Short-pulse System Research Articles

Simulation study of acoustic intermediate layer and electrical source impedance in an ultrasonic pulse system

The systematized computer simulation of the short ultrasonic pulse system is performed using a transmission line model. Optimum condition of the acoustic impedance and thickness of the intermediate layer are considered first. The effects of electrical source impedance for the ultrasonic pulse waveform are also discussed. From the simulation results, the conventional quarter wave acoustic intermediate layer does not always realize optimum pulse performance. The importance of electrical source impedance in pulse performance is also emphasized.

ERBIUM:YAG CUTANEOUS LASER RESURFACING

The short-pulsed Er:YAG laser system is an excellent ablative tool for cutaneous resurfacing. This system is most efficacious for patients with milder cutaneous involvement, including mild photoinduced facial rhytides, mildly atrophic scars, and textural changes caused by fibrosis and dermatochalasis. The Er:YAG laser cannot achieve the same dramatic clinical and histologic improvements produced with the CO2 laser but does offer some distinct advantages that make it a valuable addition to the laser surgeon's armamentarium. The Er:YAG laser, because of its higher affinity for water-containing tissues, effects a much finer level of tissue ablation. Although erbium laser resurfacing results in decreased postoperative morbidity with a shorter recovery period, it cannot effect the same degree of improvement in photodamaged skin as can the CO2 laser. Excellent results, however, can be achieved with this laser, up to 50% or more overall clinical improvement, in patients with milder photodamage and scarring (Glogau classes I and II). In darker-skinned patients, the Er:YAG laser is often the preferred treatment modality. Continued research in the field has already led to the development of longer-pulsed Er:YAG lasers, which offer a compromise between the CO2 laser and the short-pulsed Er:YAG lasers in terms of clinical benefits while maintaining the safety profile of the traditional short-pulsed system. In addition, many surgeons now use a combination approach with the CO2 and Er:YAG lasers in an effort to maximize collagen contraction in certain areas and limit postoperative morbidity. As more research is conducted within the field of cutaneous resurfacing, newer systems will be developed in the continuing effort to create the ideal laser system--one which ameliorates the signs of photoaging without risk of major side effects or significant postoperative recovery.

Stable Operation Regimes in Short-Pulse FEL Oscillators

We have investigated the stable operation regime of the short-pulse FEL oscillator system. Two different stable-focus regimes of stable operation have been identified. The one is a single-mode stable regime (SSR) and the other is a bump stable regime (BSR). The former is operated with a large cavity detuning and gives a single-mode stationary state without saturation. And the latter is operated near perfect cavity synchronism with a low current and gives a multi-mode stationary state after a transient bump of the macropulse power. Using properly defined collective phase variables, we describe the electron dynamics and the phase evolution of the optical field in the stable operation regimes.

Bunching phase evolution of short-pulse FEL oscillator system

We studied numerically the short-pulse FEL oscillator system using properly defined bunching phase θ B and Ψ B . In stable operation, we have found that the optical field “locks” the phase to π/2 at the trailing edge, which gives the maximum gain. Moreover, electrons can be detrapped from ponderomotive bucket due to the spatial variation of the optical field, and this detrapping effect is a major cause of the limit cycle oscillation of the system. The `bump’ of the output power during the amplification usually exists at the near-perfect cavity synchronism regime, which can be explained as the change of the matching condition between electron micropulse and optical pulse.

Multipulse interferometric frequency-resolved optical gating

We review multipulse interferometric frequency-resolved optical gating (MI-FROG) as a technique, uniquely suited for pump-probe coherent spectroscopy using amplified visible and near-infrared short-pulse systems and/or emissive targets, for time-resolving ultrafast phase shifts and intensity changes. Application of polarization-gate MI-FROG to the study of ultrafast ionization in gases is presented.

