Doppler velocimeter based on dual-comb absorption spectroscopy

Blood flow velocity measured by Doppler ultrasound is the relative velocity dependent on the path of the ultrasound beam, which should be influenced by its angle of incidence against the blood flow in the vessel. The angle of incidence generates varying changes in flow velocities that can be measured by the Doppler device. The aim of our study was to develop a new ultrasonic Doppler catheter which could provide a true flow velocity independently of the angle of the ultrasound beam against the flow direction, and to assess the validity of the true flow velocity obtained by a new device using the electromagnetic flowmeter. The newly developed Doppler catheter has a pair of adjoining ultrasonic crystals located on one side of the catheter at right angles to each other. Each Doppler shift, which is detected by two transducers (Δf1, Δf2) that sample the flow velocity at two closely spaced points, is used to compute two velocity measurements (V1 and V2); these are the velocities detected by the transducers. The true velocity was calculated using the following equation: V=((V1)2+(V2)2)1/2, where V = true velocity. The velocities were calculated by newly developed phase differential techniques. Using a continuous flow model, we compared the flow velocity measured by the new Doppler catheter with that assessed by an electromagnetic flow probe placed into the circuit. At between 0.42 and 4.49 l·min-1, the flow velocity measured by the new Doppler catheter (Doppler velocity) at five sampling depths was compared with the mean velocity calculated from the volumetric flow rate measured by an electromagnetic flowmeter (EMF velocity). The Doppler velocity (y) strongly correlated with the EMF velocity (x) at five sampling depths (r 2=0.99, respectively). At the maximal velocity sampling depth, the regression equation was y=1.29x+2.47 (r 2=0.99,P<0.0001,n=41, SEE=0.015). The Doppler velocity also correlated with the volumetric flow rate measured by the electromagnetic flowmeter (r 2=0.99). The flow velocity measurements using the new Doppler catheter and device we have developed can provide more instantaneous and useful information on hemodynamics.

This work investigates the error caused by nonuniformities along the line-of-sight in velocity measurement using tunable diode-laser absorption spectroscopy (TDLAS). Past work has demonstrated TDLAS as an attractive diagnostic technique for measuring velocity, which is inferred from the Doppler shift of two absorption features using two crossing laser beams. However, because TDLAS is line-of-sight in nature, the obtained velocity is a spatially averaged value along the probing laser beams. As a result, nonuniformities in the flow can cause uncertainty in the velocity measurement. Therefore, it is the goal of this work to quantify the uncertainty caused by various nonuniformities typically encountered in practice, including boundary layer effects, the divergence/convergence of the flow, and the methods used to fit the Doppler shift. Systematic analyses are performed to quantify the uncertainty under various conditions, and case studies are reported to illustrate the usefulness of such analysis in interpreting experimental data obtained from a scramjet facility. We expect this work to be valuable for the design and optimization of TDLAS-based velocimetry, and also for the quantitative interpretation of the measurements.

Tunable Diode Laser Absorption Spectroscopy (TDLAS) applies optical method to realize fast and non-contact detection. This technology has the advantages of high resolution and high sensibility, and been widely employed in atmosphere environment detection in recent years. This paper mainly introduces how TDLAS is applied to detect the vehicle exhaust, to determine the content of CO, CO2, NO and alkanes etc. Firstly, the principle of TDLAS is Lambert-Beer's law. It is based on the specific spectral fingerprint characteristics of different gases to detect the gas composition. In the same time, the light intensity is attenuated by molecular absorption, which is used to accurately analyze the density of gas. There are two ways to achieve. One is direct-absorption way, whose configuration and signal processing are simple, results have no need to be demarcated. The other is wavelength-modulation way, which has high resolution and low threshold. The selection of absorption spectral line needs not only to adapt the central wavelength of laser and responding wavelength of detector, but also to avoid the crossed spectral line of different gas to improve the precision of detection. Secondly, TDLAS system includes three parts, which are signal generation, signal detection and signal processing. Finally, TDLAS is compared with other spectral detection technologies in the aspect of sensitivity, character of working, applicable kinds of gas, in order to highlight the application scope of each technology.

