Showing papers on "Pulse duration published in 1987"

High Speed Overwritable Phase Change Optical Disk Material

Abstract: It was found that GeTe-Sb2Te3 pseud-binary amorphous alloy films showed remarkably fast switching properties to laser irradiation. By the static laser irradiation test, the film whose composition corresponded to stoichiometric compound of GeSb2Te4 were crystallized within 50ns of pulse duration at power of 8mW, whilst they could be amorphized with the same pulse duration at power of 20mW. Direct overwriting cycle test was performed on the revolving disk system for 105 times using single laser beam. CNR of more than 50dB and erasability of -22dB were obtained for linear velocity of 22m/s and overwriting frequencies of 5 and 7 MHz. The laser powers were 22 mW for recording and 10 mW for erasing. These materials will be applicable to high data rate direct overwritable disk media.

Ultrashort pulse propagation, pulse breakup, and fundamental soliton formation in a single-mode optical fiber

Abstract: The propagation of ultrashort (0.83 ps), intense dye laser pulses through a single-mode optical fiber is investigated. The input wavelength is tuned in the vicinity of the zero dispersion wavelength of the fiber. Although the input power is sufficient to generate solitons of up to the tenth order we do not observe the formation of high-order solitons. Instead, the input pulse breaks up temporally and spectrally after an initial stage of narrowing, and an ultrashort Stokes pulse is formed which shifts continuously to lower frequencies with increasing fiber length. The parameters of this pulse closely correspond to those of the fundamental soliton solution of the nonlinear Schroedinger equation. Using fiber lengths from a few meters up to 1 km the resulting pulse durations lie between 55 and 410 fs and the corresponding wavelengths between 1.36 and 1.54 μm. Numerical simulations solving a modified nonlinear Schroedinger equation including higher order dispersion and the Raman effect are in good agreement with the experimental results. It is shown that the principal soliton pulse shaping mechanisms are pulse narrowing and the soliton self-frequency shift.

Ultrasound therapy device.

Abstract: An ultrasound therapy device is provided which maintains a selected constant electrical drive power to a transducer regardless of the load on said transducer by means of an analog servo feedback circuit. The device is operable in either a continuous or pulsed mode and if operated in the pulsed mode, both the pulse period and the pulse duration can be selected by the operator. Circuitry is provided to prevent the operator from selecting a pulse duration greater than the selected pulse period. Touch pad input switches may be used to input values of the operating parameters, and the chosen parameters may be displayed on digital readouts on the front panel of the device.

Ultrasonic pulse temperature determination method and apparatus

Abstract: A first ultrasonic pulse formed by the superposition of a probe pulse on a pump pulse having a frequency lower than the center frequency of the ultrasonic probe pulse where the particle velocity of the pump pulse is in the vicinity of zero in addition to which the particle acceleration of which is at a constant portion, and a second pulse formed by the superposition of the probe pulse center frequency on the peak particle velocity portion of the pump pulse, as well as a probe pulse alone, are transmitted into a specimen before and after it is heated and the reflections of the transmitted signals from two or more reflecting points that are at different depths in the specimen are received. The received signals are subjected to frequency analysis and the spectral shift and the phase shift of the probe pulse and superposed pulses are obtained, and from these values the attenuation of the probe pulse between the differing depths is determined. Such an attenuation determination is obtained before and after heating of the specimen and the change in temperature of the specimen is determined from the obtained attenuation values on the basis of a pre-prepared table of changes in attenuation factor in accordance with changes in the temperature of the specimen.

Electrostatic precipitator power supply

Abstract: An electrostatic precipitator is supplied with a d-c voltage of variable level upon which is superimposed a series of voltage pulses. For each voltage pulse, the difference between a positive current-time area and a negative current-time area beneath a current-voltage curve is determined and maximized by changing the pulse amplitude, pulse width, pulse repetition frequency and/or the rate of pulse rise. In addition, the d-c voltage is controlled to have a value at a point of maximum slope of the function of precipitator current with respect to the magnitude of the d-c voltage.

Effect of optical pulse duration on the XeCl laser ablation of polymers and biological tissue

Abstract: Photoacoustic spectroscopy was used to measure the pulse duration dependence of the XeCl laser ablation of polyimide, polyethylene terephthalate, and post‐mortem human aorta. It was observed that the ablation threshold exhibited only a weak dependence on pulse duration. Photoablation etch depth measurements of polyimide as a function of XeCl laser fluence indicated that over a practical etch depth range of 0.1 to 1 μm per laser pulse the etch depth was independent of the pulse duration.

