Showing papers on "Pulse duration published in 1997"

Absorption of ultrashort, ultra-intense laser light by solids and overdense plasmas

Abstract: Absorption mechanisms for ultra-intense (I>10/sup 17/ W/cm/sup 2/) laser pulses incident on solids and overdense plasma slabs are discussed. We focus on the ultrashort pulse regime, i.e., where the laser pulse length is only a few to perhaps thousands of femtoseconds. Starting from well-known results at low intensity and long pulse length, we begin with absorption mechanisms such as inverse Bremstrahlung and classical resonance absorption and survey several additional absorption mechanisms significant for ultrashort, ultra-intense laser light interacting with overdense plasmas. Estimates for the fraction of laser energy absorbed by various mechanisms are given. It is found that the fraction of energy absorbed by the plasma, and the resulting electron temperatures, can depend considerably on the scale length of the plasma at the critical surface. It is also found that two-dimensional (2-D) effects greatly increase the amount of absorption into hot electrons, over the amount predicted using one-dimensional (1-D) theory. The inclusion of kinetic effects, collisionless absorption, and multidimensional effects are crucial to obtaining a complete picture of the interaction. We also review some of the experimental efforts to understand this complex process of absorption.

The effect of pulsed electric fields on biological cells: experiments and applications

Abstract: The effect of pulsed electric fields with amplitudes in the range of 100 V/cm-100 kV/cm on bacteria and aquatic nuisance species has been explored. The pulse duration was so short that heating of the biological matter could be neglected. The electrical energy required for lysing of bacteria, or stunning of aquatic species, decreases when the pulse duration is reduced. For lysing of Eschericia coli, this tendency has been proven to hold for pulsewidths as short as 60 ns. For macroorganisms, however, it was found that for pulsewidths of less than 5 /spl mu/s, the tendency is reversed: the energy required to affect the macroorganisms increases again. This minimum in energy, or maximum in efficiency, respectively, can be understood by taking the time required for electrical charging of the cell membrane into account. Applications of the pulsed electric field technique (PEFT) are in biofouling prevention, debacterialization of liquids, and in the field of medicine. A series of field tests on biofouling prevention in a cooling system with untreated water as coolant has demonstrated the economic feasibility of the electro-technology.

Pulsed mode lysis method

Abstract: An apparatus and method for the application of ultrasound to a location within the body is provided. The apparatus can advantageously operate at a pulse duration below about 100 milliseconds and in the range 0.1 milliseconds to 100 milliseconds and a pulse repetition period below about 1 second and in the range of 1 millisecond to 1 second. Duty ratios over 5 and preferably over 8 are also advantageous. Therapeutic applications of ultrasound such as for assisting in the treatment of medical conditions such as cancer and/or other ailments are also provided.

Quasi-phase-matched parametric chirped pulse amplification systems

Abstract: Use of quasi-phase-matched (QPM) materials for parametric chirped pulse amplification (PCPA) substantially reduces the required pump peak power and pump brightness, allowing exploitation of spatially-multimode and long duration pump pulses. It also removes restrictions on pump wavelength and amplification bandwidth. This allows substantial simplification in pump laser design for a high-energy PCPA system and, consequently, the construction of compact diode-pumped sources of high-energy ultrashort optical pulses. Also, this allows elimination of gain-narrowing and phase-distortion limitations on minimum pulse duration, which typically arise in a chirped pulse amplification system. One example of a compact source of high-energy ultrashort pulses is a multimode-core fiber based PCPA system. Limitations on pulse energy due to the limited core size for single-mode fibers are circumvented by using large multimode core. Limitations on pulse duration and beam quality due to multimode core are circumvented by using a PCPA scheme. Additionally, the large core of the multimode fiber facilitates cladding-pumping by inexpensive and high-power multiple-mode laser diodes.

