Showing papers on "Pulse duration published in 2000"

Pulse oximetry pulse indicator

Abstract: An intelligent, rule-based processor provides a pulse indicator designating the occurrence of each pulse in a pulse oximeter-derived photo-plethysmograph waveform. When there is relatively no distortion corrupting the plethysmograph signal, the processor analyzes the shape of the pulses in the waveform to determine where in the waveform to generate the pulse indication. When distortion is present, looser waveform criteria are used to determine if pulses are present. If pulses are present, the pulse indication is based upon an averaged pulse rate. If no pulses are present, no indication occurs. The pulse indicator provides a trigger and amplitude output. The trigger output is used to initiate an audible tone “beep” or a visual pulse indication on a display, such as a vertical spike on a horizontal trace or a corresponding indication on a bar display. The amplitude output is used to indicate data integrity and corresponding confidence in the computed values of saturation and pulse rate. The amplitude output can vary a characteristic of the pulse indicator, such as beep volume or frequency or the height of the visual display spike.

Ionized sputter deposition using an extremely high plasma density pulsed magnetron discharge

Abstract: Time resolved plasma probe measurements of a novel high power density pulsed plasma discharge are presented. Extreme peak power densities in the pulse (on the order of several kW cm−2) result in a very dense plasma with substrate ionic flux densities of up to 1 A cm−2 at source-to-substrate distances of several cm and at a pressure of 0.13 Pa (1 mTorr). The pulse duration was ∼100 μs with a pulse repetition frequency of 50 Hz. The plasma consists of metallic and inert gas ions, as determined from time resolved Langmuir probe measurements and in situ optical emission spectroscopy data. It was found that the plasma composition at the beginning of the pulse was dominated by Ar ions. As time elapsed metal ions were detected and finally dominated the ion composition. The effect of the process parameters on the temporal development of the ionic fluxes is discussed. The ionized portion of the sputtered metal flux was found to have an average velocity of 2500 m s−1 at 6 cm distance from the source, which conforms...

Energy efficient, laser-based method and system for processing target material

Abstract: An energy-efficient method and system for processing target material such as microstructures in a microscopic region without causing undesirable changes in electrical and/or physical characteristics of material surrounding the target material is provided. The system includes a controller for generating a processing control signal and a signal generator for generating a modulated drive waveform based on the processing control signal. The waveform has a sub-nanosecond rise time. The system also includes a gain-switched, pulsed semiconductor seed laser for generating a laser pulse train at a repetition rate. The drive waveform pumps the laser so that each pulse of the pulse train has a predetermined shape. Further, the system includes a laser amplifier for optically amplifying the pulse train to obtain an amplified pulse train without significantly changing the predetermined shape of the pulses. The amplified pulses have little distortion and have substantially the same relative temporal power distribution as the original pulse train from the laser. Each of the amplified pulses has a substantially square temporal power density distribution, a sharp rise time, a pulse duration and a fall time. The system further includes a beam delivery and focusing subsystem for delivering and focusing at least a portion of the amplified pulse train onto the target material. The rise time (less than about 1 ns) is fast enough to efficiently couple laser energy to the target material, the pulse duration (typically 2-10 ns) is sufficient to process the target material, and the fall time (a few ns) is rapid enough to prevent the undesirable changes to the material surrounding the target material.

Origin of retinal pigment epithelium cell damage by pulsed laser irradiance in the nanosecond to microsecond time regimen.

