Measurement capabilities of planar Doppler velocimetry using pulsed lasers

Analytical models of a spectral filter that contains iodine vapor and of the noise sources associated with charge-coupled-device (CCD) detector technology are combined with a planar Doppler velocimetry (PDV) signal analysis to evaluate the measurement capabilities of PDV for quantitative aerodynamic research and production wind-tunnel testing applications. The criteria for optimizing the filter cell and calibrating the frequency scale of its transmission function are described. The measurement uncertainty limits owing to scientific-grade CCD detector performance are then evaluated, and an analysis is developed of the scattering properties of aerosols suitable for aerodynamic flow seeding. The combined results predict that single-pulse PDV measurements with velocity measurement uncertainties as small as 2 m/s should be possible in aerodynamic test facilities for measurement distances of tens of meters.     

Frequency-locked light scattering: real-time Doppler velocimetry with closed-loop feedback control

Real-time measurement capability of a frequency-modulated filtered light-scattering- (FM FLS) Doppler velocimeter has been demonstrated. Doppler-shifted light from a frequency-modulated Ti:sapphire laser scattered from a supersonic flow is imaged through a potassium vapor cell and is detected by FM spectroscopy. The FM signal is used in closed-loop feedback control of the laser frequency to lock the Doppler-shifted scattered light to the resonance frequency of the filter. The difference between the filter resonance frequency and the laser frequency when the scattered light is frequency locked to the filter resonance is the flow-induced Doppler shift. Changes in flow velocity are tracked by changes in laser frequency, which is subsequently measured to obtain the Doppler shift. The frequency-locking capability of the technique was achieved with use of a simple analog controller. The random Doppler shift measurement errors (2varsigma) were approximately 20 MHz, which correspond to velocity measurement errors for the real-time measurement of less than 3% in a 10-Hz bandwidth.     

Minimum deviation of spatial frequency in large-particle sizing

A new method is introduced to improve the accuracy of sizing large particles in two-phase flows. This method uses a photodetector array to measure directly the spatial frequency of the light intensity scattered from a spherical particle in the measurement volume (or cross-sectional area). The effects of both the reflected and the refracted rays are considered. The phase conversion factors from the refracted and the reflected rays are used to minimize the measurement volume effect (errors) in sizing large particles. Furthermore, optimization results are compared with the results from the simulation based on the generalized Lorenz-Mie theory. A new particle-sizing system that combines the conventional and the new methods is suggested.     

频率调制多普勒全场测速实验系统测试

频率调制多普勒全场测速技术是一种基于分子滤波和多普勒频移现象的流场速度测量方法,在高速、超高速及大尺度风洞流场测量方面潜力巨大.我们设计开发了采用CCD相机作为接收探头的FM-DGV实验系统,该系统主要包括激光器、片光光学系统、碘分子滤波器、图像采集相机、频率监测单元等.基于该系统进行了谐波幅值比和转盘线速度测试实验.实验结果表明,该实验系统工作正常,速度测量误差最大值小于2m/s.     

太赫兹阵列探测器响应度校准溯源研究

提出了一种太赫兹阵列探测器响应度校准溯源方法。首先,使用阵列探测器的每一像元对太赫兹辐照场中心进行逐一扫描,识别有效像元、死像元和过热像元,并对有效像元响应值进行归一化处理,获得阵列探测器的相对辐照度响应值。其次,使用中心像元对太赫兹辐照场进行扫描测量,扫描总面积大于太赫兹光斑尺寸,保证太赫兹功率被完整测量,获得中心像元与其他有效像元的太赫兹功率响应值。最后,用标准太赫兹功率计标定高莱功率计,扩展太赫兹功率校准量限,实现微瓦级太赫兹辐射功率测量溯源,用高莱功率计测得的太赫兹辐射源总功率对阵列探测器测得的积分响应值进行校准,得到阵列探测器辐照度响应绝对值。对探测器进行测量校准和不确定度分析,测得辐照度响应度的相对扩展不确定度为Urel=20%(k=2),测量结果可溯源至国家太赫兹辐射功率标准。     

