Showing papers on "Pulse duration published in 1989"

Er:YAG laser ablation of tissue: effect of pulse duration and tissue type on thermal damage.

Abstract: The thermal damage caused by 2.94-micron Er:YAG laser ablation of skin, cornea, aorta, and bone was quantified. The zone of residual thermal damage produced by normal-spiking-mode pulses (pulse duration approximately 200 microseconds) and Q-switched pulses (pulse duration approximately 90 ns) was compared. Normal-spiking-mode pulses typically leave 10-50 microns of collagen damage at the smooth wall of the incisions; however, at the highest fluences (approximately 80J/cm2) tears were produced in cornea and aorta and as much as 100 microns of damaged collagen is found at the incision edge. Q-switched pulses caused less thermal damage, typically 5-10 microns of damage in all tissues.

Actively mode-locked semiconductor lasers

Abstract: Measurements of actively mode-locked semiconductor lasers are described and compared to calculations of the mode-locking process using three coupled traveling-wave rate equations for the electron and photon densities. The dependence of pulse width on the modulation current and frequency are described. A limitation to minimum achievable pulse widths in mode-locked semiconductor lasers is shown to be dynamic tuning due to gain saturation. Techniques to achieve subpicosecond pulses are described, together with ways to reduce multiple pulse outputs. The amplitude and phase noise of linear- and ring-cavity semiconductor lasers were measured and fond to be tens of dB smaller than YAG and argon lasers and limited by the noise from the microwave oscillator. High-frequency phase noise is only measurable in detuned cavities, and is below -110 dBc (1 Hz) in optimally tuned cavities. The prospects for novel ways to achieve even shorter pulses are discussed. >

Microcomputer-based nerve and muscle stimulator

Abstract: A transcutaneous nerve stimulation device is provided with mircocomputer-based control of pulse duration, pulse repetition rate, stimulation duty cycle and modulation of pulse parameters. The microcomputer 2 monitors battery 15 voltage and patient manipulated controls 14 that select pulse current amplitude and also provides signals used for display on a liquid crystal display 10 of all relevant stimulation parameters and warning annuciators. The microcomputer 2 is sensitive to input controls for calculation of all stimulation parameters, other than current amplitude, only when a program control switch 18 is active. When the program control switch 18 becomes inactive, the selected control information is stored in non-volatile memory 4, thereby providing electronic isolation of programming controls. The preferred embodiment of the nerve stimulation device may be used either for pain relief or to facilitate muscle contractions, which has a variety of therapeutic applications.

A quantum mechanical study of predissociation dynamics of NaI excited by a femtosecond laser pulse

Abstract: This work was stimulated by the experiments of Rosker, Rose, and Zewail (RRZ) who used a pump–probe sequence of femtosecond pulses to study the predissociation of NaI. We calculate quantum mechanically the nuclear wave function created by the pump pulse and its subsequent evolution. To make contact with the experiments we assume that, depending on its wavelength, the probe pulse is absorbed by either the free sodium atoms or by the bound molecule in the neutral state, and that the measured fluorescence induced by the probe pulse is proportional to the populations of these two ‘‘species.’’ The similarity between computed populations and the observed signal confirms this conjecture. We study how various factors, such as the pulse length and shape, the initial vibrational state, the temperature, and the diabatic coupling strength affect the populations (and thus the LIF signal). We also show that the RRZ analysis based on classical trajectories and the Landau–Zener formula agrees semiquantitatively with the exact quantum results.

Short-pulse and high-frequency signal generation in semiconductor lasers

Abstract: The application of high-speed semiconductors lasers in short-pulse generation by mode locking, gain/Q-switching, and microwave signal generation is reviewed. The underlying basic principles, current state of the art, and areas for further developments are discussed. >

A novel method for simulating laser-solid interactions in semiconductors and layered structures

Abstract: We have developed a new implicit finite difference method to simulate the interaction of intense nanosecond laser beams with semiconductors and metal-coated ceramic structures. This method is based upon a higher order implicit finite difference scheme with a smaller truncation error and is not restricted by any stability criterion, thereby allowing faster convergence to the exact solution. The temperature-dependent optical and thermal properties of the irradiated material as well as the temporal variation in the laser intensity have been taken into account. Finite difference equations have been set up for accurate determination of the temperature gradients at the liquid-solid interface, which control the melt-in and resolidification velocities. A new formulation is introduced to accomodate the effect of pulsed laser irradiation on layered composite structures (e.g. metal-coated ceramics) by incorporating the boundary conditions at the composite interface. Using this method, the thermal histories of laser-irradiated materials were predicted. The effects of variation in the pulse energy density, pulse duration and substrate temperature on the maximum melt depths, solidification velocities and surface temperatures were computed. The calculations on the depth of melting were found to be in good agreement with experimental results where complete annealing of the ion implantation damage was used as a measure of the melt depth. The surface temperatures and melt lifetimes in metal-coated ceramics were determined in order to understand the laser mixing process. Simple energy balance considerations were applied to calculate some of the effects of laser irradiation on materials. From these energy considerations, the maximum melt depths as a function of energy density, pulse duration and substrate temperature were obtained and compared with the exact solutions. The maximum surface temperatures, solidification velocities and melt lifetimes were also determined by this analytical method and compared with the detailed calculations. A good agreement between the analytical relations and the detailed numerical calculations provides an excellent guide to researchers in this field.

