Optical Fiber Sensor Technology

Prior to the early 1970s the main application for fiber optic waveguides had been for endoscopic instruments which are routinely used in medical procedures to’ look’ inside the human body. In devices of this type a bundle of optical fibers are arranged spatially such that when light from the object of interest is coupled into the endoscope, the resulting image emanating from the fiber bundle is spatially correlated with the object. The individual optical fibers used in these instruments are multimode with a relatively high optical loss. At about the same time there was a growing interest in the possibility of exploiting optics in communications systems as the potentially large bandwidth would allow significantly more simultaneous users over a single transceiver channel than the microwave links which were then being installed throughout the world. In order to achieve large bandwidth optical communications three elements are necessary:

An optical fiber is a strand of dielectric material which can trap optical radiation at one end and guide it to the other. Normally, the fiber consists of at least two optically dissimilar materials, as shown in a generalized manner in Fig. 2.1. These materials are arranged so that one material, called the cladding, completely surrounds the other. The central material, called the core, normally carries the majority of the transmitted energy. This energy is trapped in the core by reflection at the boundary surface where the core and cladding meet. Often the cladding is itself surrounded by further layers which are added mainly for mechanical strength and protection, but which are not intended to directly influence the guiding properties of the fibers. Commonly, various glasses are used for the core and cladding, but some all-plastic fibers are also used. Plastic coatings are added to fibers for mechanical protection in normal environments, but other types of coating described later in the chapter extend the range of environments in which fibers can be used. Coated fibers are normally deployed within cable structures for further protection. In addition to the fibers themselves, other components are often needed to complete a system design. These can include connectors, splices and splitters as well as more exotic devices.

With the wide range of optical fiber sensors available, and their varied demands, there is an associated wide range of sources which may be used to illuminate and energize these sensor devices. In the simplest types, such as in shutter arrangements [1], a variety of sources may be used; by contrast, in distributed optical fiber sensors using time-domain reflectometry [2], the requirement is for light from sophisticated short pulse high power lasers.

An optical sensor is a system in which some parameter characteristic of an optical signal is modulated in a reproducible and recoverable manner by a measurand. Although the transduction mechanism is optical, it is necessary to convert the optical signal to an electrical one in order that it may be processed and either recorded or displayed. This function is accomplished using a photodetector, which converts optical energy to electrical energy. The basic photodetector generally produces only a low level electrical signal, which must immediately by amplified before it can undergo further processing. The combination of a photodetector and its immediate amplification is called a receiver. The role of the receiver in an idealized optical fiber sensor system is shown in Fig. 4.1.

Of the properties of light which can be conveniently modulated, phase and polarization information are immediately lost upon entering a multimode optical fiber. This leaves intensity as the only transmittable property available for use as a modulation sensitive parameter. However, even intensity is not well conserved in an optical fiber because of variable attenuation effects. As a result the development of multimode fiber sensing is concerned with producing various amplitude or intensity modulation methods and with overcoming the problems associated with the lack of intensity conservation.*

Optical methods are some of the oldest and best established techniques for sensing chemical analytes, and have formed the basis for many chemical sensors. The development of inexpensive, high quality optical fibers for the communications industry has provided the essential component for a new technology -optical fiber sensors. In the last decade there has been considerable research effort expended in developing sensors based on optical fibers for both physical and chemical analytes, with many interesting schemes having been proposed.

Single mode fibers are used for sensing when extreme sensitivity is required or when a well defined polarization of light is needed at a remote sensing point. Most sensors which use single mode fibers are of the intrinsic type (i.e. the action of the measurand on the light occurs within the fiber itself). The sensitivity advantage of single mode fibers arises because they permit the user to construct guided wave interferometers directly from the fiber itself so as to measure small phase changes in light transmitted through the measuring region. This is achieved by comparing the phase of a light wave which has traversed a sensing path with the phase of another light wave originating from the same source but arriving via a protected, reference path. The phase difference can be measured with a sensitivity of ∼10-6 of a wavelength [1] and the pathlength for the measuring interaction can be millions of wavelengths long. This leads to a possible measurement resolution for the optical path of one in 1012! Simultaneously, the absence of free space optical paths between sources and detectors eliminates slow alignment drifts which could easily occur if bulk-optical interferometers had been used. In practice, single mode fiber sensors tend to need very stable, highly coherent sources with low phase noise in order to gain full advantage of their potential sensitivity. When such sources are used, absolute calibration of phase difference is normally not possible and a range limit arises from the periodic nature of the interferometer output. These points will be explained later in this chapter. Recently, both of these problems have been avoided by using sources emitting in a broad wavelength range, with some compromise regarding the ultimate sensitivity achievable with any particular sensor. The concluding part of the chapter will be devoted to such sensors. Another important point to understand is that this type of sensor ultimately measures optical pathlength. Anything which changes the pathlength will therefore produce a signal. Since there are a multitude of effects which can affect the optical pathlength through a fiber, great care must always be taken to reduce or to compensate for these unwanted changes.