Filtered Rayleigh scattering is an optical diagnostic technique that allows for simultaneous planar measurement of velocity, temperature, and pressure in unseeded flows. An overview of the major components of a filtered Rayleigh scattering system is presented. In particular, a detailed theoretical model is developed and discussed with associated model parameters and related uncertainties. Based on this model, results for two experimental conditions are presented: ambient room air and a Mach 2 freejet. These results include two-dimensional, spatially resolved measurements of velocity, temperature, and pressure derived from time-averaged spectra. ILTERED Rayleigh scattering (FRS), a recently developed flow diagnostic technique,1'2 achieves large suppression of background scattering allowing planar flowfield visualization and obtains quantitative measurements of velocity, temperature, and density in unseeded gaseous flows. This technique makes use of Rayleigh scattering from molecules in the flow and is driven by a high-power, narrow linewidth, tunable, injection seeded laser. When imaging the scattered light onto a charge-coupled device (CCD) camera, unwanted background scattering from stationary objects may be filtered out by tuning the frequency of the narrow linewidth laser to coincide with an atomic or molecular absorption line and by placing a cell containing the atomic or molecular species between the camera and the flow. This cell acts as a notch filter, absorbing all background scatter at the laser frequency. Scattered light that is Doppler shifted, however, passes through the filter and is imaged on the camera. Quantitative measure of flow properties is achieved by measuring the total intensity, Doppler shift, and spectral profile of the Rayleigh scattered light. The total intensity is directly proportional to density; the Doppler shift is directly proportional to velocity; and the spectral profile is a function of temperature and pressure. The scattering intensity, Doppler shift, and spectral profile are determined by passing the scattered light through the notch absorption filter and then by imaging it onto an intensified CCD camera. Because the filter absorbs light in a narrow frequency band, it converts the spectral information contained in the Doppler shift and Rayleigh profile into intensity information at the camera. By collecting data (camera pixel intensity) for varying conditions, v, T, and P may be determined. Previous work has concentrated on the use of this technique for background suppression when visualizing flows and for the measurement of velocity. The background suppression feature of FRS has been used to image flowfields that otherwise would be completely obscured by the strong scattering from wind-tunnel surfaces. The authors have used this technique to image the flowfield inside a Mach 3 inlet and to generate volumetric images of the crossing shocks and boundary layer.3 Elliott et al.4 have also used this technique to observe structures in compressible mixing layers. The use of FRS to measure velocity was initially demonstrated using scattering

Real-time breath gas analysis with high accuracy, precision and time resolution, as a promising, non-invasive, fast and reliable tool, is important in medical diagnostics. Especially stable isotopologues of carbon dioxide is applied to multiple research areas including the diagnosis of gastrointestinal diseases. Helicobacter pylori (H. pylori) is one of the most frequent bacterial infectious diseases in human beings and is now recognized as one of the key risk factors for chronic gastritis, peptic ulcers, stomach cancer and lymphoma. In contrast to traditional invasive tests, the most reliable non-invasive method in the diagnosis of the H. pylori infection is considered to be 13C-urea breath test which is implemented by measuring the 13CO2/12CO2 isotope ratio in human breath. Tunable diode laser absorption spectroscopy (TDLAS) has the advantages of fast response, low drift, good gas selectivity and high detection sensitivity, and it is very convenient to develop a high precision, real-time and online measurement system. A precision laser spectrometer for the measurement of CO2 isotope abundance in human breath (with CO2 concentration of 4%-5%) or high concentration gas is designed and evaluated based on TDLAS technology. The spectrometer contains a novel compact dense-pattern multipass cell with a small volume of 280 cm3 and an effective optical path length of 26. 4 m. The cell is in conjunction with a fiber-coupled distributed feedback diode laser operating at 2.008 μm. Wavelength modulation spectroscopy approach is used. The mass flow, pressure and temperature of the cell are actively controlled, and able to keep long-term stability. The influence of laser power fluctuation is eliminated by fitting the baseline with cubic polynomial to normalize the raw spectrum. Moving window regression is used to remove the influence of frequency drift on measuring isotope abundance. The system measurement precision is improved by wavelet denosing and Kalman filtering. The experimental results demonstrate that moving window regression method not only extends the stability time of the system but also improves the measurement precision of isotope abundance well, the wavelet denoising improves the signal-to-noise ratio by 2 times that by the method of multi-spectral average, the stability time of the system is 100 s given by Allan variance, and the measurement precision of CO2 isotope ratio is 0. 067‰ after Kalman filtering. The use of small multi-pass cell and the default of denoising devices make the system more portable and improve the real-time and online measurement performance of the system. In addition to the measurement of 13CO2/12CO2 isotope ratio in human breath, by replacing different lasers, the spectrometer can also be used to measure trace gas concentration and the stable isotope abundance of many gas molecules in atmosphere. Therefore, the spectrometer will have broad applications in the areas of medical diagnosis, carbon cycle study and environmental monitoring.

