A Three Autocollimation Method for Measuring the Refractive Index of Optical Glass

A method is presented to measure refractive index of a plane parallel plate of optical glass. We have applied focal displacement method to measure refractive index of a plane parallel plate of an optical glass having thickness of the order of millimeters. The best focus position is found by applying edge detection algorithm. We have measured LAF2 optical glass using focal displacement method and obtained the value 1.746, which is within 0.1% of the standard value 1.747 at 555 nm. In comparison to Abbe refractometer, this method is simple in terms of sample preparation, experimental set up and for measuring high refractive index of Zinc Sulfide. Using this method, refractive indices of other optical glasses are also measured.

A core exposed polymer cladded fiber sensor for refractive index (RI) measurement is presented here. The polymer cladding of the multimode silica fiber was removed from a length of 1.5 cm to act as a sensing region. The working principle of the sensor is based on total internal reflection (TIR) phenomenon. The intensity modulation of the transmitted light is measured as a function of liquid refractive index surrounding the sensing region. The sensitivity obtained for the probe is 13.99/RIU. The fabricated sensor is of low cost, easy to fabricate and have high sensitivity which makes it useful in chemical sensing.

The Z-scan method has been widely used for the estimation of the nonlinear refractive index and the nonlinear absorption coefficient of various materials which usually show quite important nonlinear behaviors. However, it still remains difficult to perform accurate measurements of small nonlinear phase shifts since the major drawbacks of the method are an important multiplicative noise and a great sensitivity to the incident beam spatial quality and to the pulses temporal profile. In order to measure accurately the nonlinear refractive index of optical glasses in the nanosecond regime we had to improve the Z-scan method sensitivity and to reduce drastically the numerous possible errors. We have developed and optimized a Z-scan experimental setup which is well-adapted for the metrology of the nonlinear refractive index.

Modern optical design requires measurement of the refractive index of optical glass to the sixth or the seventh decimal place. Reflectometry represents an interesting option to go beyond the fifth decimal place, since from the literature we know that this technique can work to the fifth decimal place. In addition to that, this method does not use a large area of the sample, a condition that helps to increase the contribution of systematic errors. The techniques used to determine the refractive index of optical materials include transmission and reflection methods. The highest accuracy transmission techniques used in refractometry are: minimum deviation and critical angle goniometry, moire deflectometry, and interferometry. The main systematic errors that limit the measuring accuracy of each of these techniques are mentioned. The techniques used for the determination of the refractive index of optical materials from measurements of the ratio of the reflectance coefficients of its surface are widely used. Many of these methods rest on simplifications of Fresnel equations at specific angles of incidence. The geometries used include: normal incidence, 45 degree(s), Brewster angle, and other angular positions related with the specific sample to be evaluated. The analyses reported in the literature contain just the parameters present in the simplified formulas obtained for the Fresnel coefficients of each specific configuration. In this work a more complete analysis of these techniques is presented.

Parameter calculations were carried of filtering devices in one of which the phenomenon of the total internal reflection was used. It is expected that these devices may be suitable for imagery of objects in the given narrow spectral bands - spectral imaging (SI) and for obtaining the spectral maps of fast flowing past processes.

The refractive index dispersion in germanate oxyfluoride glasses of GeO2-P2O5-MF2 (M=Ca, Zn), which are developed for optical fiber application, is investigated in the 0.4-4 µ m wavelength range by the minimum deviation method. The prepared glasses have a GeO2 content varying from 80 to 30 mol%. The dispersion curves for these glasses tend to shift to shorter wavelengths as the GeO2 content is decreased. Material dispersions are also derived from the refractive index measurements and the zero-material dispersion wavelengths (λ0) are found in the vicinity of 1.5 µ m. On the basis of the empirical relationship between λ0 and the minimum loss wavelength (λ0), the λ min values are located at around 1.8 µ m. A minimum loss of as low as 0.08 dB/km is expected for the present germanate glasses.

A simple method for the refractive index and dispersion measurements of transparent materials by transmissivity

Application of refractive index (RI) measurement and analysis of trace impurities with synchrotron radiation X-ray fluorescence spectrometry (SR-XRF) was investigated for the forensic discrimination of headlight glasses. Glass fragments smaller than 1 mm were collected from 17 kinds of different headlight samples for this experiment. It was found that glass fragments taken from the surface of the headlight was not appropriate for the forensic comparison and should be excluded from the RI measurement, because some of them showed inappropriate RI values different from those of the fragments from the bulk. Discrimination of headlights was carried out by comparing the range of average RI value ±2SD among 136 possible pairs of the 17 samples. The comparison of RI could distinguish 118 pairs and but not the other 18 pairs. As a result of SR-XRF analysis for the 17 sample headlights, 8 elements (Zn, As, Sr, Zr, Sb, Ba, Ce and Hf) were selected for the comparison, because these elements were considered to be effective for the discrimination. The X-ray intensities of these elements were normalized using the element which provided the highest X-ray intensity in each X-ray measurement and was followed by cluster analysis. In the dendrogram obtained, 11 of 17 pairs of data measured with 2 fragments taken from the same sample on different experimental days were classified into the nearest cluster. This result indicates that the analysis of 8 elements by SR-XRF could give useful information for the discrimination of headlight glasses. The number of indistinguishable pairs was reduced to 4 from 18 by the addition of analytical results using SR-XRF to the results of RI measurement, which demonstrates that the combination of these two methods could achieve significant improvement in the discrimination capability of headlight glasses.

