorescent background and scattering, thereby greatly increasing detection sensitivity. Using a commercially available confocal microscopy system equipped with a laser diode and time correlated single photon counting (TCSPC) detection, we are able to enhance the signal of a long-lived Ruthenium (Ru)-based probe by nearly an order of magnitude. We used 80MHz bursts of pulses (12.5ns pulse separation) repeated with a 320kHz repetition rate as needed to adequately image a dye with a 380ns lifetime. Just using 10 pulses in the burst increases the Ru signal almost 10-fold without any increase in the background signal.

Subharmonic Wave Generation at Interfaces of a Thin Layer between Metal Blocks

Subharmonic waves were significantly generated at the interfaces of aluminum blocks and an aluminum foil subjected to a 6 MHz burst incident wave. Although the fundamental wave amplitude was linearly varied with input voltage, the subharmonic waves generated at the interfaces indicated strong nonlinearity. Analyzing the relationship of the subharmonic wave generation, the applied contact pressure, and the stress of the ultrasonic wave has revealed that subharmonic components can be generated when the incident ultrasonic wave has sufficiently large stress to separate the contacting interfaces. At rough surfaces, the subharmonic peak could not be observed because a large true contact pressure prevented the interfaces from separating.

LOW-FREQUENCY TYPE III BURSTS AND SOLAR ENERGETIC PARTICLE EVENTS

We compare the ∼1 MHz type III bursts of flares associated with samples of "impulsive" and "gradual" solar energetic particle (SEP) events from cycle 23. While large gradual SEP events had much higher > 30 MeV proton intensities, the median-integrated intensities, peak intensities, and durations of the two groups of radio bursts were comparable. Thus, the median "proton yield" (peak > 30 MeV proton intensity of an SEP event divided by its associated integrated ∼1 MHz intensity) of type III bursts associated with gradual SEP events was ∼280 times larger than that for impulsive SEP events. A similar yield difference of ∼250 was observed for 4.4 MeV electron events. Only for extrapolated electron energies ∼5 keV, corresponding to the energy of the electrons that excite type III emission, does the median yield converge to the same value for both groups of events. The time profiles of ∼1 MHz bursts associated with impulsive SEP events are characteristically shorter and simpler than those associated with the gradual SEP events, reflecting the development of the second stage of radio emission in large eruptive flares. The gradual SEP events were highly associated (96%) with decametric-hectometric (DH) type II bursts versus only a 5% association rate for the impulsive events. Large favorably located ∼1 MHz type III bursts with associated DH type IIs had an ∼60% association rate with large (≥ 1 pfu) > 30 MeV SEP events versus ∼5% for ∼1 MHz bursts without accompanying DH II emission. These results are interpreted in terms of two distinct types of particle acceleration at the Sun, a flare-resident process that produces relatively few > 30 MeV protons and ∼4 MeV electrons in space and a shock process that dominates the large gradual proton events.

Wave envelopes method for description of nonlinear acoustic wave propagation

A novel, free from paraxial approximation and computationally efficient numerical algorithm capable of predicting 4D acoustic fields in lossy and nonlinear media from arbitrary shaped sources (relevant to probes used in medical ultrasonic imaging and therapeutic systems) is described. The new WE (wave envelopes) approach to nonlinear propagation modeling is based on the solution of the second order nonlinear differential wave equation reported in [J. Wójcik, J. Acoust. Soc. Am. 104 (1998) 2654–2663; V.P. Kuznetsov, Akust. Zh. 16 (1970) 548–553]. An incremental stepping scheme allows for forward wave propagation. The operator-splitting method accounts independently for the effects of full diffraction, absorption and nonlinear interactions of harmonics. The WE method represents the propagating pulsed acoustic wave as a superposition of wavelet-like sinusoidal pulses with carrier frequencies being the harmonics of the boundary tone burst disturbance. The model is valid for lossy media, arbitrarily shaped plane and focused sources, accounts for the effects of diffraction and can be applied to continuous as well as to pulsed waves. Depending on the source geometry, level of nonlinearity and frequency bandwidth, in comparison with the conventional approach the Time-Averaged Wave Envelopes ( TAWE) method shortens computational time of the full 4D nonlinear field calculation by at least an order of magnitude; thus, predictions of nonlinear beam propagation from complex sources (such as phased arrays) can be available within 30–60 min using only a standard PC. The approximate ratio between the computational time costs obtained by using the TAWE method and the conventional approach in calculations of the nonlinear interactions is proportional to 1 / N ∼ 2 , and in memory consumption to 1 / N ∼ where N ∼ is the average bandwidth of the individual wavelets. Numerical computations comparing the spatial field distributions obtained by using both the TAWE method and the conventional approach (based on a Fourier series representation of the propagating wave) are given for circular source geometry, which represents the most challenging case from the computational time point of view. For two cases, short (2 cycle) and long (8 cycle) 2 MHz bursts, the computational times were 10 min and 15 min versus 2 h and 8 h for the TAWE method versus the conventional method, respectively.

