分子过滤瑞利散射技术测量火焰温度和密度

 为了解决瑞利散射光易受米散射和背景杂散光干扰的问题,发展了结合窄线宽激光器、分子过滤器以及像增强器等技术的分子过滤瑞利散射技术。在图像诊断的基础上,依据测量的碘蒸气吸收光谱曲线,对CH₄/air预混火焰进行了诊断,获得了密度场和温度场分布。距炉面15 mm火焰中心区域处,分子过滤瑞利散射(FRS)技术测量的温度为1 827 K±84 K,密度为0.19 kg/m³,其测温结果与CARS法的测温结果基本吻合。最后分析了FRS技术测温不确定度。实验表明FRS技术具有较高的信噪比,可以定量测量温度和密度信息,有望应用于超音速燃烧流场、紊流场等复杂流场的诊断。     

Planar imaging of a laboratory flame and of internal combustion in an automobile engine using UV rayleigh and fluorescence light

Rayleigh scattering of tunable excimer laser light (193 nm and 248 nm) is used to obtain 2-D images of the distribution of total densities in a laboratory flame and in a cylinder of an automobile engine. Because the UV light is very strongly scattered, there is ample signal and there is excellent contrast of Rayleigh light against surface scattered light, even in the small volume of the engine cylinder. The laboratory flame data are converted to an image of the temperature field. The Rayleigh images are compared with those from planar laser induced predissociative fluorescence, which yield state-specific densities of selected molecules. The experimental arrangement is the same except for the selection of laser wavelength and the filtering of the radiated light.     

Direct determination of gas velocity and gas temperature in an atmospheric-pressure argon-hydrogen plasma jet

Direct gas temperature and gas velocity measurements made in the exit plane of a subsonic argon-hydrogen thermal plasma jet from high-resolution lineshape analysis of laser light scattered by the plasma are reported. The lineshapes are in general a superposition of the ion feature of the Thomson-scattered light and the lineshape of Rayleigh scattered light. In the center of the jet Thomson scattering dominates while at larger radii Rayleigh scattering dominates. Because of the complexity of the lineshapes of light scattered by multicomponent plasmas, only those that are predominantly due to Thomson scattering can in practice be analyzed for gas temperature. Gas velocity can be determined from the Doppler shift of the lineshapes relative to the frequency of the incident laser if the velocity is greater than about 50 m s⁻¹. The maximum gas temperature measured was 14,500 K±5%. The maximum gas velocity measured was 1100 m s⁻¹±3%. Temperature values and the radial velocity profile are compared with those previously obtained from a subsonic all-argon plasma jet operated under similar conditions.     

Particular features of acousto-optic visualization in scattering media upon detection of nonballistic light

Images of optically inhomogeneous objects immersed into a cell with a scattering liquid through which orthogonal laser and ultrasonic beams pass are obtained upon measuring the scattered light modulated at the ultrasonic frequency. The alternating current from a photodetector recording the optical radiation emerging from the cell was used as a parameter of the acousto-optic visualization. The quality of the visualization is analyzed in relation to the registration conditions and the scattering parameter magnitude, as well as the size and shape of three-dimensional and planar objects. Under the experimental conditions used, the positions of objects immersed into the scattering liquid are detected fairly precisely, with a satisfactory contrast and sharpness of images in the plane perpendicular to the laser beam axis, whereas, along this axis, the object positions are not determined. The experiment was performed under conditions of transition from a regime without scattering to a regime of multiple scattering.     

Simultaneous measurements of multiple flow velocity components using frequency modulated lasers and a single molecular absorption cell

Since unsteady, complex flow phenomena play an important role, optical measurements techniques are required for flow investigations, which provide simultaneous measurements of multiple velocity components. Doppler global velocimetry has this potential. It is a flow measurement technique, where the Doppler shift of scattered light is measured by a molecular absorption cell for frequency-to-intensity conversion. However, novel Doppler global velocimeters with laser frequency modulation were only used for single component measurements yet. In order to enhance such a system for the simultaneous measurement of multiple components, a concept based on frequency division multiplexing is introduced for the first time. Besides multiple lasers, only a single molecular absorption cell and a single detector unit is required. Two-component measurements of velocity profiles from nozzle flows are presented as a proof of principle. The designed measurement system provides high measurement rates of up to 20 kHz, which is three orders of magnitude higher than for typical Doppler global velocimetry setups.     