Tube model of field-inversion electrophoresis.

Abstract: The dynamics of long chain macromolecules in field-inversion gel electrophoresis is studied by Monte Carlo simulation using a reptation model which includes tube length fluctuations. A sharp minimum in chain mobility is observed as the pulse duration is varied, occurring at longer pulse periods for larger chains. Detailed examination of the dynamics shows that the reduced mobility results from the chain being trapped in long-lived compact configurations which have no net translational motion.

Transcutaneous electric nerve stimulater

Abstract: In a transcutaneous electric nerve stimulater in which a frequency of an output pulse varies with a lapse of time, a pulse width of the output pulse is increased when the frequency becomes to be smaller and is reduced when the frequency becomes to be greater. Furthermore, a pulse amplitude of the output pulse is increased when the frequency becomes to be smaller and is minimized when the frequency becomes to be greater.

Light amplifier and method of photothemolysis

Abstract: A tunable dye laser has been found particularly suited to selective photothermolysis A longer pulse duration which makes the system suitable for a wider range of applications is obtained by modifying the laser to generate a spatially noncoherent beam The optical system at each end of the laser cell, which may include a lens or spherical mirror, refocuses the aperture of the dye cell near to itself so that substantially all light emanating from the dye cell is returned to the dye cell until the light passes through one of the optic systems as a noncoherent laser beam A tunable intracavity element tunes the laser across the gain curve of the dye solution The pulse duration of the laser beam can be selected from a range of durations up to about one millisecond

Ultrahigh-intensity KrF* laser system.

Abstract: The operational characteristics of an ultrahigh-intensity subpicosecond large-aperture KrF* laser system are described. Measurements show the achievement of a focal spot diameter of less than 1.7 μm. Combined with measurements of the pulse width and pulse energy, this yields an average intensity of ~2 × 1019 W/cm2, a value corresponding to a peak electric field of ~24 (e/a02). Light sources of this nature will find application in a broad range of studies of the nonlinear properties of matter in the strong-field regime.

Classification of beam breakup instabilities in linear accelerators.

Abstract: With the use of a standard model, beam breakup growths in all types of linacs are shown to be classified by only two dimensionless parameters which depend on the beam current, beam energy, pulse length, machine length, breakup mode frequency, focal strength, and transverse shunt impedance. This classification suggests that rf cure plays a more important role in the control of beam breakup than focusing in a long pulse beam. The converse holds for a short pulse beam.

Method of and apparatus for analyzing propagation of arterial pulse waves through the circulatory system

Abstract: Method and apparatus for analyzing the propagation of arterial pulse waves in a patient's circulatory system. The apparatus comprises R-wave detecting means for outputting an R-wave timing signal at the time of detection of the R-wave component contained in a detected electrocardiographic signal. A pulse wave peak detecting means is provided for outputting a peak timing signal at the time of the detection of a selected peak of the arterial pulse wave detected in the peripheral blood vessels of a patient's circulatory system. A pulse wave propagation time counter is provided for successively counting the time interval from the input of the R-wave timing signal to the input of the peak timing signal, and a pulse wave propagation time analyzer is provided for analyzing the distribution of pulse wave propagation times and other detected biophysiological parameters, from which an effective diagnosis of the condition of the patient's circulatory system can be made.

Gain-switched pulsed operation of microchip lasers.

Abstract: Microchip lasers have produced single-frequency, gain-switched pulses with a FWHM as short as 760 psec for Nd:YAG (NdxY3−xAl5O12) and 80 psec for LiNdP4O12 (LNP).

Measurement of intense ultraviolet subpicosecond pulses using degenerate four-wave mixing

Abstract: Background-free measurement of the third-order intensity autocorrelation function of ultraviolet subpicosecond pulses is demonstrated. The method provides the pulse duration and additional information about asymmetry and other interesting structural properties of the pulses. It is possible to obtain the complete autocorrelation function from a single pulse. Measurements of subpicosecond pulses from XeCl excimer laser amplifiers reveal a distinct satellite structure, which can be attributed to a quantum beat effect of the XeCl lasing transitions. >

Vela X-1 pulse timing. II - Variations in pulse frequency

Abstract: The pulsed X-ray emission of Vela X-1 during May 1978 and December-January 1978-1979 is investigated analytically on the basis of published satellite observations. The data are compiled in tables and graphs and discussed in detail, with reference to data for the entire 1975-1982 period. Variations in pulse frequency are identified on time scales from 2 to 2600 days; the lower nine octaves are characterized as white noise (or random walk in pulse frequency), while the longer-period variations are attributed to changes in neutron-star rotation rates.