Most aircrafts is driven by chemic energy which is released in the combustion process. For improving the capability of engine and controlling the running on-time, the processes of fuel physics and chemistry need to be analysis by kinds of high quality sensor. In the research of designing and improving the processes of fuel physics and chemistry, the concentration, temperature and velocity of kinds of gas in the combustor need to be detected and measured. In addition, these engines and research equipments are always in the harsh environment of high temperature, high pressure and high speed. The harsh environment needs the sensor to be high reliability, well repetition, no cross- sensitivity between gases, and the traditional measurement system can't satisfy the metrical requirement well. Tunable diode laser absorption spectroscopy (TDLAS) analytic measurement technology can well satisfy the measurement in the harsh environment, which can support the whole measurement plan and high quality measurement system. Because the TDLAS sensor has the excellence of small bulk, light weight, high reliability and well specifically measurement, the TDLAS measurement technology has wide prospects. Different from most measurements, only a beam of laser can be pass through the measured environment by TDLAS, and the measurement equipment needn't be set in the harsh environment. So, the TDLAS equipment can't be interrupted by the measured equipment. The ability of subsistence in the harsh environment is very valuable, especially in the measurement on the subject of aerospace with environment of high speed, combustion and plasma. This paper focuses on the collecting the articles on the subject of oxygen detection of TDLAS. By analyzing the research and results of the articles, we conclude the central issues, difficulties and results. And we can get some instructive conclusions.

Development and validation of a laser-absorption-based method for in-situ and high-precision water vapor dew point temperature measurement

Temperature and OH concentration measurements by ultraviolet broadband absorption of OH(X) in laminar methane/air premixed flames

After completing this article, readers should be able to: 1. Describe the calculations used to measure fetal systemic and placental vascular impedance in the umbilical artery via Doppler ultrasonography. 2. Delineate the two primary uses of measurement of middle cerebral artery flow velocity. 3. List the fetal conditions in which Doppler flow velocity has been studied. The Doppler principle, discovered by Johann Christian Andreas Doppler in 1842, (1) was used initially in astronomy and later in the military, in various branches of industry, and more recently in medicine. Doppler shift is based on the perceived change in frequency of an energy waveform as the source and the receiver move toward or away from each other. The Doppler shift principle applies to light waves as well as sound waves. In biology, Doppler ultrasonography is used noninvasively, primarily to measure flow velocity in blood vessels. Flow velocity depends on the volume and speed of effluent relative to the diameter of the vessel. Estimating fetal vascular resistance through study of Doppler flow velocity provides a means to assess, albeit indirectly, fetal physiology and pathology. Johnson and colleagues (2) in 1965, made the earliest mention of the use of Doppler effect in obstetrics. The use of Doppler ultrasonography to investigate the pattern of waveforms in the umbilical artery during pregnancy initially was reported in 1977 by Fitzgerald and Drumm. (3) Subsequently, Wladimiroff and associates (4) reported on Doppler assessment of cerebral blood flow in the human fetus. The addition of color mapping has enhanced the utility of the Doppler effect in sonographic assessment of the fetus. The fetal umbilical artery (UA) and middle cerebral artery (MCA) have evolved as the primary targets of fetal Doppler studies (Figs. 1 and 2). Figure 1. Color Doppler view of circle of Willis. Arrows show anterior (ACA), middle (MCA), and posterior cerebral …

Transdural anastomosis (TA) from the middle meningeal arteries (MMAs) to the brain is important in maintaining cerebral blood flow in patients with Moyamoya disease. A 16-year-old adolescent girl presented with a transient ischemic attack. Preoperative cerebral angiography revealed TA from the MMA. During surgery, the MMA was exposed on the dural surface directly under the burr hole. Measurement of flow velocity in the MMA with an ultrasonic blood flowmeter revealed increased diastolic flow velocity. As the MMA was assumed to contribute to TA, a new burr hole was made to preserve the MMA, and a bone flap was prepared. Direct and indirect bypass surgeries were performed. Electrocoagulation was not applied to the MMA associated with TA, but the artery was compressed with gelfoam containing fibrin glue to achieve hemostasis. Postoperatively, no evidence of cerebral infarction was detected. Determining which of several MMAs exposed in the surgical field is associated with TA can be difficult. However, measurement of flow velocity with an ultrasonic blood flowmeter to confirm increased flow velocity in the diastolic phase is useful for confirming the presence or absence of TA from the extracranial arteries including the MMA.

