Fibre-optics, principles

IR Bond vibrations (change in dipole moment) In principle contains wealth of structural infonnation, but difficult to extract in practice Fairly sensitive and specific broadly applicable new fibre optics wiU increase suitabdity... 

Figure 11.15—Principle of a spectrophotometer fitted with an immersion probe. Monochromatic light from the spectrophotometer is guided toward an immersion cell and then brought back to the detector. The reference beam is also guided by a fibre optic.

These include such instruments as opacity monitors, turbidimeters, colorimeters, refractometers and spectrophotometers. A selection of these is described—particularly where the instrument has a more general application as an on-line process analyser and/or to illustrate a general principle of operation. It is likely that development of fibre-optic techniques (Section 6.12.4) will extend the use of this type of sensor in the future(56). 

Senior, J. Optical Fibre Communications, Principles and Practice (Prentice-Hall, London, 1985). 

As discussed later, in Section 3.4, this technique is particularly well adapted to coupling to optical fibres, so a probe that can be immersed in a polymer reaction stream may be fabricated to enable spectra to be collected in real time. The principles of total internal reflection and its application in fibre-optics are discussed in more detail in Section 3.4.1. 

The fundamental requirement of all radiation-transfer techniques in remote spectroscopy is that the radiation be transferred from the spectrometer to the sample, probe the reactions or transformations of interest and then return the modified beam of radiation to the spectrometer for the measurement of intensity at each wavelength in the spectral region of interest, all without any contributions from the transfer medium and with little loss in energy. While fibre-optics meets these requirements under favourable circumstances, there are potential artefacts and limitations, which may be understood from the principles of operation. 

The key principle in the operation of fibre-optics, and also in several of the sampling methods, is total internal reflection. This is illustrated in Figure 3.36, which shows three cases for a ray of light travelling in a medium of refractive index Kj, incident at three different angles, 6, to the interface with a medium of lower refractive index, 2-... 

The wavelength and angle dependences of the depth of penetration, d, were given earlier (Equation (3.22)), and this principle is used in the MIR for the sampling technique of ATR spectroscopy. Figure 3.13(c) illustrated this, and, for typical refractive indices of fibre core and sample of 2.5 and 1.5, respectively, and an angle of incidence of 45°, the depth of penetration of the evanescent wave is about 0.152. The effect of the evanescent wave on fibre-optic spectroscopy may be illustrated by the example of plastic-clad silica (PCS) fibre optics for remote spectroscopy in the NIR spectral region, as discussed below. 

While most measurements of tempemture in industrial processing simply involve the use of thermocouples or platinum resistance thermometers, there are some applications involving microwave fields or chemical environments for which this might not be possible. In that case a fibre-optic sensor is the instrument of choice. These sensors are often also sensitive to stmin, so they may be used to obtain a measure of pressure and local deformation. The operating principle is discussed in the following section. 

Figure 9.17 The principle of a spectrophotometer fitted with an immersion probe. Monochromatic light issuing from a spectrophotometer is guided towards an immersion cell and then returned to the detector. The route is confined by a fibre optic. Left transmission probe. Right ATR probe the sapphire prism has a refraction index greater than that of the solution. The schematic shows three reflections of the beam and its penetration into the solution (see explanation in Chapter 10, Section 10.9.3).

O. Egorov, J. Ruzicka, Flow-injection renewable fibre optic sensor system. Principle and validation on spectrophotometry of chromium (VI), Analyst 120 (1995) 1959. 

In principle the combination of fibre optics and photodiode array spectrometers shows great advantages in photokinetics. Fibre optic technology allows a wide variation of such set-ups. In the 1960s one tried to achieve a versatile spectrometer by combining modules such as light source, monochromator, cell compartment, and detector on an optical bench. This arrangement of the different components allowed the measurement of absorbance, fluorescence, and reflectance. The same can be achieved nowadays by the use of a so-called simultaneous spectrometer module, schematically presented in Fig. 4.11. 

An alternative approach to remote mid infra-red spectroscopy is presented which utilises gold-coated waveguides instead of fibre optics and which could be used for in-situ real-time process control in practically any environment. The principle objects of the paper are to describe a novel experimental set-up for remote mid infra-red spectroscopy recently assembled in laboratory, and to demonstrate the qualitative and quantitative analysis of the remote mid infra-red data using a multifunctional epoxy/amine formulation as an example. 18 refs. 

The energy is transferred down the cable as digital pulses of laser light, as against current flowing down a copper conductor in electrical installation terms. The light pulses stay within the fibre-optic cable because of a scientific principle known as total internal refraction which means that the laser light bounces down the cable and when it strikes the outer wall it is always deflected inwards and, therefore, does not escape out of the cable, as shown in Fig. 3.42. 

Blum LJ and Gautier SM (1991) Bioluminescence- and chemiluminescence-based fibre optic sensors. In Blum LJ and Goulet PR (eds.) Biosensor Principles and Applications, pp. 213-247. New York Dekker. 

Sensors which make use of the phenomenon called surface plasmon resonance (SPR) have become very popular in recent years. Recently, researchers have managed to transfer the principle to fibre optic sensors, but nevertheless... 

Principles and Characteristics Absorption spectroscopy of both vapour and liquid samples by wavelengths in the UV/VIS range, causing electronic transitions in the sample, can be used to quantify components in a mixture. Optical transmission measurements are preferred to diffuse reflectance, they provide higher sensitivity, more precision and enable monofilament fibre optics to be used. Spectroscopic (UVA IS/NIR) analysis of pellets is more complicated. 

Principles and Characteristics As already indicated in Chp. 1.2.3, Raman scattering induced by radiation (UV/VIS/NIR lasers) in gas, liquid or solid samples contains information about molecular vibrations. Raman specfioscopy (RS) was restricted for a long time primarily to academic research and was a technique rarely used outside the research laboratory. Within an industrial spectroscopy laboratory, two of the more significant advances in recent years have been the allying of FT-Raman and FTIR capabilities, coupled with the availability of multivariate data analysis software. Raman process control (in-line, on-line, in situ, onsite) is now taking off with various robust commercial instrumental systems equipped with stable laser sources, stable and sensitive CCD detectors, inexpensive fibre optics, etc. With easy interfacing with process streams and easy multiplexing with normal (remote) spectrometers the technique is expected to have impact on product and process quality. 

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