Showing papers on "Pulse duration published in 1988"

Pulsed CO2 laser tissue ablation: Effect of tissue type and pulse duration on thermal damage

Abstract: Tissue removal by infrared lasers is accompanied by thermal damage to nonablated tissue. The extent of thermal damage can be controlled by a choice of laser wavelength, irradiance, and exposure duration. The effect of exposure duration has been studied in vivo by using CO2 lasers with pulse widths that vary from 2 μsec to 50 msec. Pulse widths of 50 msec, typical of a shuttered, continuous-wave CO2 laser, produce damage regions 750 μm wide in normal guinea pig skin; the use of a 2-μseclong pulse reduced this damage zone to as little as 50 μm. Using 2-μseclong pulses, in vitro studies showed that the minimum zone of thermal damage varied significantly with tissue type. The thermal denaturation of these tissues has been studied and correlated with damage. The effect of denaturation temperature and pulse duration on the width of the damage zone is explained by a simple model.

Acoustic transient generation by laser‐produced cavitation bubbles near solid boundaries

Abstract: The acoustic transients emitted after breakdown and cavitation bubble collapse upon focusing a Q‐switch laser pulse into a liquid are investigated with special emphasis on their modifications induced by a solid boundary. For measuring the form p(t)/pmax of the pressure pulses an optical technique with a resolution of 10 ns has been developed. When p(t)/pmax is known, the pressure amplitude can be determined even when a transducer with a rise time much longer than the pulse duration is used. The duration of the transients (20–30 ns) and their pressure are nearly the same after breakdown and spherical bubble collapse. During spherical collapse, a maximum pressure of about 60 kbar is developed inside a bubble with Rmax=3.5 mm, and on average 73% of the bubble energy loss is transformed into acoustic energy. The sound emission near a solid boundary strongly depends on the normalized distance γ between the bubble and the boundary. The highest pressures at the boundary are achieved for γ→0; for γ=0.2 and Rmax =3.5 mm it has been found that p=2.5 kbar. These results are discussed with respect to the mechanisms of cavitation erosion important for hydraulic cavitation, laser lithotripsy, and ocular surgery.

Optical pulse compression using high-frequency electrooptic phase modulation

Abstract: An electrooptic method of generating and compressing optical pulses in the picosecond range is described. The method utilizes electrooptic phase modulation together with group delay/velocity dispersion and is applicable to most CW lasers. From the theoretical analysis, it is shown that normal as well as anomalous dispersion is applicable for pulse compression and that nearly transform-limited pulses can be obtained by the optimum choice of group-delay dispersion. In experiments, pulse compression was utilized for a CW Ar laser using a LiTaO/sub 3/ electrooptic phase modulator with a diffraction grating. Almost transform-limited pulses of 2.1 ps were obtained at 9.35-GHz repetition. The possibility of generating subpicosecond pulses down to several tens of femtosecond pulses is also discussed. >

Acoustic communication in the gray treefrog, Hyla versicolor : evolutionary and neurobiological implications

