Showing papers on "Pulse duration published in 1990"

Timing and scintillations of the millisecond pulsar 1937 + 214

Abstract: Pulse shapes, arrival times, and interstellar scintillations of the 1.56 ms PSR 1937 + 214 pulsar are analyzed at frequencies from 0.32 to 1.4 GHz using data obtained from April 1983 to December 1985. The narrow pulse and small period of the pulsar yield arrival times that are much more precise than those of most other pulsars. Arrival time precision for the pulsar is limited at low frequencies by distortions of the pulse shape imposed by interstellar scintillations. Arrival times at all radio frequencies are affected by variations in dispersion measure and, possibly, additional variations related to variations in angle of arrival. Uncertainties in how the pulse shape varies with frequency influence estimates of dispersion measures. The interstellar scintillation and pulse broadening of the pulsar strongly constrain the spatial power spectrum of electron density variations in the ISM. 70 refs.

Optical information recording method

Abstract: A recording apparatus overwrites an input signal (s1) having pulse duration periods and pulse spacing periods to a optical disk by irradiation of a laser beam to form recording marks corresponding to the pulse duration periods. The apparatus includes detector (10) for detecting a leading edge of the pulse duration period and for producing a start signal thereupon, another detector (11) for detecting a trailing edge of the pulse duration period and for producing a stop signal thereupon, a pattern setting circuit (3) for setting a predetermined basic pattern, and a pattern generator (12) for generating the basic pattern from its beginning in response to the start signal and for terminating the generation of the basic pattern in response to the stop signal. The apparatus further includes a circuit (9, Ia, Ib, Ic, 4) for forming a modulated signal using a full or portion of the basic pattern produced from the pattern generator (12). The laser is produced by the modulated signal.

Investigation of a millimeter-wavelength-range relativistic diffraction generator

Abstract: A theoretical and experimental study has been made of the operation of a millimeter-wavelength-range sectional relativistic diffraction generator (RDG). Microwave power transported into the atmosphere with the peak parameters of 3.5 and 4.5 GW at the wavelengths of 6.5 to 6.8 mm and 9 to 11.3 mm, respectively, has been obtained. The power at the generator output, up to a horn antenna, has reached 7 and 9 GW for each of the regimes, respectively, the electronic efficiency being equal to approximately 30%. Plasma formed in a slow-wave structure of the RDG is shown to be of considerable importance for radiation pulse duration limitation. Experiments aimed at increasing the pulse duration up to 0.7 mu s have been performed. Theoretical investigation of the beam interaction with a slow-wave structure field has shown that the generation frequency may be determined by the resonance features due to the transverse motion of beam electrons. >

Self-starting condition for additive-pulse mode-locked lasers

Abstract: We analyze self-starting in additive-pulse mode-locked lasers and find that dynamic gain saturation can play a determining role. A simple condition for self-starting is presented for comparison with experiments on different laser systems.

Self-starting additive-pulse mode-locked diode-pumped Nd:YAG laser.

Abstract: A diode-pumped Nd:YAG laser is demonstrated by using self-starting additive-pulse mode locking with a nonlinear external cavity. Pulse durations of 1.7 psec are generated at an average output power of 25 mW without the need for active amplitude or phase modulation. Design and scaling issues for this technology are discussed.

Electrical apparatus for medical treatment

Abstract: An electrical neuro-stimulator using pulse generators (10) providing a master frequency, a carrier frequency, a first modulation frequency and a second modulation frequency A pair of multivibrators (16) and (18) produce a pulse train output with an adjustable number of impulses by adjustment of the carrier frequency, which also increases the average applied power and the penetration factor Master and carrier oscillators of the pulse generator operate asynchronously and digital mixer (20) is provided for combination of these frequencies By combination of high and low-pass filter (24) and (26), the envelope of the pulse train waveform is given an increasing amplitude from its initial value during a predetermined interval A set of specially-designed pulse transformers (T1) and (T2) is provided to maintain the optimal pulse waveform shape when the tissue impedance changes, providing compensation via a reactive component

Efficient conversion of picosecond laser pulses into second-harmonic frequency using group-velocity dispersion

Abstract: A computer model has been used to investigate frequency doubling of 1-psec duration high-power pulses in potassium dihydrogen phosphate (KDP) for type-II phase matching. It has been found that group-velocity dispersion can be used to advantage by predelaying the ordinary and extraordinary polarizations appropriately in a thin KDP crystal with its axes aligned at 90\ifmmode^\circ\else\textdegree\fi{} to the main conversion crystal. In that situation power conversion g100% from the fundamental to the second harmonic can be obtained with simultaneous ``compression'' of the output pulse duration by up to a factor of 5.

Phase and group velocity matching for second harmonic generation of femtosecond pulses.

