基于传感器阵列的野外基线环境参数自动测量系统研制

为了保证野外测距精度,研制了一套环境参数自动测量系统,该系统通过在野外基线沿线布置密集的气温、气压与湿度传感器阵列,精确测量光路气温、气压及湿度等环境参数,进行空气折射率修正。采用μ-base测距仪在不同气候条件下进行验证实验,测量距离为144 m。实验结果表明,采用该系统测量环境参数,进行空气折射率修正,其修正误差引入的距离测量不确定度优于3.0×10^-7(k=2)。     

Angstrom-range optical path-length measurement with a high-speed scanning heterodyne optical interferometer

A highly accurate method of optical path-length measurement is introduced by use of a scanning heterodyne optical interferometer with no moving parts. The instrument has demonstrated the potential to measure optical path length at angstrom resolution over continuous thickness in the micrometer range. This optical path length can be used to calculate the thickness of any material if the refractive index is known or to measure the refractive index of the material if the thickness is known. The instrument uses a single acousto-optic device in an in-line ultra-stable reflective geometry to implement rapid scanning in the microsecond domain for thickness measurements of the test medium.     

Spectroscopic Measurement of Air Temperature

Optical dimensional measurements have to be corrected for the refractive index of air. The refractive index is conventionally calculated from parameters of ambient air using either Edlén or Ciddor equations or their modified versions. However, these equations require an accurate knowledge of ambient conditions and especially the temperature of air. For example, to reach an uncertainty of 10^?7 in dimensions, the air temperature has to be known at ~100 mK level. This does not necessarily cause problems in a stable laboratory environment. However, if measurements are done outdoors or in an industrial environment, variations in temperature can be very rapid and local temperature gradients can cause significant error if not taken into account. Moreover, if the required distance is long, the temperature over the whole measurement path can be impractical or impossible to determine at sufficient temporal or spatial resolution by conventional temperature measurement techniques. The developed method based on molecular spectroscopy of oxygen allows both lateral spatial and temporal overlap of the temperature measurement with the actual distance measurement. Temperature measurement using spectroscopy is based on a line intensity ratio measurement of two oxygen absorption lines, previously applied for measurements of high temperatures in flames. The oxygen absorption band at 762 nm is a convenient choice for two-line thermometry since the line strengths are practical for short- and long-distance measurements and suitable distributed feedback lasers are commercially available. Measurements done on a 67 m path at ambient conditions demonstrate that the RMS noise of 22mK, or 7.5 × 10^?5, near 293 K using 60 s measurement time can be achieved, which is to our knowledge the best reported resolution.     

A new refractometer by combining a variable length vacuum cell anda double-pass Michelson interferometer

A new refractometer with a variable length vacuum cell has been developed to eliminate errors caused by deformations in optical windows of the cell. The refractive index of air is determined by measuring the changes in the optical path difference between the air of interest and a vacuum as a function of the changes in the cell length. An optical phase modulation technique and a dark fringe detection method are used to obtain a high resolution in measuring the optical path difference by a double-pass Michelson interferometer. A combined standard uncertainty of 5×10^-9 in the measurement of the refractive index of air has been achieved     

Composition measurements of binary mixture droplets by rainbow refractometry

So far, refractive index measurements by rainbow refractometry have been used to determine the temperature of single droplets and ensembles of droplets. Rainbow refractometry is, for the first time, to the best of our knowledge, applied to measure composition histories of evaporating, binary mixture droplets. An evaluation method is presented that makes use of Airy theory and the simultaneous size measurement by Mie scattering imaging. The method further includes an empirical correction function for a certain diameter and refractive index range. The measurement uncertainty was investigated by numerical simulations with Lorenz-Mie theory. For the experiments, an optical levitation setup was used allowing for long measurement periods. Temperature measurements of single-component droplets at different temperature levels are shown to demonstrate the accuracy of rainbow refractometry. Measurements of size and composition histories of binary mixture droplets are presented for two different mixtures. Experimental results show good agreement with numerical results using a rapid-mixing model.     

Prism-pair interferometry by homodyne interferometers with a common light source for high-accuracy measurement of the absolute refractive index of glasses

A prism-pair interferometer comprising two homodyne interferometers with a common light source was developed for high-precision measurements of the refractive index of optical glasses with an uncertainty of the order of 10(-6). The two interferometers measure changes in the optical path length in the glass sample and in air, respectively. Uncertainties in the absolute wavelength of the common light source are cancelled out by calculating a ratio between the results from the interferometers. Uncertainties in phase measurement are suppressed by a quadrature detection system. The combined standard uncertainty of the developed system is evaluated as 1.1×10(-6).     