Abstract: 1. Acoustic communication in the gray treefrog,H. versicolor, was studied by analyzing the vocalizations of males and observing the phonotactic behavior of gravid females in response to pairs of synthetic stimuli, which usually simulated choices between calls of conspecific males at different temperatures or choices between calls of conspecific males and those of a sibling species,H. chrysoscelis. Calls ofH. chrysoscelis were also analyzed acoustically. 2. Pulse duty cycle (pulse duration divided by pulse period) averaged about 0.50 in the calls of both species over a wide range of temperature (Table 1). Pulse rise-time (as a percentage of pulse duration), which was also temperature-independent, was significantly longer inH. versicolor than inH. chrysoscelis (Table 1). The species difference in pulse shape was evident at a distance of 10 m from calling frogs (Fig. 1). 3. Females strongly preferred a linear approximation to the pulse shape (rise-time) typical of conspecific calls to an approximation of the pulse shape typical ofH. chrysoscelis (Figs. 1, 2A). Females did not show a preference between linear and exponential approximations of the conspecific pulse shape (Figs. 1, 2B). 4. When offered choices between synthetic calls that differed in pulse rate (pulses per s=p/s), females were usually very selective, choosing a stimulus with a pulse rate typical of a conspecific male at the test temperature over alternatives that differed by as little as 25% (Figs. 4–6). When both the call rate and pulse rate of synthetic calls were changed (Fig. 3), females showed temperature-dependent reversals in preference between 16 and 24°C (16 p/s vs 25 p/s) and between 16 and 20°C (15 p/s vs 20 p/s), but not between 20 and 24°C (20 p/s vs 25 p/s) (Table 2A–C). 5. When the call rates of alternative stimuli were the same, the pulse rate selectivity of females at 20°C was biased toward stimuli with low pulse rates (Table 2F). Females tested at 16°C rejected strongly alternatives with a high pulse rate, but females tested at 24°C did not reject strongly alternatives with a low pulse rate (Table 2E). Females tested at 24°C were also less selective than females tested at 20°C in rejecting alternatives with a high pulse rate, in the range ofH. chrysoscelis (Table 2D). Females tested at 24°C did, however, strongly reject an alternative with both a pulse rate and pulse shape typical ofH. chrysoscelis (Fig. 6). 6. Call duration and call rate were also relevant properties; changes in these variables modified preferences based on differences in pulse rate, provided that the pulse rates of both alternatives were within the range of variation produced by conspecific males over the normal range of breeding temperatures (Figs. 4–6). 7. Females showed a weak preference for synthetic calls with a bimodal spectral structure typical of conspecific males (1.1 kHz [−6 dB]+2.2 kHz) to a synthetic call with a single spectral component of 2.2 kHz. In tests of single-component stimuli of 1.9 or 2.2 kHz against alternatives of lower and higher frequencies, female preferences indicated a pattern of relative frequency sensitivity (Fig. 7) that was similar to that of an audiogram based on evoked potentials in the midbrain over the same range of frequency. 8. About 50% of the females tested responded phonotactically to a recorded call ofH. chrysoscelis when they had no other choice (Table 3). Thus, heterospecific signals were not only audible, but also behaviorally effective in the context of courtship. 9. Pattern of female preferences with respect to pulse shape and pulse rate suggest that the potential for mismating with males ofH. chrysoscelis has been an important selective force in the evolution of acoustic pattern discrimination inH. versicolor. 10. Results of this study are compared with those of other anurans and acoustic insects. Temperature-dependent shifts in temporal pattern preference, similar but less pronounced than those reported here for both fine temporal and gross temporal properties, were found in some species but not in others. 11. The pulse rate of the male's call increases linearly over a wide range of temperature (9–34°C; Gayou 1984), but female selectivity for pulse rate differs within the range of 16–24°C and is biased toward low pulse rates (Table 2). Thus, it is unlikely that both the temporal patterning of the male's call and temporal pattern recognition by the female are controlled rigidly and linearly by the same neural circuitry. 12. We discuss neurophysiological studies of temporal pattern selectivity in acoustic insects and anurans. There are several neural correlates of behavioral selectivity in gray treefrogs, but no published data concerning a neural correlate of the asymmetry in the strength of pulse rate preferences in gray treefrogs.

Compression of high-power optical pulses

Abstract: High-power compression of femtosecond pulses is demonstrated in nonlinear bulk material An energy output of ~100 μJ with a pulse duration of ≤24 fsec is produced at 630 nm, 4 orders of magnitude more energy than can be compressed in optical fibers The technique is simple and scalable to arbitrarily high pulse energies In addition, the flexibility in the choice of nonlinear media permits the compression method to be extended from the ultraviolet to the mid-infrared

The LIDAR Thomson scattering diagnostic on JET (invited)

Abstract: By combining the time‐of‐flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density are measured in a magnetically confined fusion plasma. This technique was realized for the first time on the JET tokamak. A ruby laser (3‐J pulse energy, 300‐ps pulse duration, 0.5‐Hz repetition rate) together with a 700‐MHz bandwidth detection and registration system yields a spatial resolution of about 12 cm. A spectrometer with six channels in the wavelength range 400–800 nm gives a dynamic range of the temperature measurements of 0.3–20 keV. The stray light problem in the backscatter geometry is overcome by spectral discrimination and gating of the photomultipliers. A ruby filter in the spectral channel containing the laser wavelength allows calibration of the vignetting along the line of sight by means of Raman scattering, enabling the measurement of density profiles. The low level of background signal due to the short integration time for a single spatial...

