Far-infrared laser measurement of the refractive index of polypropylene

The refractive index of polypropylene in the far infrared (FIR) is measured by means of a suitably modified laser of a FIR spectrometer. When thin polypropylene films of 12.7-microm nominal thickness are introduced in the optical cavity of a laser at the Brewster angle, the radiation ceases because of the change in the optical path of the laser beam. This change is measured from the displacement of one of the laser mirrors, which is necessary to restore the laser resonance. The refractive index of polypropylene is deduced from this measurement and from the film thickness, as obtained from an independent measurement based in pycnometry. The value obtained for the refractive index is 1.492(15) for the wavelengths between 118.834 and 251.140 microm, for a polypropylene film of 12.71(2)-microm thickness and 0.9049(7) g/cm3 density.     

Laser differential confocal lens refractive index measurement

A new laser differential confocal lens refractive index measurement is proposed, which uses the absolute zero of the differential confocal axial intensity curve to precisely identify the positions of the objective when the measurement pencil is focused on the vertex of the test lens and the reflector with or without the test lens in the measurement light-path, and then uses aberration compensation and ray tracing facet iterative calculation to obtain the refractive index of the test lens, thereby achieving the high-precision noncontact measurement of lens refractive index. The theoretical analyses and preliminary experiments indicate that the accuracy of the approach can reach about 2.5×10(-4).     

Automated tracking of liquid velocities in a refractive index matched porous medium

Abstract

Particle tracking velocimetry is applied to flow inside a porous column at Reynolds number Re = 28. The column is composed of refractive‐index‐matched solid and liquid materials, allowing seeding particles to be tracked in a laser‐illuminated axial slice. To complement earlier results acquired for 7 mm spheres, we conduct new experiments with larger 12 mm spheres. By improving the image acquisition and analysis, we are able to process the new experiments using fully automated algorithms instead of manual tracking. As a result, greater vector yields, more accurate velocity data, and a more complete spatial coverage are achieved.     

High-precision diode-laser-based temperature measurement for air refractive index compensation

We present a laser-based system to measure the refractive index of air over a long path length. In optical distance measurements, it is essential to know the refractive index of air with high accuracy. Commonly, the refractive index of air is calculated from the properties of the ambient air using either Ciddor or Edlén equations, where the dominant uncertainty component is in most cases the air temperature. The method developed in this work utilizes direct absorption spectroscopy of oxygen to measure the average temperature of air and of water vapor to measure relative humidity. The method allows measurement of temperature and humidity over the same beam path as in optical distance measurement, providing spatially well-matching data. Indoor and outdoor measurements demonstrate the effectiveness of the method. In particular, we demonstrate an effective compensation of the refractive index of air in an interferometric length measurement at a time-variant and spatially nonhomogeneous temperature over a long time period. Further, we were able to demonstrate 7 mK RMS noise over a 67 m path length using a 120 s sample time. To our knowledge, this is the best temperature precision reported for a spectroscopic temperature measurement.     

Low-coherence interferometer system for the simultaneous measurement of refractive index and thickness

We have developed a low-coherence interferometer system used for the simultaneous measurement of refractive index n and thickness t of transparent plates. Both the phase index n(p) and group index n(g) can be determined automatically in a wide thickness range of from 10 microm to a few millimeters. Two unique techniques are presented to measure n(p), n(g), and t simultaneously. One allows us to determine n(p), n(g), and t accurately by using a special sample holder, in which the measurement accuracy is 0.3% for the thickness t above 0.1 mm. In the other technique the chromatic dispersion delta n of index is approximately expressed as a function of (n(p) - 1) on the basis of measured values of n(p) and n(g) for a variety of materials, and then the simultaneous measurement is performed with a normal sample holder. In addition, a measurement accuracy of less than 1% is achieved even when the sample is as thin as 20 microm. The measurement time is also 3 min or more.     

Extraction of complex refractive index of absorbing films from ellipsometry measurement

Numerical extraction of complex refractive index of an unknown absorbing layer inside a multilayer sample from ellipsometry measurement is discussed. The approach of point by point extraction considering all points of spectroscopic data as independent data points is investigated. This problem has typically multiple solutions and the standard method consisting in fitting calculated to experimental point is likely to converge to a wrong solution if a precise guess value is not given. An alternate method is proposed, based on the determination of contours of the ellipsometric function, to provide all solutions in an as extended as wanted range of complex refractive index values. The method is tested through different kinds of sample examples. Errors relative to any of the parameters used in the sample model are calculated and discussed. This method should be helpful in many practical cases of ellipsometry data interpretation.     