基于Edlén公式空气折射率测量系统的校准研究

为了检验对激光干涉仪测量精度有很大影响的空气折射率测量系统的测试性能,提出一种可用于基于Edlén公式的空气折射率测量系统的校准方法,并且设计了专门的校准装置,该装置主要由温度、气压以及湿度测量系统组成。实验证明：取包含因子k=2时,温度在(5~40)℃区间,测量不确定度U优于0.02 ℃;气压在(66~105)kPa区间,U=18 Pa;湿度在(10~90)%RH区间,U=2.0%RH,等效于整个系统对应的空气折射率测量的相对不确定度优于1×10^-7     

Measurement of air refractive index fluctuation based on interferometry with two different reference cavity lengths

A measurement method based on interferometry with two different reference cavity lengths is presented and applied in air refractive index measurement in which the two cavity lengths and a laser wavelength are combined to generate two wavelength equivalents of cavity. Corresponding calculation equations are derived, and the optical path configuration is designed, which is inspired by the traditional synthetic wavelength method. Theoretical analyses indicate that the measurement uncertainty of the determined index of refraction is about 2.3×10^-8, which is mainly affected by the length precision of the long vacuum cavity and the ellipticity of polarization components of the dual-frequency laser, and the range of nonambiguity is 3.0×10^-5, which is decided by the length difference of the two cavities. Experiment results show that the accuracy of air refractive index measurement is better than 5.0×10^-8 when the laboratory conditions changes slowly. The merit of the presented method is that the classical refractometry can be also used without evacuation of the gas cavity during the experiment. Furthermore, the application of the traditional synthetic wavelength method may be extended by using the wavelength equivalents of cavity, any value of which can be easily acquired by changing cavity length rather than using actual wavelengths whose number is limited.     

Thermal Coefficients of the Optical Path Length and Refractive Index in YAG

The changes in optical path length and refractive index with temperature are measured at 633 nm in undoped YAG in the 85-285 K temperature range. At 100 K the change in optical path length with temperature is only ~25% of its value at 300 K; the change of refractive index with temperature is also substantially reduced at low temperatures.     

Interferometer for measuring the absolute refractive index of liquid water as a function of temperature at 1.064 µm

I describe here a new simple method for measuring the absolute refractive index of liquid water as a function of temperature at a wavelength of 1.064 μm with a Michelson interferometer. The mirror of the measuring arm is located in a water cell and displaced by a well-known distance. Compensation for a change in the optical path length in the reference arm of the interferometer is not necessary.     

高精度空气折射率测量系统设计与实现

针对商用空气折射率测量装置受到传感器采集性能和解算公式准确度的影响使得实际测量精度较低的问题,基于便携式多环境参数采集装置,设计了一套空气折射率测量系统,采集环境中的温湿度、大气压强状态信息,对3种折射率间接测量公式进行误差分析,并和商用环境补偿器进行性能对比。实验结果表明:在压强为100.17~100.21kPa,温度为21.1~21.9℃,湿度为45.9~58.0% RH的实验条件下,该测量系统的测量偏差比商用环境补偿器低2.69×10^-7。     

Effect of refractive index on the measurement of optical properties in turbid media

Continuous-wave measurement-based methods offer a rapid cost-effective way to determine optical properties in turbid media. This method requires a measure of the refractive index of the medium, which is often unknown a priori. Whereas previous studies have reported that the refractive index has little impact on the measurement of optical properties, here we show a significant effect of refractive indices on measurements, using both simulations and experiments. In addition we propose a noniterative method to determine the refractive index of the medium. This method can also provide an optimal initial guess of the optical properties for the standard iterative method for determining optical properties in turbid media. Our method is confirmed by simulations and experiments with latex spheres and Intralipid suspensions.     

High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser

A high-accuracy optical distance meter with a mode-locked femtosecond laser is proposed for distance measurements in a 310-m-long optical tunnel. We measured the phase shift of the optical beat component between longitudinal modes of a mode-locked laser. A high resolution of 50 mum at 240-m distance was obtained without cyclic error correction. The group refractive index of air is automatically extracted to an accuracy of 6 parts per million (ppm) by two-color measurement with the pulses of fundamental and second-harmonic wavelengths. Finally, an absolute mechanical distance of 240 m was obtained to within 8-ppm accuracy by use of a series of beat frequencies with the advantage of a wide range of intermode frequency, together with the results of the two-color measurement.     

Measuring optical fiber length by use of a short-pulse optical fiber ring laser in a self-injection seeding scheme

A method for measuring the length of an optical fiber by use of an optical fiber ring laser pulse source is proposed and demonstrated. The key element of the optical fiber ring laser is a gain-switched Fabry-Perot laser diode operated in a self-injection seeding scheme. This method is especially suitable for measuring a medium or long fiber, and a resolution of 0.1 m is experimentally achieved. The measurement is implemented by accurately determining the pulse frequency that can maximize the output power of the fiber ring laser. The measurement results depend only on the refractive index of the fiber corresponding to this single wavelength, instead of the group index of the fiber, which represents a great advantage over both optical time-domain reflectometry and optical low-coherence reflectometry methods.     