Abstract: Background and Objective: Selective photodamage of the retinal pigment epithelium (RPE) is a new technique to treat a variety of retinal diseases without causing adverse effects to surrounding tissues such as the neural retina including the photoreceptors and the choroid. In this study, the mechanism of cell damage after laser irradiation was investigated. Study Design/Materials and Methods: Single porcine RPE-melanosomes and RPE cells were irradiated with a Nd:YLF laser (wavelength l = 527 nm, adjustable pulse duration t = 250 nsec-3 msec) and a Nd:YAG laser (l = 532 nm, t = 8 nsec). Fast flash photography was applied to observe vaporization at melanosomes in suspension. A fluorescence viability assay was used to probe the cells vitality. Results: The threshold radiant exposures for vaporization around individual melanosomes and for ED50 cell damage are similar at 8-nsec pulse duration. Both thresholds increase with pulse duration; however, the ED50 cell damage radiant exposure is 40% lower at 3msec. Temperature calculations to model the onset of vaporization around the melanosomes are in good agreement with the experimental results when assuming a surface temperature of 150°C to initiate vaporization and a homogeneous melanosome absorption coefficient of 8,000 cm ˛1 . Increasing the number of pulses delivered to RPE cells at a repetition rate of 500 Hz, the ED50 value decreases for all pulse durations. However, the behavior does not obey scaling laws such as the N 1/4 equation. Conclusion: The origin of RPE cell damage for single pulse irradiation up to pulse durations of 3 msec can be described by a damage mechanism in which microbubbles around the melanosomes cause a rupture of the cell structure. The threshold radiant exposure for RPE damage decreases with increasing number of pulses applied. Lasers Surg. Med. 27:451‐464, 2000. © 2000 Wiley-Liss, Inc.

Effects of fish size and temperature on weakfish disturbance calls: implications for the mechanism of sound generation.

Abstract: To categorize variation in disturbance calls of the weakfish Cynoscion regalis and to understand their generation, we recorded sounds produced by differentsized fish, and by similar-sized fish at different temperatures, as well as muscle electromyograms. Single, simultaneous twitches of the bilateral sonic muscles produce a single sound pulse consisting of a two- to threecycle acoustic waveform. Typical disturbance calls at 18 °C consist of trains of 2‐15 pulses with a sound pressure level (SPL) of 74 dB re 20 μPa at 10 cm, a peak frequency of 540 Hz, a repetition rate of 20 Hz and a pulse duration of 3.5 ms. The pulse duration suggests an incredibly short twitch time. Sound pressure level (SPL) and pulse duration increase and dominant frequency decreases in larger fish, whereas SPL, repetition rate and dominant frequency increase and pulse duration decreases with increasing temperature. The dominant frequency is inversely related to pulse duration and appears to be determined by the duration of muscle contraction. We suggest that the lower dominant frequency of larger fish is caused by a longer pulse (=longer muscle twitch) and not by the lower resonant frequency of a larger swimbladder.

Generation of terawatt pulses by use of optical parametric chirped pulse amplification

Abstract: Optical parametric chirped pulse amplifiers offer exciting prospects for generating new extremes in power, intensity, and pulse duration. An experiment is described that was used to investigate the operation of this scheme up to energies approaching a joule, as a step toward its implementation at the petawatt level. The results demonstrate an energy gain of 1010 with an energy extraction efficiency of 20% and close to diffraction-limited performance. Some spectral narrowing during amplification was shown to be compatible with the time-varying profile of the pump beam and consistent with the measured recompressed pulse durations of 260 and 300 fs before and after amplification, respectively.

Switching behavior of a Stoner particle beyond the relaxation time limit

Abstract: Results of the switching properties of Stoner-like magnetic particles, subject to short magnetic field pulses, obtained by numerical investigations, are reported. The switching properties are discussed as a function of the external field pulse strength and direction, pulse length, and the pulse shape. For field pulses long compared to the precession time, the switching behavior is governed by the magnetic damping term. In the limit of short field pulses, switching properties are dominated by the details of the magnetic precession. In the latter case, the magnetic damping term is of minor importance and ultrafast switching can be achieved by choosing the right field pulse parameters. It is also possible to choose pulse parameters in order to provide switching over a wide range of applied field directions.

