Showing papers on "Pulse duration published in 1984"

Perturbation of the direction of neurite growth by pulsed and focal electric fields

Abstract: We have studied the orientation of neurite growth in the culture of embryonic Xenopus neurons in response to three types of extracellular electric fields: spatially uniform pulsed fields, focally applied steady (DC) fields, and focally applied pulsed fields. Under uniform pulsed fields, neurites showed a preferential orientation toward the cathode pole of the field in a manner similar to that previously found for DC fields. The extent of neurite orientation depended upon the duration, amplitude, and frequency of the pulse but appeared to be similar to that produced by a uniform DC field of an equivalent time- averaged field intensity. For square pulses of 5 msec duration, the minimal amplitude and frequency required to produce a detectable orientation of neurite growth over a period of 24 hr were 2.5 V/cm and 10 Hz, which correspond to a time-averaged field intensity of 125 mV/cm. Steady or pulsed focal fields were applied by passing a current through a micropipette placed near the growth cone of the neurite. Fields of negative polarity (current sink) were found to attract the growth cone, whereas fields of positive polarity (current source) were found to deflect the growth cone away from the pipette. The threshold DC current density needed at the growth cone to perturb its direction of growth within 15 min was 0.2 to 2 pA/micron2 (or 3 to 30 mV/cm); and for focal pulsed currents (pulse duration 5 msec), a typical combination of minimal pulse amplitude and frequency was 4 pA/micron2 and 10 Hz. This threshold focal current is similar to that which occurs at the synaptic cleft during active synaptic activity.

Superposed quadrature modulated baseband signal processor

Abstract: A signal processor for providing a continuous raised cosine output signal having no jitter or intersymbol-interference and with controllable main and side lobes. An NRZ input signal is converted to a double interval raised cosine pulse signal having an amplitude normalized to 1.0. Another single interval raised cosine pulse signal having a peak amplitude (A-1) is superposed with the former raised cosine pulse to provide the output signal.

Parametric generation of tunable picosecond pulses in the medium infrared using AgGaS2 crystals

Abstract: Infrared pulses were generated over the range from 1.2 to 10 μm in a single path system consisting of two AgGaS2 crystals. A high photon conversion efficiency between 10−1 and 10−3 was found depending upon the emission frequency. A bandwidth of approximately 10 cm−1 for idler pulses between 6 and 10 μm was observed. With a Nd:yttrium aluminum garnet laser pump of 20‐ps pulse duration one observes parametric pulses of 8 ps.

Field observations of electromagnetic pulse propagation in dielectric slabs

Abstract: The propagation of electromagnetic pulses in naturally occurring dielectric surface layers has been examined. Pulse duration used in field experiments reported here has been on the order of nanoseconds with pulse bandwidths in the high VHF to low UHF band. The layers were sheets of fresh water ice and granite at thicknesses ranging between .4 and 4 m. Both transverse electric (TE) and transverse magnetic (TM) modes were attempted but only the TE propagation could be interpreted. Analog recordings of wide-angle reflection and refraction (WARR) profiles were taken and recorded in a continuous graphic display. The displays allowed easy identification of phase fronts thereby facilitating study of of the dispersion of the pulses. The phase and group velocities of the wave-group packets agree well with the velocities predicted from dispersion curves derived from the modal waveguide equation. In one case the Airy phase of wave-packet propagation occurred. The best measure of the dielectric constant of the layer was the frequency of the air wave.

The effect of rupture complexity on estimates of source size

Abstract: A general technique is proposed for estimating rupture geometry from measurements of body wave durations, wherein measurements of the overall signal duration or the duration of a single pulse identified in all of the arrivals are inverted to estimate the geometry of the overall rupture or of a specific subevent, respectively. The complexity of a waveform is determined by comparing the overall signal duration to the duration of the most prominent pulse in the waveform; this complexity is used to estimate the ratio of the average rupture velocity to the subevent rupture velocity. The shear waves radiated by eight moderate (3.6≤ML≤4.9) multiply recorded aftershocks of the 1975 Oroville, California, earthquake are analyzed using measurements of the corner frequency, the characteristic frequency, the pulse duration, and the signal duration. Comparison of these four different estimates of source size indicates that the corner frequency and the characteristic frequency are more sensitive to the dominant subevent size than to the overall event size. The difference between the estimates of source size determined from the signal duration measurements and the spectral measurements increases as the rupture complexity increases.