一种用多色光作为激发源产生拉曼光谱的新算法

探讨用多色光代替激光作为拉曼光源的新型拉曼光谱仪的可能性。根据拉曼光谱原理, 并通过数学分析, 发现当用多色光照射在样品上时, 所得到的散射光经散射频率校正后在不同频率上的强度分布可以写成样品的Raman-Rayleigh联合散射谱和激发光光源的功率谱的卷积。利用傅里叶变换算法,有可能从多色光照射样品所得到的散射光谱中导出样品的拉曼光谱。基于上述原理,可能发展出不用激光的新一代拉曼光谱仪。     

基于边缘技术的大气激光通信原理研究

介绍了利用边缘技术检测激光频率信号，实现大气激光通讯的原理。描述了利用碘分子滤波器作为边缘滤波器的原理性实验系统，并介绍和分析了实验结果。基于边缘技术的通讯系统拥有很窄的光学带宽和很高的激光频率稳定性，系统的光学信号带宽在400MHz以内，并且接收系统仅对激光信号的频率敏感，可以剔除大气中绝大部分的干扰。对于原理实验的不足之处，文中进行了初步的探讨。     

Doppler velocimetry in a supersonic jet by use of frequency-modulated filtered light scattering

A new nonintrusive velocimetry diagnostic that combines the sensitivity of frequency-modulated (FM) absorption spectroscopy techniques and the spectral discrimination afforded by atomic-vapor absorption filters is presented. Doppler-shifted light from a FM Ti:sapphire laser scattered from a supersonic flow is imaged through a potassium-vapor cell and is detected by FM spectroscopy. The difference between the potassium resonance frequency and the laser frequency when the scattered light is in resonance is the flow-induced Doppler shift.     

Ultraviolet filtered Rayleigh scattering temperature measurements with a mercury filter

We report the development of ultraviolet filtered Rayleigh scattering as a diagnostic tool for measurements of gas properties. A frequency-tripled narrow-linewidth Ti:sapphire laser illuminates a sample, and Rayleigh scattered light is imaged through a mercury-vapor absorption filter. Working in the ultraviolet improves the signal-to-noise ratio compared with that previously obtained in the visible as the result of an enhanced scattering cross section as well as the nearly ideal properties of the mercury filter. Tuning the laser through the absorption notch of the filter is a means of probing the scattering line shape, which contains temperature information. Temperature measurements of air are shown to have uncertainties of less than 3%.     

Iodine stabilised He-Ne Laser

A single mode He-Ne laser operating at 6328 Å is used with an iodine cell in the cavity to detect the absorption components of iodine falling within the gain curve of the laser line. Experimental details are given for locking the frequency of the laser line with one of the hyperfine components of the iodine absorption line, using a servo-control system. The system uses the technique of detecting the first and third harmonics of the modulation frequency.     

Imaging flow structure and species with atomic and molecular filters

Rayleigh and Raman scattering have not been widely used for flow field and combustion imaging because of their very low scattering cross-sections and because of interference from strong background light. With the use of sharp cut-off, atomic and molecular filters this background can be rejected while high throughput is maintained. The sharp cut-off edge of the filters provides for the possibility of using them for high-resolution spectral discrimination. Consequently, Rayleigh and Raman imaging are now becoming feasible and have the promise of providing quantitative images of temperature, velocity, pressure, density, species, and nonequilibrium phenomena. Because Rayleigh and Raman scattering arise from all molecules and are not affected by quenching, quantitative images can be taken of all dominant molecular species as well as thermodynamic and transport properties of complex flows, weakly ionized plasmas, and combusting fields.     

