The use of fibre-optics

Simply stated, fibre-optic cable allows for the transfer of light from one point to another without going in a straight line, and as such their use in process analytical applications is highly seductive. In a likely NIR process application there may be multiple sample stream take-offs, each with different sample characteristics, located in positions spread around a process unit, with the only apparently convenient analyser location some distance away. 

The maximum input acceptance angle for a fibre-optic cable is determined by the critical angle for total internal reflection, which is given by Snell s law. 

Typical values for low-hydroxyl silica fibres are in the range of 0.22-0.3. 

The throughput of an optical fibre depends on a number of factors including  

All of these factors need to be considered against the cost, the robustness, and the amount of light available to couple into the fibre from the NIR process analyser. 

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The use of TRDs has been facilitated by the use of fibre-optic cable assemblies (Section 6.12.4). This permits sensing in confined spaces as well as enabling the sensor to see round corners and into regions containing nuclear or electromagnetic radiation 36 . Low levels of source visibility can be overcome by employing two-colour or ratio sensors which measure radiation at two different wavelengths simultaneously. Such instruments can be used where dust or dirt obscures the source from the sensor. 

The main advantages of the use of fibre optic as sensors are the following ... 

Schwartz et al. evaluated the use of fibre-optic probe dispersive Raman and PLS calibration to study crystallisation in a hanging drop experiment. They could show that the lysosome concentration could be monitored during the experiment through the concentration phase, nucleation and crystal growth [27], In later experiments, they were able to utilise in-line Raman data to control the concentration of lysozyme to affect the crystallisation path and thereby obtaining the desired large single crystals [28], The same setup was used for crystallisation of apoprotein. Also in this case the Raman data were used to control the evaporation and thus the supersaturation for optimised crystal size [29]. 

The initial studies described above were conducted on Raman microspectrometers on extracted human teeth. In order to translate this research from the laboratory bench to the dental chair side, the next step it is to develop dedicated systems for clinical use. A key element to this development is the use of fibre-optic probes to allow measurements in vivo. We recently reported a study in which optical fibres were used for PRS measurements [48], Although not yet fully realized into a dental probe, this study demonstrated the design and feasibility of acquiring parallel- and cross-polarized Raman spectra via a bifurcated optical fibre whose distal terminal has the two fibres aligned vertically for simultaneously collecting spectra from the two polarization channels on a 2D CCD array. Simultaneous data acquisition will allow for more rapid measurement times in vivo. 

Many technical problems must be solved to enable routine use of Raman spectroscopy as an in vivo clinical tool. The employment of Raman spectroscopy in situ for the study of most tissues necessitates the use of fibre optics to deliver and return the signal from the tissue site. There have been a number of fibre probe designs considered and this is an active area of research [57-60, 10]. Clinical applications to date include GI tract [61], urology [62], lung [63], stomach [64] and cervix [65]. 

A block-scheme of this apparatus is presented in Fig. 2.12, A and B. Compared to that given in Fig. 2.11, an adequate improvement is the use of fibre optic cable which conducts the light from Hg-Xe arc lamp to the reflected light microscope. The thickness is determined by the microinterferometric technique of Scheludko-Exerowa (see Section 2.1). 

The use of fibre optics should also prove to be very interesting. The formation of imprinted matrices directly on the optic fibre (Sections 20.2.5.6. and 20.2.5.7.) should overcome the shortcomings of Kriz s prototype (Section 20.2.5.1.). Thin membranes of MIP should vastly increase mass-transfer rates, reducing incubation times as well as obviating the need for physical entrapment of polymer particles. 

Use of NIR energy as the mode of information transport provides the potential for high signal throughput and the use of fibre optic cables over long distances for... 

The temperature dependences of many optical properties of the resin, e.g. fluorescence and Raman scattering (the ratio of Stokes to anti-Stokes intensities), provide an opportunity to use this as a way of monitoring temperature by comparison with a known standard material. Other systems are based on the properties of the fibre itself or a deliberately added dopant rather than the resin being probed. Table 6.4 shows the commercially available temperature probes that are based on optical phenomena and the use of fibre-optics (Fernando and Degamber, 2006). 

Lessons learned in the use of fibre optic sensors for civil engineering monitoring , in Present and Future of Health Monitoring, P. Schwesinger,... 

The absence of external wiring, important not only from the aesthetic point of view, but also allowing the use of fibre optic sensors for applications that require clean surfaces, as in aeronautical structures. 

Further develoiment work is continuing to make the camera smaller and more versatile. The chief areas being looked at are the use of fibre optics or diode lasers to enable the bulk of the He-Ne laser to be removed from the camera and the use of alternative scanning systems, such as rotating mirrors/prisms or electro-optic devices to enable the scan rate to be increased. 

An important development has been the use of fibre-optic transducers for gas detection, particularly of explosive gases. A significant limitation in this area is the fibre transmission spectrum, which for silica fibres extends only to about 1.8 fim, whereas most of the standard fingerprint absorptions used to identify simple molecules (C0,CH4,C2H6,N20,etc.) occur in the mid-IR region (2 pim to 6 /im). Thus overtone spectroscopy is used, in which the higher harmonics of these fundamental absorptions (21-23) are measured between 1.2 and 1.7 jum. Absorption coefficients are low for the CH4 harmonic the absorption coefficient is 9.3 x 10 Vppi i - The lower explosive limit for CH4 is 5% and the higher explosive limit 15%. Detections of 0.8% (or 400 mm) of the lower explosive limit has been achieved with fibre-optic cavities (24). 

Trials were undertaken successfully for the use of fibre optics to illuminate loco cab instruments and subsequently fitted on all the locos needing them. 

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