Optical fibre transducers

Figure 13.2 Schematic of instrumentation for use with optical fibre transducers. S, source M, modulator MC, monochromator L, lens T, optical chemical transducer Z), detector A, amplifier R, readout. (MC and L are optional).

Figure 13.3 Optical fibre transducer, based (a) on bifurcated optical fibre bundle with reagent phase at the end of the fibre (b) on a single optical fibre with reagent phase at the end of the fibre (c) with reagent phase coated on the outside of the single optical fibre. (Reproduced with permission from reference 7).

Unlike potentiometric devices, optical fibre transducers do not require an external reference simple integral referencing is carried out optically by the use of the ratiometric method. In this method, a part of the conducted light that is not affected by the measurement variable is used to correct for other optical variations. A high degree of stability can be achieved in this manner. 

Many optical fibre chemical sensors employ optical fibre transducers that are interfaced with immobilized chemical reagent(s). At the chemical transducer end, active chemical/biochemical reagents are held on supporting polymeric matrices and covered by membrane materials in certain applications. The following sections review some developments in this field. 

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

All the most relevant transducers such as MOSFET, CMOS, Surface Plasmon Resonance device, Optical Fibre, ISFET, will be covered in some detail including their intrinsic operating mechanisms and showing their limitation and performance. Shrinking effects of these transducers will also be commented on. 

M Brenci, Thermochromic transducer optical fibre temperature sensor, SPIEProc. 515, 155 (1984). 

Figure 9.2 Schematic diagram of reaction vessel and associated components65. 1, Pressure transducer 2, pressure relief valve 3, sample addition/removal port 4, optic fibre thermometer 5, cold-finger 6, retaining cylinder 7, PTFE reaction vessel 8, top flange 9, magnetic stirrer bar.

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. 

Recent developments of optical-fibre chemical sensors are based on the use of immobilized chemical reagents (reagent phase) interfaced with the optical fibres (3,7). The reagent phase provides the selective chemistry by which chemical information pertaining to the analyte is converted into spectroscopic information. The fibre optic transducer converts this spectroscopic information into an electrical signal. 

Optical fibre chemical transducers may involve the use of either optical fibre bundles or single optical fibres. Fibre bundles can be bifurcated so that separate fibres transmit incident and detected radiation (Figure 13.3a). In optical transducers employing fibre bundles, the optical fibres observe only the part of the reagent phase that fall within both the cone of incident fibres and the cone of detection fibres. The chemical transducer can be interfaced at the common end of the fibre bundle. A similar configuration is adapted when using two single optical fibres in the construction of a sensor, where one fibre transmits the incident radiation and the second fibre the detected radiation. 

When employing a single optical fibre, the chemical transducing element can... 

Sakamoto Y, Takai K, Takashima I, Imada M (1996) Electrical resistance measurements as a function of composition of palladium-hydrogen(deuterium) systems by a gas phase method. J Phys Condens Matter 8 3399-3411 Schroeder K, Ecke W, WUlsch R (2009) Optical fibre Bragg grating hydrogen sensor based on evanescent-field interaction with palladium thin-film transducer. Opt Lasers Eng 47(10) 1018-1022 Sharma AK, Gupta BD (2007) On the performance of different bimetaUic combinations in surface plasmon resonance based fiber optic sensors. J Appl Phys 101(9) 093111... 

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. ... 

Trettnak W., Leiner M.J.P., Wolfbeis O.S., Fibre-optic glucose sensor with a pH optrode as a transducer, Biosensors 1988 4 15. 

W. Trettnak, M.J. Leiner and O.S. Wolfbeis, Optical sensors. Part 34. Fibre optic glucose biosensor with an oxygen optrode as the transducer, Analyst, 113(10) (1988) 1519-1523. 

Trettnak W, LeinerMJP, Wolfbeis OS. Fibre-optic glucose sensor with apH optrode as the transducer. Biosensors 1989, 4, 15-26. 

The classic solution is the application of current and voltage transformers, usually as a cast resin embedded type. As a certain disadvantage of inductive transducers, their small frequency range may be considered. Other solutions for current sensors are Rogowski coils (with active or passive integrators) or the fibre-optic current sensor, based upon the Faraday effect [20]. 

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