Laser‐induced bulk damage in SiO2 at 1.064, 0.532, and 0.355 μm

Abstract: Single‐ and multiple‐pulse laser‐induced bulk damage of fused and crystalline SiO2 has been investigated. We studied the dependence of the intensity required to produce macroscopic damage upon wavelength, focal spot size, pulse duration, and pulse repetition frequency. It is found that the material must undergo some undetected microscopic change prior to macroscopic damage which increases the probability of damage with increasing number of pulses. The multiple pulse damage depends strongly on spot size, occurring in far fewer pulses at larger spot sizes than at small ones, and displays a significant dependence on pulse length and repetition frequency. Possible mechanisms are discussed.

80-MW photoconductor power switch

Abstract: The application of photoconductors to fast rise time, high‐power switching is discussed. We report the successful switching of a 100‐kV system to generate a 1.8‐kA, <5‐ns rise time, 200‐ns duration electrical pulse in a 25‐Ω load using a single photoconductor switch excited by a Q‐switched Nd: glass laser. The photoconductor was a 2.5‐cm‐long bar of single‐crystal, high‐resistivity silicon with a 0.5×0.5 cm cross section. Only a depth of about 1 mm of one side was used for conduction.

Laser triggering of a 500-kV gas-filled switch: A parametric study

Abstract: We have investigated neodymium: yttrium aluminum garnet (Nd:YAG) laser triggering of a 500‐kV, SF6‐insulated gas switch for a range of laser parameters. Laser wavelengths of 266 nm and 1064 nm with nominal pulse lengths of 2‐ and 4‐nsec full width at half maximum (FWHM) were used to trigger the switch. The switch was triggered by focusing the laser to form a breakdown arc in the gas between the electrodes. Subnanosecond jitter in the operation of the switch was obtained for 266‐nm laser pulse energies as low as 5 mJ. Results obtained with the Nd:YAG laser are compared to earlier data in which the switch was triggered with a krypton‐fluoride laser operating at 248 nm with a pulse length of 20‐nsec FWHM. Our experimental results show that ultraviolet laser triggering gives results which are a dramatic improvement over infrared laser triggering in terms of lower jitter and smaller change in delay time with variations in applied voltage. We also see indications that the optimum laser pulse length is equal to or greater than the closure time of the switch.

Breakdown in air‐filled microwave waveguides during pulsed operation

Abstract: A theoretical investigation is made of the threshold condition for breakdown in air‐filled high power microwave waveguides during pulsed operation. The analysis concentrates on a parameter range characterized by microwave frequencies in the GHz range and pressures larger than 100 Torr. In particular, explicit analytical expressions are obtained for the dependence of breakdown field on pulse length and pulse repetition frequency.

Effects of laser field statistics on coherent anti-Stokes Raman spectroscopy intensities.

Abstract: The effects of intensity fluctuations in the frequency-doubled output of a multimode Nd:YAG pump laser on coherent anti-Stokes Raman spectroscopy (CARS) signal generation have been investigated in a crossed-beam, two-color CARS experiment. Measurements of signal intensity as a function of time delay (small compared with pulse length) in one pump beam show a maximum at zero delay. In measurements on a single unresolved transition, however, the nonresonant signal is enhanced by almost a factor of 2 more than the resonant signal. Our results demonstrate the non-Gaussian nature of the laser field statistics and introduce new considerations into the analysis of experimental CARS spectra.

Generation of relativistic photoelectrons induced by excimer laser irradiation

Abstract: For the first time, photoelectrons from a metal surface irradiated by a pulsed excimer laser were accelerated to relativistic energy. The energy, the current density, and the pulse duration of the accelerated photoelectrons were 0.41 MeV, 0.5 A/cm2, and 20 ns, respectively.