Frequency-domain lifetime imaging methods at unilever research

Fluorescence lifetime imaging methodology has been successfully implemented at Unilever Research in a frequency-domain manner. The experimental rig constructed comprises a wide-bandwidth electrooptic modulator operating on a CW argon-ion laser. The modulated excitation with a typical upper modulation frequency limit of 200 MHz falls on macroscopic samples and the resultant scattered light or fluorescence emission is then imaged onto a custom gain-modulatable image intensifier and slow-scan CCD camera combination. Phase adjustment of the image intensifier relative to the laser modulator is achieved by the RF function generator driving the intensifier. Both homodyne and heterodyne (500-Hz) strobing modes are employed to generate a double image stack (scattered light reference and fluorescence emission) comprising an image sequence as a function of instrumental phase difference. These image stacks are analyzed by Fourier least-squares methods to yield lifetime images by both phase delay and normalized demodulation. Correct operation of the apparatus is deduced from the direct imaging of a quencher-induced lifetime variation of BODIPY disulfonate over a range of concentrations. A typical industrially relevant sample, comprising an investigation of the lifetime aspects of human dental enamel autofluorescence at 50MHz modulation frequency, is given. This shows that there are real emission lifetime decreases of about 0.5 nsec in white-spot lesion areas compared to the surrounding sound enamel.     

Compact all-solid-state continuous-wave single-frequency UV source with frequency stabilization for laser cooling of Be<Superscript>+</Superscript> ions

A compact setup for generation, frequency stabilization, and precision tuning of UV laser radiation at 313 nm was developed. The source is based on frequency quintupling of a C-band telecom laser at 1565 nm, amplified in a fiber amplifier. The maximum output power of the source at 313 nm is 100 mW. An additional feature of the source is the high-power output at the fundamental and the intermediate second- and third-harmonic wavelengths. The source was tested by demonstration of laser cooling of Be⁺ ions in an ion-trap apparatus. The output of the source at the third-harmonic wavelength (522 nm) was used for stabilization of the laser frequency to molecular iodine transitions. Sub-Doppler spectroscopy and frequency measurements of hyperfine transitions in molecular iodine were carried out in the range relevant for the Be⁺ laser cooling application.     

Spin-flip Raman scattering in CdS and High resolution spectroscopy near 5145 Å

Spin-flip Raman scattering is observed in CdS, from electrons bound to donors, using the 5145 Å line of an argon-ion laser. A high improvement is provided by the use of an iodine vapour cell in the scattered beam to filter the stray light at excitation frequency. Using the spin-flip line as a magnetically tuneable source a high resolution absorption spectrum is obtained for I₂, near 5145 Å. A technique is suggested to improve the accuracy in the determination of the g factor, and to measure low g values and g-factor anisotropies.     

Intermittent frequency offset lock of a symmetric three-mode stabilized He-Ne laser to an iodine stabilized He-Ne laser

There is a need for an intense, unmodulated single-frequency stabilized laser light that guarantees absolute optical frequency in a rapid laser calibration or an ultra-high resolution interferometer. To obtain such a light, we developed a new laser system that uses an intermittent frequency offset lock of a symmetric three-mode stabilized He-Ne laser to an iodine stabilized He-Ne laser. The proposed laser system provides two operational modes: (1) independent and (2) slave mode. In the independent mode, frequency of the three-mode laser is stabilized via control of frequency difference between two intermode beats. The resultant output is a single longitudinal mode light of maximum intensity that locates at the top of the gain curve. Frequency instability of 8X10^-12 (at a sampling time of 100 s) which is better than conventional stabilized lasers is attained in the independent mode. Slow optical frequency drift during the independent mode is periodically corrected by the offset lock to the iodine stabilized laser (slave mode), resulting in accurate reset of the frequency drift. After reset of the frequency deviation, the three-mode laser is again operated in the independent mode. Due to such intermittent offset lock, duty factor of the iodine stabilized laser was reduced to a few % of continuous operation.     