Enhancement of keV x-ray emission in laser-produced plasmas by a weak prepulse laser

Abstract: X‐ray conversion efficiency in a 0.53‐μm laser‐produced copper plasma is studied in the 1.5–5.0 keV range as a function of laser pulse duration (τL) with a laser intensity of 1×1014 W/cm2. The efficiency increases as τ1.3L at pulse lengths of less than 400 ps. For a 200‐ps pulse duration, an enhancement of the conversion efficiency is observed with the use of a prepulse. The efficiency is found to be proportional to the scale length of a preformed plasma. Enhancement of a factor 3 is observed for the shots with a prepulse.

Excited-state dynamics and temporally resolved nonresonant nonlinear-optical processes in polydiacetylenes

Abstract: Degenerate four-wave mixing experiments (along with saturated absorption experiments) were carried out in and below the excitonic-state resonance in polydiacetylenes. A self-consistent analysis of the data was obtained with a two (electronic) level molecularlike model. The excitonic-state lifetime was determined to be 1.8 psec, and an initial excited-state process with a 100-fsec time constant was resolved. The time constant of the nonresonant third-order nonlinear-optical susceptibility was determined to be less than the resolution of the four-wave mixing experiment, -300 fsec, which was limited by the optical pulse duration.

The pulsed dye laser: its use at 577 nm wavelength.

Abstract: The ability to increase laser-induced cutaneous blood vessel damage selectivity is dependent upon optimizing various intrinsic laser parameters. Varying the laser emission wavelength, pulse duration, and/or incident energy dosage allows the clinician potential improvement in the laser treatment of cutaneous small blood vessel processes such as port-wine hemangioma and telangiectasia. The pulsed dye laser at 577 nm and a pulse duration of 300 to 400 microseconds, incorporates desired laser parameters into its design. Clinical evaluation of the therapeutic outcome with this laser reveals favorable results while significantly minimizing potential adverse side effects.

Pulse-duration memory effect and deformable charge-density waves.

Abstract: When repeated identical voltage pulses are applied to a material with a sliding charge-density wave, the current response synchronizes with the end of the applied pulse. We discuss how this observation can be understood using the model of classical deformable charge-density waves. The effect provides insight into unusual behavior that may be typical of many multivariate dynamical systems.

Theory of the fiber Raman soliton laser

Abstract: The sync-pumped fiber Raman soliton ring laser is analyzed. The dependence of pulse width on the gain and gain saturation and of the carrier frequency on the length of the ring is determined. The frequency shift caused by the noninstantaneous response of the nonlinear medium is determined, and it is found that bandwidth limiting is necessary to contain it. Limits on achievable pulse width are given.

Method of controlling damping force of damper

Abstract: The damping force of a damper on a motor vehicle is automatically controlled dependent on the running condition of the motor vehicle and the road condition in which the motor vehicle travels. An acceleration of vertical vibration of the motor vehicle body is detected by a sensor, and a binary pulse signal indicative of a variation in the acceleration is produced. Then, a first signal is generated on the basis of whether the pulse duration of the binary pulse signal is at least a prescribed value or smaller than the prescribed value. Thereafter, a second signal is produced indicating the magnitude of the absolute value of the acceleration. Finally, the damping force of the damper is selectively adjusted to hard, normal, and soft settings based on a combination of the first and second signals.

Far infrared spectroscopy with subpicosecond electrical pulses on transmission lines

Abstract: Optically generated and detected electrical pulses on transmission lines in the subpicosecond range have frequencies extending up to 1 THz, thereby covering the far infrared region of the spectrum from 0 to 30 cm−1. We have studied the propagation of these short pulses through a section of the transmission line covered with erbium iron garnet which shows distinct absorption lines in the far infrared at low temperatures (2–30 K). The absorption and dispersion of the garnet modify the shape of the pulse, and the absorption spectrum is obtained by Fourier transforming the propagated pulse shape.