Abstract: We theoretically analyze a method for matching group velocities of fundamental and second harmonic femtosecond pulses during phase matched frequncy doubling by predispersing the fundamental pulse with a prism. The method permits improved conversion efficiency by allowing crystal lengths of several millimeters without sacrificing second harmonic pulse duration. Second harmonic pulse energy and duration are analyzed for beta-BaB(2)O(4), and limiting experimental factors are discussed. The results show that the method is most advantageous for incident pulses between 0.1- and 1.0-ps duration and microjoule and higher energies and that second harmonic pulse duration and conversion efficiency are not highly sensitive to optical misalignments of the order of 1 degrees .

Regulated AC power system energized by variable speed prime mover

Abstract: A three phase alternator is driven by a variable speed prime mover, and its rectified output is pulse width modulated by a gate controlled H-bridge power switch circuit. Predetermined pairs of power switches are gated on and off according to programmed duration and timing instructions contained in a read only memory. In a first embodiment, a sine wave output is produced by successive pulse trains. The first pulse train has alternating positive and negative constant amplitude pulses, with the widths of positive pulses varying sinusoidally and the widths of negative pulses being constant. The second pulse train has alternating positive and negative pulses of constant amplitude, with the pulse widths of the negative pulse components varying sinusoidally, and the pulse widths of the positive components being constant. The components of the first and second pulse trains have an energy distribution which corresponds to one cycle of a sine wave having a fixed frequency. The pulse width modulated waveform is filtered to produce a smooth, low distortion sine wave output waveform. In a second embodiment, the on and off states, and the duration and timing of the on and off states, are controlled to produce a pseudo sine output power wave having an RMS value of 120 VAC at 60 Hz.

Biosonar behavior of mustached bats swung on a pendulum prior to cortical ablation

Abstract: 1. The biosonar signal (pulse) of the mustached bat, Pteronotus parnellii parnellii, has four harmonics (H1-4), each consisting of a long constant-frequency component (CF1-4) followed by a short frequency-modulated component (FM1-4). As the bat approaches a target, it systematically modifies its pulses to optimize the extraction of information from the echoes. These behavioral responses include 1) Doppler-shift (DS) compensation in which the bat adjusts the frequency of its pulses to correct for the DS in the echoes. This maintains the echo CF2 at a frequency to which the bat's cochlea is very sharply tuned, slightly above the CF2 frequency of the bat's pulses when it is at rest (Frest, approximately 61 kHz); 2) echo intensity compensation, in which the bat lowers its pulse intensity as it approaches a large target, thus maintaining the echo intensity within a suitable range for auditory processing; and 3) and 4) duration and rate adjustments, in which the bat first increases its pulse duration to facilitate target identification, then shortens its pulse duration while increasing its pulse rate to facilitate target analysis. 2. We examined these responses, especially DS compensation, by swinging bats on a pendulum toward a large target over a distance of 3.6 m. Eight bats were given 15-30 swings per day for 6-25 days. 1) On 97% of all swings the bats showed strong DS compensation as the pendulum approached the target. They did not show DS compensation on the backswing. 2) On 40-50% of all swings, the bats clearly displayed the other responses. The bats typically increased their pulse intensity a small amount early in the pendulum swing, then decreased pulse intensity by as much as 18 dB as the target was more closely approached. They increased their pulse intensity during the backswing. 3) Pulse duration increased from approximately 20 to 23 ms early in the forward swing, decreased to approximately 18 ms as the target was more closely approached, and then increased to 20 ms by the end of the backswing. 4) The instantaneous repetition rate increased from approximately 17 pulses/s at the start of the forward swing to approximately 28 pulses/s near the target, then decreased to approximately 10 pulses/s by the end of the backswing. Pulses usually occurred in trains of 1-2 pulses, with longer trains occasionally occurring near the target. 3. The maximum DS on the pendulum was 1.34 kHz, and the maximum DS compensation was 146 +/- 98 (SD) Hz less than this value.(ABSTRACT TRUNCATED AT 400 WORDS)

Micron-sized droplets irradiated with a pulsed CO 2 laser: measurement of explosion and breakdown thresholds

Abstract: Measurements of minimum CO(2) laser fluence required to explode or disintegrate 10-60 microm radius droplets of water, ethanol, diesel (hexadecane), CCl(4), bromoform, and ethyl bromide are reported. Threshold fluences range from 0.4 J cm(-2) for 10-microm radius ethanol drops to 20 J cm(-2) for 30microm bromoform drops. Threshold fluences for water droplets are ~3 J cm(-2) independent of drop size. Comparison of the measurements to calculations of laser fluence required for considered absorbing droplets to reach superheat temperature is in good agreement for cases where liquid material properties are known, suggesting that superheating of droplets is the dominant mechanism causing explosion/disintegration. Measured droplet-induced laser breakdown thresholds are considerably higher than explosion thresholds and have less dependence on droplet size and composition. The highest breakdown threshold values are for water drops, which range from 150 to 280 J cm(-2) (0.9-1.7 x 10(9)W cm(-2)) compared with 670 J cm(-2) (4.0 x 10(9) W cm(-2)) for clean air breakdown for the laser pulse length and spot size.