High temporal resolution optical instrument

Abstract: An optical time-domain reflectometry (OTDR) system comprises an oscillator, a phase shifter, an amplifier, an optical pulse generator, an integrated electro-optical circuit, a detector, a timer, a converter, and an output device. The OTDR system can be used to determine which surface of a photographic lens is impairing transmissivity the most. When the amplifier is enabled, a signal from the oscillator starts the timer and triggers the pulse generator. The timer is a time-to-digital circuit with analog interpolation. The resulting pulse is transmitted through the integrated electro-optical circuit and into the lens being evaluated. The strongest reflection from the lens is detected and the resulting detection signal stops the timer. The timed duration is expressed by the converter as a distance which can be used to identify the lens surface of interest. The result is displayed and/or stored on the output device. An electrical waveform from the oscillator is conveyed along a transmission line defined by electrodes of the integrated electro-optical circuit. This waveform introduces phase distortion in the optical pulse conveyed along an optical path of the integrated electro-optical circuit. The phase shifter is programmed so that these phase distortions are approximately quadratic over the pulse duration, so that the frequency of the pulse is "swept" linearly. The integrated electro-optical circuit includes a dispersive grating which introduces time delay as a function of frequency. The combination of sweeping and dispersing results in a pulse which is compressed relative to its original form. The compressed pulse allows more precise identification of the surface causing the strongest reflection so that modern multi-element lenses can be effectively evaluated.

Mode locking of a continuous wave Nd:glass laser pumped by a multistripe diode laser

Abstract: The performance of a continuous wave actively mode‐locked Nd: phosphate glass laser longitudinally pumped by a multistripe diode laser is described. The laser operates at 1.054 μm; the pump threshold and the slope efficiency are found to be 120 mW and 11%, respectively. The shortest pulse duration is 7 ps, which appears to be approximately twice as short as predicted by the theory of amplitude modulation mode locking. We explain the improved performance by additional frequency modulation due to the nonlinear index of the active material.

Device and method to measure a short radiation pulse or an electric pulse

Abstract: A device for measuring a single short pulse includes at least one measuring unit (D1, Dn) comprising a conductive line (L) connected to a set of photoconductors (A, B1-Bp), the line and photoconductors being placed between two nonconductors forming a single support in which the length of the line (x1) separating the photoconductors two-by-two is equal to the product of the propagation speed on the line with respect to the duration of the pulse concerning the number of measurement points, the lifetime of the majority carriers of the photoconductors being selected as being equal to or less than 10% of the duration of the pulse. The device makes it possible to obtain a temporal analysis or the autocorrelation of the pulse which may be an electromagnetic radiation or ionizing pulse or an electric pulse.

Interferential mode locking: Gaussian pulse analysis.

Abstract: A simple analysis of the interference between two Gaussian pulses is presented. It is shown that the resulting pulse may be shorter than the initial ones if the interference phase is properly adjusted. The compression effect results from the difference of the durations of the interfering pulses and from their phase modulation. This mechanism may be implemented in mode-locked lasers, either by adjunction of a coupled cavity or use of a pulse-shaping Michelson interferometer as the output coupler. Gain-bandwidth-limited pulses may then be obtained. The model provides analytical expressions for pulse duration.

Observation of enhanced x‐ray emission from long‐pulse‐width laser‐produced plasmas

Abstract: Experiments investigating the pulse‐width dependence of thermal x‐ray conversion efficiencies (hν<1.5 keV) in laser‐heated gold plasmas are described. The results show that the instantaneous ratio of the emitted x‐ray flux to the laser energy deposition rate increases throughout a 4‐ns laser pulse. The studies were carried out using single arms of the 10‐beam Nova laser facility. The irradiance was ∼4–5×1014 W/cm2 in the target plane as we varied the laser pulses’ FWHM from 2 to 4 ns. The laser pulses were nominally flat‐topped and contained between 1 and 2 kJ of (1)/(3) μm light. Time‐resolved plasma radiation was monitored with a broadband, streaked x‐ray spectrograph that has a roughly 30‐ps time resolution and channels that are roughly 100–150 eV wide. One‐dimensional numerical models run with the lasnex code produce a conversion efficiency that is nearly constant throughout the laser pulse. We discuss various approximations made in the one‐dimensional models and conclude that none of them are a likel...