Battery charging apparatus and method

Abstract: A battery charger and associated method for charging a DC battery utilizes a step-down transformer with high leakage reactance, and means for rectifying and regulating the step-down transformer output to continuously supply a constant charging current to the battery. Feedback from a current sensing means in series with the battery is provided for determining proper regulation. A discharge current controller in parallel with the battery contains a load resistor and switching means electrically responsive to a pulse train generator. The discharge current is applied to the battery in a rapid succession of brief pulses throughout the entire charging cycle, with the discharge current pulses being superimposed over the charging current. The discharge current pulse duration is selected utilizing the pulse train generator, between 50 and 100 microseconds with a time period on the order of twenty times the pulse duration. The amplitude of the discharge pulses, fixed by selection of the load resistor, is approximately three times the magnitude of the average nominal charging current. The rapid application of high amplitude current for an extremely brief pulse duration induces electromagnetic forces within the battery. A turbulence is produced which advantageously scrubs the battery plates to remove hydrogen bubbles, so the entire plate surface is available for the redeposition of ions.

Analytic study of pulse broadening in dispersive optical fibers

Abstract: It is demonstrated that for an optical pulse propagating along an optical fiber the rms pulse width varies parabolically with distance, irrespective of initial pulse form and frequency chirp variation. Furthermore, the result is true to arbitrary dispersive order and should prove a very useful tool in determining the information-carrying capability of long-distance optical-fiber transmission systems.

Intensity, pulse width, and polarization dependence of above-threshold-ionization electron spectra

Abstract: Some difficulties in above-threshold-ionization experiments (mainly the intensity measurements and space-charge effects) are discussed. New results obtained in a space-charge-free regime are reported. The intensity dependences of the spectra show a clear departure from the predictions of lowest-order perturbation theory. At short pulse duration (50 psec) the peaks appear red shifted, and the effect is amplified in circular polarization. A possible interpretation of this effect in terms of ponderomotive potentials is proposed.

Current rise‐time limitation of the large volume x‐ray preionized discharge‐pumped XeCl laser

Abstract: Sixty‐six joules has been extracted from a 22‐l active volume, 5‐atm laser mixture, in a 180‐ns (FWHM) optical pulse. The best efficiency obtained under these conditions was 0.8%, based on the available energy stored in the double Blumlein. For the laser chamber geometry used in this experiment it was essential that the surface track path between the positive high‐voltage electrode and any ground be a minimum of 3 times the interelectrode separation. The small‐signal gain under these operating conditions was 6.0% cm−1 and the measured absorption at the peak of the gain was 0.4% cm−1. We observe that for both volumetric scaling and pulse length scaling the current rise time is of critical importance.

Efficient generation of the second harmonic of a nanosecond CO2 laser radiation pulse

Abstract: Efficient (49 ± 5%) second harmonic generation was demonstrated for the first time in the infrared. The frequency of CO2 laser radiation with a pulse duration of ~2 nsec and an intensity of up to 1GW/cm2 was doubled in an unbleached ZnGeP2 crystal. The internal energy efficiency was ~80%.

Pulsed-laser-generated rare-gas plasma as a light source in the vacuum ultraviolet.

Abstract: The plasma generated by focusing a 20-Hz, 330-mJ, Nd:YAG laser pulse in a rare gas at pressures from 1 to 6 bars is proposed as a source of a VUV continuum giving as many as 109 photons nm−1 sec−1 near 170 nm. Its stability makes it suited to VUV spectrophotometry. The pulse duration, ranging from 20 nsec for Ne to 40 nsec for Kr, should also permit its use in time-resolved spectroscopy.

Determination of carrier capture cross sections of traps by deep level transient spectroscopy of semiconductors

Abstract: The conventional method of determining trap capture cross sections by deep level transient spectroscopy (DLTS) of semiconductors involves observing the DLTS peak amplitude as a function of filling pulse duration. Pulses that are inconveniently short and time‐consuming experimental measurements may be required. A method is proposed that involves observing the DLTS peak amplitude change as the observation rate window (t2−t1) is varied for a fixed‐duration filling pulse. This gives consistent results for the capture cross section and its dependence on temperature. Analysis of the technique is given and a comparison between theory and experiment is presented for two electron traps in bulk n‐GaAs. For the trap at EC‐ 0.215 eV, σ∞ is 2.3×10−15 cm2 and the activation energy from σ(T)=σ∞ exp(−Eσ /kT) is 0.116±0.003 eV. For the trap at EC ‐0.35 eV, the cross section σ∞ is 1.0×10−13 cm2 and the activation energy is 0.200±0.025 eV. Large variations of cross section with temperature such as these have been reported ...

