Showing papers on "Pulse duration published in 1992"

Short pulse generation using multisegment mode-locked semiconductor lasers

Abstract: Mode-locked semiconductor lasers which incorporate multiple contacting segments are found to give improved performance over single-segment designs. The functions of gain, saturable absorption, gain modulation, repetition rate tuning, wavelength tuning, and electrical pulse generation can be integrated on a single semiconductor chip. The optimization of the performance of mode-locked lasers in terms of material parameters, waveguiding parameters, electrical parasitics, and segment length is discussed experimentally and theoretically. >

Nanosecond and femtosecond excimer laser ablation of fused silica

Abstract: Ablation of fused silica using standard excimer lasers (20–30 ns pulse duration at 193, 248, and 308 nm) and a short pulse laser system (500 fs at 248 nm) is reported. Ablation rates range from several hundred nm/pulse (193 nm or fs-laser) up to about 6 μm/pulse (308 nm). The performance of the ablation is found to depend not only on wavelength and pulse duration but also on the existing or laser induced surface quality (e.g., roughness) of the material. Special ablation phenomena are observed. At 193 nm and moderate fluence (3 J/cm2) ablation takes place at the rear side of a plate without affecting the front side, whereas at higher fluence normal ablation at the front side occurs. At 248 nm (standard excimer) the existence of two consecutive ablation phases is observed: smooth ablation at low rate is followed by explosive ablation at high rate. Using fs-pulses smooth shaped holes are formed during the first pulses, whereas high pulse numbers cause the development of a ripple structure in the ablation craters.

Self-focusing and Raman scattering of laser pulses in tenuous plasmas.

Abstract: The propagation of laser pulses of intensity which is large enough for relativistic self-focusing to occur is strongy affected by Raman instabilities. Both large and small angle scattering are important. The latter is the most severe since it couples with relativistic self-focusing leading the pulses to acquire significant axial and transverse structure in a time of the order of the self-focusing time. This prevents smooth self-focused pulses propagation for distances longer than the Rayleigh length except for pulse duration of the order of the plasma period.

Pair production in a strong wake field driven by an intense short laser pulse.

Abstract: A preliminary investigation is presented of electron-positron pair production by means of plasma electrons accelerated by relativistic velocities in a strong wake field. The propagation distance of the plasma wake field, which is determined by the depletion of the short laser pulse due to wake-field generation, is much larger than the pulse length. For this case, the total number of electron-positron pairs produced is independent of the plasma concentration. For achievable parameters of the laser pulse, the total number of pairs may be quite significant ([similar to]10[sup 6][ital Z2], where [ital Z] is the nuclear charge of the plasma ion).

Method of treating pigmented lesions using pulsed irradiation

Abstract: A cutaneous pigmented lesion is treated by irradiation with laser light of between 345 and 600 nm and preferably about 500 nm wavelength. The fluence is between 1 and 10 J/Cm2 and preferably between 2 and 4 J/cm2 . The pulse duration is less than 1 μsec and preferably less than 500 nsec. A 3 mm diameter spot is illuminated.

Laser-induced molecular alignment probed by a double-pulse experiment

Abstract: The authors have studied the multielectron dissociative ionization of CO using a linearly polarized YAG laser delivering 1015 W cm-2 at 1064 nm, with a pulse duration of 30 ps. By firing two identical laser pulses, with crossed polarizations and a time delay of 800 ps, they show that an intense laser field forces all the molecules to align along its polarization vector. The molecular confinement increases with the dissociation threshold energies of the decay paths involved. Surprisingly enough, the different molecular decay paths observed with a 30 ps pulse release the same 'magic' kinetic energies as already observed with 100 fs, 600 fs and 2 ps pulse durations.

Spatial and temporal transformation of femtosecond laser pulses by lenses and lens systems

Abstract: A Fourier-optical analysis of the transformation of ultrashort light pulses by lenses is given. Inserting the material dispersion up to second order, we find a coupling between the temporal, spectral, and spatial properties of the light pulse. In general, this coupling leads to a drastic increase in pulse duration and width of the spatial intensity distribution in the focal plane of the lens, which can be avoided with the use of achromatic lens doublets. The results are discussed for Gaussian-shaped input pulses.