Erbium fibre soliton laser

Abstract: Soliton pulses of 2-3.5 ps duration have been generated at 1.56 mu m from a mode-locked erbium-doped fibre laser. The shortest pulses recorded (1.2 ps in duration) were generated in conjunction with a low level pedestal component.

Effects of pulse length and pulse strength on transfection by electroporation.

Abstract: The relative importance of pulse field strength E and pulse length tau 1/2 (half decay time of an exponential decay pulse) on the stable transfection frequency for HeLa or HUT-78 cells was investigated. Cells were transfected with plasmids containing the promoter and drug resistant genes pRSVgpt or pRSVneo by electroporation. The stable transfection frequency was assayed using the marker rescue technique. The transfection frequency increases with increasing values of E tau 1/2. For a given pulse length, the transfection frequency is proportional to the power of the pulse (E2 tau 1/2). Pulses with half decay times of 2.2 to 4.6 ms appear to be more efficient than 0.275 to 0.31 ms for stable transfection of HeLa cells.

Experimental investigation of a pseudospark‐produced high‐brightness electron beam

Abstract: A high‐brightness electron beam produced by a six‐gap pseudospark chamber operated in 5–25 kV voltage and 30–70 mTorr pressure ranges is experimentally investigated. The electron beam of current 150 A at an average energy of 20 keV and pulse duration of 10 nsec is extracted with a repetitive frequency of ∼10 Hz. The electron‐beam current scales linearly with the breakdown voltage and about 20% of the stored energy is converted into the total beam energy. The time‐integrated rms emittance of the electron beam is measured, and a typical value is found to be 65 mm mrad, yielding a normalized brightness of the beam Bn≊2×1010 A/(m2 rad2).

A high-power, traveling wave tube amplifier

Abstract: High-power X-band traveling-wave tube amplifiers (TWTs) have been fabricated and tested. The tubes have gains ranging from 13 to 35 dB at 8.76 GHz and output powers ranging from 3 to 100 MW. The amplifiers are driven by the interaction of a slow space-charge wave, propagating on an electron beam, with an electromagnetic wave supported by the structure. The electron beam, which is produced from a magnetic-field-immersed field-emission cathode, has an energy of 850 keV, a current in the 1-kA range, and a pulse duration of 100 ns. The amplifiers are designed to operate as narrow-band devices in the TM/sub 01/ mode. A report is presented on the amplifier characteristics, and their performance is compared with calculated performance using conventional TWT theory. The scaling of the gain and bandwidth with the beam current are approximately as expected from theory, but the absolute magnitude of the gain is somewhat greater than expected. >

Design and performance of a multiterawatt, subpicosecond neodymium glass laser

Abstract: A tabletop Nd: phosphate glass laser system capable of producing subpicosecondpulses with a peak power > 8.5 TW is described. Relay imaging and optimization of the chirped-pulse amplification technique, including spectral shaping and careful attention to beam quality, produce a transform-limited, 800-fs compressed pulse with an energy of 6.8 J in a near-diffraction-limited spatially uniform beam.

Flashlamp‐Pumped Pulsed Dye Laser Therapy for Poikiloderma of Civatte

Abstract: The use of pulsed yellow light from a flash-lamp-pumped pulsed dye laser has provided a successful form of therapy to remove both the telangiectasias and irregular pigmentation associated with poikiloderma of Civatte. The short pulse duration and specificity of the interaction between yellow light and hemoglobin permit this technique to be performed on the thin skin of the neck without textural changes or scarring. Furthermore, use of the large beam diameter of this laser system allows for rapid treatment of large areas in a short period of time.

Phase-locked loop with pulse-duration modulation fine frequency control

Abstract: A phase-locked loop includes an oscillator controlled by means of a switching network and a microprocessor which generates, in response to the output of a phase detector, two groups of output signals. A first group (Q1 . . . QN) is for adjusting the frequency of the oscillator in steps by selectively switching in frequency determining elements, and a second group (P1 . . . PM) for feeding a pulse duration modulator. The pulse duration modulator produces a control signal for a frequency determining minimum element of the switching network. The control signal has a duty cycle indicative of the frequency determination contribution by the minimum element.

Picosecond laser system continuously tunable in the 0.6-4-microm range.

