Optical Fibre Sensor

Optical fibres behave as wave guides for light. The original ones were made of glass but now polymeric materials are mostly used as they are cheaper than glass and metal wires. 

The light can propagate along the fibre usually by totally internal reflection (TIR) as indicated in fig. 18. TIR is controlled by the angle of incidence and also by the refractive indices of the media involved in the propagation. 

From Snell s Law, sin(0j) m = sin(0j) nr. We have TIR when sin(Oj) nr/rii, while we will have refraction and reflection when sin(0j) nr/ni. In practical cases properties of light, such as phase, polarization and intensity, can be modulated inside the wave guide by a given measurand, which is interacting, for instance, with a CIM lying within the penetration depth for the evanescent field of the light localized near the external guide surface. 

Integrated guides can also be used as a chemical sensor. Fig. 19 shows a Mach-Zehnder interferometer where one of the two branches has been covered by palladium, a catalytic metal for H2 [4]. The output phase change measurement determination of parts per million of H2 has proven to be possible. 

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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. 

Bevenot X., Trouillet A., Veillas C. et.al., Hydrogen leak detection using an optical fibre sensor for aerospace applications, Sensors Actuators B 2000 67 57-67. 

Bioluminescence and chemiluminescence are very powerful analytical tools, since in addition to the direct measurement of ATP, NAD(P)H or hydrogen peroxide, any compound or enzyme involved in a reaction that generates or consumes these metabolites can be theoretically assayed by one of the appropriate light-emitting reactions. Some of these possibilities have been exploited for the development of optical fibre sensors, mainly with bacterial bioluminescence and with luminol chemiluminescence. 

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. 

The main performances of batchwise and flow luminescence-based optical fibre sensors are summarized in Tables 2 and 3. As it can be shown... 

Table 2. Performances of batchwise and flow injection analysis (FIA) bioluminescence-based optical fibre sensors.

Baldini F., In vivo monitoring of the gastrooesophageal system using optical fibre sensors, Anal. Bioanal. Chem. 2003 375 732-743. 

Cherif K., Hleli S., Abdelghani A., Jaffrezic-Renault N., Matejec V., Chemical detection in liquid media with a refractometric sensor based on a multimode optical fibre, Sensors 2002 2 195-204. 

This aspect makes it clear why optical fibre sensors for medical applications have been characterised by a remarkable development after the introduction of optical fibres. The first invasive optical fibre oximeter which was described in 19641, can be considered one of the first optical fibre sensors. Since then, many sensors have been proposed which find application in different biomedical areas, ranging from cardiovascular and intensive care to angiology, gastroenterology and ophthalmology. Some of these are still at the prototype level, whereas others are already available on the market. 

The intravascular system Paratrend 7 was distributed by Diametrics Medical up to 2003 and has been the last one available on the market which assures an intravascular monitoring of pH, p02 and pC02, using an optical fibre sensor designed for arterial placement. 

Although pH optical fibre sensors have been already developed for blood analysis, they were not designed to reach, through a catheter, the vascular system of the brain. A pH sensor was developed for this purpose at the... 

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. 

Figure 9. Clinical test on a patient response curve of the optical fibre sensor and of Tonocap.

An investigation of the rapid changes of gastric C02 observed, possibly using the new optical fibre sensor, could lead to a better understanding of gastrointestinal physiology. 

Baldini F., Falai A., De Gaudio A.R., Landi D., Lueger A., Mencaglia A., Scherr D., Trettnak W., Continuous monitoring of gastric carbon dioxide with optical fibres, Sensors and Actuators B 2003 90 132-138. 

The nonlinear optical and dielectric properties of polymers find increasing use in devices, such as cladding and coatings for optical fibres, piezoelectric and optical fibre sensors, frequency doublers, and thin films for integrated optics applications. It is therefore important to understand the dielectric, optical and mechanical response of polymeric materials to optimize their usage. The parameters that are important to evaluate these properties of polymers are their dipole moment polarizability a, hyperpolarizabilities 0... 

J. Dakin and B. Culshaw, eds. Optical Fibre Sensors, Vols. 1 and 2, Artech House, London (1988). 

T. Bosselmann, A. Reule, and J. Schroder, Fibre-optic temperature sensor using fluorescence decay time, Proc. of 2nd Conf. on Optical Fibre Sensors (OFS 84), SPIE Proceeding 514, 151-154 (1984). 

A. L. Harmer, Fibre optic sensors for offshore process control instrumentation, Proc. Optical Fibre Sensors Conference (OFS 86), Informal Workshop at Tusukuba Science City, 1986, VII. Pub Institute of Electronics and Communication Engineers of Japan, Tokyo, 1986. 

Deboux BJC, Lewis E, Scully PJ, Edwards R (1994) Optical Fibre Sensors Conference, Glasgow, p 6... 

Guthrie, A.J., Narayanaswamy, R. and Russell, D.A. (1988). Application of Kubelka-Munk diffuse reflectance theory to optical fibre sensors. Analyst 113,457-461. Narayanan, S. (1989). Principles and Applications of Laboratory Instrumentation. ASCP Press, Chicago, pp. 192-195. 

Lyons, W. B. Lewis, E. Neural Networks and Pattern Recognition Techniques Applied to Optical Fibre Sensors. Trans. Inst. Measurm. Control 2000, 22, 385 104... 

Rella R, Capone S, Siciliano P, Spadavecchia J, Ciccarella G (2004) Proc SPIE-Int Soc Opt Eng 5502 (Second European Workshop on Optical Fibre Sensors, 2004) 435 t38... 

Hale Z M and Payne F P 1994 Fluorescent sensors based on tapered single-mode optical fibres Sensors Actuators 17 233 0 Luo S and Walt D R 1989 Avidin-biotin coupling as a general method for preparing enzyme-based fiber-optic sensors Anal. Chem. 61 1069-72 Li L and Walt D R 1995 Dual-analyte fiber-optic sensor for the simultaneous and continuous measurement of glucose and oxygen Anal. Chem. 67 3746-52 Tan W, Shi Z-Y, Smith S, Bimbaum D and Kopelman R 1992 Submicrometer intracellular chemical optical fiber sensors Science 258 778-81 Tan W, Shi Z-Y and Kopelman R 1992 Development of submicron chemical fiber optic sensors Anal. Chem. 64 2985-90... 

K. T. Grattan and B. T. Meggitt, Optical Fibre Sensor Technology. Chemical and Environmental Sensing, Vol. 4. Boca Raton, FL CRC Press/Chapman Hall,... 

Body movement, posture Piezoresistive strain/ pressure sensors, accelerometers, gyroscopes, optical fibre sensors Body kinematics Dependent on motion to be analysed... 

Phase modulation as optical fibre sensors, or interferometers... 

Besides these functions, safety aspects are vital in developing these types of MRl compatible devices. One should avoid electric cables, metallic parts and prevention of bacteriologic and viral cross-patient contamination. One of the main advantages of optical fibre sensors is their immunity to electromagnetic radiations. 

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