Simultaneous measurement of refractive index and thickness of birefringent wave plates

A nondestructive measurement system based on a position sensing detector (PSD) and a laser interferometer for determining the thickness and refractive indices of birefringent optical wave plates has been developed. Unlike previous methods presented in the literature, the proposed metrology system allows the refractive index and thickness properties of the optical plate to be measured simultaneously. The experimental results obtained for the e-light and o-light refractive indices of a commercially available birefringent optical wave plate with refractive indices of n(o)=1.542972 and n(e)=1.552033 are found to be accurate to within 0.004132 and 0.000229, respectively. Furthermore, the experimentally derived value of the wave plate thickness deviates by no more than 0.9 microm from the analytically derived value of 453.95 microm. Overall, the experimental results confirm that the proposed metrology system provides a simple yet highly accurate means of obtaining simultaneous measurements of the refractive indices and thickness of birefringent optical wave plates.     

Dispersion-insensitive measurement of thickness and group refractive index by low-coherence interferometry

We describe a low-coherence interferometric technique for simultaneous measurement of geometric thickness and group refractive index of highly dispersive samples. The technique is immune to the dispersion-induced asymmetry of the interferograms, thus overcoming limitations associated with some other low-coherence approaches to this simultaneous measurement. We use the experimental configuration of a tandem interferometer, with the samples to be characterized placed in an air gap in one arm of the measurement interferometer. Unambiguous, dispersion-insensitive measurements of critical group-delay imbalances in the measurement interferometer are determined from the optical frequency dependence of interferogram phases, by means of dispersive Fourier transform spectrometry. Sample thickness and group refractive index are calculated from these group delays. A thickness measurement precision of 0.2 mum and group index measurement accuracy of 5 parts in 10(5) across a wavelength range of 150 nm have been achieved for BK7 and fused-silica glass samples in the thickness range 2000 to 6000 mum.     

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.     

Experimental measurement of the refractive index of biological tissues by total internal reflection

We discuss the refractive-index measurement of biological tissues by total internal reflection. The methodology of the measurement is illuminated comprehensively, and an experimental setup, combined with a data processing program, is developed correspondingly. Refractive indices of typical tissue samples are measured by use of the developed methodology. The agreement of our measurements with the reported results shows the validity of our scheme, which has the potential for being a simple, quick, and low-cost practical means for determining the refractive index of a turbid medium. Moreover, an empirical formula for evaluating the refractive index of Intralipid suspensions with different concentrations is also presented according to experimental measurements.     

Concentration measurement of refrigerant/refrigeration oil mixture by refractive index

Refrigeration oil having good miscibility with refrigerant is generally used in refrigeration units. Precise measurement of the mixing ratio of refrigerant to refrigeration oil is required for a sufficient understanding of the refrigeration cycle. In this paper, refractive index is chosen as a property which indicates the mixing concentration of the refrigerant/oil mixture. A laser displacement sensor is used to detect a change of optical path which changes according to the refractive index of test medium. The refractive indices of pure refrigerant, pure refrigeration oil and refrigerant/refrigeration oil mixture are measured with several combination of refrigerant/oil. It is found that the difference of refractive index between the refrigerant and the oil is sufficient for the measurement of the mixing concentration of refrigerant/oil mixture, and that the refractive index of the refrigerant/oil mixture changes almost linearly according to the mixing concentration. These data will be utilized for development of an in situ sensor in refrigerant compressors.     

