Optical fibres problems

PMMA has not been able to compete in the field of compact discs, the market having gone to the polycarbonates (see Chapter 20). It is, however, suitable for optical data storage using large video discs. Large-scale acceptance in the field of optical fibres has been held back by problems of obtaining material of an acceptable level of purity. 

Over the last several years, the number of studies on application of artificial neural network for solving modeling problems in analytical chemistry and especially in optical fibre chemical sensor technology, has increase substantially69. The constructed sensors (e.g. the optical fibre pH sensor based on bromophenol blue immobilized in silica sol-gel film) are evaluated with respect to prediction of error of the artificial neural network, reproducibility, repeatability, photostability, response time and effect of ionic strength of the buffer solution on the sensor response. 

The increasing use of optical fibre in the telecommunications network will, ultimately, require all-optical signal processing to exploit the full bandwidth available. This has led to a search for materials with fast, large third order optical nonlinearities. Most of the current materials either respond in the nanosecond regime or the nonlinearity is too small (1-3). Organic materials are attractive because of their ultra-fast, broadband responses and low absorption. However the main problem in the materials studied to date, e.g. polydiacetylenes (4) and aromatic main chain polymers (5), has been the small nonlinear coefficients. 

Those based on the fluorescence lifetime, rather than intensity (e.g. the Ipitek system), allow one to avoid problems of light loss and other factors that could affect calibration, as discussed above. Assessment of tilted Bragg gratings and long-period gratings on optical fibres has shown them to be a probe of cure of the resin as well as being both temperature- and strain-sensitive (Buggy et al, 2007). The complexity of the response of these and fibre-optics based Fabry-Perot interferometers to strain, temperature and refractive index makes it necessary to employ combinations of sensors if measurements of all of these properties are required separately. 

In TCSPC systems, light is often to be eolleeted from a small spot in a sample and transferred to a small-area deteetor, the slit of a monoehromator, or an optical fibre. Most optical systems for TCSPC do not use strongly oblique beams so that eoma, astigmatism, and curvature of the image field are usually not problems. However, to obtain a high colleetion effieieney, high f numbers must be used, and problems arise from spherical and chromatical aberration. 

Attenuation is caused by a combination of absorption and scattering within the glass. Extrinsic attenuation is due to poor processing or fabrication techniques, and may be due to artefacts such as bubbles, particles, impurities and variable fibre dimensions. These problems have been eliminated in modem optical fibre manufacture. Intrinsic attenuation is a property of the pure material itself and cannot be removed by processing. It is the ultimate limit on the performance of the fibre and mainly arises firom two factors, Rayleigh scattering and lattice vibrations. 

In spite of these problems, the technique of chemical sensing based on the use of optical fibres has great potential. Their success not only requires the development of appropriate reagent phases but also the understanding of the principles and mechanism of their operation, and in the investigation of new sensor designs. 

The case that an uncoated silica optical fibre is laminated by two rigid plates is considered, as shown in Fig. 9.5. Since the silica optical fibre is a rigid medium, this contact problem can be simplified to line force loading. Then the close form of the stress solutions can be obtained with the help of the plane stress assumption. Since the electric field in a single mode fibre is concentrated in the core region, the solution in the centre is approximately selected to represent the stress of a single mode fibre under lateral compression. The stress in the core can be expressed as ... 

The main advantages of optical fibres are a low weight and a small volume. In addition, in fields of high electricity there are no transmission problems, as there is no problem with electrical grounding. 

If we have now accepted the basic tenet that both the measurement and its quality must match the problem, and we have, and if also the problems are becoming more numerous and sophisticated it follows that the diversity and sophistication of the measurement techniques will continue to increase. Preversely, this will not be in the hardware of the instruments which may well become, increasingly, standard arrays of photodiodes, optic fibre bundles, piezo transducers and conductance electrodes. Rather the difference will be in the software. The basic configuration of the instruments cannot change much. ... 

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