Optical Fiber Sensor Technology

‘Making light work’ has been one of the objectives of mankind for many centuries. Even before a clear understanding of the nature of light itself emerged in the work of Newton, Young, Huygens and others, mankind put light to work to create fire, to illuminate the darkness, to understand the seasons, to communicate, or even to try to predict the future. The story of the exploitation of light is indeed a story of the involvement of ‘many hands’ — who knows who the first human was to use a burning glass to light a fire, to manipulate a bright object to deflect the sun’s rays to pass a message, or to use a flame to shine light into the darkness and so extend the working day? Since then many hands have indeed been involved.

Optical techniques for chemical analysis are well established. Actual sensors based on these techniques, as opposed to proposed systems described in the literature, are now attracting considerable attention because of their importance in applications such as environmental monitoring, biomedical sensing and industrial process control. In many instances these sensors exploit the specific advantages made available by optical fiber technology, as fiber optic chemical sensors (FOCS) can benefit from, for example, the geometric versatility, low attenuation and electrical noise immunity of optical fibers. In this chapter, the emphasis throughout is on the developments which have occurred to produce a series of FOCS systems and devices, although many of the methods described can be transferred, sometimes with considerable advantage, to planar waveguide configurations. Alternatively, they can operate successfully without the use of optical fibers at all through open air-path systems, an option which is often neglected in the general promotion of optical chemical sensors.

The sol-gel process is a well-known method for chemical synthesis of numerous ceramic and glassy materials. This method typically involves the hydrolysis and condensation of metal alkoxide precursors to form gels which are later densified at much lower temperatures than are required by conventional ceramic processing techniques [1]. The versatility of this process is evidenced from the silicate-based systems, which have been developed into a wide variety of final products ranging from catalytic supports [2] and photochromic glasses [3] to planar waveguides [4] and fiber optic preforms [5]. The process can be tailored to yield materials of the desired composition and physical properties in the form of powders, fibers, thin films and monoliths [6]. One of the advantages of using the sol-gel process is a high degree of purity and homogeneity due to mixing at near molecular levels. Furthermore, due to the low processing temperatures, amorphous compositions which are unstable if produced by melting can be made by the sol-gel route.

In the more than 30 years since their first conception by Polanyi [1], significant effort has been devoted to the development of fiber optic sensors for biomedical applications. In the interest of cohesion, this chapter will mainly focus on sensors for determining chemical analytes of biomedical interest, with only a brief discussion of influence and physical parameter sensors; the reader is referred to chapters elsewhere in this volume for comprehensive treatments of these topics. Because of Baldini and Mignani’s recent review [2] and other publications [3–6], a comprehensive review of the field would be redundant. Instead, we shall focus on selected recent sensors and sensor technology of interest to the biomedical community from the standpoints of both research and clinical practice. Finally, we shall consider the possible reasons why fiber sensors have apparently had only a modest impact in biomedicine, and what their future prospects might be as the technology evolves.

Fiber optics is best known today for its widespread and ever growing applications in communication networks around the world, and since the early 1980s, the application of fiber optics in measurement and sensor systems has begun to emerge.

Integrated optical devices use optical signals for their operation in an analogous way to electrical currents in electronic circuits. Electrical connections such as wires are replaced by optical waveguides, and semiconductor integrated circuits are replaced by optical circuits. Figure 6.1 shows a schematic representation of an integrated optical sensor, which illustrates the principal components of such devices and also some of the advantages of optical circuitry. Many volumes provide a detailed description of integrated optic theory and technology, and in this section only a brief overview of the most important aspects will be given [1, 2].

There are many areas of industry where temperature measurement is essential, including electricity generation, plastics and glass production and molding, a wide range of manufacturing processes in the metallurgical industries, food production and processing, and many areas of biomedical engineering and process chemistry. Indeed, it is difficult to consider an industry where temperature measurement is not important somewhere in the process.

The development of fibers doped with materials through which luminescence is generated has a long history, which follows upon the original proposal by Snitzer [1] in the early 1960s of the use of an optical fiber geometry for lasers and optical amplification, employing 300 μm core diameter neodymium doped fiber. Subsequent development of this concept has been the major reason for the production of a range of different types of optical fiber, doped with appropriate materials to show fluorescence. Only ions that belong to the lanthanide group of rare earths have been observed to allow laser oscillation in a fiber geometry, as these ions possess a number of distinct features which make them attractive for use as a laser medium. These benefits apply equally (but under less rigorous conditions) to their use as fluorescent sources where the need to generate stimulated emission is not present, and so strong, noncoherent fluorescent light arising from spontaneous emission is the primary output. The fiber geometry, however, provides a very useful constraint on the propagation of the light beam, and so an intense directional emission can be achieved from a fluorescing fiber when monitoring the light emerging at either end. The discussion of the use of luminescent fibers in sensing in this chapter excludes the considerable use of optical fiber in lasers and amplifiers, either as sources for optical sensing or for other purposes, such as optical communications and in optical fiber laser sensors themselves. Both these topics have been discussed in some detail, the former by Langford [2] and the latter by Kim [3]. Further, the use of other luminescent phenomena such as are produced by nonlinear effects, typically Raman and Brillouin scattering, are also not discussed in this section, as again they have been considered extensively elsewhere, for example in the work of Grattan and Zhang [4] and Rogers [5].

Liquid crystal (LC) devices and optical fibers are both technologies which have matured over the past 20 years, with each year bringing new advances in these exciting areas. Although the primary applications for each of these technologies are rather disparate, with liquid crystals used primarily for display devices and optical fibers for telecommunications purposes, they have in common one aspect — light. Optical fibers serve to transport light from one location to another, and the material properties of liquid crystals modulate some aspects of light, such as the polarization state, the spectral distribution, the intensity or other aspects. Coupled with the numerous ways in which it is possible for external quantities (the measurands) to affect the material properties (and hence the optical properties) of liquid crystals, it becomes a natural step to seek to combine the two technologies for sensing applications.

White light, or low-coherence interferometry, is a technique which dates back to 1913 and the work of Benoit et al. [1]. A detailed theoretical analysis of a simple interferometer, given by Born and Wolf [2], shows that with two plates in series, if the light reflected backwards and forwards between them is neglected, the output intensity of the white light transmitted by both plates distributes itself in such a way that there is a central white fringe in the pattern produced, with colored maxima and minima on either side, and uniform illumination further away. Such fringes are called fringes of superposition. The technique was reapplied to optical fiber sensing in 1983 by Al-Chalabi et al. [3] in an experiment where two Mach—Zehnder interferometers were coupled together to form a multiple remote interferometric sensor system, and in multimode form by Bosselman and Ulrich in 1984 [4]. The essential background to the subject was detailed by Meggitt [5] in an earlier volume, and this work discusses advances and new techniques applied in recent years.