Continuous-wave System Research Articles

Stochastic model of the deep penetration laser welding of metals

Deep penetration laser welding of metals generates a keyhole surrounded by a molten region which forms the weld on freezing. Mathematical models of the process produce relationships between the process parameters to provide an effective simulation of the process and a guide to optimal operating conditions both for continuous wave (cw) and pulsed systems. It is usual when pursuing such an analysis to assume the existence of a suitably averaged keyhole structure. To assume a steady-state keyhole with suitably averaged properties, however, constitutes a considerable idealization. Actually, the keyhole manifests itself as a writhing, twisting entity. This characteristic arises from many different instabilities that can and do inevitably come into play. They represent the reaction of the material being welded to the laser beam. It is even more necessary to consider such reactions to the laser beam when pulsed lasers or cw lasers with pulsed modulation are considered. The details concerning the characteristics of specific instabilities are not considered here. Instead, a stochastic description of some aspects of the laser welding process is provided when the laser operates in the cw case. The applicability of current mathematical models is thus extended giving increased insight into the nature of the laser welding process.

Fabry-Pérot enhancement of external electro-optic sampling

This paper describes the Fabry-Perot enhancement of external electro-optic sampling. Previous reports of Fabry-Perot enhancement have been exclusively on continuous wave systems. Using a picosecond dye laser and a standard electro-optic probe tip an increase is shown both in the sensitivity and the responsivity of the picosecond probing system. The results show that for a picosecond system considerable advantage can be gained by use of the Fabry-Perot effect.

Detectivity of shot‐noise‐limited interferometric systems measuring weak gaseous absorption

Three alternative interferometric-calorimetry systems that can be used to detect extremely small concentrations of trace gases are compared for their sensitivity limits due to both shot noise and vibrational noise. The three systems are (1) a pulsed system based on excitation by a repetitively pulsed heating laser; (2) a continuous-wave (cw) acoustic system, based on excitation by a cw heating laser whose intensity is modulated at an acoustic resonance frequency of the absorption cell; and (3) a cw isobaric (optical homodyne) system, based on a cw heating laser whose intensity is modulated at a frequency that is well below the acoustic cutoff of the absorption cell. It is shown that the shot-noiselimited sensitivities of the three systems are comparable, but that the potential presence of vibrational noise strongly favors the pulsed system.

Objective Quantification of the Continuous-Wave Venous Doppler Examination

Objective: To develop a quantitative, objective method to diagnose deep venous thrombosis (DVT) with the continuous-wave (CW) Doppler system. Design: Case control study. Setting: Vascular Laboratory of the Surgical Services, Veteran's Affairs Medical Center, La Jolla, California, USA. Patients, participants: Thirty-five control subjects and the limbs of 50 patients with suspected DVT were examined. Main outcome measures: Phlebography, CW Doppler ultrasound. CW Doppler output was recorded from the popliteal, superficial femoral and common femoral veins. The response to standardized calf compression was analysed. Results: The most useful information included the Doppler velocity increase, decay time and deceleration. Because the determination of decay time does not require calibration of the Doppler system, it is the preferred parameter. Conclusions: A decay time longer than 1.05 s is a very good discriminant between normal control and DVT limbs.

SEVERE INTRACTABLE FACIAL PAIN ATTENUATED BY A COMBINATION OF 830 nm DIODE LOW REACTIVE-LEVEL LASER THERAPY AND STELLATE GANGLION BLOCK:

A 59 year-old woman fell accidentally, and sustained bruising to her shoulder and left cheek two years before presenting at our pain clinic. She suffered from episodes of intermittent, sharp pain of the left cheek for three months after the accident, and this was then augmented by a continuous dull pain seven months after the accident. The usual diagnostic cranial examinations revealed no anatomically abnormal findings. As medication with antidepressants did not effectively attenuate the pain, she was referred to our clinic. We suspected that she was suffering from neuropathic pain secondary to her trauma, and we treated her with diode laser low reactive-level laser therapy (LLLT) using a GaAlAs 830 nm continuous wave system with an output power of 150 mW. Because the therapy was not effective, trigger point injection of an anesthetic (bupivacaine) was applied, which suceessfully attenuated the sharp pain, Subsequent combined treatment of conventional stellate ganglion block and antidepressant medication (desipramine) gave her a warm sensation on the pain region following LLLT. Three months after her admission, the intractable pain was successfully attenuated by the combination of diode LLLT and stellate ganglion block. As neuropathic pain is related to various mechanisms of neurotransmission and neuromodulation, a combination of treatment methods is recommended for successful attenuation of this type of intractable and difficult to treat pain.