Theory and simulation on the threshold of water breakdown induced by focused ultrashort laser pulses

Abstract: A comprehensive model is developed for focused pulse propagation in water. The model incorporates self-focusing, group velocity dispersion, and laser-induced breakdown in which an electron plasma is generated via cascade and multiphoton ionization processes. The laser-induced breakdown is studied first without considering self-focusing to give a breakdown threshold of the light intensity, which compares favorably with existing experimental results. The simple study also yields the threshold dependence on pulse duration and input spot size, thus providing a framework to view the results of numerical simulations of the full model. The simulations establish the breakdown threshold in input power and reveal qualitatively different behavior for picoand femto-second pulses. For longer pulses, the cascade process provides the breakdown mechanism, while for shorter pulses the cooperation between the self-focusing and the multiphoton plasma generation dominates the breakdown threshold.

Residual stress improvement in metal surface by underwater laser irradiation

Abstract: Laser shock processing of water-immersed material was developed for improving the residual surface stress of metal components. The process changes the stress field from tensile to compressive by means of impulsive pressure of laser-induced plasma generated through the ablative interaction of the intense laser pulse with the material. The plasma, generated by the irradiation of second harmonic of a Q-switched Nd:YAG laser (SH-YAG, λ = 532 nm) on an SUS304 test piece, was directly observed by imaging the plasma radiation with a gated image intensifier and a charged coupled device (CCD) camera. Comparing the observed image to the plasma expansion velocity calculated with an analytical model, we deduced that about 20% of the plasma internal energy would represent the thermal energy. The calculation of the plasma pressure with this result showed that it exceeded 2 GPa in water and the yield stress of SUS304, when a typical laser pulse of the SH-YAG impinged on a water-immersed SUS304 test piece. A residual compressive stress exceeding 200 MPa was built over 200 μm in depth, by scanning the SH-YAG focused on a spot of 0.75 mm diameter with a power density of 60 TW/m2 and a pulse duration of 5 ns.

High Harmonic Generation from Ultrafast Pump Lasers

Abstract: Using an ultrafast pump pulse affects the spectral and temporal characteristics of high order harmonics in an unexpected and fortuitous way. Calculations of spectra for rare gases using 10100fs, 800nm pulses show that as the pump pulse length decreases the highest harmonics become correspondingly shorter while their conversion efficiencies increase dramatically. Especially significant is that we find these highest harmonics have phase characteristics that allow for the possibility of compression to subfemtosecond time scales. {copyright} {ital 1997} {ital The American Physical Society}

Inactivation of escherichia coli suspended in liquid egg using pulsed electric fields

Abstract: Liquid egg inoculated with Escherichia coli was exposed to a 26 kV/cm pulsed electric field with 2 and 4 μs pulse duration, 1.25 and 2.50 Hz pulsing rates, up to 100 pulses/unit volume, and stepwise and continuous recirculation treatment schemes while maintaining a bulk temperature below 37C. The inactivation of E. coli was a function of the pulse duration and the number of pulses. The destruction of Escherichia coli in liquid egg followed a first order kinetic and the treatment was more effective when the applied pulses were of a 4 its pulse duration. A 6D reduction was obtained for viable E. coli using both pulsing rates and treatment schemes with no protein coagulation.

Shielding properties of laser-induced breakdown in water for pulse durations from 5 ns to 125 fs

Abstract: The shielding effectiveness of laser-induced breakdown from focused, visible laser pulses from 5 ns to 125 fs is determined from measurements of transmission of energy through the focal volume. The shielding efficiency decreases as a function of pulse duration from 5 ns to 300 fs and increases from 300 fs to 125 fs. The results are compared with past studies at similar pulse durations. The results of the measurements support laser-induced breakdown models and may lead to an optimization of laser-induced breakdown in ophthalmic surgery by reduction of collateral effects.

0.2-TW laser system at 1kHz.

