On line interferometric investigation of the neck propagation phenomena of stretched polypropylene fibre

On line interferometric investigation of the neck propagation phenomenon of stretched fibre is carried out using an automatic multiple-beam Fizeau fringes technique. It was observed that under such a deformation condition, a neck deformation is formed and propagated steadily towards the gripped ends of the stretched sample. The neck propagation was recorded carefully during the course of the tests by means of a CCD camera. The fringe shift profile due to the neck formation was analyzed during the propagation stage. A polypropylene (PP) sample was stretched until a neck deformation is formed at room temperature. The 3D time-refractive index profile is investigated to clarify the fixed profile of propagation. The obtained microinterferograms are clarifying the fixed neck profile during the propagation of necking phenomena. The speed of neck propagation was calculated. The dependence of the propagation on the drawing speed and draw ratio was investigated.     

Detecting and avoiding the necking deformation along polypropylene fibre axis using the fringe pattern analysis of multiple-beam microinterferometry

A fibre-drawing device attached with the system for producing multiple-beam Fizeau fringes in transmission is used to optimize the optical properties during the cold drawing of polypropylene (PP) fibres. This system is automated for interference pattern analysis. Two drawing processes for the PP fibres are applied and investigated. The first one is fast drawing in which the necking deformation is predicted and the other is the slow (step) drawing in which the necking can be avoided. The refractive index profiles (n and n) of PP fibres are determined at different positions along the fibre axis during the fast and slow drawing processes. The fibre interference patterns are automatically digitized and stored in a computer storage media. The slow drawing technique for PP fibres is recommended to overcome the deformation difficulties along the fibre axis due to necking during the drawing processes. Microinterferograms in case of light vibrating parallel and perpendicular to the fibre axis are given for illustration.     

Detection of necking deformation along polypropylene fibres axis at low draw ratios using multiple-beam microinterferometry

A modified drawing device attached with an automated system for producing multiple-beam Fizeau fringes in transmission is used to study the mechanical deformation along polypropylene (PP) fibres at low draw ratios. Two drawing processes are used for drawing PP fibres. In the first process (fast drawing), the necking deformations are formed at draw ratios from 1.2 to 1.9 along the fibre axis. While in the other process (slow/step drawing), these deformations disappeared. The refractive indices calculated at different positions along the fibre axis during the fast- and slow-drawing processes. To overcome the formation of necking deformation along the PP fibres during fast-drawing process, the slow-drawing technique is recommended. Microinterferograms are given for illustration.     

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.     

Characterization and estimation of refractive index profile of laser-written photopolymer optical waveguides

In this study, channel waveguides fabricated in photopolymer films by direct-writing using a low-power CW laser, are used as phase objects in a simple plane-wave diffraction setup, and the refractive index modulation profiles of the waveguides are characterized using the recorded diffraction patterns. Index profiles are modeled by piece-wisely combining two Gaussian functions representing the central and the tail regions. Measured diffraction patterns are matched with patterns generated using the model. This simple model makes it possible to design various channel waveguides embedded into polymer substrates. The proposed model is tested on three distinctive waveguide profiles written on the same Acrylamide/Polyvinyl Alcohol based photopolymer with different exposures.     

Determination of the refractive index profile of a symmetric fiber preform by the transport of intensity equation

The refractive index profile of an axially symmetric fiber preform is determined by using the transport of intensity equation. In this method the preform is immersed in an index-matching liquid, and a collimated light beam impinges on it laterally. The intensity distributions of the transmitted light are measured on two close parallel planes inside the preform core. From the recorded intensity distributions, the deflection function is calculated by the transport of intensity equation. The refractive index profile is obtained by means of Abel inversion. Also, for comparison, the refractive index profile of the preform is measured by the focusing method and the results are in agreement with less than 3% error.     

Multi-mode opto-thermo-mechanical stretching system for determination of 3D refractive index along the axis of stretched and/or heated fibres

A multi-mode opto-thermo-mechanical stretching system was modified to study the changes in the 3D of optical and structural properties of stretched fibre along its axis. The structural deformation of isotactic Polypropylene, (iPP), fibres was studied at different draw ratios. The modified system coupled with Pluta polarising interference microscope was used to determine the variation of the birefringence in three dimensions during stretching process. Using this modified system, the multi-necking was detected. Subfringe analysis technique was used to determine the phase distribution of the obtained microinterferograms, which were given for illustration.     

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.     

Determination of the refractive index of fibres using the modified area method: I. Homogeneous fibres

The mathematical formulae for determining the mean refractive indices and birefringence of homogeneous fibres are modified. This modification depends on calculating the area under the fringe shift and cross-sectional area of homogeneous fibres having nearly circular cross-sectional area taking the refraction of the incident light beam by the fibre into consideration. The mathematical analysis for two-beam and multiple-beam interferometric techniques is dealt with. These formulae are applied to polypropylene fibres with draw ratios of 3.5 and 4 using the two techniques. A comparison between the results when using the new formulae and when neglecting the refraction has been studied. The diffraction of a He–Ne laser beam is used for determining the shape of the cross-sectional area of these fibres. Microinterferograms are given for illustration.     

Construction of the refractive index profiles for few-mode planar optical waveguides

We present a nondestructive technique to predict the refractive index profiles of isotropic planar waveguides, on which a thin gold film is deposited to as the cladding. The negative dielectric constant of the metal results in significant differences of effective indices between TE and TM modes. The two polarized modes and a surface plasmon resonance (SPR) with abundant information of the surface index can be used to construct the refractive index profiles of single-mode and two-mode waveguides at a fixed wavelength.     