From above threshold ionization to statistical electron emission: the laser pulse-duration dependence of C60 photoelectron spectra

Abstract: The photoelectron spectra of C60 ionized using a 790 nm laser with pulse durations varying from 25 fs to 5 ps have been determined. For 25 fs pulses, in the absence of fragmentation, the ionization mechanism is direct multiphoton ionization with clear observation of above threshold ionization. As the pulse duration is increased, this becomes dominated by a statistical ionization due to equilibration among the electronic degrees of freedom. For pulse durations on the order of a ps coupling to the vibrational degrees of freedom occurs and the well-known phenomenon of delayed (ms) ionization is observed.

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation.

Abstract: We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency-resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction.

Nano-oxidation of silicon surfaces by noncontact atomic-force microscopy: Size dependence on voltage and pulse duration

Abstract: Local oxidation of silicon surfaces by noncontact atomic-force microscopy is an emerging and promising method for patterning surfaces at the nanometer scale due to its very precise control of the feature size. Here, we study the voltage and pulse duration conditions to generate a motive of a given height with the minimum lateral size. We find that for a fixed tip–sample separation, the combination of short pulses and relatively high voltages (∼20 V) produces the highest height:width ratio. The application of relatively high voltages produces a fast growth rate in the vertical direction while the lateral diffusion of oxyanions is inhibited for short pulses. The above results are applied to generate lines of tens of microns in length with an average width at half maximum of about 10 nm.

Pulsed source scanning interferometer

Abstract: A pulsed light source in used conjunction with a ramping scanning mechanism for phase-shift and vertical-scanning interferometry. The pulse length and the scanning velocity are selected such that a minimal change in OPD occurs during the pulse. As long as the duration of the pulse is shorter than the detector's integration time, the effective integration time and the corresponding phase shift are determined by the length of the pulse, rather than the detector's characteristics. The resulting minimal phase shift produces negligible loss of fringe modulation, thereby greatly improving signal utilization during phase-shifting and vertical-scanning interferometry.

Transient response in high-resolution Brillouin-based distributed sensing using probe pulses shorter than the acoustic relaxation time

Abstract: We perform numerical simulations on a model describing a Brillouin-based temperature and strain sensor, testing its response when it is probed with relatively short pulses. Experimental results were recently published [e.g., Opt. Lett. 24, 510 (1999)] that showed a broadening of the Brillouin loss curve when the probe pulse duration is reduced, followed by a sudden and rather surprising reduction of the linewidth when the pulse duration gets shorter than the acoustic relaxation time. Our study reveals the processes responsible for this behavior. We give a clear physical insight into the problem, allowing us to define the best experimental conditions required for one to take the advantage of this effect.

Passively mode-locked high-power Nd:YAG lasers with multiple laser heads

Abstract: We discuss power scaling of passively mode-locked lasers using multiple laser heads in the resonator. We experimentally compared two different approaches for the cavity design, both using three side-pumped Nd:YAG laser heads. We obtained a record-high average output power of up to 27 W with close to diffraction-limited beam quality, a pulse duration of 19 ps, a pulse energy of 0.5 μJ, and 23 kW peak power. Single-pass second-harmonic generation in a 10-mm-long LBO crystal yields 16.2 W of 532-nm radiation.

Chemical and physical side effects at application of ultrashort laser pulses for intrastromal refractive surgery

Abstract: Focusing femtosecond (fs) laser pulses into transparent media, such as corneal tissue, leads to optical breakdown, generation of a micro-plasma and, thus, a cutting effect inside the tissue - provided the intensity at the focus exceeds a threshold which depends on the pulse duration. For fs pulses, the corresponding threshold pulse energy is reduced by some orders of magnitude compared with ns or ps pulses. At a low pulse energy, thermal and mechanical damage to surrounding tissue is minimized, enabling a highly precise cutting effect inside the tissue while leaving anterior layers unaltered. In the following, investigations concerning aspects of the interaction of ultrashort laser pulses with aqueous media are presented: in particular, detection of pressure transients and bubble formation. For the first time - to our knowledge - the content of the resulting gas bubbles was analysed by gas chromatography, giving evidence of molecular hydrogen. Secondly, the potential of three-dimensional cutting effects within the corneal stroma for refractive surgery applications was evaluated in vitro on freshly enucleated porcine eyeglobes. Laser pulses with a duration of 200-300 fs and energies of 1.5-600 µJ were provided by a mode-locked Ti:sapphire laser with subsequent chirped pulse amplification.

