Preface The present book is an introduction to the rapidly emerging field of fiber optic sensors that is having significant impact upon areas such as guidance and control, structural monitoring, process control and medicine. The book will enable the reader to Understand the benefits of fiber optic sensor technology Know the basic principles upon which the fiber optic sensors rely Become familiar with various physical, chemical and biological parameters that can be measured using fiber optic sensors Understand how this technology is being used today and how it is likely to impact future systems and products.

1.1 Introduction Since 1990s optoelectronics and fiber optics technology underwent significant advances due to innovations in the telecommunications, semiconductor and consumer electronics sectors. The developments of compact disk (CD) players, laser printers and the high performance and reliable optical fiber communication are the result of these advances. The optical fiber sensor technology is a direct outgrowth of the revolutions taken place in optoelectronics and fiber optic communication industries. Many of the components associated with these industries are used for optical fiber sensor applications. In the beginning the main hurdle in the development of sensors was the cost of components such as optical sources and detectors. With the time, components prices are falling and the improvements in fiber, laser and photodetector technology have taken place. It is expected that as the time progresses the optical fiber sensors will replace the conventional devices for the measurements of various physical, chemical and biological parameters such as rotation, acceleration, electric and magnetic fields, temperature, pressure, acoustics, vibration, position, strain, humidity, viscosity, pH, glucose, heavy metals, gases, viral infection, pollutants etc.

2.1 Introduction In a fiber optic sensor the heart of the sensor is an optical fiber. An optical fiber in its simplest form is a cylindrical symmetric structure consisting of a central ‘core’ glass/plastic of uniform refractive index and of diameter 4-600 µm. It is surrounded by a ‘cladding’ glass/plastic of slightly lower refractive index. To provide the mechanical and environmental protection to the fiber, the cladding is covered with an external plastic coating (seeFig. 2.1). The light in the fiber propagates by bouncing back and forth from the core-cladding interface. The design of the fiber optic sensor depends critically on the choice of optical fiber. Telecommunication fibers are inexpensive but may not be the right choice for sensors. Some of the fiber optic sensors dictate the use of application-specific optical fibers. In this chapter we shall first describe ray analysis for the light propagation in the fiber. The modes propagation, numerical aperture of the fiber, fiber characteristics and different kinds of optical fibers used for sensing will be described later.

3.1 Introduction Optical sources are a key element in many fiber optic systems. This component converts the electrical signal into a corresponding light signal that can be injected into the fiber. Several factors such as wavelength, source output and spectral bandwidth are important when selecting light source to achieve the best performance of the sensor. The optical sources used for sensors range from hot filament lamps to semiconductor lasers. These sources can be placed into two groups - incoherent and coherent sources. The tungsten lamp and light-emitting diode (LED) are some of the incoherent sources while the lasers come in the category of coherent sources. The LED is normally preferred if the multimode fiber is used for the sensor. If the optical power requirement is high then the laser with multimode optical fiber is used. For the sensors based on interference phenomenon, coherent sources with single mode fibers are used. In the case of distributed sensing, short pulse high power lasers are used. In a laboratory demonstration experiment, a visible laser such as a simple He-Ne laser may be the preferred source while in commercial applications a compact and rugged source such as laser diode or an LED is preferred. Before using the source it is very essential to know its maximum power output, wavelength of emission, linewidth, radiation pattern and modulation bandwidth. The purpose of this chapter is to provide the physics of some of the light sources and their important operating characteristics that are used in sensors.

4.1 Introduction Apart from an optical source, the optical detector is another important component of the optical fiber sensor. The optical detector converts the optical signal to an electrical one. The signal is then processed either to record or display. There are various kinds of optical detectors but the commonly used in sensors are semiconductor diodes. In some of the cases photomultiplier tube has been used. In this chapter we shall describe the basic principles of optical detectors and their operational characteristics.

5.1 Introduction Evanescent field absorption spectroscopy is a powerful well-established laboratory technique for chemical analysis. The technique utilizes the evanescent wave penetration at the boundary between two dielectric media in conditions of attenuated total reflection (ATR). One of the media, a thin slab-shaped non-absorbing crystal, is called a waveguide, while the other medium of lower refractive index is the absorbing sample being studied. At each reflection at the boundary of the crystal and the sample, the penetration of the evanescent wave of the guided ray into the absorbing sample gives rise to a reduction of the power propagating in the crystal. The degree of absorption depends on the amplitude of the evanescent field in the sample medium and the number of reflections within the waveguide. The former increases dramatically for incident angles approaching the crystal-sample critical angle while the latter increases with the decrease in the thickness of the waveguide. The number of reflections can also be increased by increasing the length of the waveguide. Thus, to achieve maximum absorption, an unclad optical fiber is well suited for evanescent wave absorption spectroscopy. This is because, unlike the crystal waveguide, the narrow core diameter of the fiber provides a large number of reflections per unit length. Further, the use of an optical fiber to transmit light to a section of its core in contact with an absorbing sample offers the possibility of remote sensing or in-line analysis. These advantages, together with the benefits of simplicity, reliability, flexibility and relatively low cost have motivated a number of research groups all over the world to investigate evanescent wave spectroscopy at the surface of an unclad optical fiber. This chapter will be devoted to the fiber optic sensors utilizing evanescent field absorption spectroscopy.