Switching system for single antenna operation of an S-band FMCW radar

Increasing interest is being shown in frequency modulated continuous wave (FMCW) radars due to their advantages of compact size, low peak power requirement, good range resolution and low interference to other systems. A major disadvantage of FMCW systems is the need to use two separate, well shielded antennas for transmission and reception. Several schemes have been devised to enable FMCW radars to operate with a single antenna. The authors describe a ground based S-band ocean surface remote sensing FMCW radar that has been adapted for single antenna operation by alternately switching the single antenna between the transmitter and the receiver. Even though this frequency modulated interrupted continuous wave (FMICW) system is no longer a true continuous wave radar, the essential properties of the FMCW waveform are preserved, provided some conditions on the switching speed and FMCW parameters are met. The purpose of the radar is to deduce ocean wave and current parameters by measuring ocean surface velocities. The radar was tested operationally by comparing ocean surface data from both the single antenna (FMICW) and dual antenna (FMCW) systems recorded under similar conditions.

Measurement of blood flow by near infrared spectroscopy: a validation study : A.D. Edwards, C. Richardson, P. van der Zee, C. Elwell, J.S. Wyatt, M. Cope, D.T. Delpy and E.O.R. Reynolds, Departments of Paediatrics, and Medical Physics and Bioengineering, University College and Middlesex School of Medicine, London, U.K.

Functional near-infrared spectroscopy (fNIRS) is an optical neuroimaging method that detects temporal concentration changes of oxygenated and deoxygenated hemoglobin within the cortex, so that neural activation can be inferred. However, even though fNIRS is a very practical and well-tolerated method with several advantages particularly in methodically challenging measurement situations (e.g., during tasks involving movement or open speech), it has been shown to be confounded by systemic compounds of non-cerebral, extra-cranial origin (e.g. changes in blood pressure, heart rate). Especially event-related signal patterns induced by dilation or constriction of superficial forehead and temple veins impair the detection of frontal brain activation elicited by cognitive tasks. To further investigate this phenomenon, we conducted a simultaneous fNIRS–fMRI study applying a working memory paradigm (n-back). Extra-cranial signals were obtained by extracting the BOLD signal from fMRI voxels within the skin. To develop a filter method that corrects for extra-cranial skin blood flow, particularly intended for fNIRS data sets recorded by widely used continuous wave systems with fixed optode distances, we identified channels over the forehead with probable major extra-cranial signal contributions. The averaged signal from these channels was then subtracted from all fNIRS channels of the probe set. Additionally, the data were corrected for motion and non-evoked systemic artifacts. Applying these filters, we can show that measuring brain activation in frontal brain areas with fNIRS was substantially improved. The resulting signal resembled the fMRI parameters more closely than before the correction. Future fNIRS studies measuring functional brain activation in the forehead region need to consider the use of different filter options to correct for interfering extra-cranial signals.

Controlling the Relative Position Using Coherent Ultrasonics

Abstract By both measuring the amplitude and the phase angle of the received ultrasonic echo (using eg. quadrature mixers) the resulting coherent system can be made very sensitive to changes in range. Two receivers also makes interferometric measurements possible. In this paper we describe how this sensitivity for range variations and for angular variations can be exploited for both monitoring and for control of a robot gripper. In 0.002 seconds (using a continuous wave system) a change of 0.01 cm in range can be detected as well as a rotation of 0.3 degrees. The same sensitivities in range and angular variations are obtained with a pulsed coherent system although the measuring time will be longer. The main results in this paper are: - A summary of how the steps in the assembly sequence can be monitored using coherent ultrasonic receivers inside the gripper - Experimental results on how signal feedback can be used to compensate for detected deviations - On compound effects by also using a light beam across the finger tips It should be stressed that coherence is crucial for the results in this paper. The type of coherent system described should be contrasted with inexpensive systems that transmitts short pulses and where the detection is based on just thresholding the amplitude. Also, coherent systems suppresses noise.

The effects of angiotensin II on Doppler signals from a soft-tissue tumour.

A 10 mHz continuous-wave Doppler system has been used to detect blood flow within a sarcoma implanted in a rabbit's ear. The effects of vasoconstriction produced by intravenous angiotensin II were studied. Criteria are described that enabled differentiation of the Doppler signals from the tumour from those of the adjacent central artery. The effect of vasoconstriction upon these criteria is also described.