Design of a high-efficiency short-pulsed positron beam system

A conceptual design of a new pulsing system for slow positron beams is described. Although the principle resembles the conventional pulsing system using rf power, the present method applies ideal waveform to the moderator for adjusting the time-of-flight of positrons using a fast arbitrary waveform generator. Requisites for the new design are the monochromaticity (less than about 1 eV) of the initial dc positrons and the ultra-fast precise waveform to be applied to accelerate the positrons. It is shown that these requisites are within the present status of art of positron technology and electronics. A preliminary result on the monochromaticity of the positron beam is also presented.

Vee dipoles with resistive loading for short-pulse ground-penetrating radar

The worldwide crisis due to land mines has given rise to a need for better detection technologies. One means of detecting buried land mines is the short-pulse ground-penetrating radar (GPR). A short-pulse GPR system requires broadband directive antennas. Resistively loaded Vee dipoles, with either constant or linearly tapered conductivity, offer a simple means of achieving such antennas. Vee dipoles have greater directivities than conventional linear dipoles, and the resistive loading can greatly reduce unwanted signals (clutter) due to reflections from the open ends of the dipole arms and multiple reflections between the antenna and the surface of the ground. This clutter can obscure real targets. In most cases, the Vee dipole with a linearly tapered conductivity offers the best performance. © 1996 John Wiley & Sons, Inc.

A model fitting approach to the broad band measurement of ultrasonic wave velocities in thin samples of engineering material

This paper presents a novel broad band short-pulse ultrasonic system for the simultaneous measurement of compression and shear acoustic wave velocities in thin solid slabs. Using a goniometer stage a test sample is immersed in a water cell between two axially aligned thick ultrasonic transducers. Compression and shear wave reverberations are generated in the test material and the resulting compression waves in the water on the receiver side are received and digitized. Each wave reverberation can be separately time resolved and time-of-flight (TOF) measurements can be made. A time domain model of the wave system in the solid is presented with comparisons with existing frequency domain models. The measured arrival times of the successive reverberations are input to a computer algorithm derived from the time domain model which, using a parameter fitting routine, calculates the acoustic wave velocity for both the compression and shear reverberations in the sample. The problem of interference between successive reverberations in the time domain is greatly reduced due to the wide bandwidth/short pulse operating mode possible with acoustically thick transducers.

The current status of laser angioplasty: coronary and peripheral results

It is pointed out that during the past decade considerable efforts have been devoted to the use of lasers coupled to fiber-optic catheter delivery systems to open atherosclerotic arterial narrowings in peripheral and coronary arteries. Continuous-wave lasers (argon and Nd:YAG) are able to create an opening, but because of thermal injury they have had a medically unacceptable high incidence of restenosis. Specially designed excimers and short pulsed near-IR systems have been shown to cause minimal thermal injury. In order to achieve a sufficiently large vessel lumen, however, a balloon dilatation (PTA/PTCA, or percutaneous transluminal peripheral and coronary angioplasty) is often done after the laser treatment, so that there is a double injury to the blood vessel wall, by the laser and by the balloon. Better understanding of the biology of the tissue reaction to injuries of various etiologies has led to the suspicion that shock wave injury has replaced the thermal injury with these new systems, since restenosis rates are still higher than for balloon dilatation alone. Some subsets of disease may have been detected recently for which the current laser systems offer some improvement in results compared to balloon dilatation. Many technical problems remain to be solved to achieve widespread benefit and use.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Borehole radar applied to characterization of fracture zones

A new short-pulse radar system (RAMAC) developed by ABEM AB has now been in operation for three years during which more than 100 km of borehole logging has been performed. The bulk of the surveys have been in granites and gneisses.The RAMAC system operates at centre frequencies in the interval 20 to 60 MHz. At those frequencies single-hole reflection ranges of 50 to 150 m are normally obtained in gneissic and granitic rock. Cross-hole ranges have in some cases exceeded 300 m. The large probing range in combination with resolution of the order of a few metres makes borehole radar a unique technique for investigation of fracture zones in crystalline rock.Case histories illustrate application of the RAMAC system in three different configurations (single-hole reflection, cross-hole reflection, and cross-hole tomography) and demonstrate how combination of these three can yield consistent 3D models of fracture zones and other structures.

