Measuring the refractive index of crude oil using a capillary tube interferometer

A method for measuring the refractive index of low-transparent crude oils using a capillary tube interferometer is described. The method is based on analyzing the resulting transverse interference fringe patterns in terms of their positions with respect to the lens/capillary tube interferometer. The refractive indices of seven blended crude oils of low transparency were measured with accuracy of up to six decimal digits and were related to the API gravity of the oils. The ray tracing inside the capillary tube is explained and the transverse bell-shaped interference fringes are interpreted.     

Polynomial approximation method for tomographic reconstruction of three-dimensional refractive index fields with limited data

We propose a polynomial approximation method (PAM) for reconstruction of three-dimensional refractive index fields by interferometric tomography using limited data. Based on the assumption that the fields to be reconstructed are usually smooth and can be decomposed into a finite order of (orthogonal) polynomials, a set of linear equations can be constructed using both the measured projection data and the Radon transform of the basis functions. By solving these equations, the least-squares solutions of expansion coefficients can be obtained and then substituted back to yield the desired fields. Numerical results have demonstrated that the proposed method is fast, robust to noise and can achieve satisfactory results for refractive index fields with limited projection views and large opaque objects.     

X射线在折射率连续变化等离子体介质中的传播

 从Maxwell方程出发，得到了X射线光束在非均匀等离子体介质中传播的类衍射积分解，并从理论和数值模拟上着重讨论了折射率梯度效应对光束参数的影响。结果表明，在等离子体电子密度分布较高的区域，梯度效应明显，它直接影响光束的光强分布、衍射效应和光束的偏转程度。     

Photonic crystal refractive index sensing based on sandwich structure

An ultracompact refractive index sensing based on the sandwich structure of photonic crystal is presented and demonstrated with solutions of water, methylsilicone and silicone. The performances of the sensor are analyzed theoretically by using Fabry-Perot cavity mode, the change in opposite direction between the transmission of the refractive index sensing and its quality factor is derived. The experimental results are consistent with the results of theoretical analysis.     

Automatic determination of refractive index profile, sectional area, and shape of fibers having regular and/or irregular transverse sections

A new method based on a mathematical model and a computer program is suggested to determine the refractive index profile of fibers having regular and/or irregular transverse sectional shape. Microinterferogram of both multiple-beam Fizeau fringes and the duplicated image from two-beam interference microscope are used for the determination of refractive index profile, cross-sectional area and shape of three different types of fibers. To confirm the suggested model, the calculated area and the shape of the transverse section of these fibers are compared with those results obtained using conventional methods.     

Measurement of refractive index variation of liquids by surface plasmon resonance and wavelength-modulated heterodyne interferometry

In this study an alternative method based on surface plasmon resonance is proposed for in-situ monitoring of variation in the refractive index of a test sample. A wavelength-modulated light source and an unequal-path-length optical configuration heterodyne interferometer are used to detect the phase difference change, which can then be used to estimate the change in the refractive index of a test sample. The experimental results demonstrate a phase stability of 0.02°. The resolution power of the refractive index is 1.5 × 10^− 6 RIU. This method has several advantages over previously used methods such as simple optical setup, easier operation in real time, and low cost.     

Measurement of small differences in refractive indices of solutions with interferometric optical method

Based on the principles of both surface plasmon resonance (SPR) and heterodyne interferometry, an optical method for measuring small differences in refractive indices of solutions was proposed. On a specially designed probe, two light beams are incident on both reference and test solutions. The phase differences between the p- and s-polarizations of each reflected light under SPR condition are measured simultaneously with heterodyne interferometric technique. The phase values are substituted into special equations derived from Fresnel's equations. Finally, the difference between the refractive indices of these two solutions can be estimated. The feasibility of this method was demonstrated and the measurement sensitivity of refractive index can reach a value of at least 8.57×10⁻⁷. This method should bear the merits of a simple structure, easy operation, high sensitivity and rapid measurement.     

