Optical fibres results

The attenuation of an optical fibre results from physical phenomena either occurring within the fibre or coming from the environment. This is the sum of light lost by scattering in the fibre, absorption by the fibre materials, leakage of light out of the core due to environmental factors (e.g. microbends). Scattering and absorption losses dominate in every fibre. 

In connectors light source to optical fibre, power losses result from different space characteristics of the light source and the fibre. Each light source emits a beam in its typical shape whereas a fibre can accept only these rays within the acceptance cone. Some typical losses are presented in Figure 2. 

Another approach, developed in our laboratory, consists of the compartmentalization of the sensing layer25"27. This concept, only applicable for multi-enzyme based sensors, consist in immobilizing the luminescence enzymes and the auxiliary enzymes on different membranes and then in stacking these membranes at the sensing tip of the optical fibre sensor. This configuration results in an enhancement of the sensor response, compared with the case where all the enzymes are co-immobilized on the same membrane. This was due to an hyperconcentration of the common intermediate, i.e. the final product of the auxiliary enzymatic system, which is also the substrate of the luminescence reaction, in the microcompartment existing between the two stacked membranes. 

A laboratory characterisation was carried out, during which the performances of the optical fibre sensor were carefully verified and compared with those of the Tonocap. The results showed the capability of the optical fibre sensor to detect CO2 correctly even in the presence of rapid changes of the order of 1 minute. 

First clinical results were obtained by using a combined catheter which included both the optical fibre sensor and the Tonocap balloon (Figure 8) A typical result obtained on an intensive care patient, is shown in Figure 9. In the graph the tracing of the end-tidal CO2 (EtCC>2), i.e. the CO2 concentration in the expiration at the end of the expiratory phase, and the values of the arterial CO2 (PaC02), obtained from blood samples drawn from the patient, are also shown. As expected, a rapid CO2 peak was detected only by the optical fibre sensor, and was not seen by Tonocap (as in the measurements carried out on volunteers). Moreover, the optical fibre sensor seems to follow better the end-tidal CO2. 

It is important to model a correlation spectroscopy system, firstly to predict performance, and also to aid the very important choice of optical filter (or choose the best LED or super-luminescent optical fibre source to give the optimal spectral output), in order to achieve the best detection performance or best selectivity possible. The length of the cells and the pressure of gas (or gas concentration) are also important parameters (although all simulated results described below are based on use of lm long cells). 

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 -. ...

These observations, along with similar results obtained with a variety of complexing ligands, presented convincing evidence that a lead-sensing optrode could be made. The 3 mole % Pb(DVMB)2Br2 complex (2 mole % DVB) was bound by in situ copolymerisation on a 400 pm optical fibre surface. The fibre was then used in the set-up schematically presented in Fig. 19.10. The luminescence... 

Figure 3.42. The arrangement of excitation (lex) and emission (lem) beams from optical fibres for remote fiuorescence or Raman spectroscopy. The resulting analytical volume is shaded. Reproduced with permission from Boisde and Harmer (1996). Copyright 1996, Artech House.

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