Special optical fibres

So the function of special optical fibres for sensing is to produce a sensitive response to changes in the fibre surroundings. Such requirements on optical hardware as durability to the analyte, transparency (i.e. minimum optical losses) in a wide spectral range and common availability should be pointed out. Related to the these requirements, the choice of the fibre material as well as of the fibre coating and fibre structure belong to fundamental tasks in the design of fibre-optic sensors. 

Special optical fibres have been intensively investigated during recent years because of their potential wide-range use for on-line monitoring of material properties or processes in a number of areas of human activity (environment protection, food industry, medicine etc.) Their technology can be considered an integral part of the team-work on optical fibre sensors development. Despite special optical fibres represent a unique and often indispensable tool for a variety of sensor applications, special fibre production still represents only a small fraction of the market. Probably it is because of their low consumption (in comparison with standard telecommunication fibres), the need for much more advanced know-how and lower reproducibility. 

Optical fibres can be used in the transmittance and ATR mode (a special ATR application is the remote sensor), and even in the reflectance mode. The development of special optical fibres for transmission, transflection or diffuse reflectance measurements favours on-line analysis of problematic product streams and reaction mixtures (solutions, suspensions, emulsions, melts, solids). Both quartz and fluoride (ZrF4-based) glass fibres are used, with the former having poor transmission characteristics above 2000 nm. 

Special optical fibres can also be produced by a modified jacketing method [270]. A tapered glass preform is produced by suction casting of the core material into a tube made of the cladding material. This initial preform is then drawn off and loosely joined to circular positioned outer tube glasses to form a secondary preform, which is used for the fibre drawing process. This method allows the production of multi-layer fibres with large refractive index steps. 

Optical fibres were originally studied and developed for the transmission of information in telecommunications and have been used for building of long-haul as well as local networks for years. Such kinds of optical fibres have to be of ultra-low optical losses, standard size, resisting to temperature changes in a large interval or to chemical influence of their surroundings, etc. In contrast to these standard widely used fibres, the special ones are... 

In the manufacture of plastics in forms such as minute particles, special illumination is needed—typically, the combination of a halogen lamp with optical fibres. Recently, equipment of high illuminating capacity using a short arc has been marketed for work of this nature. 

Figure 6.69 gives an example for an optical current sensor. The light path is wound around a current-carrying conductor equidirectionally with the azimuthal magnetic field of the current. The rotation of the plane of the electric vector is not detectable on its own and is converted to light intensity variations by a polarizer/analyser combination. A photo diode is used as a light intensity detector. The optical sensor itself is installed in the - e - compartment, the electronics shall be protected in an adequate type of protection, e.g. in a small flameproof - d - enclosure or in encapsulation - m -. In the special case of an energy distribution system with combined - e - and - d - compartments, the optical fibres may enter the d-compartment to the electronics inside via bushings complying with d -standards EN 50018 or IEC 60079-1 respectively (Fig. 6.70). The evacuation of the sensors into the e-compart-ment results in additional available space in the more expensive d-compart-ment, compared with increased safety - e -. ...

Initially fibre optic chemical sensors were simply devices for modifying the light path of a conventional spectrometer. The optical fibres were just used to conduct light to and from the sample. Sometimes conventional colorimetric reactions were carried out on the sample, and specialized probes could be used in conjunction with the optical fibres to make absorption measurements (for example). Such devices can be described as photometric or spectroscopic transducers. 

These syringe applicators with very fine needles may apply very small dots or drops down to one hundredth of a milligram. They are widely used in electronics for surface mounting, chips bonding and in jewellery and watches. The application of the adhesive may be controlled with a special camera or an optical fibre detector. The viscosities of the adhesives may range from 100 mPas (cyanoacrylates) to 20,000 mPas (epoxies, silicones). 

Laser chemistry can also be used for purifying certain chemicals. The production of ultrapure silicon for the semiconductor industry and for producing solar cells is of special interest. For optical fibres the purest possible Si02 is desirable. A suitable starting material is silane gas. Arsine (ASH3) and phosphine (PH3) are typical impurities. On radiation of the natural gas mixture with an ArF excimer laser (A=193nm) the impurities are quickly dissociated. The technique has a good production potential and the cost for purification could be quite reasonable. Since the cost of the laser photons will always be an important factor it is likely that laser-induced... 

Optical couplers are used to split, combine or distribute light beams to and from optical fibres. The material of the coupler must have a coefficient of thermal expansion close to that of the optical fibres, excellent dimensional stability at elevated temperatures, high modulus, and extremely low mould shrinkage to allow very narrow tolerances. A special grade of glass-fibre-filled LCP has been developed for this application. " ... 

The scope of UV analysis of dissolved polymer/additive matrices is thus quite restricted and mainly limited to special cases in which the additive package is known, e.g. the determination of Irganox 1098 in GFR-PA4.6 after dissolution in H2SO4/HNO3. Fibre-optic dissolution analysis by means of a UV diode array spectrometer is well known. In comparison to IR spectroscopy, UV spectrophotometry is better equipped to provide quantitative data. 

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