New approaches to the treatment of vascular lesions

Abstract: Background and Objective The pulsed dye laser was developed based on the concept of selective photothermolysis. By using a wavelength of light well absorbed by the target and pulse duration short enough to spatially confine thermal injury, specific vascular injury could be produced. Study Design/Materials and Methods Although the pulsed dye laser revolutionized the treatment of port wine stains (PWS) and a variety of other vascular lesions, the ideal thermal relaxation time for the vessels in PWS is actually 1–10 ms, not 450 μs of the original pulsed dye laser machines. These original theoretical calculations recently have been proven correct in a study that used both an animal vessel model and in human PWS. Results Longer wavelengths of light, within the visible spectrum, penetrate more deeply into the skin and are more suitable for deeper vessels, whereas longer pulse durations are required for larger caliber vessels. Conclusion A variety of lasers recently have been developed for the treatment of vascular lesions which incorporate these concepts into their design, including pulsed dye lasers at 1.5 ms, a filtered flash-lamp pulsed light source with pulse durations of 1–20 ms, several 532-nm pulsed lasers with pulse durations of 1 ms to as high as 100 ms, long pulsed alexandrite lasers at 755 nm with pulse durations up to 20 ms, pulsed diode lasers in the 800 to 900 nm range, and long pulsed 1064 Nd:YAG sources. Lasers Surg. Med. 26:158–163, 2000. © 2000 Wiley-Liss, Inc.

Extreme ultraviolet interferometry measurements with high-order harmonics

Abstract: We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2–1020 electrons/cm3 are measured.

A fast pulsed power source applied to treatment of conducting liquids and air

Abstract: Two pilot pulsed power sources were developed for fundamental investigations and industrial demonstrations of treatment of conducting liquids. The developed heavy-duty power sources have an output voltage of 100 kV (rise time 10 ns, pulse duration 150 ns, pulse repetition rate maximum 1000 pps). A pulse energy of 0.5-3 J/pulse and an average pulse power of 1.5 kW have been achieved with an efficiency of about 80%. In addition, adequate electromagnetic compatibility is achieved between the high-voltage pulse sources and the surrounding equipment. Various applications, such as the use of pulsed electric fields (PEFs) or pulsed corona discharges for inactivation of microorganisms in liquids or air, have been tested in the laboratory. For PEF treatment, homogeneous electric fields in the liquid of up to 70 kV/cm at a pulse repetition rate of 10-400 pps could be achieved. The inactivation is found to be 85 kJ/L per log reduction for Pseudomonas fluorescens and 500 kJ/L per log reduction for spores of Bacillus cereus. Corona directly applied to the liquid is found to be more efficient than PEF. With direct corona we achieve 25 kJ/L per log reduction for both Gram positive and Gram negative bacteria. For air disinfection using our corona pulse source, the measured efficiencies are excellent: 2 J/L per log reduction.

Process Characterisation of Pulsed Nd:YAG Laser Seam Welding

Abstract: A wide range of pulsed laser welding parameters was identified. These include average peak power density (APPD), peak power, mean laser power, traverse speed, pulse repetition rate, duty cycle, pulse energy, spot size, and pulse duration. The type of laser beam temporal pulse shape studied was a rectangular power pulse. The effects of pulsed laser welding parameters on heat flow, weld dimension, and weldability are investigated. The study shows that weld quality is principally affected by APPD, mean power, and traverse speed, of which APPD is the most critical process parameter. A processing map containing the APPD effects is constructed as a guide for producing good welds.