Application of a phase-conjugate Brillouin mirror to generation of high-quality variable-duration KrF pulses

Abstract: Phase conjugation by Brillouin scattering in SF/sub 6/ gas has been implemented on a two-pass KrF amplifier to provide near-diffraction-limited output pulses having a very low background level of amplified spontaneous emission. The pulse duration is variable between 1 and 10 ns by simply attenuating the input pulse. These pulses have also been used for the generation of much shorter duration pulses by truncated Brillouin scattering at the surface of cyclohexane. >

Frequency modulation mode-locking and Q-switching of diode-laser-pumped Nd:YLF laser

Abstract: Using frequency modulation (FM) mode-locking techniques, and pumping with a 500 mW laser diode we have developed a Nd: YLF (yttrium lithium fluoride) laser operating at 1053nm that produces 49 mW average power in pulse durations of 9ps. When this system is Q-switched the energy within the 90 ns pulse envelope is 17µJ. The average mode-locked pulse duration within this envelope is 21 ps giving rise to a peak power of 22 kW.

Multiterawatt excimer-laser system

Abstract: A peak power of 4 TW has been obtained in KrF with an output energy of 1.5 J in 390 fsec from an electron-beam-pumped amplifier. The energy of the amplified spontaneous emission was as low as 1.8%. The pulse width was measured by the newly developed third-order autocorrelation technique based on the XeF C–A transition. Amplifiers increased the initial pulse width of 210 fsec to 390 fsec. The process of pulse-width broadening was investigated in detail. It was confirmed that the two-photon process contributes to small absorption in CaF2 by the luminescence of self-trapped excitons. A terawatt-class KrF laser system with a moderate repetition rate was developed for applications to multiphoton processes.

Nonlinear resonant absorption effects on the design of resonance fluorescence lidars and laser guide stars

Abstract: Saturation effects may significantly reduce the backscattered signal expected for resonance fluorescence lidar systems. Saturation arises when the laser energy density within the layer is large enough to significantly alter the population densities of the atomic states within the layer. These altered state populations lead to nonlinear absorption of the laser energy resulting in reduced rate of fluorescence and increased rate of stimulated emission. The level of saturation is determined by the laser pulse length, pulse energy, beam-width, and linewidth. Analysis reveals that the saturation effects can be characterized by two parameters: the saturation time τs and the rms laser linewidth Δωrms.The saturation time is the characteristic time of stimulated emission and is an indication of the level of saturation. The saturation time depends on pulse energy, pulse length, and beamwidth. The laser linewidth is important since it determines the effective absorption cross section of the atoms and also the percentage of atoms illuminated for an inhomogeneously broadened absorption line. The impact of saturation on lidar design is also examined. Design examples, including lidar systems for atmospheric research and laser guide stars for adaptive imaging applications in astronomy are studied in detail.

A study of the transient response of position-sensitive detectors

Abstract: A distributed RC transmission-line approximation is used to analyze the transient response of a conventional Position Sensitive Detector (PSD). We develop a generalized mathematical model for different kinds of excitations illuminated on the PSD. The closed form expressions derived allow expedient evaluation of candidate PSDs capable of yielding fast time-resolved information for the spatial position measurement of short duration beams. Based on these calculations, it is shown that for very short pulse excitations having duration shorter than the time constant (Tc) of the PSD, the characteristic delay time (Tch) must be known for distortionless measurement of the spatial position. It is also found from calculated results, for the pulse duration ( T p T c ) 0.02 that, Tch is 0.26. Model predictions compare favorably with observations.

Coupled cavity modelocking of a Nd:YAG laser using second harmonic generation

Abstract: The modelocking of a Nd:YAG laser using second-harmonic generation in an external cavity as the only pulse shortening process is reported. The output of a long pulse Nd:YAG laser was simultaneously modelocked and Q-switched. The average pulse duration was 30-50 ps and the peak power was in the range 60-160 kW

Method and apparatus for print image enhancement

Abstract: A method for enhancing a displayed image of a display apparatus which produces a digitized image in a dot matrix format comprising: generating a first digital data signal comprising unitary signal pulses representative of continuous line portions of a character to be displayed; and generating a second digital data signal having sets of pulses corresponding to each unitary signal pulse in the first data signal, each set of pulses in the second data signal comprising a leading pulse and a trailing pulse, the leading pulse in each set of pulses being of equal duration and equal interval spacing from the immediately following pulse, the trailing pulse in each set of pulses being of equal duration and of equal interval spacing from the immediately preceding pulse.

KrF laser system with corrected pulse front and compressed pulse duration

Abstract: A high-power KrF laser system is presented providing 45 mJ, 150 fs pulses at the position of a target. Pulse front distortion is avoided by specially designed refractive optics. Pulse compression is done after final amplification using a simplified compressor.