Real-time measurement of mixing ratio of refrigerant/refrigeration oil mixture

The mixing of refrigeration oil with refrigerant in a refrigeration cycle has great influence on cycle performance. A sampling method is the most general way to measure the mixing ratio of refrigerant and refrigeration oil. Since the sampling method is time-consuming and reduces the amount of refrigerant and oil in the cycle, a real-time measurement is desirable. In this study, a refractive index measurement was applied to measure the mixing ratio of refrigerant/oil mixture. A laser displacement sensor was used to detect any change in optical path which results from changes of the refractive index of refrigerant/oil mixture. For the practical application of real-time measurement of the oil circulation ratio (OCR) in the refrigeration cycle, a correlation between the refractive index and the mixing ratio was developed. In addition, the changes of the refractive index in a range of a few percentages of the oil concentration and under subcooled conditions were measured. Finally, a transient measurement of the OCR in a practically operating refrigeration cycle was carried out successfully.     

论纳米光栅测量技术

作者及其同事们经过20多年努力将计量光栅技术提高到了纳米量级和亚纳米量级，并进行了大面积推广应用，形成了一整套纳米光栅测量技术，该文为对这套技术的综合论述．首先回顾了刻线技术的发展历史，指出中国古代曾作出杰出贡献．归纳了发展计量光栅技术的5个阶段和4项内容．给出纳米光栅的定义和两个特点，并通过作者和同事们的光栅制造过程说明，纳米光栅实质上是一种刻录、固化到光栅基体上的光波波长．它为纳米测量领域提供了一种新途径、新方法，与激光干涉仪等现有纳米测量方法相比，具有自己独特的优点．文中讨论了纳米光栅的读取技术与信号处理技术，提出了纳米光栅细分误差的错位测量法．还讨论了与纳米光栅相关的纳米机械，介绍了作者和同事们的科研成果，圆柱、导轨等的机械精度已经进入了纳米量级．最后讨论了纳米光栅与激光干涉仪以及高等级量块的比对结果．     

Temperature-Dependent Refractive Index Determination from Critical Angle Measurements: Implications for Quantitative SPR Sensing

Temperature-dependent measurements of surface coverage and interfacial kinetics remain relatively unexploited in thin-film sensing applications that rely on optical surface-sensitive techniques such as surface plasmon resonance spectroscopy (SPR). These techniques are inherently sensitive to the optical properties of the bulk solution in contact with the thin film; therefore, quantitative thin-film sensing requires accurate refractive index data for bulk solutions at the conditions of interest. The refractive index for bulk solutions depends strongly on temperature, solution composition, and optical excitation wavelength. In this paper, we demonstrate the use of critical angle measurements for accurate, independent determination of the refractive index of bulk solutions and present results for different experimental conditions of solution temperature, solution concentration, and excitation wavelength. We also examine the implications of incorrect accounting of the bulk solution for the case of two-color SPR sensing of ultrathin organic films. This sensing technique, which depends inherently on the contrast in the dispersion of the refractive index of the film and the bulk solution, can be over 1 order of magnitude more sensitive than single-color SPR measurements. Critical angle measurements can be implemented in conjunction with SPR measurements and will be invaluable for thin film sensing application in which the bulk refractive index varies during the experiment, for example, in temperature-dependent SPR measurements, or for applications in which the solution refractive index is not known.     

Optical path length fluctuations in the atmosphere

The effect of atmospheric fluctuations on measurements of distance has become increasingly important as laser interferometers become more widely used. The present study evaluates the precision with which dimensions can be monitored by interferometry involving optical path lengths in the open air under good atmospheric conditions. The size and time scale of optical path length fluctuations over distances of one meter, and correlations between fluctuations in nearby optical paths, are measured and analyzed.     

Digital Hilbert transformation for separation measurement of thicknesses and refractive indices of layered objects by use of a wavelength-scanning heterodyne interference confocal microscope

A wavelength-scanning heterodyne interference confocal microscope quickly accomplishes the simultaneous measurement of the thickness and the refractive index of a sample by detection of the amplitude and the phase of the interference signal during a sample scan. However, the measurement range of the optical path difference (OPD) that is obtained from the phase changes is limited by the time response of the phase-locked loop circuit in the FM demodulator. To overcome this limitation and to improve the accuracy of the separation measurement, we propose an OPD detection using digital signal processing with a Hilbert transform. The measurement range is extended approximately five times, and the resolution of the OPD is improved to 5.5 from 9 microm without the electrical noise of the FM demodulator circuit. By applying this method for simultaneous measurement of thickness and the refractive index, we can measure samples 20-30-microm thick with refractive indices between 1 and 1.5.     

Development of an interferometer for measurement of the diffusion coefficient of miscible liquids

A common-path interferometer (CPI) system was developed to measure the diffusivity of transparent liquid pairs by real-time visualization of the concentration gradient profile. The CPI is an optical technique that can be used to measure changes in the gradient of the refractive index of transparent materials. The CPI is a shearing interferometer that shares the same optical path from a laser light source to the final imaging plane. Molecular diffusivity of liquids can be determined by use of physical relations between changes in the optical path length and the liquid phase properties. The data obtained by this interferometer are compared with similar results from other techniques. This demonstrates that the instrument is reliable for measurement of the diffusivity of miscible liquids and allows the system to be compact and robust. It can also be useful for studies in interface dynamics as well as other applications in a low-gravity environment.