Fiber bragg reflectors prepared by a single excimer pulse

Abstract: Narrow-line, permanent Bragg reflection gratings have been created in Ge-doped silica-core optical fibers by interfering beams of a single 20-ns pulse of KrF excimer laser light. Of the fibers studied, the highest reflectance value of ~2% was observed with a linewidth (FWHM) of 0.1 nm, which corresponds to a 2-mm grating length with an index modulation of ~3 x 10(-5).

Apparatus for defibrillation pretreatment of a heart

Abstract: The present invention is an apparatus to apply an electrical pretreatment of pulses to a fibrillating heart that begins the process of organizing the action of the chaotically contracting myocardial cells, so that the defibrillating waveform applied after the pretreatment can accomplish its task with less energy than would otherwise be required. The same electrodes are employed for both pretreatment and defibrillation. In one preferred embodiment a long-duration, low-voltage pretreatment pulse is generated directly by battery and voltage-conversion circuitry, and is followed by the defibrillation pulse delivered from the previously charged capacitor. In variations on the embodiments, the capacitor is charged during the pretreatment pulse, or between the pretreatment and defibrillation pulses. In other embodiments, pretreatment includes a train of pulses of appreciably lower energy than the defibrillation pulse, and with inter-pulse time adjusted so that necessary capacitor recharging can be accomplished, so that the same capacitor (a component of dominant size) can be used for all pretreatment pulses and for the defibrillation pulse as well. Wide-ranging variations of pulse duration, height spacing and waveform are included, as well as selected polarity inversions.

Theory of short-pulse gain saturation in semiconductor laser amplifiers

Abstract: Carrier heating and spectral hole burning are shown to have a strong influence on the amplification of ultrashort pulses using semiconductor laser amplifiers. Approximate analytical expressions for the effective saturation energy in different pulsewidth regimes are derived. >

Apparatus and method for phase resolved fluorescence lifetimes of independent and varying amplitude pulses

Abstract: A flow cytometer measures phase fluorescence lifetimes by the phase shift of a reference signal and an emission from a particle or cell in a flow chamber (17). An acoustic optic modulator (13) modulates laser light with a sinusoidal wave of a predetermined frequency to excite particles or cells. Detectors respond to emissions of individual particles or cells in the form of an output signal pulse at the predetermined frequency. The output signal pulse is divided into equal pulses with each at the modulation frequency, the same amplitude and fidelity and amplitude. One part of the divided pulse is stripped of its envelope to pass the width thereof and out of band components are rejected. A variable amplifier passes a portion of the pulse above a preset level. A delay line sets a central part of the signal at a predetermined point in time. A circuit limits the attenuated one part. A double balance mixer multiplies and the relates the limited signal with a reference signal to determine the phase shift. The next step attenuates the signal to a preset level so it can be limited, then filtered to remove harmonics above the modulation frequency and an envelope below. A double balance mixer multiplies the limited signal with a reference signal and produces a multiple signal and determining the relative phase shift of the emissions.

Disordered garnets Ca3(Nb, Ga)5O12:Nd3+- prospective crystals for powerful ultrashort-pulse generation.

Abstract: Disordered garnets Ca3(Nb, Ca)5O12:Nd3+ grown by the Czochralski method are used in a pulsed picosecond laser to generate 5–10-ps pulses in passive mode-locking trains of 120–180 ns and 1.6 mJ of energy at 1.06 μm. The evolution of the pulse parameters along the train is investigated. Direct time-resolved measurements allow us to determine the positive linear chirp of the laser frequency, which causes the pulse spectral broadening of Δν = 50–60 cm−1. The chirped pulses are compressed in the external grating compressor. The measured pulse duration of 2 ps is limited by the time resolution of the streak camera.

Gallium arsenide: A new material to accomplish passively mode‐locked Nd:YAG laser

Abstract: A passively mode‐locked Nd:YAG laser using a semi‐insulating GaAs is accomplished. Ultrashort pulses, shorter than 10 ps, with an energy of 10 μJ per pulse are obtained. The dynamics of the pulse formation is described.

Generation of high-peak-power 20-fs pulses from a regeneratively initiated, self-mode-locked Ti:sapphire laser

Abstract: We report the generation and measurement of 804-nm pulses with durations as short as 20 fs and with peak powers as high as 500 kW from a regeneratively initiated, self-mode-locked Ti:sapphire laser. Pulse duration is shown to decrease, and spectral content to increase, as intracavity power is increased. Control of intracavity focusing and a high-modulation-depth, acousto-optic modulator allow the intracavity power to be maximized. Cavity cubic phase error is minimized by correct design and placement of a group-velocity-dispersion-compensating prism pair.