In order to be able to implement the signal processing techniques discussed in Chapter 5 a means is required of measuring changes in one or more of the parameters describing the optical beam: amplitude, phase, direction and frequency of the light wave. Temporal modulation of one, or more, of these parameters enables information to be encoded onto or extracted from the optical wave. For example, optical communications systems often use amplitude modulation of the light to encode information combined with modulation of the optical frequency to enable multiplexing and demultiplexing of a number of different signals. In single mode fiber optic sensor systems we are generally using interferometry to transduce very high frequency electric field oscillations (10¹⁴–10¹⁵Hz in the visible) to intensity modulations (Chapter 7). Measurands then induce a change in the optical phase, frequency or polarization state of the beam. Optical fiber modulation techniques are therefore required to either encode information or extract information from the fiber guided beam.

The use of optical interferometric techniques in optical fiber sensor applications allows access to the high resolution and large dynamic range that is associated with these methods. Conventional interferometric fringe-counting techniques allow micrometer displacement resolutions and the additional use of phase-tracking methods increases the attainable down to the nanometer scale.

In a semiclassical description of light propagation in dielectric media, the optical electric field drives the atomic/molecular oscillators of which the material is composed, and these oscillators become secondary radiators of field; the primary and secondary fields then combine vectorially to form the resultant wave. The phase of this wave (being different from that of its primary) determines a velocity of light different from that of free space, and its amplitude determines a scattering/absorption coefficient for the material.

Optical fiber sensors have been researched now for a number of years and a wide body of knowledge has been accumulated as witnessed by other chapters in this book. Although much of the initial development of these sensors was technology-driven, the most successful examples of fiber sensors are those where one or more of the often-cited benefits of fiber sensors brings a fundamental advantage to a particular application. For example, the fiber gyroscope has been able to compete on cost with the laser gyroscope and yet retain some of the advantages of the latter, e.g. zero spool-up time and complete elimination of moving parts. More generally, certain industries have noted the benefits that all-dielectric sensors could bring, in particular the gas and electricity supply industries, where the removal of electrical sensors has significant and specific advantages. In both cases, these are industries where statutory requirements on safety and security of supply (passed on to suppliers in the form of requirements for very long term product guarantees) have forced a certain degree of caution in the introduction of new technology.

For over a decade, intensity modulation has remained one of the most extensively investigated forms of optical signal modulation for sensing applications [1-10]. The simple reason for the extensive and diversified usage of this modulation scheme is a multitude of potential benefits that include the inherent simplicity, reliability, flexibility and relatively low costs. Although intensity-modulated optical fiber sensors have been fabricated in many different designs and with varying degrees of complexity, the essential building blocks of a simple optical fiber sensor system are depicted in Fig. 12.1. Light from an optical source, such as a light-emitting diode (LED), is coupled into an optical fiber for transmission to the optical sensor where it can be modulated in accordance with the state of a measurand. When using reflection-mode sensing the modulated optical signal can be retroreflected into the same optical fiber for transmission to the photo-detector [9]. However, in transmission-mode sensing a second optical fiber is normally used for the transmission of the modulated signal to the photodetector.

Research in optical fiber sensors (OFS) began in earnest in 1977 and for some years subsequently fiber optics was regarded as a sensor technology with great potential likely to emerge in the late 1980s as a generic series of sensors capable of determining all common measurands. Unfortunately, despite the acknowledged advantages of OFS, primarily intrinsically safe operation in many hazardous environments and freedom from electromagnetic interference (EMI), the predictions have yet to be realized and OFS have made very little market penetration. The problem is seen as one of technology transfer; potential users lack confidence in sensors which depend upon, as yet, unproven optical measuring methods and this uncertainty is unlikely to be assuaged until it has been shown that OFS can match the performance and reliability of established sensors under industrial conditions. Additionally, all-optical transducers cannot, using current technology, incorporate intelligence in the sensing head, a practice which has become increasingly popular in the last few years because of the enhanced rangeability and greater user flexibility attributed to the sensor [1].

Optical fiber measurement devices offer several important advantages for power system application. Amongst the parameters which may be measured conveniently with optical fiber techniques are current, voltage, temperature and strain/ pressure. Of the various optical fiber devices which have been developed for such measurements, probably the most advanced is that which seeks to measure electric current, and this chapter will review the methods which have been explored in this development.

Accurate and reliable temperature measurement is a very important consideration in a number of areas of industrial and scientific activity. In many cases both contact and noncontact techniques have been applied to the measurement of temperature in a wide variety of industrial processes. The importance of temperature measurement can be viewed simplistically from the investment internationally in temperature sensors. Estimates of world-wide sales of temperature sensors run to approximately one billion dollars per year, a figure that could be increased several times when associated technology e.g. the measurement system, is added. Fiber optic temperature sensors represent devices with the capability of operation in hazardous environments, or with inflammable materials and it is in particular in these areas where such sensors have their greatest potential for their applications.

In this chapter the mode of operation and performance of the most advanced external fiber optic sensor systems for velocity, vibration, acceleration and displacement will be discussed. These may be grouped under the following headings: