Continuous-wave Beam Research Articles

Study of the Cycle-to-Cycle Variation of In-Cylinder Flow in a Motor Engine.

An experimental study investigating cyclic variation of the internal air motion was made in a motor engine having a cup-in-piston combustion chamber. The engine used for this study had a transparent quartz liner and a transparent acryl piston. The continuous wave beam of a 4 watt argon ion laser was formed into a thin light sheet to define the measurement plane. The scattered light from the particles which were supplied to the intake air stream, were recorded by a NAC E-10 high-speed camera and the bulk flow fields inside the combustion chamber were measured at both planes parallel and vertical to the piston crown by means of an image processing technique. This study reveals for the first time the continuous cycle-resolved two-dimensional bulk flow fields inside the combustion chamber around the compression TDC. Also, comparison between cycle-resolved and ensemble-averaged flow patterns showed that ensemble-averaged flow patterns may become artificial ones if there exists a large effect of the cyclic variation. Moreover, by examining the rms of the cyclic variation component and the ratios of the rms to ensemble-averaged velocity with/without swirl conditions, it was found that the ratios of the rms to ensemble-averaged velocity can be used for evaluating the effect of the cyclic variation on the flow fields.

Injection locking a laser-diode array with a phase-conjugate beam

We use a mutually pumped phase conjugator to guide the output beam of a single-mode laser diode into the active region of a high-power laser-diode array. This injected beam locks the frequency of the array, causing it to emit a single, 1.45 times diffraction-limited, continuous-wave output beam containing 85% of the array’s total output power. Phase-conjugate injection dramatically improves the coupling into the laser array, so that less than a milliwatt of injected power is sufficient to lock all the array’s 450-mW output to the frequency of the master laser.

Photocarrier spreading at a p-n junction

Continuous wave and pulsed optical beam induced current measurements were carried out on n-InP/n-InGaAs/n-InP double heterostructures which contain large areas of diffused InGaAs p-n junctions. The flat cw photocurrent and illumination-position independence of the transient photocurrent response, observed when the optical source spot illuminates a floating diode, demonstrates the effect of photocarrier spreading in a p-n junction. The very long photocarrier spreading length observed at low optical power may be an important parasitic coupling mechanism which should be considered in the isolation-design of integrated optoelectronic devices.

Acoustic streaming generated by a focused Gaussian beam and finite amplitude tonebursts

Theoretical models for calculating the on-axis acoustic streaming velocity generated by a continuouswave (CW) and pulsed finite amplitude ultrasonic beams have been developed. Using these models, the on-axis streaming velocity is calculated for unfocused linear plane wave ultrasonic beams, focused linear Gaussian beams, and unfocused ultrasonic tonebursts of finite amplitude. The numerical results show that the on-axis streaming velocity is significantly enhanced by the nonlinear harmonic generation of finite-amplitude ultrasonic tonebursts.

振動による空間曲線管内の長い連続線状弾性体の輸送

A very long elastic beam, which is a general model of a long optical fiber, inserted into a long spatially curved tube by means of vibration is investigated. Analytical models and equations are established for the forced transversal movement, the axial movement and the flying motion of the continuous wave beam. A kind of recurrence strategy is successfully developed for the forced transversal vibration problem of a very long beam floating on a vibrating surface. The detailed motion behavior of the whole response, called the "subperiodic motion", and the phenomena of changing tension force and different wavelengths are revealed through numerical solutions. Fundamental effects of some basic parameters on the motions of the system are discussed. Appropriate ranges for both the frequency and the amplitude of vibration are discovered to be very narrow. The reasons for the troublesome "stopping-up" phenomenon are clarified. Experimental studies confirmed the theoretical analyses presented.

Propagation and stability of nonlinear surface waves

The propagation of three-dimensional continuous-wave Gaussian optical beams focused onto an interface between a nonlinear cladding and a linear substrate is studied numerically. Both Kerr-law and saturable self-focusing nonlinearities are considered. The Gaussian input beam profile closely matches the corresponding stationary stripe nonlinear-surface-wave field distribution. Saturable nonlinear media are shown to lead to an increase in the stability of stripe nonlinear surface waves.

Modulational instability in nonlinear periodic structures: Implications for ‘‘gap solitons’’

A nonlinear periodic structure can convert an input continuous wave beam into a train of pulses. This modulational instability implies that the nonlinear spatial resonance (‘‘gap solitons’’) of distributed feedback structures are generally unstable. Stability is assured only for low coupling strengths or large detunings from the Bragg condition.