Generation of High Resolution Radar Range Profiles and Range Profile Auto-Correlation Functions Using Stepped-Frequency Pulse Train

Abstract: : This report describes some of the issues involved in using stepped-frequency radar pulse trains to obtain high resolution target range profiles. Radar returns from stepped-frequency pulse trains may be processed either coherently or noncoherently. Coherent processing consists of taking the Fourier transform of the N coherently-detected (complex) pulse returns from each range cell, where N is the number of pulses in the train. The output of the transform is the high resolution range profile of the scatterers within that range cell (i.e., within the span of a single pulse width). Noncoherent processing consists of taking the Fourier transform of the squared-magnitudes of the pulse returns from a given range cell and yields the auto-correlation function of this range profile. In either case, the resulting range resolution (of both the profile and its auto-correlation function) is determined by the total bandwidth (i.e., the frequency spread) of the pulse train. The stability requirements of the variable frequency source used to generate the stepped frequencies of the pulse train are determined for both the coherent and noncoherent processing cases.

Electron-beam-controlled atomic Xe infrared laser

Abstract: A study was made of a high-power high-pressure electron-beam-controlled laser utilizing the 5d-6p and 7p-7s transitions of the Xe atom and having an active volume of 9 liters. The dependences were obtained of the electrical parameters of an electron-beam-controlled discharge and of the energy parameters and spectral composition of the output radiation on the type of buffer gas, the discharge circuit parameters, and the composition and pressure of the active mixture. When the medium was pumped by an electron beam alone, having a current density of ~ 6 A/cm2 and a pulse duration of O.7 μsec, the laser output energy was ~ 6/J (efficiency ~ 1.5%), but for electronbeam- controlled pumping it was roughly 20 J. The physical lasing efficiency, in terms of the electric field, was ~5%. An analysis was made of the mechanism of the establishment of the active medium conductivity and of the population kinetics of the active transition levels.

Kinetics of picosecond pulse generation in semiconductor lasers with bimolecular recombination at high current injection

Abstract: The kinetics of generating ultrashort light pulses by gain switching unbiased semiconductor lasers emitting relaxation oscillations is theoretically modeled and described using phase portraits. Biomolecular recombination processes and realistic injection current pulse shapes are incorporated in the model. Approximate analytical solutions of the rate equations are derived for high current injection. Laser pulse widths, pulse peak power, electrical to optical pulse delay times, and time difference to subsequent relaxation oscillations are computed. Their dependence on injection current to threshold current density ratio ( J/J_{t} ) and on material and laser design parameters is explicitly derived and is in good agreement with experiment. In particular the remarkable observation that the laser pulse width is broadly independent of the injection current rise and fall time can thus be understood.

Self‐sustained picosecond pulse generation in a GaAlAs laser at an electrically tunable repetition rate by optoelectronic feedback

Abstract: We demonstrate that applying optoelectronic feedback to a high‐speed, self‐pulsing semiconductor laser is an effective and practical means of generating picosecond optical pulses (∼10–20 ps) at a very high repetition rate, between 1 to 5 GHz, which can be electrically tuned. The optical pulses are very stable both on a short term basis with a frequency stability of one part in 105, and on the long term basis as a result of the absence of critical optical alignment. This laser system is potentially very useful in high‐speed electro‐optic signal processing, optical multiplexing, or laser ranging.

D.C. magnetic properties of metallic glasses after flash annealing

Abstract: In this study, flash annealing involves passing a well shaped d.c. current pulse of known duration, through a straight strip of metallic glass. The rapid temperature rise (∼ 5000 K s-1), the high final temperature ( \sim500-600\deg C), the short pulse duration (112 ms), and the rapid cooling at the end of the pulse (∼500 K s-1) combine to achieve a thermal cycle unobtainable in a conventional furnace treatment. This study shows that a lower value of coercive field, H c , can be achieved after flash annealing than is possible in a furnace treatment. The uniaxial anisotropy, K u , is also reduced by flash annealing. A discussion of the results is given in terms of stress relief, creep enhanced relaxation and the total degree of relaxation attainable. A novel method of heat treating metallic glass cores is outlined, as an extension of this work.