Vaginosonographic velocimetry of both main uterine arteries by visual vessel recognition and pulsed Doppler method during pregnancy

In 88 pregnant women (7 to 40 weeks' gestation) we performed Doppler velocimetric studies of the uterine artery on its course through the parametrium by means of the newly developed, frontally radiating duplex sector scanner. This device enabled us to visually recognize the vessel and apply the more refined pulsed wave system. In 71 of 88 women, we were able to obtain measurements from both uterine arteries. We found significant differences in the maximum systolic to minimum diastolic frequency shift (AB ratio) up to 4 between the right and left vessels in the first and second trimester. In the third trimester, the differences almost disappeared (mean ± SD AB ratio difference 0.4 ± 0.3) in these uncomplicated pregnancies. We also report our mean values (first, second, and third trimesters) of the AB ratio and the pulsatility index as an average from the right and left side measurements. Both values declined during the course of pregnancy (AB ratio, 5.5, 2.9, and 2.1; pulsatility index, 2.5, 1.7, and 1.3). We conclude that, with some experience, this scanner has the advantage of visually recognizing the vessel under study and allows measurements in both main uterine arteries. The average of the right and left measurements provides a better summary of uterine perfusion. Due to the significant right/left difference in the AB ratio, unilateral measurements of the uterine artery may give erroneous results during the first and second trimesters. Theoretically, the pulsed system provides a “cleaner” signal, but the values are comparable to those of devices that apply a continuous wave system.

Role of laser and thermal ablation devices in the treatment of vascular diseases

Since the first coronary angioplasty in 1977, both the number and complexity of interventional procedures have grown dramatically. Continuous-wave and pulsed lasers may further extend the capabilities of balloon angioplasty. Fiberoptic catheters may be used to transmit continuous-wave laser energy to ablate plaque via thermal mechanisms. Pulsed laser systems (such as the excimer) are technologically more complex than the continuous-wave systems, but may prove superior in small vessels given their ability to ablate plaque with minimal associated effects. On the other hand, modifications of the fiber-optic tip, such as the placement of a metal cap, have yielded even better results than current bare fiber systems. Such laser thermal techniques have proved a useful adjunct to balloon dilatation in peripheral vessels, but further research is necessary to determine their effect on coronary arteries. New, nonlaser technologies, however, may provide simpler power sources for thermal angioplasty. Although balloon angioplasty remains the cornerstone of interventional vascular therapy, new technologies should help to further expand the indications for nonsurgical interventions.

The pulsed ultrasonic cross-correlation flowmeter for two-phase flow measurement

The pulsed ultrasonic cross-correlation system is attractive for measuring the dynamic flow characteristics of two component liquid/gas mixtures because the sensors are invasive but do not extend into the pipe and, in contrast to previous continuous wave systems, have a uniform spatial sensitivity due to the absence of standing waves. The potential for measuring the flow characteristics in industrial processes including oil/gas risers is demonstrated by the measurement of the double peak cross-correlation function caused by the 'saddle' velocity distribution encountered in these risers. A theoretical model for the measurement of bubble velocity is developed and supported by experimental results.

Free radical production in amniotic fluid and blood plasma by medical ultrasound.

Evidence is reported that continuous wave and long pulse ultrasonic systems can induce relatively large amounts of free radicals in biological fluids in vitro under conditions similar to those currently in use for therapeutic applications. The operating conditions and approximate threshold intensities under which these instruments can produce free radicals are described.

In vitro color flow, pulsed and continuous wave Doppler ultrasound masking of flow by prosthetic valves

On the hypothesis that Doppler ultrasound fails to penetrate prosthetic valves, an in vitro flow simulation system was constructed in a large water tank. Conventional pulsed, continuous wave and Doppler color flow systems were used to detect flow in tubing placed diagonally within the water and maintained by a continuous pump. After control periods of flow detection within the tubing, six different prosthetic valves were interposed on a stage between the transducer and the tubing. In comparison with control measurements, detection of flow within the tubing was impossible when the Doppler beam traversed the central occluding ball of the Starr-Edwards Silastic prosthesis by any modality. Marked reduction in the detection of the Doppler signal was noted for the Starr-Edwards stellite prosthesis with only slight improvement in the ability to detect the flow signals through the central occluding discs of the Björk-Shiley, Hall-Kastor and St. Jude valves. In distinction to the other valves, the ability of Doppler ultrasound to detect flow behind the cusps of the Carpentier-Edwards heterograft was similar to that during the control period. An understanding of flow masking should improve the clinical utility of Doppler methods for investigating prosthetic valve dysfunction.

A frequency averaging method to improve sea-state measurements with a HF skywave radar

A frequency averaging method which can improve the quality of averaged spectra of HF signals scattered by the sea is described. The method is compared to the classical single frequency pulse-radar. Using the skywave radar of Valensole, the statistical independence of signals scattered by the sea at close frequencies is experimentally verified. Spectra of signals reflected by an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F2</tex> ionospheric layer are presented to illustrate the method. The possibility of further reducing interference by introducing the method in frequency modulated continuous wave (FMCW) systems is discussed.