Determination of the refractive index profile of polymer optical fiber preform by the transverse ray tracing method

In graded-index polymer optical fiber (GI-POF), the refractive index profile is an important parameter in defining its bandwidth. However, direct determination of the refractive index profile of GI-POF is difficult due to its extreme thinness. In this study, the refractive index distribution of the GI-POF is indirectly determined by measuring the refractive index distribution of the GI-POF perform by applying the transverse ray tracing method to a simplified measurement system that we developed.In this system, a parallel tabular ray is irradiated transversely to a GI-POF preform. The transverse ray from the preform is then projected on a screen, and its digital image is processed to calculate the refractive index distribution. The calculation is based on a transverse ray simulation, a computer program that we developed in which the refractive index distribution of the preform is determined by comparing the displacement of the transverse ray projected on the screen with the actual measurement.The accuracy of this new measurement method is validated by comparing the refractive index distribution of a GI-POF preform with the refractive index distribution measured by the conventional method using an interferometer. We find that the refractive index distribution measured by this novel method agrees well with that measured by the conventional method.     

High-performance refractive index sensor based on guided-mode resonance in all-dielectric nano-silt array

We propose a high-quality refractive index sensing by utilizing the characteristic guided-mode resonance (GMR) excited in a simple one-dimensional all-dielectric nano-slit array. We demonstrate a figure of merit up to 12000, which is higher than that achieved in most plasmonic refractive index sensors. We show that high-quality sensing performance can be sustained over a broadband range in near infrared region with relatively large variations in both grating depth and angle of incidence. Such a GMR-based all-dielectric sensing device with the ease of fabrication is expected to hold great promise for realizing broadband refractive index sensors with high performance and compactness.     

Automatic determination of refractive index profile of fibers having regular and/or irregular transverse sections considering the refraction of light rays by the fiber

A new model, using non-destructive two- and/or multiple-beam interferometric techniques, is suggested for measuring the refractive index profile of fibers having regular and/or irregular cross-sectional shape taking into consideration the refraction of the light rays by the fiber. The proposed model is applied for three different fibers having different cross-sectional shapes and different refractive index profiles. These fibers are PPT, homogeneous fiber, with circular cross-section, graded index optical fiber of circular cross-sectional shape and Dralon fiber of irregular cross-section. To validate the proposed model it is used, firstly, to calculate the index profile for a standard PPT fiber. Secondly, the calculated results for the irregular Dralon fiber and GR-IN optical fiber are compared with that calculated using other conventional method. From this comparison, we recommend that the refraction must be taken into account to obtain accurate results especially for birefringent fibers and graded index optical fibers.     

A theorem on refractive index as a function of wavelength

A theorem upon refractive index as a function of wavelength is derived starting from the Taylor expansion around the origin of this function. Our result involves the above index, the group index, and the coefficient of material dispersion all extrapolated to zero wavelength.     

A generalized formulation for the refractive index at the geometric-optics limit

Starting from the concept of rapidity considered as generalization of the group velocity, we establish a generalized mathematical expression for the refractive group index of a material medium with little refraction at sufficiently small wavelengths, this index becoming approximately the refractive index at the geometric-optics limit and the phase velocity being approximately equal to the group velocity at the above limit. As main result, we derive an approximate relationship for the refractive index of a medium of small refraction at the geometric-optics limit. This medium is found to be left-handed at sufficiently small wavelengths.     

Mathematical modeling and simulation of refractive index based Brix measurement system

Optical techniques developed for sensing and analysis purposes have been used in various fields. An attempt has been made to design optimization of refractormetric based method for the measurement of Brix. Mathematical modeling and simulation of the proposed system has been performed. From the study it is seen that mathematical model may help for getting the better performance to developed experimental model. Optimization of various constructional parameters including selection and location of source, prism and detector, position of source, angular position and height of source from prism plane, divergent angle of source, refractive index of prism, size of prism, the location of detector to pickup the optimum reflected light, refractive index of sample, critical angle, choice of suitable prism. The various possibilities of mounting position of source, detector, prism are studied. The mathematical model is developed considering the above mentioned parameters.     