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.     

Automatic fringe analysis of two-beam interference patterns for measurement of refractive index and birefringence profiles of fibres

Automatic fringe pattern analysis is a powerful and inexpensive digital image-processing technique. Two-beam polarizing interference Pluta microscope [Pluta M. Opt Acta 1971;18:661, Pluta M. J Microsc 1972;96:309] is automated by the computer-aid via CCD camera and digital frame grabber. Software program is prepared to deal with the duplicated (separated and overlapped) Microinterferograms produced by two-beam polarizing microscope. It also gives an accurate and fast automatic measurement of refractive index and birefringence profiles for fibres. In this paper, the refractive index and birefringence profiles of two different types of fibres, basalt and polypropylene (PP) fibres are presented. A new method to determine the birefringence profile of fibres from non-duplicated microinterferogram is suggested. The cold drawing process for PP fibres is studied.     

变波长逆WKB方法确定单模渐变平面光波导的折射率轮廓

将渐变波导模式方程(WKB积分方程)化为分段积分,以波导某一模式在不同波长下的转折点为分段点,当波长相差很小时,相应的转折点相差也很小,可在各个分段积分中作折线近似,从而从理论上推出确定波导轮廓数据的递推式.以所得轮廓必须满足光滑条件为判据,最后定出波导的轮廓.该方法尤其适用于单模渐变波导,而且无需事先假设待定轮廓的函数形式.本文对双曲止割和抛物线轮廓的理想波导进行了计算机模拟,结果证明该方法的精度达到10~(-3)甚至于更高.而且理论上具有分割愈密,精度愈高的优点.     

The effect of mechanical drawing on optical and structural properties of nylon 6 fibres

The Pluta polarizing double-refracting interference microscope was attached to a mechanical drawing device to study the effect of cold drawing on the optical and structural properties of nylon 6 fibres. The microscope was used in its two positions for determining the refractive indices and birefringence of fibres. Different applied stresses and strain rates were obtained using the mechanical-drawing device. The effect of the applied stresses on the optical and physical parameters was investigated. The resulting optical parameters were utilized to investigate the polarizability per unit volume, the optical orientation factor, the orientation angle and the average work per chain. The refractive index and birefringence profiles were measured. Relationships between the average work per chain and optical parameters at different strains rates were determined. An empirical formula was deduced for these fibres. Micro-interferograms are given for illustration.     

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.     

透射式微分干涉显微镜测量光纤折射率分布

提出一种由两组偏光棱镜组成的透射式微分干涉系统,用来测量光纤的折射率分布。系统利用旋转检偏器对测量光束进行调制,从相位分布和光程分布中计算出折射率分布。介绍了测量原理、测量公式、系统组成及典型应用。用该系统对光纤折射率进行实际测量,给出渐变折射率光纤的二维分布,及折射率沿轴向的分布图,同时还在NR 9200光纤测试仪上进行了测试。结果表明,系统原理正确,结构简捷,具有足够分辨率和测量精度,抗干扰能力强,不破坏光纤,还可测量包层和芯层的直径,发现光纤加工过程中的缺陷,特别适合加工、焊接等工艺过程中的检测。     

Measuring method for the refractive index profile of optical glass fibres

A new method for measuring the refractive index profile of optical glass fibres is described. This simply monitors the reflection of a focused laser beam from a polished fibre end face. Detailed tests on the accuracy and resolution of the method are described and the index profiles of several fibres are given as an example of its feasibility.     

Surface plasmon resonance device with imaging processing detector for refractive index measurements

This work presents a new surface plasmon resonance (SPR) instrument based on Kretschmann configuration. This device acquires and processes the reflected SPR image of the sample by using a webcam together with an image processing algorithm that transforms the RGB image in numeric values and correlates the integrated intensity with the refractive index of the solution. Experimental signals were compared with theoretical values and it was found an excellent agreement. In addition, the applicability of the instrument was tested by measuring the refractive index of solutions in a continuous flow mode. Excellent stability and sensitivity of the signal were found in the presence of small changes of the refractive index.     

Photonic crystal fibers with high index infiltrations for refractive index sensing

We report on the experimental demonstration of a refractive index sensing technique based on the photonic bandgap (PBG) shift in two types of solid-core photonic crystal fiber (PCF) structures with a series of high refractive index infiltrations. Bandgap edge and central wavelength with a significant red-shift are observed with the increase of infiltration material index. The measurement results from both structures agree well with the frequency scaling law, which helps to simplify the process of sample preparation in PCF-based sensors using the infiltration method. Refractive index resolution up to 10⁻⁶ refractive index unit (RIU) can be achieved experimentally, which is two orders of magnitude higher than that in PCF-based long-period gratings.     

The use of defocussed position of a Ronchi grating for evaluating the refractive index of lens

This report describes a Ronchi-grating shearing interferometric collimation method for determining the refractive index of a lens. A new formula has been deduced for the refractive index of the lens, which is independent of lens parameters. This newly developed device has two lenses. One of them collimates the light from a point source while the other acts as a decollimator. The point source acting as a test target is situated in the front focal plane of the collimator. The test lens is placed inside a glass cell in the back focal plane of the collimator. The Ronchi grating is exactly placed in the back focal plane of the decollimator, and a fringe-free space is observed in the common region. When the test lens is immersed in a liquid cell, it disturbs the focal point and hence interference fringes are observed. The combined system consisting of the test lens and the decollimator is moved to bring the focal point on the Ronchi grating. Thus, the difference between the two settings gives the amount of defocusing.