High speed tunable thermic modulation of far infrared waves

This paper proposes a new method for the high speed temporal modulation of infrared wave beams reflected from thin films of semiconductor plasma containing free carriers. The physical foundations of such a method, based on the non-linear and bi-stable properties of free carriers in these films, show possibilities of the elaboration of high-speed tunable modulators and polarizers. The bi-stable conditions are tunable by means of a magnetic field. The reflection of a continuous wave beam from the film with a tunable reflection coefficient leads to pulse shaping, pulse duration being as small as 10−9 s.

Noninvasive vibration measurement system and method for measuring amplitude of vibration of tissue in an object being investigated

A system and method for measuring the acoustically induced vibrations within a living organism. The object is ensonified to set it into low frequency vibration. A continuous wave beam of ultrasonic energy is transmitted along one path and focused at the tissue to be investigated. A focused transducer receives the reflected beam along a second axis such that the intersection of the transmitted and received beams define a small, concentrated region positioned at the object under investigation. The received beam is phase modulated by the amplitude of vibration of the object producing sidebands whose amplitude can be ratioed to the amplitude of the high frequency peak to determine the absolute amplitude of the low frequency vibration.

Requirements for high-surface electric fields for high-brightness FEL injectors

For FEL (free-electron-laser) operation, the advantages of photocathodes over thermionic cathodes for the emission of electrons with the required properties are presented. Simulation codes have been used to design a photoinjector prototype for testing the highest surface electric fields an RF gun cavity could sustain at 144 MHz. From the available information on different RF systems operating at high fields (continuous wave or pulsed beams), the authors infer that 15 MV/m should be a realistic value for the peak surface field. However, their photoinjector system is designed to study higher field limits as they are of crucial importance for FEL operation with the required characteristics. >

Long minority hole diffusion length and evidence for bulk radiative recombination limited lifetime in InP/InGaAs/InP double heterostructures

Minority hole diffusion length, lifetime, and mobility for undoped InGaAs were measured at 297 and 77 K in metalorganic chemical vapor deposited InP/InGaAs/InP double heterostructures using continuous-wave and pulsed optical beam induced current techniques. A hole diffusion length Lp=140 μm, a lifetime τp=18.5 μs, and a mobility μp=425 cm2/(V s) were obtained at room temperature. To the best of our knowledge, these properties are the highest reported values for InGaAs. The measured carrier lifetime and its temperature dependence are consistent with bulk radiative recombination in undoped InGaAs.

Optical instabilities in sodium vapor

We present here instabilities observed in the transverse profile of a continuous-wave light beam that crosses a sample of sodium vapor twice by means of a feedback mirror. The wave front and the intensity of the beam are spatially modulated by the intensity-dependent dispersive and absorptive action of the vapor. Experimental evidence indicates that the vapor acts as a phase-conjugate mirror, thus providing an active cavity when it is coupled with the feedback mirror. The instabilities develop as a consequence of sideband generation at the resonant modes of this cavity. The transverse-mode profiles of these sidebands are analyzed experimentally.

Side-lobe suppression for ultrasonic imaging arrays

Abstract Equations describing the focal depth two-way continuous wave beam patterns of a linear array which is focussed on transmit and receive are analysed. By using different size apertures on transmit and receive, it is possible to place the minima of one beam pattern near the side-lobe of the other and achieve side-lobe suppression. The optimum side-lobe response occurs when the transmit aperture is ≈ 0.72 (or 1 0.72 ) of the receive aperture. Simulations for pulsed mode operation of a 2 MHz synthetically focussed linear array, using this technique, are included. The implications for other imaging techniques such as confocal scanning acoustic and optical microscopy are discussed.

Self-amplitude-modulation of optical pulses in nonlinear dispersive fibers.

We consider propagation of optical pulses in the anomalous dispersive region of single-mode fibers after including the effects of Kerr-type nonlinearity. It is found that the pulse develops an internal structure with deep amplitude modulation. We show that this self-amplitude-modulation of pulses is related to the modulation instability of continuous-wave optical beams in nonlinear dispersive fibers. The initiation of this phenomenon does not require an external probe or spontaneous emission. Numerical results show that self-phase-modulation broadens the power spectrum enough to encompass the frequency at which self-amplitude-modulation provides the maximum gain. The amplification of these sidebands manifests as a breaking of the optical pulse into several subpulses. For negligible loss, the pulse breakup may also be viewed as the internal structure of a high-order soliton. However, modulation instability appears to provide a simpler interpretation and explains the features of numerical simulation and experiments.