Real-Time Measurement of Microwave Parameters and EM Fields

A technique for making fixed-frequency or wide-band real-time measurements of reflection coefficients, transmission coefficients, and fields in open or closed structures at UHF and microwave frequency is given. The basic system uses two isolated channels, which originate from a common CW source, and one channel is single-sideband modulated. These two signals are then added and mixed in a (homodyne) detector, and the detected signal at the modulation frequency displays the desired amplitude and phase information in real time simultaneously and independently. The sources of inherent phase and amplitude errors are considered, including those due to the unwanted sidebands created by imperfect modulation. Operation on the linear portion of the characteristic curve of a balanced detector tends to reduce these errors and increase the dynamic operating range. The system sensitivity should approach -110 to -120 dBm, and dynamic operating ranges of the order of 100 to 110 dB are possible.     

The development of a portable spectrophotometer for noncontact color measurement

A portable spectrophotometer for noncontact color measurement was developed to display spectral distribution and tristimulus values of a color simultaneously. It comprises electronic control circuits, control firmware, and an optical mechanism with a photo detector array. Essentially, a light beam is reflected or transmitted by a measured object into a device that disperses the light beam into its spectrum on a photo detector array. Then, the photo detector module generates a series of signals as functions of light intensity and produces their corresponding digital values reflecting light intensity. These digital data are processed by the electronic control circuits and control firmware to directly provide the spectral distribution and standard color values. The measured results of the developed system were also compared with those obtained by a similar instrument on the market.     

Accurate Doppler angle estimation for vector flow measurements

Traditional Doppler methods measure only the axial component of the velocity vector. The lack of information on the beam-flow angle creates an ambiguity that can lead to large errors in velocity magnitude estimates. Different triangulation techniques so far have been proposed, which basically perform multiple measurements of the Doppler frequency shift originating from the same region. In this work, an original approach is introduced, in which two ultrasound beams with known relative orientation are directed toward the same vessel, but only one of them is committed to perform a Doppler measurement; the second (reference) beam has the specific task of detecting the beam-flow angle. The latter goal is obtained by accurately identifying the achievement of the target 900 reference-beam-to-flow angle through the inspection of the backscattered Doppler signal spectrum. In transverse flow conditions, in fact, such spectrum is expected to be centered on the zero frequency, and even small deviations from the desired 900 orientation cause noticeable losses of spectral symmetry. Validation of the new method has been performed through experimental tests, which show that the beam-flow angle can be estimated with high accuracy (rms errors lower than 1 degree), and repeatable velocity magnitude measurements are possible. A procedure for automatically tracking the desired orientation by the reference beam is also introduced and shown suitable for implementation in steerable linear array transducers.     

Uncertainty analysis of absolute concentration measurement with continuous-wave cavity ringdown spectroscopy

To evaluate the uncertainty of concentration measurement using cavity ringdown spectroscopy, we analytically derived expressions for uncertainty for parameters, such as temperature, laser frequency, and ringdown time deviation, from the model equation. The uncertainties that are due to systematic errors in a practical cavity ringdown system were assessed through an experimental study of the PQ(35) transition in an A band of molecular oxygen. We found that, except for the line strength that is regarded as a reference value independent of the measurement, the laser frequency jitter is the largest uncertainty source in the system. Some practical requirements for minimizing the uncertainty in concentration measurements are discussed. We also demonstrated determination of the line strength of the PQ(35) transition line of oxygen to be 8.63(3) x 10(-27) cm(-1) with a relative uncertainty of less than 0.4%.     

High-precision velocimetry: optimization of a fabry-perot interferometer

We present the optimization of a Fabry-Perot velocimeter designed to measure speed at a few millimeters per second with a relative uncertainty of 10(-8). We focus on the accuracy and the optimization of the Fabry-Perot, with a review of the uncertainties related to the geometry, the beam shape, and the Doppler frequency measurement. These errors are quantified to ensure that the required accuracy is reached. We then describe the practical implementation and show the results.     