The Application of a Plasma Erosion Opening Switch to a Nanosecond Generator at the Power Level of 1010 W

This paper is devoted to experimental studies of a short-pulse (80 ns) inductive system with a coaxial plasma erosion opening switch (PEOS), operating at the 2-5 × 1010 W level. Scalings of the PEOS and ion diode characteristics with different parameters (PEOS plasma density and velocity, PEOS electrode geometry, load impedance, type and strength of an external magnetic field) were carried out. It was seen that for the most efficient energy and power switching to the load by the PEOS, the following conditions are preferable: high velocity and low density of the plasma flow, negative polarity of the inner PEOS electrode, coincidence of the switch current and injected plasma flow directions, the absence of an external magnetic field, and the presence of an additional self-field in the PEOS region. Power enhancement of a factor of 3 and pulse shortening by a factor of 2 were obtained under optimal conditions.

A High-Power Tunable Short-Pulse XeCl Laser

A high-power, tunable short-pulse XeCl laser-amplifier system was constructed as a pumping source of the coherent vacuum ultra-violet (VUV) radiation. In this system, tunable radiation from a frequency-doubled dye laser pumped by a short-pulse XeCl laser was amplified by two XeCl lasers. The output radiation was tunable over 307.6 nm-308.6 nm and the maximum peak power was about 30 MW with a pulse duration of 0.9 ns FWHM. The output beam had a bandwidth of 4-6 pm and a almost diffraction limit beam divergenceof 0.6× 10-3 rad.

Temperature prediction and control in pulsed fusion-hybrid neutron multipliers

A numerical algorithm was developed for the one-dimensional unsteady-conduction heat-transfer equation to model the effects of volume heating in fusion hybrid-system neutron multipliers of slab geometry for any fusion-core pulsing scenario. The algorithm was shown to be highly accurate in all cases where it could be compared to the exact solution. The algorithm was compared to a previously developed simple analytic expression for predicting average multiplier temperatures over a wide range of fusion-core pulsing parameters. For most pulsed fusion-core scenarios, the analytic expression agreed with the numerical algorithm results to within a few percent or less. In the worst case, which corresponded to theta pinch-type operation (where the heat-pulse length is small relative to the multiplier thermal time constant and the time between pulses is large relative to the time constant), the analytic expression was accurate to within 10–15%. Thus, for most applications of temperature estimation and prediction, the analytic expression is adequate. A simple nomogram was developed to allow the average slab temperatures to be calculated easily as functions of the heat-pulse length, cooling time, and slab thermal time constant. As the computations showed, for those cases in which high slab average temperatures were due to short pulse lengths and long times between pulses, large average temperature decreases required slab thermal time constants that were small relative to the heating-pulse length and, therefore, very thin slabs. Finally, it was shown that, for long-pulsed systems, allowing high-intensity volume heating of the slab near the faces only, with no internal heat generation over the bulk of the interior, permitted the average temperature to be reduced by an order of magnitude relative to the uniform volume heating with the same total slab power generation. For short-pulsed systems with a large energy input per pulse (such as impact fusion and theta pinch), where the heat generated does not have time to diffuse throughout and from the slab during the heating pulse and where the temperature profiles at the end of the heating pulse would thus mirror the heat generation profiles, minimum peak temperatures are obtained by heating the volume uniformly.