Te-Ge-Sn-Au Phase Change Recording Film For Optical Disk

Abstract: Te-Ge-Sn-Au thin films were studied for phase change type rewritable disk media, in order to obtain the fast crystallization speed and the thermal stability. Films were prepared by co-evaporation method. It was found that through static record/erase measurements the threshold crystallizing pulse duration of Te60Ge4SnllAu25 film was only 1 usec at 2 mW of laser power; that is less than one tenth compared with that of Te80Ge5Sn15 film, while the threshold amorphizing laser power of them were almost the same, 6 mW at 0.2 psec of pulse duration regardless of Au concentration. Its crystallizaton temperature of about 130°C was enough to maintain the good thermal stability. Through DSC, X-ray and electron diffraction studies, the first appeared crystalline state in crystallization process, corresponding to the drastic change in optical property, showed only one phase of metastable simple cubic(SC) structure. It was concluded that the appearance of the SC structure made the crystallization speed higher. The obtained thin film was a candidate for the simultaneously erasable and recordable material.

Short pulse amplification in the presence of absorption

Abstract: Energy amplification of short pulses in the presence of distributed absorption is analyzed and discussed. A short pulse is defined as a pulse length that is much shorter than the lasing state lifetime. It is shown that the energy density of a short pulse cannot be amplified indefinitely in the presence of nonsaturable absorption. In an active medium having a small signal gain g0 and a nonsaturable absorption α, the maximum attainable energy density is (Esat g0/α), where Esat is the medium saturation energy. High‐energy densities are obtained at the expense of the extraction efficiency. An amplifier design can be optimized by choosing the appropriate parameters such as small signal gain and active medium gain length.

A method for time resolved neutron spectroscopy on short pulsed fusion neutron sources

Abstract: A method is presented which yields time-resolved spectral distributions of fusion neutrons by an extension of time of flight spectroscopy. The signals of various neutron detectors placed at optimized distances from the source are used. The evaluation is done by a computer assisted Laplace transformation procedure. Applying this method it is possible to measure time resolved neutron spectra of pulsed neutron sources (pulse duration 50 ns to 1 mu s). Five to ten photomultiplier signals in a distance range of 5-100 m are necessary. Time resolution better than 10 ns and energy resolution in the range of 100-300 keV is possible. An experimentally obtained time-resolved neutron spectrum is shown. It was taken at the POSEIDON plasma focus and exhibits characteristic features, such as an emission of a double neutron pulse and different energy distributions in the pulses.

PWM control for power converter system utilizing pulse dropping at minimum pulse widths

Abstract: A pulse generator (including a microcomputer) for supplying a pulse pattern to semiconductor devices of a power converter (such as an inverter or a converter) using the pulse width control scheme is provided with function of dropping pulses so that the pulse pattern may form a predetermined function provided that the pulse width is shorter than a limit value (Limit). As a result, the input and output waveforms of the power converter can be made sinusoidal and can be released from the constraint of the minimum pulse width of power conversion semiconductor devices.

A survey of the acoustic output of ultrasonic Doppler equipment.

Abstract: Measurements of the acoustic output generated by a variety of clinical ultrasonic Doppler instruments have been carried out. The instruments surveyed include continuous wave Doppler units for cardiovascular investigations, fetal monitors, stand-alone pulsed Doppler equipment and 'duplex' scanners working in Doppler mode. Acoustic measurements have been made using a calibrated PVDF membrane hydrophone, and a milliwatt radiation force balance. Almost all the pulsed Doppler and duplex systems investigated could generate spatial-peak, temporal-average intensities in water which exceeded 100 mW cm-2, with a maximum of 825 mW cm-2 measured. These intensities were also reached by some continuous-wave Doppler systems, particularly those incorporated into duplex scanning systems. Fetal monitoring equipment was found to operate typically at lower intensities. Doppler units have been found to show a very wide variation in pulse length, pulse repetition frequency and spatial peak pressure, and a wider range of pulse average intensities than previously reported. Units were found to vary considerably in their control of total acoustic power whilst operating parameters such as gate width and range were altered.