Time-resolved investigations of laser-induced shock waves in water by use of polyvinylidenefluoride hydrophones

Abstract: Laser light from a Q‐switched Nd:yttrium‐aluminum‐garnet laser (λ=1064 nm; pulse duration=20 ns; pulse energies up to 150 mJ) focused into water creates shock waves by rapidly expanding microplasmas. Using piezoelectric, thin‐film polyvinylidenefluoride (PVDF) as a transducer, a broadband hydrophone (100‐MHz bandwidth) was developed to investigate underwater shock waves. The electrical signal is analyzed with respect to reflections of the shock wave within the transducer and the input impedance of the measuring device. The shock waveform is determined, its peak pressure ranging to kbars (108 Pa), decreasing with r−1.12 and increases by the square root of the laser pulse energy. The time resolution of the hydrophone (4 ns) is sufficient to determine the plasma dimensions and the number of shock waves generated by a single laserpulse. Both vary statistically, primarily because of contaminations in the fluid. Because of the length of the region containing plasmas, different peak pressures are found in the di...

Femtosecond optical pulse amplification

Abstract: A number of techniques have been developed for amplification of optical pulses of approximately 100-fs duration. These amplifiers span a wide range of operating parameters from kilowatt to gigawatt peak powers and from 10 Hz to megahertz repetition rates. Amplification of femtosecond pulses has also been demonstrated at several wavelengths including visible, near-infrared, and ultraviolet regions. Several problems arise when amplifying short optical pulses to very high intensities. The problems are discussed and the state of the art of femtosecond optical pulse amplification is reviewed. >

Single-shot measurement of subpicosecond KrF pulse width by three-photon fluorescence of the XeF visible transition.

Abstract: The intensity of the XeF C–A transition induced by a subpicosecond KrF laser is shown to have a cubic dependence on KrF laser intensity. The third-order autocorrelation technique for measuring the duration of a single KrF subpicosecond pulse has been developed utilizing this visible transition. A pulse width of 220 fsec was successfully measured with a high contrast of ~10. The visible fluorescence is more useful to researchers than vacuum-UV fluorescences. Furthermore, this simple technique may be applied over a wide UV wavelength region from 204 to 306 nm.

Uptake of fluorescence-labeled dextrans by 10T 1/2 fibroblasts following permeation by rectangular and exponential-decay electric field pulses.

Abstract: The uptake of fluorescence-labeled dextrans by adherent 10T 1/2 murine fibroblasts following electric field pulse application was used as a criterion for the efficiency of electropermeation. The cells in monolayers were permeated by immersing a coaxial electrode in culture dishes. The percentage of cells which exhibited fluorescence uptake following electric field pulse application was measured at independently varying pulse field strength and pulse length. Dextrans with molecular weights equal to or higher than 41,000 dalton require higher field strength or longer pulse time to penetrate the cells. There is no detectable advantage of using a rectangular pulse against using an exponential decay pulse of similar power. The uptake was proportional to the product of the pulse amplitude and duration over the experimental range of 40-950 microseconds and 0.1-14.5 kV/cm. Cell survival decreases at the upper end of this range. The result provides a direct comparison of electric parameters which so far have not been standardized with regard to cell electropermeation.

Multiphoton ionisation from a short-range potential by short-pulse lasers

Abstract: The time-dependent Schrodinger equation is solved numerically for an electron which is bound in a one-dimensional short-range potential and exposed to a strong laser pulse. Total ionisation, and photoelectron spectra as a function of laser intensity, frequency and pulse length are calculated. There are features due to multiphoton resonances, which appear and disappear as the laser intensity changes during the pulse, similar to those observed recently in experiments. Results from a quasisteady state (complex-coordinate Floquet) calculation compare well with those from the time-dependent solution. The quasi-energies are shown to be complicated functions of laser intensity and frequency. For the short-range potential considered, new bound states appear as the ponderomotive barrier deepens the potential.

Thermal lensing measurement and compensation in a continuous-wave mode-locked Nd:YLF laser.

Abstract: Thermal lensing effects in a cw-pumped Nd:YLF rod have been characterized, permitting the optimization of the laser resonator. In the mode-locking regime, a well-designed system generates an average output power similar to that of a typical Nd:YAG laser with, however, an increased stability and pulse durations that are significantly shorter.

Evidence that stimulated Raman scattering in laser-produced plasmas is an absolute instability.

Abstract: We report a sequence of experiments in which 3-..mu..m-thick CH foils were irradiated by one arm of the Nova laser with square pulses of 0.35-..mu..m light. The laser intensity, pulse duration, and spot size were varied to produce plasmas in which the dependence of stimulated Raman scattering (SRS) on laser intensity and on density-gradient scale length could be independently evaluated. The traditional, convective-amplifier model of SRS fails, by many orders of magnitude, to explain either the magnitude or the scaling of these data. We suggest that SRS is absolutely unstable in these plasmas.

An investigation of the complexity of the earthquake source time function using dynamic faulting models

Abstract: By constructing more than 30 numerical models of spontaneous earthquake rupture on heterogeneous faults, the complexity of the far-field pulse shapes (the source time function), that is, the pulse consisting of several distinct subpulses, is investigated The influence of multiple strength barriers and/or dry friction on the far-field waveforms has been studied using the simple model of a square fault with uniformly spaced square barriers A central asperity is allowed to break to initiate the dynamic process It is found that the pulse duration is shorter the greater the density of barriers (ratio of barrier size to barrier spacing) The presence of friction has been found to have three different consequences First, the slipping area is reduced giving rise to a shorter pulse duration Second, the rate of slip propagation is reduced which increases the pulse duration Third, the frictional stress drop, if present, increases the amplitude of the pulse The interaction of these three effects together with the effect of the barriers results in different pulses depending on the relative role of these effects Breaking of multiple asperities with relatively short time delay between their fracturing produced a single pulse with ripples in its rising portion, the pulse duration depending on the total time required for the asperities to break Breaking of, say, two asperities with sufficient time delay between them produced a double pulse which is in accord with the asperity model of complex earthquakes In one case of a fault with barriers and friction, an additional peak was observed in some directions from the source, the peak form and amplitude being essentially the same as those of the peaks due to breaking of asperities One may conclude that each subpulse observed on a complex far-field pulse is not necessarily interpretable as being due to the failure of an asperity on the fault Furthermore, for the subpulses which can be related to asperity fracture, the duration is not determined by the size of the corresponding asperity but rather by the physical conditions on the fault surrounding the asperity

Twin pulsed arc welding system

Abstract: The invention provides a gas-shielded arc welding system using two side-by-side electrodes each of which is provided with current pulses superimposed on a respective background current level to maintain the desired controlled spray transfer mode of the weld metal without overheating effects. The pulses are supplied to the two arcs out of phase with one another to minimize interaction of their magnetic fields, but this alone is not sufficient to assure stability of the two arcs. In the system of the invention, one arc is designated as the master arc and the other as the slave arc. The master arc is stabilized by means of a feedback loop controlling its master pulse frequency. Each master pulse triggers a respective slave pulse after a preset time delay, in this way keeping corresponding master and slave pulses in synchronism. This is not necessarily sufficient to stabilize the slave arc and independent stabilization is therefore provided for it by a feedback loop controlling its pulse duration or its background current, but preferably by controlling both simultaneously.

Laser-intensity scaling experiments in long-scalelength, laser-produced plasmas

Abstract: An experimental technique is demonstrated that allows variation of the average laser intensity by more than two orders of magnitude while producing much smaller changes in the other parameters that determine the laser–plasma interactions. By irradiating exploding‐foil targets with 0.35 μm laser light in flat‐topped pulses of variable duration, the high‐power Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)] produced plasmas with electron temperatures of order 1 keV and with scale lengths of the (radial and axial) electron‐density gradients of order 1000 laser wavelengths. By using a constant target thickness and systematically decreasing the pulse length and spot size as laser intensity increased, the changes in the temperature and in the scale lengths were minimized. The time‐resolved spectrum of the Raman‐scattered light was used to measure both the electron temperature and the maximum density of the expanding plasmas. In this paper, these measurements are compared to both 1‐D models and 2‐D simulations. ...

Phase correction of femtosecond optical pulses using a combination of prisms and gratings.

Abstract: The performance of a combination of prisms and diffraction gratings as phase correctors is calculated numerically. Special attention is given to the regime where the pulse duration is 10 fsec or less.

Catheter-mediated electrical ablation: the relation between current and pulse width on voltage breakdown and shock-wave generation.

Abstract: Voltage waveform breakdown is characteristic of barotraumatic shock-wave generation during electrical catheter ablation of cardiac arrhythmias. The purpose of this investigation was to avoid barotrauma by defining, in vitro, the limits of pulse amplitude and pulse width for rectangular constant-current pulses that do not result in voltage breakdown and subsequently to determine what pulsing frequency is safe for use when high-energy trains of pulses are used. Electric pulses were delivered with a variable waveform modulator with a wide dynamic range and bandwidth capable of delivering pulses of 30-10,000-mu sec duration with amplitudes of up to 25 A. Cathodal pulses were delivered to a 6F catheter immersed in fresh anticoagulated bovine blood warmed to 37 degrees C to stimulate the milieu of a catheter in the chambers of the human heart. The maximum pulse amplitude that could be delivered without incurring voltage waveform breakdown varied inversely with pulse duration. Pulses of 30 mu sec broke down at currents above 24 A (2,500 V). Pulses of 10,000-mu sec duration broke down at 1 A (250 V). The maximum safely delivered energy for a single pulse was 2.5 J for pulses of 80-120 mu sec. Peak power for single pulses was maximum at 50-55 kW with 30-50-mu sec pulses. Charge delivery for single pulses was maximized at 9 mC with long, 10,000-mu sec duration pulses. To deliver an electrical pulse with energy significantly greater than 2.5 J without incurring voltage breakdown, trains of pulses were delivered where each pulse in the train had previously been shown to be free of voltage breakdown.(ABSTRACT TRUNCATED AT 250 WORDS)

Theory of pulsed excitation of nonlinear distributed prism couplers

Abstract: The coupling of high-power optical pulses into integrated-optics waveguides exhibiting nonlinearities is treated theoretically. A general result is that the coupling efficiency varies with pulse energy. Furthermore, the output pulses are distorted relative to the input pulses. For input pulses symmetric in time about their peak amplitude, the output pulses are asymmetric and symmetric for nonlinearities with turn-off times slower and faster, respectively, than the pulse width. Coupling efficiency and pulse distortion are also evaluated for interference between slow and fast nonlinearities of opposite sign. Finally, the dependence of the temporal pulse profile on the ratio of the pulse width to nonlinearity relaxation time is examined.

Effect of laser pulse duration on short wavelength emission from femtosecond and picosecond laser‐produced Ta plasmas

Abstract: The efficiency, duration, and spectral content of the emission from laser‐produced Ta plasmas in the 10–71 nm spectral region have been measured for laser pulse durations ranging from 100 fs to 70 ps. Efficiencies from 0.3% for 100 fs pulses to 2.65% for 70 ps pulses, x‐ray pulse durations from less than 10 ps for 100 fs excitation pulses to 105 ps for 70 ps excitation pulses, and spectral content peaking in the 17–35 nm region were measured for 1 mJ pulses at intensities from 1011 to 5×1014 W/cm2.

Digital pulse generator having a programmable pulse width and a pulse repetition interval

Abstract: A technique for accurately controlling both the pulse repetition interval and pulse width of a pulse signal generator which uses a crystal oscillator to maintain a very accurate time base. Two separate digital counters clock-in the clock pulses. When the desired number of clock pulses are registered by the first counter, a first digital comparator generates a start pulse which resets the first counter and triggers an output flip-flop. The change of state in the flip-flop enables the second counter to begin its count. When the desired number of clock pulses are registered by the second counter, a second digital comparator generates an end pulse which resets the second counter and triggers the flip-flop a second time. The second change of state of the flip-flop disables the second counter until the first comparator generates a new start pulse. The new start pulse toggles the flip-flop and the entire process is repeated continuously to generate at the output of the flip-flop a periodic pulse train having the desired pulse width with the leading edges separated by the desired pulse repetition interval.