Limits of pulse shortening in solitary lasers.

Abstract: The influence of dispersive pulse-broadening effects on femtosecond pulse formation in solid-state lasers has been investigated. Empirical formulas are derived from computer simulations, which permits the estimation of the magnitude of performance-limiting effects in practical solid-state systems.

Corneal surgery device and method

Abstract: A laser-based method and apparatus for corneal surgery. The present invention is intended to be applied primarily to ablate organic materials, and human cornea in particular. The invention uses a laser source which has the characteristics of providing a shallow ablation depth (0.2 microns or less per laser pulse), and a low ablation energy density threshold (less than or equal to about 10 mJ/cm2), to achieve optically smooth ablated corneal surfaces. The preferred laser includes a laser (102) emitting approximately 100-50,000 laser pulses per second, with a wavelength of about 198-300 nm and a pulse duration of about 1-5,000 picoseconds. Each laser pulse is directed by a highly controllable laser scanning system (104).

Tissue ablation through water with erbium: YAG lasers

Abstract: The authors report the dynamics of vapor cavities generated by 200 mu s long Er:YAG laser pulses under water and in gelatin. Acoustic transients were detected at the beginning of the laser pulse and when the cavity collapsed. Cavity expansion and collapse, and the associated acoustic transients are possible ablation mechanisms with the Er:YAG laser. Shortening of the pulse duration is suggested to minimize long range underwater tissue damages. >

Digital generation of high time-bandwidth product linear FM waveforms for radar altimeters

Abstract: A method is described and demonstrated of generating linear FM waveforms of very high time bandwidth product (in excess of 10/sup 5/) with range sidelobe levels more than adequate for future generation radar altimeters. The technique is extremely flexible, and the pulse length and bandwidth are easily varied by changing the parameters of the digital circuitry. An analysis is developed to relate the level of phase and amplitude errors to the permissible range sidelobe level, showing that considerably greater phase errors can be tolerated than for conventional pulse compression radars, and the validity of this analysis is confirmed by experiment.

Radio tele-communication device having function of variably controlling received signal level

Abstract: A level changing circuit is provided in a reception signal path for a radio signal and a level detection circuit and a pulse width modulation circuit are provided in a gain control circuit. A level corresponding to the amplitude level of a signal processed by a digital demodulation circuit is detected by the level detection circuit and a pulse signal whose pulse width is modulated according to the detected signal level is generated from the pulse width modulation circuit. Then, a gain control signal having a D.C. level corresponding to the pulse signal is generated and the gain of the level changing circuit is controlled according to the gain control signal, thereby making it possible to control the level of the received signal.

Method for optimal pulse defibrillation using an implantable defibrillator

Abstract: The model that is developed in the present invention is based upon the pioneering neurophysiological models of Lapicque and Weiss. The present model determines mathematically the optimum pulse duration, d p , for a truncated capacitor-discharge waveform employed for defibrillation. The model comprehends the system time constant, RC, where R is tissue resistance and C is the value of the capacitor being discharged, and also the chronaxie time, d c , defined by Lapicque, which is a characteristic time associated with the heart. The present model and analysis find the optimum pulse duration to be d p =(0.58)(RC+d c ). Taking the best estimate of the chronaxie value from the literature to be 2.7 ms, permits one to rewrite the optimum pulse duration as d p =(0.58)RC+1.6 ms. The present invention makes use of the mathematical definition of optimum pulse duration by storing in the control circuitry of the defibrillation system the actual measured value of the particular capacitor incorporated in the system. The optimized-pulse prescription of this invention can be applied to a monophasic waveform, or to either or both of the phases of a biphasic waveform.

Control of stirling cooler displacement by pulse width modulation of drive motor voltage

Abstract: The displacement of a Stirling cycle cryocooler (10) is controlled as a function of temperature by controlling the amplitude of the fundamental component of an AC signal which is applied to the motor (14) at its operating frequency. A pulse train (Fig. 4) is generated, having a frequency which is a harmonic of the operating frequency. The duty cycle of the pulse train (Fig. 4) is modulated (Fig. 8) between 50 % and 100 % as a function of the temperature. The modulated pulse train is applied to the motor (14) during one-half of the load's operating period and the complement (M) of the pulse train is applied to the load during the other half of its operating period. Modulating the duty cycle of the pulse train as a function of temperature variably controls the amplitude of the functional component of the drive voltage (32) and therefore variably controls the displacement of the motor and, as a consequence, of the cryocooler piston (16).