玻璃材料折射率及色散的高准确度测量

使用德国Triotics公司生产的SpectroMaster HR UV-VIS-IR型折射率测量设备,对FK5玻璃块的e、d、g、F′和C′谱线的折射率值进行高准确度测量,并修正至标准环境条件下的折射率值。用得到的折射率值进行曲线拟合,验证Cauchy色散公式和Sellmeier色散公式。     

Ionization effect on arc plasma's optical diagnosis by the measurement of the refractive index

The effect of arc plasma ionization on its temperature diagnosis by the measurement of the refractive index is discussed. The refractive index of arc plasma in two conditions is compared: 1) only the first ionization is considered and 2) both the first and second ionizations are considered. In order to facilitate plasma temperature reconstruction, two corresponding refractive index models are deduced. For the sake of making this study universal, both the monatomic and dual-atomic molecule arc plasmas are chosen as typical examples for theoretical deduction and analysis. A condition, which can be adopted to estimate whether the second ionization should be considered in temperature reconstruction, is proposed. Finally, an argon arc plasma is chosen as an example for experiment, and the experimental results match well with the theoretical analysis. This study is crucial to arc plasma's optical diagnosis, which is based on the measurement of the refractive index.     

Simultaneous measurement of thickness and refractive index based on rotation incidence angle

A method for simultaneous measurement of geometric thickness and refractive index of an optical wafer is presented. By using a fiber optic Mach–Zehnder interferometer (MZI) with a free space, the transmission spectrum of a MZI for the optical wafer at different incidence angle is interrogated, and the geometric thickness and the refractive index of the optical wafer are measured simultaneously. With the transmission spectrum, we can obtain a clear interferogram with a high visibility no matter how small the measurement range of the refractive index. Therefore the proposed technique possesses a broad measurement range and low cost. The experimental results show that the maximum errors of the geometric thickness and the refractive index are only 0.007?mm and 0.008, respectively, and that a broad measurement from 1.316 to 3.503 can be achieved.     

Extrinsic fiber-optic Fabry-Perot interferometer sensor for refractive index measurement of optical glass

A simple fiber-optic sensor based on Fabry-Perot interference for refractive index measurement of optical glass is investigated both theoretically and experimentally. A broadband light source is coupled into an extrinsic fiber Fabry-Perot cavity formed by the surfaces of a sensing fiber end and the measured sample. The interference signals from the cavity are reflected back into the same fiber. The refractive index of the sample can be obtained by measuring the contrast of the interference fringes. The experimental data meet with the theoretical values very well. The proposed technique is a new method for glass refractive index measurement with a simple, solid, and compact structure.     

The syncang method for simultaneous measurement of film refractive index and thickness

The synchronous angle method (SYNCANG) for the simultaneous determination of the refractive index and film thickness of thin dielectric films is especially useful for researchers involved in integrated optics.The method involves the measurement of the synchronous angles θ_5E and θ_5M (see Fig. 1) required to couple transverse electric (TE) and transverse magnetic (TM) modes into light guides, and the solution of the resulting pair of simultaneous transcendental eigenvalue equations yields n₁ and W.The sensitivity of the method has been studied, and it is shown that n₁ may easily be resolved within 0.002 per minute of arc, considering θ_5E and θ_5M as having equal errors. W may be resolved within 0.02 μ (200 Å) per minute of arc, depending primarily on the proximity of the synchronous guide angle θ₁ to 90°. The method does not require a step in the film, and may be easily reduced to measuring n₁ knowing either the TE or the TM synchronous angle and W.     

Automated measurement of aggregate properties: Part 2—flakiness index

Aggregate shape is considered to be a key parameter for concrete and bituminous mixtures design. Generally, this parameter is determined by calculating a ratio between the different dimensions of the particles and by comparing this ratio to a flatness or elongation limit. The standardized methods used for this purpose vary among countries, but all of them are slow and lack precision. In order to increase the rapidity and accuracy of aggregate shape determination, a model based on a two-dimensional image analysis approach has been developed and implemented in the French VDG 40 videograder, to determine the flakiness index referenced to in European standard EN 933-3. The parameters of the model have been determined from an extensive experimental database, involving manual shape characterization of 122 samples, each of one containing about 500 particles. The predictive performance of the model has been assessed through supplementary data (including materials from other quarries). A very good agreement between experimental and estimated flakiness indexes was found.     

Simple polarimetric approach to direct measurement of the near-surface refractive index in graded-index films

In the standard M-line method for the characterization of graded-index film, an analytical curve is fitted to the waveguide mode measurements and extrapolated to provide the refractive index in the zero-depth limit. Here we review our polarimetric approach to a direct near-surface measurement, which complements the M-line method. Also, we present its new and more straightforward version, which is applicable to existing samples and does not require masking before ion exchange.