By means of computer simulation, the influence of absorption index, κ, on the accuracy of determining the refractive index, n, of optical materials by the minimum deviation method (prism method) is studied at different measuring angular accuracies from 5'' to 0.1''. The n values within 1.0-4.0 and κ values within 0-0.1 are considered. It is shown that (1) except for the far-infrared region, the error in determining n by formula (1)commonly used in the minimum deviation method, i.e., neglecting absorption, is less than the error due to the accuracy of measuring the prism and minimum deviation angles; (2)the difference between n values calculated with and without absorption taken into consideration is, in most cases, negligible throughout the optical range; (3)however, the total error in determination of n is higher when absorption is taken into account than that in the case when absorption is neglected. The further the increase in accuracy of measuring the angles and sensitivity of radiation detectors, the more the proposed calculation technique will allow one to optimize the prism angle value and to calculate more correctly the values of n and error in its determination.

1.55 μm silicon-based reflection-type waveguide-integrated thermo-optic 2 × 2 switch

Describing the phenomenon of total internal reflection in terms of a reflection coefficient of unit magnitude, we found that not only can propagating plane waves be total internally reflected at the planar interface of two dissimilar, homogeneous, isotropic dielectric–magnetic mediums, but evanescent plane waves can also be. The refracting medium must be the optically denser of the two mediums for total internal reflection of an evanescent plane wave to occur.

Thermodynamical fluctuations impose random noise on the position of optical components. It is predicted that this thermal noise will limit the sensitivity of interferometric gravitational-wave detectors in their most sensitive frequency band. Thermal noise originating from optical coatings was first considered in the context of interferometric gravitational wave detectors. Its true significance was, however, only revealed after Y. Levin introduced a new method in 1998 to calculate the resulting phase noise of a laser beam reading out the position of a coated mirror. A result of this analysis is that the reflective optical coatings introduce a particularly large portion of thermal noise. ¶ As a consequence, coating thermal noise is expected to prevent the detection of the standard quantum limit; a limitation to the sensitivity of an interferometric measurement caused by quantum fluctuations in the optical field. Elimination of the coating thermal noise will increase the likelihood of the successful observation of the standard quantum limit, thus enabling the investigation of quantum noise in the regime of optical squeezing. This project investigated a means to eliminate the effects of coating thermal noise, with the design and characterisation of a highly reflective coating-free mirror. This mirror utilised the phenomenon of total internal reflection and the Brewster angle to reflect light without the use of coatings. The dimensions of the mirror were governed by its expected implementation in an experiment to measure the standard quantum limit. ¶ The design of the coating-free mirror undertaken as the initial part of this project is presented in detail. Once a CFM had been created according to this design, its spatial dimensions were measured. The weight of the mirror is 0.43 ± 0.01 g, well within the design goal of 0.5 g. ¶ In order to analyse the reflectivity of the coating-free mirror it was incorporated, together with a high quality conventional mirror, into a triangular ring cavity. This cavity was stabilised to the laser frequency by the Pound-Drever-Hall technique. This enabled the interrogation of the stable cavity properties by an AM-sideband transfer scheme. The reflectivity of the mirror was analysed for optimum rotational alignment and as a function of its rotational alignment angle. The maximum reflectivity deviated from the expected value calculated from the mirror design. Most of the excess loss was attributed to scattering due to surface roughness at the points of total internal reflection and a necessary deviation from the Brewster angle due to the geometry of the cavity combined with the flat front face of the coating free mirror. ¶ For optimum alignment a cavity finesse of about 4000 was measured, corresponding to a reflectivity of the coating free mirror of 99.89%. Thus, the objective of creating a highly reflective lightweight coating-free optic was achieved. The obtained reflectivity can be further increased by using a substrate that is super polished at the faces of total internal reflection.

A new method based on self-mixing interference (SMI) for refractive index measurement by means of simply frequency analysis is proposed. The optical phase shift is implemented by translating a transparent object through a motorized stage with a setting velocity. Then, the refractive index is determined by the frequency of SMI fringes. A relative repeatability of refractive index estimated in 1.0 x 10 -4 is achieved experimentally. This method has found that there exists an optimal velocity to achieve minimum error of refractive index measurement.

Agilent Technologies' approach to photonic cross-connect switching combines a silica-based planar lightwave circuit chip with a silicon-based actuator chip. The former contains two intersecting arrays of waveguides, with trenches etched into each crosspoint. The latter contains a matching pattern of electrically addressed resistive elements, and the two chips are hermetically sealed together. In the default state, the trenches, along with the rest of the sealed space between the two chips, are filled with a liquid whose index matches the waveguides. Optical signals passing along any guide simply pass on through. However, by activating a resistive element, a bubble can be created at that crosspoint, so that total internal reflection occurs at the side wall of the corresponding trench, and switching is achieved. Recent performance data of prototype 32x32 optical switches, including insertion loss, polarization dependent loss, switching time, and vibration sensitivity will be presented. Some of the intrinsic advantages of our technology will be discussed. One example is the built-in alignment of the waveguides and the trench walls at which switching occurs, which means that no optical realignments are necessary during operation. Another example is the potential for very low crosstalk, characteristic of the underlying phenomenon of total internal reflection.

We present a novel noncontact optical method for absolute measurement of refractive index and thickness of optically transparent media. The method is based on a simple dual-confocal fiber-optic sensor design. It includes two independent confocal channels consisting of two identical apertureless fiber-optic-type confocal microscopes constructed by use of a single 2x2 fiber coupler. A geometrical-ray model is used to obtain the analytical dependence between the sample's refractive index and its thickness. The measurement method provides high accuracy in spatially locating the specific imaging points that correspond to the backreflected intensity peaks of the confocal responses. Thus, a simultaneous measurement of the sample refractive index and thickness is achieved.