Method and apparatus for measuring the intensity and phase of an ultrashort light pulse

Abstract: The pulse shape I(t) and phase evolution x(t) of ultrashort light pulses are obtained using an instantaneously responding nonlinear optical medium to form a signal pulse. A light pulse, such a laser pulse, is split into a gate pulse and a probe pulse, where the gate pulse is delayed relative to the probe pulse. The gate pulse and the probe pulse are combined within an instantaneously responding optical medium to form a signal pulse functionally related to a temporal slice of the gate pulse corresponding to the time delay of the probe pulse. The signal pulse is then input to a wavelength-selective device to output pulse field information comprising intensity vs. frequency for a first value of the time delay. The time delay is varied over a range of values effective to yield an intensity plot of signal intensity vs. wavelength and delay. In one embodiment, the beams are overlapped at an angle so that a selected range of delay times is within the intersection to produce a simultaneous output over the time delays of interest.

Time-gated imaging through dense-scattering materials using stimulated Raman amplification

Abstract: An apparatus for the time-gated optical examination of objects that are p of, embedded in or viewed through dense scattering materials is disclosed. In a preferred embodiment, the apparatus comprises: a light source for producing a first light pulse and a pump light pulse, each of the first and pump light pulses having a preselected pulse width at a preselected pump wavelength; a Raman generator for shifting the wavelength of the first light pulse to a preselected Stokes wavelength to form Stokes light; a first optical train for transmitting the Stokes light through the dense scattering material to obtain Stokes light comprised of a relatively small, image-bearing signal component including unscattered Stokes light or Stokes light with minimal deviation due to scattering, light followed in time by a relatively large amount of light scattered by the dense scattering material; a Raman amplifier; an optical system for transporting the Stokes light from the dense scattering material to the Raman amplifier; a variable delay path disposed between the light source and the Raman amplifier for varying the transit time of the pump pulse to cause the pump pulse to overlap in time with the signal component of Stokes light at the Raman amplifier, the Raman amplifier only amplifying the signal component of Stokes light that is overlapped in time by the pump pulse; and a two-dimensional detector for detecting the amplified signal component of Stokes light.

Study of different ablation models by use of high-speed-sampling photography

Abstract: In our study we investigated the ablation characteristics of an aqueous dye solution with a defined absorption coefficient, irradiated by short (8 ns) and long (100 microsecond(s) ) pulses from a Nd:YAG laser (wavelength: 1064 nm). The experimental technique was schlieren photography with a second Nd:YAG laser at 532 nm as a light source and with a variable delay between the two laser pulses. With a special arrangement of the laser beams and the sample effects below and above the surface of the liquid could be simultaneously observed. We could distinguish three ablation mechanisms, depending on the pulse duration and the incident fluence. With short pulses and a fluence below the vaporization threshold the tensile pulse from the bipolar thermoelastic wave, propagating from the liquid-air interface into the sample, caused rupture and spallation of the liquid. At fluences generating a surface temperature in excess of 100 degree(s)C the short pulses caused explosive vaporization, characterized by shock wave emission both in air and in liquid. At the same fluence the long pulses caused slow vaporization, meaning that vapor and liquid ejection started during the laser pulse and was less violent than with the 8 ns pulses.

Temporal and spatial reflectivity of focused beams in stimulated Brillouin scattering for phase conjugation.

Abstract: The buildup and decay of the sound-wave gratings in stimulated-Brillouin-scattering cells were calculated taking into account the spatial dependence in the propagation direction of the focused laser beams. The incident intensities were varied up to the saturation values. The influence of the different parameters such as focal length, cell length, pulse duration, and phonon lifetime is discussed.

Method and apparatus for unpiling pulses generated by piled-up scintillation events

Abstract: A signal containing piled-up pulses from a scintillation camera head is sampled at a high frequency. Samples are summed to provide a time-integration of pulses which are sampled. When pulses pile up, the tail of the first pulse can be determined by extrapolation, permitting the first pulse to be energy-validated. The overlapping tail of the first pulse and the head of the second pulse are also time-integrated, and the area of the tail of the first pulse is subtracted from the time-integration so obtained. It is then possible to validate the second pulse.