6.1 Introduction There are a variety of intrinsic and extrinsic fiber-optic sensors which use absorption spectroscopy indirectly for the detection of various chemical species. These include pH, metal ions, oxygen in water etc. In such sensors a suitable reagent is immobilized on a solid support. The support can be the core of the fiber or some optical material attached at the end of the fiber. Further it is optically transparent to allow transmission of the light signal from the interface and should remain inert to the chemical reaction being analyzed. The analyte (or the chemical species) when comes in contact of the reagent affects the absorption properties of the reagent. In other words, in these sensors, the reagent acts as a chemical transducer for analytes that are not directly measurable by optical techniques. Following conditions for the use of reagents in sensors must be met:

7.1 Introduction There are numerous wavelength dependent phenomena that may be used to modulate the wavelength of incident light source. These are wavelength dependent absorption, luminescence, dispersion, interference and scattering. Wavelength modulated sensors use a broadband source, a wavelength modulator or measurand (i.e. analyte), a form of spectrometer (using prisms, gratings or interference filters), a detector (single photodiode or array) and signal processing unit. Silicon photodiode is cheap and can be used for detection but it limits the detection wavelengths to the region 400 to 1050 nm. The sensitivity or the resolution of the wavelength modulated sensor depends on the characteristics of the detector, the inherent resolution of the spectrometer, mechanical stability of the modulation unit and the spectrometer, and the capability of the signal processing unit. However, wavelength modulated sensors do not require any form of referencing like intensity modulated sensors. In intensity modulated sensors, loss in intensity occurs due to connectors and splices, microbending, mechanical creep and misalignment of light sources and detectors due to environmental effect. Further the fluctuations in light source can affect the performance of the sensor. Thus, for intensity modulated sensors one needs referencing to cancel out these losses. In some intensity modulated sensors two wavelengths are used. One wavelength is used to cancel out all of the losses while the second is used to sense the measurand. Wavelength detection is relatively easy if the spectral changes result from changes in chemical indicator dyes, fluorescence, phosphorescence, or black-body radiation. In these cases the incident and emitted wavelengths are far apart and therefore detection is easy. If the spectral change occurs due to mechanical movement, then the wavelength filtering mechanisms must have sufficient resolvable spectral points. The reasons for using fluorescence in the fiber optic sensors are well known. Fluorescence is intrinsically more sensitive than absorbance and is more flexible in that a great variety of analytes and physical parameters are known to change the emission of particular fluorophores. The fluorescence based sensors utilize the change in the intensity, lifetime or colour of fluorescence as the affected parameter by the measurand. That means, any interpretable observation of fluorescence can be made the basis of a sensor. In this chapter we shall first describe luminescence phenomenon and then various physical and chemical fiber optic sensors based on wavelength modulation and fluorescence phenomenon.

8.1 Introduction Interferometric or phase modulated sensors have received special attention because these are much more sensitive than intensity modulated sensors and can be used over a much larger dynamic range. The sensors rely on the interferometric technique. Generally, the sensor employs a coherent light source such as a laser and two single mode optical fibers. Light is split and coupled into both the fibers. If the measurand perturbs the optical properties of one of the fibers relative to the other, a phase shift occurs. The change in phase can be measured very precisely. For the measurement a sophisticated electronic processing equipment is needed. Thus the interferometric sensors are expensive compare to intensity modulated sensors. There are four kinds of interferometer configurations: Mach-Zehnder, Michelson, Fabry-Perot and Sagnac interferometers. The Mach-Zehnder, Michelson and Sagnac interferometers are the two beam interferometers while in the case of Fabry-Perot interferometer multiple beam interference occurs. In this chapter we shall first describe interference phenomenon and then different kinds of fiber optic interferometers. In the end we shall describe a number of sensors based on interference phenomenon or phase modulation.