Dual laser diode array emission coherently summed in an external cavity

The output power from two ten-element laser diodes arrays has been efficiently summed in an external cavity to obtain a coherent, continuous wave beam. In this technique, laser emission from each front surface antireflection coated diode array is stimulated by injection of emission from a second such array. This method was also applied to single-element diode lasers. The output beam characteristics such as spectral distribution, far-field and near-field beam patterns, and overall efficiency have been measured.

Photorefractive deflection technique using parallel and overlaping probe and pump beams

The present work provides an experimental study of a pulsed photorefractive technique to diagnose the interior of an irradiated region. Results obtained in a mixture of SF6 and Ar are presented to illustrate the experimental technique. Three distinct areas have been studied, (i) The two beams are parallel and their optical axes are coincident (collinear). At this particular position, the probe beam suffers a defocusing effect that is used to align collinearly two laser beams with a precision of ±3 μm. (ii) The two beams are parallel but slightly off axis. In this case, the probe beam suffers a deviation that is the sole result of a rarefaction in the irradiated region followed by the normal thermal equilibration. (iii) The two beams are parallel and slightly overlapping. The probe beam suffers a deviation similar to the preceding one. Moreover, it sees an initial outward-propagating acoustic (density) wave. It is in this particular region that the acoustic perturbation appears. Finaly, the gas mixture used under our experimental conditions is a good candidate for the collinear alignment of several laser beams. The only restrictions are that the pump laser must produce a beam with a symmetrical intensity profile, the other laser beams must be continuous-wave beams, and the gas mixture must be transparent to the latter.

Doppler Color Flow Imaging: A New Technique for Visualization and Characterization of the Blood Flow Jet in Mitral Stenosis

We used Doppler color flow imaging, a new noninvasive technique for mapping of intracardiac blood flow, to visualize and characterize the blood flow jet in 42 patients with mitral stenosis. Color flow imaging provides information about the direction of blood flow, its velocity, and the presence of turbulence. Although we found various jet configurations, most frequently the jet was centrally and apically directed and had a "candle flame" appearance (a central blue zone surrounded by hues of yellow and orange). The blood flow jet can be used to guide the positioning of the continuous-wave Doppler beam parallel to the blood flow; thus, the accuracy of the Doppler data can be enhanced. This new technology has promising potential for other clinical applications in cardiology.

Simple analytic approximation to continuous-wave on-resonance beam reshaping

We present a simple, analytically tractable, generalized aperture model for treating the stripping of a laser beam in a medium with saturable absorption. The model departs from previous treatments in allowing a soft transition from transparency to opacity, which yields a physically realistic diffraction-free amplitude encoding for the laser beam and thereby reduces unphysical fringes in subsequent beam evolution. The model is applied to the problem of cw exactly resonant self-focusing in a vapor of two-state atoms. It is confirmed that for strong beam saturation and absorption that is not too strong, beam propagation can be well modeled as a single stripping–diffraction sequence. A simple interpretation of the on-axis enhancement features is given.

A New Formulation for Linear Accelerator Design

We define three basic, calculable and measurable parameters. With these parameters we derive expressions for the radio frequency induced, beam induced, and loaded section voltages and average section gradients. Unlike the present well known expressions these alternate expressions are valid for continuous wave, pulsed, and single bunch beams, for both lossy and lossless sections and for both standing wave and travelling wave sections. We use these alternate expressions to maximize the efficiency and the gradient for a given peak power.

Field patterns of pulsed, focused, ultrasonic radiators in attenuating and nonattenuating media

A theoretical method which can be used to calculate the pressure field patterns of pulsed, focused, ultrasonic radiators in attenuating and nonattenuating media is discussed in this paper. The underlying principle involved is the superposition of continuous wave beams to form pressure pulses. The method is adapted to the circumstances associated with using a hydrophone to measure field patterns. Experimentally obtained hydrophone signals are then compared to theoretical predictions. The field patterns of four transducer-pulser combinations are investigated. A medium with tissue-mimicking acoustical properties is used to attenuate the ultrasonic beam in these studies. The theory compares favorably with experiment whether an attenuating medium is present or not. However, the theory fails at positions very close to the transducer face (∠1 cm) and when significant nonlinear effects occur during the transmission of the pulse through the medium. This work may have significant applications in research devoted to designing ultrasonic transducers for particular studies, determining dose profiles of medical ultrasonic machines, and analyzing the ultrasonic signals backscattered from within patients.