The newly developed airborne radio-echo sounding system of the AWI as a glaciological tool

AbstractSince 1994 the Alfred Wegener Institute (AWI) has operated an airborne radio-echo sounding system for remote-sensing studies of the polar ice caps in Antarctica and in Greenland. It is used to map ice thicknesses and internal layernigs of glaciers, ice sheets and ice shelves, and is capable of penetrating ice thicknesses of up to 4 km. The system was designed and built by AWI in cooperation with Aerodata Flugmeßtechnik GmbH, Technische Umversitat Hamburg-Harburg and the Deutsches Zentrum fur Luft- und Raumfahrt e.V. The system uses state-of-the-art techniques, and results in high vertical (5 m) as well as along-track (3.25 m) resolution. The radar signal is a 150 MHz burst with a duration of 60 or 600 ns. The peak power is 1.6 kW, and the system sensitivity is 190 dB. The short backfire principle has been adopted and optimized for antennae used on Polar2, a Dormer 228-100 aircraft, resulting in an antenna gain of 14 dB each. Digital data recording allows further processing. The quality of the recorded data can be monitored on screen and as online analogue plots during the flight.

A combined analysis of Hα, microwave and hard X-ray bursts of the flare of 1993-10-02

We present the results of multi-wave observations of the Class 1N/C6.5 flare of 1993-10-02. We compared monochromatic images of the flare, two-dimensional spectra around Hα, microwave bursts at 2840 MHz and hard X-ray burst data. We derived the intensity curves of different kernels in the Hα image, compared them with the time profiles of the microwave and hard X-ray bursts, gave the evolution of the brightness field of the chromosphere flare region, determined the magnetic field configuration from the magnetograms and hence proposed a possible interpretation of the generation and heating of the flare.

Correlation of ultrasound-induced hemolysis with cavitation detector output in vitro

A 20-MHz passive acoustic detector was used to quantify the amount of transient acoustic cavitation occurring in a sample exposed to intense pulsed ultrasound. A dilute suspension of human erythrocytes with and without a microbubble echo-contrast agent was exposed in vitro to 500 W/cm 2 (SPPA) ultrasound of center frequency 1 MHz and tone burst duration 20, 100, 200, 500 and 1000 μs at a pulse repetition frequency of 20 Hz. Inertial cavitation occurring within the sample, as measured by the temporal average of the detector output, correlated well with hemolysis, suggesting that violent bubble collapse is responsible for cell damage. The result also raises the prospect of cavitation monitoring as a possible predictor of adverse bioeffects when echo-contrast agents are used clinically.

Spectral Characteristics of a Coronal Oscillation with a Period of the Order of 10 MS

AbstractA coronal oscillation with a changing period was observed with the acousto-optic radio spectrometer of Yunnan Observatory. The bandwidth of the oscillation was about 10 MHz. It accompanied radiation which lasted for about 500 ms. A corresponding burst was found on a radio spectrogram obtained at Weissenau, Germany. A 245 MHz burst was reported at the same time and we found a bright spot on the Hα film records of Yunnan Observatory. In this paper, we present a series of spectra of the oscillation at 10 ms intervals. In the spectra the oscillation appears in the form of a 3.5 MHz drift of radio flux with frequency.