Abstract: We have developed a 1-kHz repetition-rate Ti:sapphire laser system that can simultaneously generate high peak and average powers of 0.2 TW and 4 W, respectively. The laser system generates 4-mJ energy pulses with a 20-fs pulse width. We eliminated thermal lensing in the system by cooling the Ti:sapphire crystal to 125 K. The output 20-fs pulses were fully characterized by use of the new technique of transient-grating frequency-resolved optical gating. We demonstrate experimentally that the pulse duration at the output is limited only by fifth-order dispersion.

Propagation of damselfish (Pomacentridae) courtship sounds

Abstract: Many damselfishes perform a courtship behavior known as the signal jump, in which they rise in the water column and then rapidly swim downward while producing a pulsed sound. Pulsed sounds produced during the signal jump of the damselfish Dascyllus albisella were analyzed to determine whether they were correlated with the signal jump distance or speed, and how they changed with propagation. No statistical relationship was found between signal jump speed or distance with the number of pulses, pulse period, or change in the peak frequency of pulses in a call. If echoes were present in the sound, the change in echo delay would likely have been too small for damselfish to detect.Sounds attenuated with distance such that the signal-to-noise ratio decreased from 17–25 dB at 1 to 2 m to 5–10 dB at 11 to 12 m. It is unlikely that D. albisella can detectsounds at or beyond 11–12 m from the sound source, based on noise masking data from other fishes. Pulse period is least affected by propagation when compared to peak frequency, pulse duration, interpulse interval, and coefficient of variation of pulse amplitudes within a call. These results suggest that the sound produced during the signal jump functions over short distances and that the pulse period provides the most reliable basis for signal identification.

A Saturated X-ray Laser Beam at 7 Nanometers

Abstract: A saturated nickel-like samarium x-ray laser beam at 7 nanometers has been demonstrated with an output energy of 0.3 millijoule in 50-picosecond pulses, demonstrating that saturated operation of a laser at wavelengths shorter than 10 nanometers can be achieved. The narrow divergence, short wavelength, short pulse duration, high efficiency, and high brightness of this samarium laser make it an ideal candidate for many x-ray laser applications.

Laser-based microfocused x-ray source for mammography: feasibility study.

Abstract: A laser-produced plasma (LPP) x-ray source with possible application in mammography was created by focusing a laser beam on a Mo target. A Table-Top-Terawatt (TTT) laser operating at 1 J energy per pulse was employed. A dual pulse technique was used. Maximum energy transfer (approximately 10%) from laser light to hot electrons was reached at a 150 ps delay between pulses and the conversion efficiency (hard x-ray yield/laser energy input) was approximately 2 x 10(-4). The created LPP x-ray source is characterized by a very small focal spot size (tens of microns), Gaussian brightness distribution, and a very short pulse duration (a few ps). The spectral distribution of the generated x rays was measured. Images of the focal spot, using a pinhole camera, and images of a resolution pattern and a mammographic phantom were obtained. The LPP focal spot modulation transfer function for different magnification factors was calculated. We have shown that the LPP source in conjunction with a spherically bent, high throughput, crystal monochromator in a fixed-exit Rowland circle configuration can be used to created a narrow band tunable mammography system. Tunability to a specific patient breast tissue thickness and density would allow one to significantly improve contrast and resolution (exceeding 20 lp/mm) while lowering the exposure up to 50% for thicker breasts. The prospects for the LPP x-ray source for mammographic application are discussed.

Wideband acoustic transducer

Abstract: A transducer according to various aspects of the present invention provides high fractional bandwidth with relatively low degradation of the pulse duration and sensitivity. The transducer includes a back matching layer behind the transducer material. The back matching layer is characterized by an impedance selected to transmit a selected portion of the backwards propagating acoustic energy to an absorption layer. The remaining acoustic energy is reflected in the desired direction of propagation. As a result, the transducer provides enhanced bandwidth without excessive loss of sensitivity or increase in pulse duration.