1 550 nm全光纤激光多普勒测振系统研制

针对目前国内激光测振系统价格昂贵、使用不便的问题,采用1 550 nm波段成熟的窄线宽光源和光纤元器件研制了一套低成本的全光纤激光测振系统原理样机。此原理样机光路部分采用马赫?泽德干涉仪结构,搭建了外差式激光干涉光路,参考光被40 MHz的声光调制器调制,与测量光在光电探测器表面发生干涉,产生原始的激光多普勒信号;信号解调部分采用相位解调法对原始激光多普勒信号进行解调,得到振动目标的运动特性,包括位移、速度和加速度信息。采用本单位的振动标准装置对其性能进行了测试,实验结果表明：在10 ~ 2 000 Hz的中低频振动范围内,1 550 nm全光纤激光多普勒测振系统峰值位移、速度和加速度的测量误差在-0.6% ~ 0.7%内。该系统在中低频段具有较高的测量准确度,且成本相对较低、操作便捷,具有技术借鉴价值。     

Enhancing time-stamp counter phase modulation measurements

This paper demonstrates that digital signal processing techniques can enhance the quality of phase modulation measurements produced by a time-stamp (phase digitizing) frequency counter. A typical time-stamp counter utilizes a digital divider to reduce signal frequency to the desired sample rate. Unfortunately, division also reduces phase modulation to the point where useful information may be obscured by counter measurement uncertainty (jitter). An analogy between an analog-to-digital converter (ADC) and a time-stamp counter predicts that the counter induced modulation can be modeled as random noise which is white in phase. The noise magnitude is directly related to the instrument's resolution specification. Fourier analysis, subject to some restrictions, can compute the power spectra of phase or frequency modulation, revealing even low level responses. A number of techniques can be used to reduce the amount of counter induced noise that appears on time domain plots of phase and frequency modulation. Experimental data, generated by a prototype counter, illustrates the type of results that can be expected from Fourier analysis and various noise reduction techniques     

Determination of physiological levels of glucose in an aqueous matrix with digitally filtered Fourier transform near-infrared spectra

A procedure is described for the measurement of clinically relevant concentrations of glucose in aqueous solutions with near-infrared (NIR) absorbance spectroscopy. A glucose band centered at 4400 cm-1 is used for this analysis. NIR spectra are collected over the frequency range 5000-4000 cm-1 with a Fourier transform spectrometer. A narrow-band-pass optical interference filter is placed in the optical path of the spectrometer to eliminate light outside this restricted range. This configuration provides a 2.9-fold reduction in spectral noise by utilizing the dynamic range of the detector solely for light transmitted through the filter. In addition, a novel spectral processing scheme is described for extracting glucose concentration information from the resulting absorbance spectra. The key component of this scheme is a digital Fourier filter that removes both high-frequency noise and low-frequency base-line variations from the spectra. Numerical optimization procedures are used to identify the best location and width of a Gaussian-shaped frequency response function for this Fourier filter. A dynamic area calculation, coupled with a simple linear base-line correction, provides an integrated area from the processed spectra that is linearly related to glucose concentrations over the range 1-20 mM. The linear calibration model accurately predicted glucose levels in a series of test solutions with an overall mean percent error of 2.5%. Based on the uncertainty in the parameters defining the calibration model and the variability of the magnitudes of the integrated areas, an overall uncertainty of 7.8% was estimated for predicted glucose concentrations.     

一种新型红外多组份气体传感器

针对红外多组份气体分析存在的问题,采用窄带红外滤光片、光锥、热释电探测器阵列组合技术,研制了一种新型多组份气敏传感器。通过更换滤光片组件功能模块,可实现不同测量领域不同种类气体的定量分析。该传感器具有结构新颖、简单可靠、成本低等特点。实验结果表明,SO2和CO2的最大相对测量误差分别为8.3%和1.2% 。     

Modeling of direct detection Doppler wind lidar. I. The edge technique

Analytic models, based on a convolution of a Fabry-Perot etalon transfer function with a Gaussian spectral source, are developed for the shot-noise-limited measurement precision of Doppler wind lidars based on the edge filter technique by use of either molecular or aerosol atmospheric backscatter. The Rayleigh backscatter formulation yields a map of theoretical sensitivity versus etalon parameters, permitting design optimization and showing that the optimal system will have a Doppler measurement uncertainty no better than approximately 2.4 times that of a perfect, lossless receiver. An extension of the models to include the effect of limited etalon aperture leads to a condition for the minimum aperture required to match light collection optics. It is shown that, depending on the choice of operating point, the etalon aperture finesse must be 4-15 to avoid degradation of measurement precision. A convenient, closed-form expression for the measurement precision is obtained for spectrally narrow backscatter and is shown to be useful for backscatter that is spectrally broad as well. The models are extended to include extrinsic noise, such as solar background or the Rayleigh background on an aerosol Doppler lidar. A comparison of the model predictions with experiment has not yet been possible, but a comparison with detailed instrument modeling by McGill and Spinhirne shows satisfactory agreement. The models derived here will be more conveniently implemented than McGill and Spinhirne's and more readily permit physical insights to the optimization and limitations of the double-edge technique.     

