Refractive index variation

The schlieren microscope is able to detect refractive index variations to six decimal places. Any small difference in optical path (index difference, film thickness, etc) is very precisely detected by the schlieren microscope, especially in the Dodd modification. It is, in effect, a darkfield method. The specimen is illuminated with light in a portion of the illuminating cone and that direct light is masked in the conjugate back focal plane of the objective (Fig. 3). The only light to pass through this plane is refracted, reflected, or diffracted by the specimen. 

Miniaturized and robust optical fiber sensors capable of accurate and reliable measurement of refractive index of the surrounding environment have attracted tremendous interest in recent years. One of the driving forces for the development of these fiber optic devices is their broad applications in chemical sensing. When placed in the liquid solution or gas mixture, these fiber sensors can detect the chemical composition change by monitoring its refractive index variation. These... 

In many cases, only the relative refractive index change is of interest and the range of refractive index variation is small so the phase shift is less than 2n. In this case, the phase ambiguity issue can be avoided. The relative refractive index change can be calculated based on the spectral shift of the interferogram. 

The above measurement results also included the error contribution of the temperature cross-sensitivity of the device. From Fig. 7.11, the temperature dependence of the device was 0.074 nm °C Based on (7.6), the temperature crosssensitivity of the device was less than 3.2 x 10 6 RIU °C. Therefore, the total temperature cross-sensitivity-induced measurement error was about 2.8 x 10 4 RIU in Fig. 7.13 over the temperature variation of 87°C. The temperature dependence of the device was small and contributed only about 2.3% to the total refractive index variation over the entire temperature range. 

Kim, Y. Paek, U. Lee, B. H., Measurement of refractive index variation with temperature by use of long period fiber gratings, Opt. Lett. 2002, 27, 1297 1299... 

In Sect. 15.2 we concluded that refractive index variation in the aqueous cover media shifts the effective refractive index of the waveguide modes. Thus, by monitoring on-line the effective refractive index, the refractive index change can be followed. This is the basic principle of waveguide sensing. 

More detailed analysis shows that the application of the low refractive index substrates not only increases the penetration depth into the cover media, but also since the mode profile is reversed places a larger portion of the electromagnetic power flowing in the waveguide structure into the cover media, thus increasing the sensor sensitivity for refractive index variations in the cover solution9 15. 

Fig. 5.34 Refractive index variation of 450°C annealed nanotubular titania film, and for comparison a glass (Corning 2947) substrate, in the range 380 to lOSOnm. The Ti02 film has an average refractive index in the visible range of 1.66.

The fiber optic sensors utilize an extrinsic Fabry-Perot interferometer to spectrally modulate light in proportion to pressure, temperature, or refractive index variations. Because they are based on spectral modulation instead of amplitude modulation, they are not affected by such common problems as fiber bending, connector losses, and aging. 

In Sect. Ill, the photoinduced refractive index variation in organic materials is discussed. The four elementary processes necessary for a PR effect are considered photogeneration, transport, and trapping of charged carriers, as well as the EO effect. 

FIGURE 31. The refractive index variation in the PMPS film by UY-light irradiation using different light sources (a) excimer lamp (308 nm) and (b) mercury-arc lamp (185, 254, 303 nm). (Reprinted from Ref. 134.)... 

The development of phase masks using the refractive index variation due to the UV-photodecomposition of the polysilane was examined.134 The refractive index due to UV-photodecomposition of PMPS changes from 1.70 to 1.56. The refractive index variation in PMPS films by UV-light irradiation is shown in Figure 31. 

Finally, under burning conditions, the refractive index variations produced by the gas temperature gradients cause deflections of the beams, hence drop-outs of-the photodetector signal, but visibility measurements are still possible. 

The parameters e0, A, A0 were adjusted to retain continuity with the actual spectrum used, and to fit the refractive-index variations in the visible region published by other authors.94 At low temperature, we had... 

External mass-transport systems require either the addition or removal of material to create the desired pattern of refractive-index variation. The use of the latter method is by far the more common and we discuss two techniques for accomplishing this the use of solvents to dissolve portions of a film, and selective evaporation of a fraction of the material in the film. 

The next evolution in ABS technology was the need to produce a transparent ABS. Existing ABS was opaque owing to the scattering of light by the rubber domains. While producing smaller domains would make the system clear, it led to a loss of impact strength. The answer was to modify the refractive index of the components so that the various phases were less optically different. A fourth monomer , methyl methacrylate, was used to minimize the refractive index variation in the ABS and a clear impact-resistant thermoplastic named Cyclolac CIT was achieved [20]. 

There is a host of other intriguing phenomena associated with the structure and dynamics of stars, which we only list here. The inhomogeneous monomer density distribution in Fig. 2 is responsible for temperature and/or solvency variation in analogy to polymer brushes attached on a flat solid surface [198]. In fact, multiarm star solutions display a reversible thermoresponsive vitrification (see also Sect. 5) which, in contrast to polymer solutions, occurs upon heating rather than on cooling [199]. Another effect is the organization of multiarm stars in filaments induced by weak laser light due to action of electrostrictive forces [200]. This effect was recently attributed [201] to local concentration fluctuations which provide localized-intensity dependent refractive index variations. Hence, the structure factor speciflc to the particular material plays a crucial role in the pattern formation. 

For higher concentrations, a number of corrective factors have to be introduced, as for example the refraction index variation related to the concentration of the analyte. One way to correct this effect is to substitute the value of ex of the Beer-Lambert equation by e n/(n2 +2)2, where n is the value of the refraction index. Usually, this correction is not very high for concentrations lower than 0.01 M. Another effect that can distort the Beer-Lambert law linearity may be the use of a polychromatic radiation. This problem, encountered with instruments using filters (photometers), obliges the use of instruments allowing the selection of narrower wavelength ranges by means of monochromators (spectrophotometers), and are, therefore, more expensive. 

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