Machining of transparent materials using an IR and UV nanosecond pulsed laser

Abstract: Channels are traditionally machined in materials by drilling from the front side into the bulk. The processing rate can be increased by two orders of magnitude for transparent materials by growing the channel from the rear side. The process is demonstrated using nanosecond laser pulses to drill millimeter-sized channels through thick silica windows. Absorbing defects are introduced onto the rear surface to initiate the coupling of energy into the material. Laser drilling then takes place when the fluence exceeds a threshold. The drilling rate increases linearly with fluence above this threshold. While UV light drills about four times faster than IR light, the pulse length (in the nanosecond regime) and the pulse repetition rate (in the 0.1–10 Hz range) do not greatly influence the drilling rate per pulse.

Laser-diode-pumped Cr 4+ ,Nd 3+ :YAG with self-Q-switched laser output of 1.4 W

Abstract: By use of a laser diode as a pump source, a self-Q-switched laser from a Cr,Nd:YAG crystal is demonstrated. The output Q–switched traces are very stable, the threshold pump power is 3.5 W, the pulse duration is 50 ns, and the slope efficiency is as high as 20%. In addition, the pulse width remains constant while the pulse repetition rate varies with pump power.

Modeling of laser-induced breakdown in dielectrics with subpicosecond pulses

Abstract: Theoretical study of ultrafast laser induced damage by short pulses (τ<1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker–Planck type equation for free electron production is discussed in detail. The calculation shows that the collisional avalanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equation are performed and the most critical ones are identified. From these tests we obtain valuable information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxation of electron plasma, a three body recombination rate is included. Thus, the temporal behavior of the electron density due to a single pulse is treated, as well as the case of exposure to two laser pulses with a time delay between them. The model is only partially successful in reproducing the recent experimental data. Effect of the presence of a linear decay term and optical defects on the damage threshold is considered in the context of the rate equation input.

Ultrafast solid-state lasers

Abstract: Today's ultrafast all-solid-state lasers continue to demonstrate unsurpassed performances in terms of pulse duration, pulse repetition rates, average power and wavelength range. Optical pulses in the 5-femtosecond range are produced by a variety of methods. Although different in technical detail, each method relies on the same three key components: spectral broadening due to the nonlinear optical Kerr effect, dispersion control, and ultrabroadband amplification. The shortest pulses generated to date all rely on chirped mirrors for dispersion compensation. A major limitation in chirped mirror design arises due to interference between light reflected at different penetration depths inside the minor structure. This results in residual oscillations in the group delay dispersion (GDD) which ultimately limits pulse shortening Unfortunately, there is always a trade-off-between GDD-oscillations and reflection bandwidth. The double-chirped mirror technique (DCM) reduced GDD oscillations and resulted in the sub-6-fs pulses. Novel DCM designs result in a sufficiently large reflection bandwidth that could, in principle, support 4-fs pulses. The technique of Kerr lens mode-locking, successful with Ti:sapphire, has not performed so well in directly diode-pumped lasers. Semiconductor saturable absorber mirrors (SESAMs) were a breakthrough resulting in the first demonstration of self-starting and stable passive mode locking of diode-pumped solid-state lasers with an intracavity saturable absorber. The design freedom of SESAMs has allowed us systematically to investigate the stability regime of passive cw mode-locking with an improved understanding and modeling of Q-switching instabilities. Simple design guidelines allowed us to push the frontiers of ultrafast solid-state lasers.

Ponderomotive streaking of the ionization potential as a method for measuring pulse durations in the XUV domain with fs resolution

Abstract: We present an alternative method for measuring ultrashort extreme-ultraviolet pulses that can be synchronized with an intense infrared pulse. The method, based on photoionization of a target atom by the extreme-ultraviolet pulse in the presence of the infrared pulse, has a potential accuracy of close to 1 fs and is susceptible to single-shot operation. It is demonstrated on harmonic 15 of a titanium:sapphire laser. The minimum pulse duration that can be measured is limited only by the frequency of the radiation used for the ponderomotive shift of the ionization potential (3 fs in the case of the titanium:sapphire fundamental).