通道间延迟时间差的测量不确定度

介绍了用正弦波拟合法评价测量系统通道间延迟时间差不确定度的分析和评定过程。讨论了几个主要的不确定度来源,包括信号源误差、采集序列的谐波失真、噪声及非谐波失真、抖动、软件拟合运算误差等。结合一个实例,给出了通道间延迟时间差测量不确定度的评定结果。该过程可用于相应计量标准的不确定度分析与评定。     

改进的相控阵多普勒计程仪波束形成技术

目前相控阵式多普勒计程仪的基阵体积比活塞式多普勒计程仪已经缩减了很多,该技术只需通过一个收发共用的平面基阵实现信号的发射和接收,可以同时形成4个方向的波束,大大减小了基阵的尺寸,同时还消除了声速变化对测速的影响。但在中深度和大深度多普勒计程仪设备中,换能器基阵的体积仍然限制了其适装的载体种类。以多普勒计程仪小型化思想为核心,分析了密排相控阵多普勒计程仪波束形成技术,对多普勒计程仪进行集成化、小型化设计。针对密排相控阵波束形成技术,比较其发射指向性、声源级、信噪比、系统复杂度与稀疏相控阵的差异。分析表明,在相同频率、相同发射功率以及相同尺寸的条件下,密排相控阵的收发联合响应较稀疏相控阵提高近6 dB。同时在相同测速作用距离的条件下,利用密排相控阵技术可以减少换能器阵元数,从而减小换能器尺寸,达到小型化的目的。     

Filter Transmittance Measurements in the Infrared

We have set up a novel direct detection system to measure filter transmittances over an attenuation range of at least 5 decades, with relative combined standard uncertainties as low as 0.5% (1σ) per decade, in the 9 μm to 11 μm spectral region. This system, using an apparatus originally designed for a heterodyne measurement of transmittance, achieves higher accuracy at the expense of a reduced dynamic range. This independent measurement of transmittance allows verification of the heterodyne technique. Our system uses a source modulated at 30 MHz and a specially constructed high dynamic range and high accuracy lock-in amplifier capable of operation at the modulation frequency. The high modulation frequency and narrow bandwidth of the system allow thermal background radiation to be suppressed and high accuracy to be achieved. We correct for the non-ideal natures of the detector and attenuators. In particular, the detector position is scanned to reduce the effect of its spatial nonuniformity and the deflection of the transmitted beam caused by the nonparallel surfaces of the filter. We discuss the sources of systematic errors and the methodology to reduce their contribution.     

一种共路补偿激光漂移的直线度测量方法

在激光基准长距离、高精度直线度测量中,空气扰动是影响其测量精度的一个主要因素。提出了一种简单的共路补偿空气扰动造成激光漂移的方法。角锥棱镜作为位置敏感器,四象限探测器探测角锥棱镜逆向反射光所携带的直线度误差,二维PSD和透镜用来监测空气扰动所带来的角度漂移。实验结果表明,通过补偿后,空气扰动所带来附加位置误差可以减少大约75%。     

Fiber-optic temperature sensor used for oil well based on semiconductor optical absorption

This paper describes an optical-fiber sensor developed for temperature measurement under offshore oil well conditions. The sensor exploits the displacement of the optical absorption edge occurring in semiconductors under the influence of temperature variation as a result of temperature-induced energy shifting of conduction band extrema. The structure of the sensor and the measurement principle are described. The common-path reference measurement and node type error compensation technologies are developed. And, the detailed theoretical analysis indicates that the proposed sensor system can effectively improve measurement errors caused by light fluctuation, difference and variation of the detector responsiveness, circuit magnification times, and so on. It proves that this sensor system can be applied under long-term formidable conditions with fairly good measurement stability.