Abstract: A continuously tunable picosecond laser system has been developed that operates in two modes: cw (100 MHz) and pulsed (10 Hz). In either mode the tuning range is approximately the same: from 0.6-4 microm. The pulse duration can also be selected: either short (typically 5 ps) or long (typically 50 ps) pulses are available. The average output power is in the milliwatt range for both repetition rates. In addition, both modes of operation are simultaneously available, and they are mutually synchronized. The system is based on a mode locked high power Nd:YLF laser which synchronously pumps a dye laser and seeds a Nd:YLF regenerative amplifier. Frequency mixing and parametric generation/amplification in KTiOPO(4) are used to obtain the large tunability.

Pulsed delivery of laser energy in experimental thermal retinal photocoagulation

Abstract: Retinal lesions produced with a pulsed laser beam of 1-20 kHz frequency and 10-100% duty cycle were compared with lesions produced with a continuous wave (cw) laser of the same peak power and total energy. Photocoagulation was applied to the retina of three black pigmented rabbits using krypton red laser (647.1 nm) equipped with an acousto-optical modulator to convert cw laser emission to a pulsating beam. An optical fiber fed the laser beam into an optical system delivering a collimated beam of predetermined divergence; the animal's eye focused this beam to a 50-pm spot on the retina. Peak power was kept constant at 0.2 W, and energy was kept constant at 20 mJ. After 7 months the animals were sacrificed and retinal tissue examined by light microscopy. The central section of each lesion was identified and photographed. For lesions with the same energy per pulse and the same pulse duration, the most influential factor, in the frequency range of 1-20 kHz, appeared to be the duty cycle: the smaller the duty cycle, the smaller the lesion, and vice versa. In other words, the shorter the time interval between consecutive pulses, the larger were the pulsed laser lesions.

Effects of near-infrared laser and superluminous diode irradiation on Escherichia coli division rate

Abstract: An investigation is conducted of the effect of a 1300-nm continuous-wave (CW) laser diode 950-nm modulated superluminous diode which had a fixed average power density of 1.2*10/sup 3/ W/m/sup 2/ at all variable repetition rates of 2, 26, 700, 1000, and 5000 Hz and had an equivalent pulse duration of 410, 31.1, 1.15, 0.82, and 0.16 ms, respectively. The effect of both diodes on the rate of Escherichia coli WP2 division was examined. It was found that the radiation of the CW mode of the 1300-nm laser diode increased the division of E. coli in the dose range of 0.9-9.0 J/cm/sup 2/. The 950-nm pulsed irradiation increased or inhibited the division rate of bacteria depending on the pulsing frequency and/or pulse duration. >

Generation of picosecond pulses with a gain‐switched GaAs surface‐emitting laser

Abstract: Pulses shorter than 4 ps (deconvolved) have been obtained by optically gain switching a GaAs multiple quantum well vertical‐cavity surface‐emitting laser with a picosecond dye laser. Pulse width and relative peak delay were measured as a function of pump power. A theoretical model of the large signal response agrees well with the measured data. The model predicts the minimum achievable pulse width and pulse delay for this device structure. Experimental results and calculated values indicate that very high modulation rates are possible with vertical‐cavity surface‐emitting lasers.

Dependence of the XeCl laser cut rate of plaque on the degree of calcification, laser fluence, and optical pulse duration.

Abstract: A XeCl laser with an optical pulse duration of 35 ns was used to determine the cut depth per laser pulse of postmortem human aorta as a function of laser fluence for four main categories of plaque development. The data indicate that the cut depth per pulse progressively decreases as the degree of calcification increases even at very high (100 mJ/mm2) laser fluences. A comparison was made between the XeCl laser cut rate data obtained using the 35-ns duration laser pulses to data obtained using 200-ns duration pulses for each of the four plaque types. As the degree of tissue calcification increased higher XeCl laser fluences were required for the long pulse case to achieve the same cut depth per pulse as that observed using the shorter pulse duration.

Use of lasers to break down objects for removal from within the body

Abstract: Calculi, stones, or calcified tissue are broken down for removal from within a body using laser pulses that are delivered via an optical fiber and have a wavelength and pulse duration selected to break down the object into smaller particles without delivering energy sufficient to cause damage to other tissue.

High peak power picosecond pulse generation from AlGaAs external cavity mode‐locked semiconductor laser and traveling‐wave amplifier

Abstract: Thin active region single stripe aluminum gallium arsenide traveling‐wave amplifiers have been used to generate and amplify 15 ps optical pulses to nearly 50 mW average powers and to peak powers in excess of 3 W. No appreciable temporal pulse distortion is observed after amplification. These peak power levels should prove to be sufficient to induce nonlinear optical effects in fibers.

Amplification of femtosecond pulses in Er/sup 3+/-doped single-mode optical fibres

Abstract: Femtosecond pulse amplification in Er/sup 3+/-doped single-mode optical fibres has been experimentally investigated. The effect of amplified pulse compression from 80 to 55 fs has been observed. The amplified pulse energy was evaluated as 5 nJ. Maximum gain for the amplified signal was 26 dB.< >