Novel optical devices based on the tunable refractive index of magnetic fluid and their characteristics

As a new type of functional material, magnetic fluid (MF) is a stable colloid of magnetic nanoparticles, dressed with surfactant and dispersed in the carrier liquid uniformly. The MF has many unique optical properties, and the most important one is its tunable refractive index property. This paper summarizes the properties of the MF refractive index and the related optical devices. The refractive index can be easily controlled by external magnetic field, temperature, and so on. But the tunable refractive index of MF has a relaxation effect. As a result, the response time is more than milliseconds and the MF is only suitable for low speed environment. Compared with the traditional optical devices, the magnetic fluid based optical devices have the tuning ability. Compared with the tunable optical devices (the electro-optic devices (LiNbO₃) of more than 10 GHz modulation speed, acoustic-optic devices (Ge) of more than 20 MHz modulation speed), the speed of the magnetic fluid based optical devices is low. Now there are many applications of magnetic fluid based on the refractive index in the field of optical information communication and sensing technology, such as tunable beam splitter, optical-fiber modulator, tunable optical gratings, tunable optical filter, optical logic device, tunable interferometer, and electromagnetic sensor. With the development of the research and application of magnetic fluid,a new method, structure and material to improve the response time can be found, which will play an important role in the fields of optical information communication and sensing technology.     

Introduction of equilibrium-constant method into flame's temperature reconstruction from the measurement of refractive index

The Equilibrium-Constant Method is introduced to flame's temperature reconstruction for the combustion region from the experimentally measured refractive index. The mole fractions of species composition are described as the functions of equivalence ratio, temperature and pressure. Based on which, the dependence of the mole fractions on temperature and pressure can be obtained once the equivalence ratio is known. Finally, the temperatures which are reconstructed based on the method proposed by us are compared with those of obtained by previous method, as well as the imperfectness of the proposed method is analyzed and discussed.     

The measurement of refractive index profile and aberration of radial gradient index lens by using imaging method

In this article, through the linear fit of objective distance and reciprocal of magnification in different imaging locations, we use imaging method to measure the central refractive index n(0) and focusing constant α of radial gradient index (GRIN) lens and obtain 10⁻³ measurement accuracy of n(0). At the same time, under distortion, the fourth-order refractive index coefficient and aberration of GRIN lens are derived.     

Large refractive index with vanishing absorption in optical fibers

We propose and demonstrate a novel scheme for realizing large refractive index with vanishing absorption in a three-level quantum system consisting of the energy levels of Er³⁺-doped ZrF₄–BaF₂–LaF₃–AlF₃–NaF (ZBLAN) optical fiber. It is found that the refractive index of the probe field can be controlled via the coherent coupling field and incoherent pumping field. The switching from negative refractive index to positive refractive index and manipulation of the value of the index of refraction while maintaining vanishing absorption of the probe field can be achieved via tuning the detuning of the coherent coupling field. Our scheme may provide some new possibility for technological applications in fiber communication.     

Characterization of fiber Bragg grating for maximum reflectivity based on modulation depth of refractive index

This study proposed the modeling of fiber Bragg grating as a sensor for the optimum reflectivity with minimum discontinuities at the end of center wavelength. Recent advancement in optical sensor technology, make it necessary to generate the optimum results with minimum error. In this analysis, dependence of grating output spectra on various parameters has been studied for minimum bandwidth, side lobes and maximum reflectivity. In previous studies effect of length and other parameters has been studied. In this paper author has included the effect of modulation depth also. Study shows that grating length and modulation depth of refractive index of core are very critical factors which significantly alter the grating characteristics. Here low absorption third window of optical communication is used for the sake of low attenuation. All the simulation has done using GratingMOd software using transfer matrix method.     

Full-field refractive index measurement with simultaneous phase-shift interferometry

Using the phenomenon of total internal reflection and a beam splitting device, a technique of simultaneous phase-shift interferometry is proposed for measuring the full-field refractive index. Because this method applies a beam splitting device that mimics the characteristics of beam splitting and phase modulation, four interferemetric images of various phase distributions can be simultaneously captured. Therefore, this setup can avoid errors caused by non-simultaneous capturing of images and offers the benefits of high stability, ease of operation, and real-time measurement. Furthermore, using the phenomenon of total internal reflection, the phase difference between p- and s-polarized light varies considerably with the refractive index of a tested specimen. This can substantially increase the measurement resolution. The feasibility of this method is verified using an experiment, and the measurement resolution can be higher than 3.65 × 10⁻⁴ RIU.