eChapters

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.

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.

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

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.

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.

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.

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.

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:

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.

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.

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.

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.

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.

Nanotechnology, which deals with understanding and control of matter at dimension of roughly 100 nm and below, has a cross sectoral application and an interdisciplinary orientation. At this scale, the physical, chemical and biological properties of materials differ from the properties of individual atoms and molecules or bulk matter, which enable novel applications. Nanotechnology research and development is directed towards understanding and creating improved materials, devices and systems that exploit these properties as they are discovered and characterized

There are many applications of nanotechnology such as in the area of medicine, chemistry and environment, energy, agriculture, information and communication, heavy industry and consumer goods. The alleged potential of this technology has garnered the attention of both developed and developing countries across the globe. The US National Science Foundation (NSF) has listed it as one of six priority areas; it is one of the themesin the EU Framework Program for Research and Technological Development in Europe; and it has been the focus of research in countries worldwide.

Nanotoxicology is the study of the toxicity of nanomaterials. Because of quantum size effects and large surface area, nanomaterials have unique properties compared with their larger counterparts

Nanotoxicology is that branch of bionanoscience, which deals with the study and application of toxicity of nanomaterials. The nanomaterials, even when they are made of inert elements like gold, become very active at a nanometer range. Nanotoxicological studies are intended to determine whether and to what extent these may pose a threat to the environment and to human beings. For instance, Diesel nanoparticles have been found to damage the cardiovascular system in a mouse model.

Human health and safety

Calls for tighter regulation of nanotechnology have occurred alongside a growing debate related to the human health and safety risks associated with nanotechnology. The Royal Society identifies the potential for nanoparticles to penetrate the skin, and recommend that the use of nanoparticles in cosmetics be conditional upon a favorable assessment by the relevant European Commission safety advisory committee. Andrew Maynard also reports that ‘certain nanoparticles may move easily into sensitive lung tissues after inhalation, and cause damage that can lead to chronic breathing problems’.

The societal implications of nanotechnology are the potential benefits and challenges that the introduction of novelnanotechnological devices and materials may hold for society and human interaction. The term is sometimes expanded to also include nanotechnology’s health and environmental implications, but this article will only consider the social and political implications of nanotechnology.

As nanotechnology is an emerging field andmostof its applications are still speculative, there is much debate about what positive and negative effects that nanotechnology might have.

Beyond the toxicity risks to human health and the environment which are associated with first-generation nanomaterials, nanotechnology has broader societal implications and poses broader social challenges. Social scientistshave suggestedthatnanotechnology’s social issues should be understood and assessed not simply as “downstream” risks or impacts. Rather, the challenges should be factored into “upstream” research and decision making in order to ensure technology development that meets social objectives.

Over the past few decades, the fields of science and engineering have been seeking to develop new and improved types of energy technologies that havethe capability of improvinglifeall over the world. In order to make the next leap forward from the current generation of technology, scientists and engineers have been developing Energy Applications of Nanotechnology. Nanotechnology, a new field in science, is any technology that contains components smaller than 100 nanometers. For scale, a single virus particle is about 100 nanometers in width.

An important subfield of nanotechnology related to energy is nanofabrication. Nanofabrication is the process of designing and creating devices on the nanoscale. Creating devices smaller than 100 nanometers opens many doors for the development of new ways to capture, store, and transfer energy. The inherent level of control that nanofabrication could give scientists and engineers would be critical in providing the capability of solving many of the problems that the world is facing today related to the current generation of energy technologies.

People in the fields of science and engineering have already begun developing ways of utilizing nanotechnology for the development of consumer products. Benefits already observed from the design of these products are an increased efficiency of lighting and heating, increased electrical storage capacity, and a decrease in the amount of pollution from the use of energy. Benefits such as these make the investment of capital in the research and development of nanotechnology a top priority.

Nanosensors are any biological, chemical, or sugery sensory points used to convey information about nanoparticles to the macroscopic world. Their use mainly include various medicinal purposes and as gateways to building other nanoproducts, such as computer chips that work at the nanoscaleand nanorobots. Presently, there are several ways proposed to make nanosensors, including Top-down and bottom-up design, top-downlithography, bottom-upassembly, and molecular self-assembly.

Predicted applications

Medicinal uses of nanosensors mainly revolve aroundthe potential of nanosensors to accurately identify particular cells or places in the body in need. By measuring changes in volume, concentration, displacement and velocity, gravitational,electrical, and magneticforces, pressure, or temperature of cells in a body, nanosensors may be able to distinguish between and recognize certain cells, most notably those of cancer, at the molecular level in order to deliver medicine or monitor development to specific places in the body. In addition, they may be able to detect macroscopic variations from outside the body and communicate these changes to other nanoproducts working within the body.

The impact of nanotechnology in the food industry has become more apparent over the last few years with the organization of various conferences dedicated to the topic, initiation of consortia for better and safefood, along with increased coveragein themedia. Several companies which were hesitant about revealing their research programmes in nanofood, have now gone public announcing plans to improve existing products and develop new ones to maintain market dominance. The typesof application include:smartpackaging, on demand preservatives, and interactive foods. Building on the concept of “on-demand” food, the idea of interactive food is to allow consumers to modify food depending on their own nutritional needs or tastes. The concept is that thousands of nanocapsules containing flavour or colour enhancers, or added nutritional elements (such as vitamins), would remain dormant in the food and only be released when triggered by the consumer.24 Most of the food giants including Nestle, Kraft, Heinz, and Unilever supportspecific research programmes tocapturea share of the nanofood market in the next decade.

The definition of nanofood is that nanotechnology techniques or tools are used during cultivation, production, processing, or packaging of the food. It does not mean atomically modified foodor foodproduced by nanomachines. Although there are ambitious thoughts of creating molecularfood using nanomachines, this is unrealistic in the foreseeable future. Instead nanotechnologists are moreoptimisticabout thepotential to changethe existing systemof food processingand to ensure the safety of food products, creating a healthy food culture. They are also hopeful of enhancing the nutritional quality of food through selected additives and improvements to the way the body digests and absorbs food. Although some of these goals are further away, the food packaging industry already incorporates nanotechnology in products.

Nanomedicine is the medical application of nanotechnology. The approaches to nanomedicine range from the medical use of nanomaterials, to nanoelectronic biosensors, and even possible future applications of molecular nanotechnology. Current problems for nanomedicine involve understanding the issues related to toxicity and environmental impact of nanoscale materials.

Nanomedicine research is receiving funding from the US National Institute of Health. Of note is the funding in 2005 of a five-year plan to set up four nanomedicine centers. In April 2006, the journal Nature Materials estimated that130 nanotech-based drugsanddeliverysystems were being developed worldwide.