9.1 Introduction Many physical parameters such as pressure, temperature, electric current etc. can be measured using polarization-modulated sensors. In these sensors modulation of state of polarization of light in the sensing region by an external stimuli takes place. A variety of physical effects influence the state of polarization of light. These phenomena are Faraday effect, electro-optic effect and photo-elastic effect. Polarization modulated sensors, in some cases, offer significant advantages not only over conventional sensors but also over widely used fiber optic interferometric sensors. These sensors require control over the state of polarization of light entering the sensing region of the fiber. Thus, for these sensors, highly birefringent fibers or single mode fibers are used. In this chapter we shall first discuss magneto/electro-optic effects and then shall describe sensors for the measurement of electric current/ magnetic field, voltage, pressure, temperature etc. utilizing polarization modulation.

10.1 Introduction There are very few frequency modulated fiber optic sensors. This is because frequency modulation of light occurs under a limited range of physical conditions. The principal one is based on Doppler effect. There are few other circumstances under which the frequency of light is modulated. These include luminescence and Raman scattering. Luminescence has been included in chapter 7under fluorescence based sensors. In this chapter we shall focus on the sensors based on Doppler effect and Raman scattering. In particular, we shall discuss the measurement of blood flow velocity, colloidal particle motion and Raman spectrum using optical fibers.

11.1 Introduction In previous chapters we have described fiber optic sensors based on intensity and phase modulations. These sensors have some problems in practical applications that need to be solved. Some of these are fluctuation of light source power, loss in couplers, scattering and absorption effects that affect measurement performance, temperature drifts and vibrations. In comparison to intensity and phase modulated sensors fiber Bragg grating (FBG) sensors have following advantages: High resolution Insensitive to intensity fluctuation because the signal obtained from an FBG sensor is encoded directly in wavelength domain. Provision of well localized sensing region Linear output as a function of the measurand The capability of multiplex many sensors along one fiber

12.1 Introduction Among a variety of optical evanescent wave techniques, spectroscopies based on the resonant excitation of surface plasmons have matured into a widely accepted surface analytical method used in different fields. Liedberg et al1 were the first who demonstrated the exploitation of surface plasmon resonance (SPR) for chemical sensing. Since then the SPR sensing principle has received much attention. In this chapter we shall first describe the principle of this technique and then few sensors that have utilized this phenomenon.

13.1 Introduction Distributed optical fiber sensors are those sensors which monitor a single measurand at a large number of locations or continuously over the path of the optical fiber. The measurand can be a physical or a chemical quantity. The distributed optical fiber sensor generally consists of a common light source, optical fiber to carry light to the sensing probes and a return fiber to carry modulated light to a photodetector. These sensors can utilize any kind of light modulations described earlier. The advantages associated with distributed sensing are reduced number of components and hence the cost, reduced weight and cabling, and electrical passivity. The applications include the measurements of temperature, pressure, strain, current and chemical concentration. If the measurand is monitored continuously along the fiber length then the sensor is called intrinsic distributed sensor. If it is measured at a finite number of locations then the sensor is called quasi-distributed sensor. The quasi-distributed sensor has a number of advantages over intrinsic distributed sensor. In quasi-distributed sensor, spatial resolution is not as important as in intrinsic distributed sensor provided that individual sensors can be clearly resolved. Further, in quasi-distributed sensing, the sensor elements can be tailored in terms of responsivity and range to meet the specific needs of the application. In this chapter we shall describe various kinds of distributed sensing approaches and few optical fiber distributed sensors.

14.1 Introduction In this chapter we shall describe various simple and easy to fabricate fiber optic sensors. Most of these sensors are based on intensity modulation. Intensity modulated optical fiber sensors have been extensively studied in comparison to sensors based on other modulation techniques. This is because of their several advantages such as simplicity, reliability, flexibility and relatively low cost. Intensity modulated sensors make use of multimode fibers. Multimode fibers have large core and high numerical aperture that make the coupling of light into the fiber easier. The large core is also better suited to propagate high power laser beams. Further, the plastic-clad fused silica multimode fibers are available. Plastic-clad silica multimode fibers have an additional advantage. Their plastic cladding can be removed easily permitting access to the core. The evanescent field in the unclad part has been used for various kinds of chemical and biochemical sensors (see chapter 5). Above all, these fibers are cheap. This chapter is devoted to simple and easy to make both intrinsic and extrinsic type sensors. Sensors utilizing bare-ended fibers have also been described.

Index Acceleration sensor 215, 216 Acceptance angle 15,16 Acoustic sensor 175, 252-253 Aluminium sensor 115, 116, 150, 151 Ammonia sensor 116, 117