Thermal nonlinear refraction in dye solutions: a study of the transient regime

Abstract: The efficiency of thermal nonlinear refractive effects in the nanosecond regime is discussed. A single-beam Z-scan and a two-color time-resolved Z-scan experiments demonstrate the influence of the geometric configuration and the pulse duration on the photoinduced index variation. We present a theoretical analysis of thermal-lens formation that takes into account acoustic wave propagation and thermal diffusive effects. It permits us to quantify the index variation during the pulse and to define a new figure of merit for thermal nonlinear refractive effects.

Method for measuring RF pulse rise time, fall time and pulse width

Abstract: A method for measuring the rise/fall time and pulse width of RF pulses using multi-purpose, commercial-off-the-shelf test devices, such as an RF signal down converter, a digitizer and a signal processor. The method is based on digitizing the RF signals and developing an average sample pulse waveform. The rise/fall time and pulse width are then calculated from data points on the sample waveform. The method is suitable for real-time calculations of the rise/fall time and pulse width.

Diode-pumped thin-disk Yb:YAG regenerative amplifier

Abstract: . A passively modelocked 750-fs Yb:YAG oscilla-tor is used as the seed laser for a diode-pumped thin diskYb:YAG regenerative amplifier. Pulse energies of 180mJ areobtained at repetition rates of up to 750Hz,and120mJ pulseswere achieved at 1kHz. The amplified pulse duration was2 : 3ps, resulting in a pulse peak power of 78MW.Thein-crease in pulse duration during amplification is attributed togain narrowing in the amplifier material rather than to intra-cavity dispersion.PACS: 42.60; 42.55; 42.65Compact sources of ultrashort laser pulses at microjoule tomillijoule energy levels are of interest for several applica-tions, such as medical surgery, industrial machining, or non-linear frequency conversion. The development of powerfullaser diodes, matching the absorption wavelengths of manysolid state materials, made it possible to build compact andefficient modelocked oscillators and regenerative amplifiers.Typical pulse durations of diode-pumped passively mode-locked oscillators range from below 100fs to a few ps [1].However, most experiments on diode-pumped regenerativeamplifiers have been performed in the regime of 10 ps andmore. For example, pulse energies of 90mJ and 750 mJ wereextracted from cw diode-pumped Nd:YLF regenerative am-plifiers with 2Wand 15 W of pump power, respectively[2,3].Furthermore, 2

14-fs pulse generation in Kerr-lens mode-locked prismless Cr:LiSGaF and Cr:LiSAF lasers: observation of pulse self-frequency shift

Abstract: Kerr-lens mode-locked Kr-laser-pumped Cr:LiSGaF and Cr:LiSAF lasers containing only two newly developed low-loss chirped mirrors instead of conventional dielectric resonator mirrors and generating pulses of widths as small as 14-fs with as much as 100 mW of average output power are reported We report what is to our knowledge the first experimental observation of a pulse self-frequency shift in crystalline active media that are characteristic of such short pulses at megawatt intracavity peak powers We also believe this phenomenon to be one of the significant limiting factors for pulse duration in femtosecond Cr:LiSAF-type lasers

Apparatus and method for femtosecond pulse compression based on selective attenuation of a portion of an input power spectrum

Abstract: An apparatus for, and method of, decreasing a temporal duration of an optical input pulse having an input power spectrum associated therewith, the apparatus and method operative on at least unchirped optical input pulses. The apparatus comprises: (1) a zero-dispersion pulse shaper for receiving the optical input pulse and spectrally spreading the input pulse to produce a spectrally-spread pulse and (2) a modulator array interposed within the zero-dispersion pulse shaper for receiving the spectrally-spread pulse and selectively attenuating a portion of the power spectrum of the spectrally-spread pulse by a predetermined amount to produce a selectively-attenuated spectrally-spread pulse, the zero-dispersion pulse shaper focusing and recombining the selectively-attenuated spectrally-spread pulse to produce an output pulse having a broader power spectrum than the input pulse, the output pulse further having a temporal duration less than the input pulse, regardless of whether the input pulse is chirped.