Temperature is an important indicator for assessing the state of combustion. With the development of technology, the application of noncontact temperature measurement techniques is becoming increasingly widespread. The tunable diode laser absorption spectroscopy (TDLAS) technique is noninvasive, highly sensitive, and capable of continuous measurement. However, the TDLAS technology has limitations in terms of spatial resolution, and existing studies are homogeneous in terms of the selection of characteristic spectral lines. In this paper, two absorption lines of oxygen (O2) were innovatively selected. A 64-element array sensor was used to acquire signals. A standard high-temperature tube furnace was used to calibrate the sensor, and the mathematical relationship between the integral absorbance of the absorption lines and temperature was obtained. Finally, the accuracy and stability of the calibration results were verified. This study is important for the further investigation of temperature distribution in high-resolution combustion fields.

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. It needs to combine with a long absorption path in the ambient trace-gas measurements. TDLAS has a significant advantage not only in sensitivity but also in rapidity of response. It has been widely used in many atmospheric trace-gases detection, ground trace-gas detection and gas leakage detection. On-line signal processing is necessary to most trace gas monitor. In this paper, data processing method for retrieval of gas concentration, as well as the wavelength alignment algorithm based on correlation detecting technique is described, realizing the aim of wavelength alignment between the standard signal spectral line and the measurement signal spectrum. So the precision of the retrieving system is improved satisfactorily.

The measurement of flow velocity and particle size remains an important issue in granular -flow dynamics and can provide important basis to better understand the physics in granular material, particularly when it impacts on a structure. In this study, laboratory chute experiments were performed with quartz-glass particle materials to investigate the characteristics of granular shock developed upstream of a cylinder generated by granular flow impacts. A time series of flow images recorded by a camera has been analyzed and processed using the digital image processing methods such as the gray processing, the image binarization, the image corrosion and expansion, and the generative adversarial networks, with a goal of obtaining flow velocity and particle size in the granular shock area. The experimental results reveal that the granular-flow velocity grows with increasing slope angle. The granular shock thickness shows a general increase with the growing number of particles in the shock area, and the number of particles demonstrates an inversely proportional to increasing Froude number, providing a potential method for determining the particle size of dense granular flow. The findings of this study could help to better understand the shock dynamics of granular flow impacting on an obstacle.

Chapter 11 - Laser instrumentation and measurement

The use of optical circular multipass absorption cells (CMPAC) in an open-path configuration enables the sampling free analysis of cylindrical gas flows with high temporal resolution and only minimal disturbances to the sample gas in the pipe. Combined with their robust unibody design, CMPACs are a good option for many applications in atmospheric research and industrial process monitoring. When deployed in an open-path configuration, the effects of inhomogeneities in the gas temperature and composition have to be evaluated to ensure that the resulting measurement error is acceptable for a given application. Such an evaluation needs to consider the deviations caused by spectroscopic effects, e.g., nonlinear effects of temperature variations on the intensity of the spectral line, as well as the interaction of the temperature and concentration field with the characteristic laser beam pattern of the CMPAC. In this work we demonstrate this novel combined evaluation approach for the CMPAC used as part of the tunable diode laser absorption spectroscopy (TDLAS) reference hygrometer in PTB's dynH2O setup for the characterization of the dynamic response behavior of hygrometers. For this, we measured spatially resolved, 2D temperature and H2O concentration distributions, and combined them with spatially resolved simulated spectra to evaluate the inhomogeneity effects on the line area of the used H2O spectral line at 7299.43 cm-1. Our results indicate that for dynH2O, the deviations caused by the interaction between large concentration heterogeneities and the characteristic sampling of the beam pattern of the CMPAC are three orders of magnitude larger than deviations caused by small temperature heterogeneity induced spectroscopic effects. We also deduce that the assumption that the "path-integrated" H2O concentration derived with the open-path CMPAC setup represents an accurate H2O area average in the flow section covered by the CMPAC in fact shows significant differences of up to 16% and hence does not hold true when large H2O concentration gradients are present.