Transient infrared spectroscopy on the picosecond time scale by coherent pulse propagation

Abstract: A novel time‐resolved IR spectroscopy is accomplished studying coherent propagation of ultrashort pulses at low‐intensity level. A detailed theoretical discussion is presented for electric dipole interaction between a resonant coherent pulse and a molecular transition. Homogeneous broadening (single line) and also more realistic situations with several adjacent transition frequencies are considered. Drastic changes of the pulse wings are predicted depending on several important parameters; e.g., absorption length αl, pulse duration tp, and a molecular dephasing time T2. It is shown that coherent superposition of neighboring transitions leads to a beating phenomenon, which is readily observable for αl≲1 and T2/tp>1. The theoretical results are substantiated by experimental data on HCl gas in the pressure range 0.5 to 3 bar. Tunable pulses of 4 ps produced by a parametric generator system are used and two ultrafast IR light gates. Quantitative agreement between theory and experiment is obtained. The dephasi...

Depolarization-contraction coupling in short frog muscle fibers. A voltage clamp study.

Abstract: Short muscle fibers (1.5 mm) were dissected from hindlimb muscles of frogs and voltage clamped with two microelectrodes to study phenomena related to depolarization-contraction coupling. Isometric myograms obtained in response to depolarizing pulses of durations between 10 and 500 ms and amplitudes up to 140 mV had the following properties. For suprathreshold pulses of fixed duration (in the range of 20-100 ms), the peak tension achieved, the time to peak tension, and contraction duration increased as the internal potential was made progressively more positive. Peak tension eventually saturates with increasing internal potentials. For pulse durations of greater than or equal to 50 ms, the rate of tension development becomes constant for increasing internal potentials when peak tensions become greater than one-third of the maximum tension possible. Both threshold and maximum steepness of the relation between internal potential and peak tension depend on pulse duration. The relation between the tension-time integral and the stimulus amplitude-duration product was examined. The utility of this relation for excitation-contraction studies is based on the observation that once a depolarizing pulse configuration has elicited maximum tension, further increases in either stimulus duration or amplitude only prolong the contractile response, while the major portion of the relaxation phase after the end of a pulse is exponential, with a time constant that is not significantly affected by either the amplitude or the duration of the pulse. Hence, the area under the tension-response curve provides a measure of the availability to troponin of the calcium released from the sarcoplasmic reticulum in response to membrane depolarization. The results from this work complement those obtained in experiments in which intramembrane charge movements related to contractile activation were studied and those in which intracellular Ca++ transients were measured.

Field observations of electromagnetic pulse propagation in dielectric slabs

Abstract: The propagation of electromagnetic pulses in naturally occurring dielectric surface layers has been examined Pulse duration used in field experiments reported here has been on the order of nanoseconds with pulse bandwidths in the high VHF to low UHF band The layers were sheets of fresh water ice and granite at thicknesses ranging between 04 and 4 m Both transverse electric (TE) and transverse magnetic (TM) modes were attempted but only the TE propagation could be interpreted Analog recordings of wide-angle reflection and refraction (WARR) profiles were taken and recorded in a continuous graphic display The displays allowed easy identification of phase fronts thereby facilitating study of the dispersion of the pulses The phase and group velocities of the wave-group packets agree well with the velocities predicted from dispersion curves derived from the modal waveguide equation In one case the Airy phase of wave-packet propagation occurred The best measure of the dielectric constant of the layer was the frequency of the air wave

Time-dependent semiclassical theory of gain-coupled distributed feedback lasers

Abstract: A semiclassical treatment of the time-dependent behavior of a gain-coupled, distributed feedback laser (DFL) has been developed. This treatment takes into account the field propagation within the DFL, and is therefore capable of predicting its behavior even for extremely short pump pulses. Comparisons are made to existing steady-state and transient theories where they are valid, showing good agreement, and new predictions are made of the behavior in the short pump pulse regime. It is found that the emitted pulse duration is dominated by the transit time through the pumped region. By studying the evolution of the field distribution inside the cavity, insight can be gained into the operation of the DFL. The effect of spatial hole burning in a gain-coupled DFL is treated and found to be small.