Regions and Boundaries for Diffracting Shock Wave Systems

AbstractThis paper examines, as completely as possible, the wave systems that may occur when a plane shock wave i diffracts over a concave corner. For a perfect gas, a particular wave system is specified by three parameters (γ, ϵi, ωo), which are respectively the ratio of the specific heats γ, the inverse strength ϵi of i, and its angle of incidence ωo. The topology of the (ϵi, ωo) plane is studied for a given γ (gas), and regions or domains corresponding to each wave system are delineated. Exact expressions are given for the boundaries of many of these regions. Although it is plausible that the boundaries represent the transition conditions between the wave systems, a detailed analysis shows that the situation is sometimes more complicated than might be expected. A new transition criterion, which may be valid for diffraction over flexible surfaces, is presented. For very weak shocks (ϵi &gt; 0.8328 in air), it is found that Mach reflexion cannot exist, and in its place a continuous wave system is indicated.

Continuous-wave versus range-gated pulsed Doppler power frequency spectrum analysis in the detection of carotid arterial occlusive disease.

Two types of ultrasonic Doppler velocity metering devices currently used in the detection of extracranial carotid artery disease, the continuous-wave (CW) and the range-gated pulsed (RP) Doppler systems, were compared in the present study. Power frequency spectrum analysis (PFSA) was performed on 130 carotid arterial bifurcations with a CW Doppler and 81 carotid arteries with an RP Doppler system. All results were compared with angiographic findings. The frequency bandwidth at 50% peak power (f50%), a quantitative index for defining spectral broadening, detected stenoses equal to or greater than 50% diameter reduction with 93% sensitivity, 92% specificity, and 92% accuracy with the CW system. With the RP Doppler, the same degree of stenosis was identified with 94% sensitivity, 93% specificity, and 93% accuracy. Compared with angiographic classification into 0-24%, 25-49%, and 50-99% diameter reduction categories, CW Doppler PFSA and an 85% overall accuracy, and the RP Doppler overall accuracy was 86%. CW Doppler also correctly identified 15 of 16 internal carotid artery (ICA) occlusions; 8 of 8 ICA occlusions were correctly identified with the RP Doppler. Thus, both techniques detected carotid artery disease with comparable results. For research and ease of operation, an RP Doppler system with a variable sampling volume appears to be most desirable. However, a standard CW system is superior if utility and cost-effectiveness are of prime importance.

High field RF superconductivity: To pulse or not to pulse?

Experimental data on the behavior of superconductors under the application of RF fields of amplitude comparable to their critical fields are sporadic and not always consistent. In many cases the field level at which breakdown in superconducting RF cavities should be expected has not been clearly established. Tests conducted with very short ( \sim1\mu s) RF pulses indicate that in this mode of operation fields close to the critical values can be consistently reached in superconducting cavities without breakdown. The advantages and disadvantages of the pulsed method are discussed compared to those of the more standard continuous wave (cw) systems.

Doppler echocardiographic studies in sequential atrioventricular pacing.

Six patients with sequential atrioventricular pacemakers were studied by Doppler echocardiography. A commercially available continuous wave system (transcutaneous aortovelography) was used and the transducer was placed in the suprasternal notch and angled to obtain peak aortic arch blood flow velocity. Changes in Doppler peak velocity have been previously shown to correlate closely and reliably with changes in hemodynamically measured stroke volume/cardiac output in the same patients following interventions. In all patients, the pacemaker was programmed from DVI mode to VVI (heart rate was kept constant) and then back to DVI, with the Doppler transducer held in a fixed position all the time, and peak velocities measured after the patient had been in any particular mode for at least 1 minute. Five of 6 patients showed reduction in peak velocities ranging from 13-25% (mean 18.4%) when the pacing mode was switched from DVI to VVI. Doppler peak velocity remained unchanged in 1 patient. Doppler echocardiography represents a simple, noninvasive method for estimating increments in stroke volume/cardiac output obtained with sequential AV pacing.

Analysis of discrete wave system by optical ray operators

AbstractThe optical ray operators which are used for description and analysis of continuous‐variable wave systems are extended to the discrete‐variable wave systems, and several formulas for discrete wave systems and sampling operators are derived. The discrete wave system relates to diffraction by diffraction gratings. For the discrete wave systems with periodicity in case of a finite number of sampled values, the condition of completeness is derived in relation to the discrete Fourier transform. The result of the analysis is applied to the discussion of the recursive convolution and the derivation of the convolution theorem. The ray operators are applied further to two types of Fresnel transforms to describe wave propagation. The ray operators are also used for analysis of super‐resolution in the presence of both continuous and discrete wave systems.