A short-pulse electromagnetic transponder for hole-to-hole use

We have made hole-to-hole observations through nearly 20 m of granite using an electromagnetic transponder (an active reflector) in one borehole and a single-hole short-pulse radar in another. We found that the transponder is inexpensive, operationally simple, and effective in extending the capability of a short-pulse borehole radar system to allow hole-to-hole operation without requiring timing cables. A detector in the transponder senses the arrival of each pulse from the radar (which may be millivolts in amplitude); each pulse detection triggers a kilovolt-amplitude pulse for retransmission. The transponder “echo” may be stronger than that of a passive reflector by a factor of as much as 120 dB. The result is an increase in range capability by a factor which depends on attenuation in the medium and hole-to-hole wavepath geometry. Single-hole reflection-mode echoes are still available at times prior to the transponder pulse arrival. The transponder is helpful in yielding velocity information, because the radar-transponder distance is known and the echo time is observed. Field tests have demonstrated that the transponder is a useful alternative to employing timing cables in some short-pulse hole-to-hole measurement situations.

Measurement of time dependent optical gain using frequency modulation spectroscopy

Frequency modulation (FM) spectroscopy is demonstrated to be an attractive tool for gain analysis. It offers the three major features: a) potential high sensitivity in absolute gain measurement, b) high spectral resolution allowing the determination of the spectral gain profile, c) high temporal resolution capable of monitoring gain in short-pulsed systems. As an example, results of a gain analysis of the 6328 å line in a He-Ne discharge are presented.

Microwave short-pulse generator for bed-level detection

Describes the design and testing of a short-pulse microwave radar system which is simple both in design and operation. The radar can be used for measuring with accuracy targets which are very near the measurement system. A very simple step-recovery diode circuit is used to generate the radar pulses. Experimental testing has been conducted in the laboratory using metal and limestone targets.

The plasma z-pinch effect on the I-V characteristic of fast discharge flash tubes

Our empirical results show that the I-V relationship of flash lamps operating in the preionized mode with peak current density higher than 2 kA/cm2 follows Marotta’s formula V=K0i0.85, and reduces to Gonz’s equation V=K0i0.5 for current density lower than 0.6 k A/cm2. At a high-current, short-pulse system, the inductance Lf and the equivalent negative resistance dLf/dt across the flash tube becomes more significant than the inductance of the external circuit. The plasma z-pinch equation can successfully solve these time-variation terms.

Early Termination of Flashlamp Pumped Dye Laser Pulses by Shock Wave Formation

The problem of early termination of laser action in flashlamp pumped dye lasers is investigated. It is found that in high power, short pulse duration systems shock waves are created in the dye solution. Index inhomogeneities in the laser medium are created, and these lead to the termination of laser action. The velocity of propagation of the disturbance is measured. Some possibilities concerning the origin of these shock waves are examined. A design scheme that overcomes the effect of the disturbance in short pulse (</=5 microsec) systems is described.

Short-pulse optical communications experiments

Experiments in short-pulse optical communications have been performed at high data rates (107bit/s) with M-ary modulation formats and, using a subnanosecond internally gated receiver, at very high data rates (2 × 108bit/s) with binary modulation formats to experimentally verify the potential of short-pulse low-duty-cycle direct-detection formats. High resolution TV pictures and error rate data are given for two M-ary formats, pulse interval modulation and pulse position modulation, and error rate data and gated optical receiver characteristics are given for the 200-Mbit/s pulsed gated binary modulation. Results demonstrate the capability of short-pulse laser-communication systems to discriminate against background light and to efficiently convey information. Subnanosecond gating of the receiver was achieved in the 200-Mbit/s experiments. In the M-ary experiments, as many as 12 bit/pulses were transmitted, using 4095 digitally selected 1 -ns time slots. High-quality TV pictures were transmitted at average detected signal to background ratios of less than 0.01.

Nanosecond Laser Imaging

Selective imaging of objects in range is demonstrated using a giant pulse ruby laser illuminator synchronized with a high speed Kerr cell camera. Elimination of film exposure due to backscatter in a turbid atmosphere is accomplished. Analytic expressions for film exposure due to gated target returns and backscatter luminance are derived. Data obtained for exposures over a 300 meter clear atmospheric path and a 30 meter turbid atmospheric path, α≈ 6.7 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> meter <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , agree with predicted results. The technique is a useful operational mode for short range pulsed laser imaging systems under adverseseeing conditions.