Thermal properties of thin insulating layers using pulse transient hot strip measurements

Abstract: A transient method for measuring thermal conductivity and thermal diffusivity has been developed for studies of insulating solid surface layers with thicknesses down to a few micrometers. The method is based on a procedure by which a string of square pulses, via an ac‐coupled circuit, is applied to the hot strip (deposited thin film), which is acting both as a heat source and a sensor of the temperature increase of the substrate. By performing transient experiments over short times (selecting a short pulse duration), it is possible to limit the depth under the hot strip beyond which the thermal properties of the substrate does not influence the recorded thermal properties. A series of experiment with probing depths ranging from 4 to 10 μm has shown that the thermal properties measured with this technique agree very well with independently measured values provided the pulse duration is small compared to the pulse period. Because of the small probing depth and the size of the thermal probe this novel techni...

Effect of the pulse length on the sensitivity of lead azide to laser radiation

Abstract: The existence of characteristic internal scales for the duration of any physical phenomenon imparts a special significance to investigations of the effect of temporal factors on the process under study. Unavoidably reflected in the relationships found, this circumstance aids both in the identification of, and in the deeper understanding of the mechanisms for the phenomenon. This work was undertaken to specify the assumptions used in modeling the laser initiation of explosives resulting from the appearance of a thermal instability in the neighborhood of an absorbing microinhomogeneity of submicron size R /sub o/ . The fundamental characteristics of the heating centers are evaluated on the basis of an experimental study of the dependence of the threshold energy for initiation of lead azide on the duration of the exciting pulse.

Synchronization tracking in pulse position modulation receiver

Abstract: A clock pulse generator for decoding pulse position modulation in an optical communication receiver is synchronized by a delay tacking loop which multiplies impulses of a data pulse by the square-wave clock pulses from the generator to produce positive impulses when the clock pulse is of one level, and negative impulses when the clock pulse is of another level. A delay tracking loop integrates the impulses and produces an error signal that adjusts the delay so the clock pulses will be synchronized with data pulses. A dead-time τd provided between data pulses of an interval τp in the data pulse period τ. When synchronized, the average number of positive impulses integrated will equal the average number of negative impulses over the continuous stream of data pulses.

Capacitor charging FET switcher with controller to adjust pulse width

Abstract: A switching power supply includes an FET full bridge, a controller to drive the FETs, a programmable controller to dynamically control final output current by adjusting pulse width, and a variety of protective systems, including an overcurrent latch for current control. Power MOSFETS are switched at a variable frequency from 20-50 kHz to charge a capacitor load from 0 to 6 kV. A ferrite transformer steps up the DC input. The transformer primary is a full bridge configuration with the FET switches and the secondary is fed into a high voltage full wave rectifier whose output is connected directly to the energy storage capacitor. The peak current is held constant by varying the pulse width using predetermined timing resistors and counting pulses. The pulse width is increased as the capacitor charges to maintain peak current. A digital ripple counter counts pulses, and after the desired number is reached, an up-counter is clocked. The up-counter output is decoded to choose among different resistors used to discharge a timing capacitor, thereby determining the pulse width. A current latch shuts down the supply on overcurrent due to either excessive pulse width causing transformer saturation or a major bridge fault, i.e., FET or transformer failure, or failure of the drive circuitry.

Pulse width modulation pulse generator

Abstract: A modulating wave signal whose phase abruptly changes is compared with a carrier signal to generate a pulse width modulation pulse. The carrier signal is a sine wave signal having a frequency n times as high as that of the modulating signal and synchronized with the modulating signal. A phase of the carrier abruptly changes by a factor of n as the phase of the modulating signal abruptly changes. Thus, a pulse with modulation pulse which causes an output voltage of a pulse with modulation inverter to be sinusoidal is generated even if the phase of the modulating wave signal abruptly changes.