Numerical analysis of spatial evolution of the small signal gain in a chemical oxygen–iodine laser operating without primary buffer gas

A chemical oxygen–iodine laser (COIL) that operates without primary buffer gas has become a new way of facilitating the compact integration of laser systems. To clarify the properties of spatial gain distribution, three-dimensional (3-D) computational fluid dynamics (CFD) technology was used to study the mixing and reactive flow in a COIL nozzle with an interleaving jet configuration in the supersonic section. The results show that the molecular iodine fraction in the secondary flow has a notable effect on the spatial distribution of the small signal gain. The rich iodine condition produces some negative gain regions along the jet trajectory, while the lean iodine condition slows down the development of the gain in the streamwise direction. It is also found that the new configuration of an interleaving jet helps form a reasonable gain field under appropriate operation conditions.     

Rheo-acoustical study of the shear disruption of reversible aggregates. Ultrasound scattering from concentrated suspensions of red cell aggregates

Shear-induced disruption of reversible aggregates or clusters in a concentrated suspension is investigated by ultrasound backscattering in the low shear regime. Fractal aggregates are considered as non-Brownian scatterers much smaller than the wavelength with acoustic properties close to those of the surrounding liquid, so that the attenuation of the coherent field is weak and multiple scattering can be neglected. The concept of variance in local particle volume fraction is used to deduce a first-order expression of the ultrasound scattering cross section per unit volume for Rayleigh scatterers in a dense suspension. On the basis of a scaling law for the shear-induced disruption of aggregates, the shear stress dependence of the ultrasonic scattered intensity from a dense suspension of clusters is derived. In a second part, the shear breakup of hardened red blood cell aggregates is investigated in plane-plane flow geometry by ultrasound scattering. Rheo-acoustical experiments are analyzed within the framework of the self-consistent field approximation and the scaling laws currently used in microrheological models. Finally, the ability of ultrasonic, light reflectometry and viscometry methods to provide quantitative information about red blood cell aggregation and membrane adhesiveness is discussed.     

Light scattering from a fluid with a stationary temperature gradient

The spectrum of light, scattered from a fluid with a stationary temperature gradient, is calculated on the basis of fluctuating hydrodynamics. Explicit expressions are obtained for the spectrum of the scattered light which is no longer symmetric around the frequency of the incident light. In particular the difference in height and intensity of the Brillouin lines is given. Furthermore the shift in the position of the maximum of the Rayleigh line is calculated.     

Fluctuations in doubly scattered laser light

Fluctuations in laser light, doubly scattered by brownian particles, were analysed by measuring the spectral noise power of the photodetector current. Scattering took place at two spatially separated systems of spherical particles. Analytic expressions for the field and intensity correlations are derived. The analytic expressions for the spectrum of the intensity fluctuations of the doubly scattered laser light demonstrate that the frequency dependence of the spectrum depends strongly of the geometry of the experimental arrangement. This is not the case for singly scattered light where in good approximation the spatial and temporal correlations can be separated analytically.Our measurements show that the noise spectrum of the doubly scattered radiation may have the same frequency dependence as the spectrum of the singly scattered light. However, there are conditions where the frequency dependence of the noise of the doubly scattered light diverges markedly from that of the singly scattered light.     

Scattering of surface waves by a sub-surface crack

The scattering of Rayleigh waves by a two-dimensional sub-surface crack, which is perpendicular to the free surface of an elastic half-space, is investigated. The boundary-value problem for the scattered field is stated, and reduced to an uncoupled system of integral equations which are solved numerically. At large distances from the crack the scattered field is shown to consist of outgoing Rayleigh waves and cylindrical body waves. Graphs of the variation of the amplitude and phase of the forward and backward scattered Rayleigh waves with the frequency and the geometrical parameters of the crack are presented. The stress intensity factors at both crack tips have also been obtained.