Equalization of series-connected cells of a battery using controlled charging and discharging pulses

Abstract: A charge pulse (200A) is applied to the battery (B). The open circuit voltage of each cell (C1-CN) is then measured during a first rest period (210A). A depolarization pulse (220A) is then applied to the battery. The open circuit voltage of each cell is then measured during a second rest period (210B). The open circuit voltages for the first and second rest periods for each cell are compared to yield a voltage difference (DELTA Y). This voltage difference is then compared with a threshold voltage (V THRESHOLD). If the voltage difference is greater than the threshold voltage then the cell is being charged too rapidly, or is being overcharged, so one or more of the charge cycle parameters are adjusted. Some of the charge cycle parameters are, for example, the charge pulse current amplitude, the charge pulse duration, the number of consecutive charge pulses, the depolarization pulse current amplitude, the depolarization pulse duration, the number of consecutive depolarization pulses, the duration of the rest periods, or a combination of these. Multiple charge pulses (200A, 200B) and depolarization pulses (220A, 220B, 220C) may also be used. These parameters may be adjusted for the battery as a whole and/or for each cell individually. The adjustment to the charge cycle parameters may also be made on a "worst case" or a "best case" cell basis, with this cell controlling the charge cycle parameters for all the cells. Several techniques for adjusting the charge cycle parameters are disclosed.

Dispersion-managed soliton dynamics.

Abstract: We obtain and solve algebraic eigenvalue equations that predict the dependence of the pulse energy of a dispersion-managed soliton on pulse duration, chirp, and dispersion-map parameters. We demonstrate that a variational ansatz based on a Gaussian pulse shape remains useful even when the actual pulse shape is not Gaussian, and we show that the enhancement factor saturates as the pulse duration decreases.

Delta-sigma pulse width modulator control circuit

Abstract: A delta sigma pulse width modulator control circuit uses a delta sigma modulator as a first stage to create a sequence of pulses representing an input control signal. A pulse width modulator accumulates the sequence of pulses and defines a pulse width modulated output signal from the accumulated pulses. The pulse width modulated signal is given a randomly generated offset to the time of pulse value transition and an adjacent pulse value matching technique is used to reduce harmonically related noise generation.

Compact 1000 pps high-voltage nanosecond pulse generator

Abstract: A compact high-voltage nanosecond generator is described with pulse repetition rate of up to 1000 pps. The generator includes a 30-/spl Omega/ coaxial forming line charged by a built-in Tesla transformer with high coupling coefficient, and a high voltage (N/sub 2/) gas gap switch with gas circulating between the electrodes. The maximum forming line charge voltage is 450 kV, the pulse duration is /spl sim/4 ns, and its amplitude for a matched load is up to 200 kV. The generator has been applied to create powerful sources of ultrawide-band electromagnetic radiation and nanosecond microwave pulses.

Superradiance in a Short-Pulse Free-Electron-Laser Oscillator

Abstract: We demonstrate that superradiance (SR) is a general feature of free-electron-laser (FEL) devices, incuding high gain amplifiers as well as low gain oscillators, and that SR pulses and spiking, due to synchrotron instabilities, have the same origin. In particular, we present a direct observation of SR from an FEL oscillator. The ultrashort SR pulses are characterized by a peak intensity and pulse duration which depend, respectively, on the square and the inverse square root of the electron current. The spectral brightness, $B$, defined as the ratio between the efficiency and the relative rms spectral width, is constant and equal to $0.86$.

Cantilever magnetometry in pulsed magnetic fields

Abstract: The technique of cantilever magnetometry is shown to be functional in pulsed magnetic fields. Employing micromachined single crystal silicon cantilevers and capacitance detection, we demonstrated a utilizable sensitivity to magnetic moment of 2.5×10−12 Am2 in magnetic fields to 36 T, representing an improvement of more than a factor of 10 over competing technologies. Torque magnetization measurements on microcrystals of anisotropic superconductors are presented as evidence of the feasibility of the technique in long pulse magnets of pulse duration ∼0.1–1 s.