Fiber Optic Sensors

The combination of fiber optics with sensitive nano-films offers great potential for the realization of novel sensing concepts. Miniatured optical fiber sensors with thin films as sensitive elements could enable new fields of optical fiber sensor applications. Thin films work as sensitive elements and transducer to get response and feedback from environments, while optical fibers are employed to work as signal carrier. In this chapter fiber optic sensors based on nano-films are reviewed.

In the past couple of years, lossy mode resonance (LMR) phenomena has attracted the attention of researchers with its promising benefits in the field of fiber optic sensing. LMR based sensors have become a useful tool in sensing applications ranging from physical sensing to biosensing in a short span of time. In addition to sensing, LMR phenomena can also be utilized as wavelength filters for communication purposes. LMR based sensors are able to work independently of the specific polarization of light for sensing operations. Also, unlike evanescent wave and surface plasmon resonance (SPR) based sensors, the sensitivity of these LMR sensors does not get affected by the geometrical parameters of fiber and primarily depends on the thickness of thin film material. Till date, various geometries of fiber probes such as straight, D-shaped, tapered etc., have been explored. Bending and tapering of multimode fiber based LMR sensors improve the detection accuracy without affecting their sensitivity. However, in single mode fiber based LMR sensors the side polishing and tapering of fibers improve both the detection accuracy and sensitivity. Another method to improve the sensitivity is by using two LMR supporting thin film layers of higher refractive index instead of one. This chapter describes the theory and developments made in the field of LMR based fiber optic sensors for various sensing applications. Finally, future scope of the LMR sensing technology and possible research in this emerging area are suggested.

Since the unveiling of optical fiber technology in the field of plasmonics-based optical sensors, a lot of advancements have been witnessed. This chapter discusses a detailed mechanism of the technique of surface plasmon resonance (SPR) applied in optical fiber sensors. Some selected research works in the area of plasmonics-based fiber optic sensors reported in last 25–30 years along with future scope of work are also discussed.

Plasma-based techniques are widely applied for well-controlled deposition, etching or surface functionalization of a number of materials. It is difficult to imagine fabrication of novel microelectronic and optoelectronic devices without using plasma-enhanced deposition of thin films, their selective etching or functionalization of their surfaces for subsequent selective binding of chemical or biological molecules. Depending on the process parameters, i.e., generator frequency and power, composition of gases, pressure, temperature, and applied substrates, different effects of the process can be obtained. The chapter discusses current trends in application of plasma-based techniques for fabrication of novel optical sensing devices. Fabrication of materials with different structure (from amorphous to crystalline, porous, and multilayers), optical properties (absorption, refractive index), and surface activity, as well as their processing are reviewed. Application of the plasma methods enhancing sensing properties of various optical fiber sensing structures, namely long-period gratings, intermodal interferometers based on photonic crystal fiber, sensing structures based on lossy mode resonance or stacks of nano-films are given as examples and are discussed.

Along this chapter, a different point of view on the sensing uses of optical fibre is shown, focusing on its applicability to medical diagnostics. A wide majority of the fibre-optic-based structures described in this book can be used in medicine. But there are some challenges for fibre-optics to be fulfilled in the future when talking about healthcare. First, current society strongly demands day-by-day applications. This means, technologies that permit their use ‘on the go’ such as wearables or their integration in our smartphones. Second, due to the reduced dimensions of the optical fibre, there is an increasing interest in introducing it into the body, as it occurs when using catheters or fibrescopes. Moreover, the need for analysing biological substances makes it crucial to use reduced size devices that permit their interaction with the biomolecules, once the samples have been extracted from the patient. And of course, all the addressed applications must be achieved in order to search for cheap devices that work under exigent conditions. The main goal of this chapter is to search for those applications that will lead us to use fibre-optics for self-healthcare and diagnose patients in the next times.

The use of ionising radiation in a wide range of areas, from industrial processes to medical applications, has become increasingly significant in recent years. Radiation dosimetry, the measurement of absorbed dose delivered by ionising radiation, is fundamental to these radiation processes, ensuring tight control on radiation processes and the safety of personnel. As such it is the focus of much recent research to develop novel dosimeters and to improve dosimetry systems, be it increasing the sensitivity, providing real-time measurements or significantly reducing the costs. The inherent properties of optical fibres lend themselves to be used with great success in monitoring ionizing radiation. Optical fibres provide the means whereby real-time in situ monitoring of radiation doses can be realised, and offer numerous advantages over conventional methods, such as electrochemical and semiconductor sensors. Of significant advantage is the optical fiber’s immunity to electromagnetic and chemical interferences and their ability to monitor remotely, whereby the sensor can be placed several hundred metres from the control electronics. This means that they can be employed in harsh environments, such as in high-radiation-level areas in the vicinity of a nuclear reactor or gamma sterilisation facility. The small dimensions and light weight of optical fibres also provide for many opportunities for such sensors in personal dosimetry and medical applications. This chapter will discuss some of the key optical fibre based sensing techniques used in radiation dosimetry and outline their implementation in the different application areas of ionising radiation.

A development of gas concentration sensing systems based on a photonic bandgap fiber (PBGF) is described. Several types of PBG fibers of various parameters and core diameters ranging from 10.9 to 26.25 microns have been designed and tested. The capillary gas flow rate within the fiber has been simulated and measured. A new method for cutting the fiber using focused ion beam in a vacuumed chamber for fine milling was tested to obtain the required angle of the fiber’s end, to avoid the destruction of the cladding structure and to create a novel low-loss splice for use between PBGF and the conventional solid-core fiber. The measurement results obtained using proposed systems for selected types of gases are presented. The experimental results clearly indicated a high overlap between the propagating light and filled gas inside the PBGF. Therefore, these studies can contribute to highly sensitive gas sensing, higher accuracy of wavelength references, and other applications.

Following the modern technological needs requiring highly increased safety and standards in structures (especially civil) located in densely populated areas with increased seismic activity or other safety critical perturbations, various technologies have been developed, aiming towards improved monitoring requirements, needs for structural performance evaluation and increased safety in general. Fibre-optic technologies provide a lot in the field of structural monitoring as a basis for condition assessment before, during or after a random (e.g. earthquake), human-imposed (e.g. blast) or other operational (e.g. increased load) event. This chapter provides an overview of the structural health monitoring concept and particular requirements per application area, the monitoring systems currently available on the market and a thorough analysis of the fibre optic technologies available today. The chapter starts with the definition of structural health monitoring in terms of the specific industrial needs for monitoring and sensing. It then presents a detailed analysis of the fibre-based monitoring solutions available, their concept of operation and operational (measuring) characteristics and capabilities and closes with a presentation of typical fibre optic installation examples where fibre optics are installed for structural health monitoring.

This paper presents the performance improvement of Brillouin distributed temperature sensor (BDTS) using deconvolution algorithm. We have analysed three different OTDR techniques in terms of spatial resolution improvement using Fourier regularized deconvolution (FourRD) algorithm. The effects of coherent Rayleigh noise (CRN) on temperature and spatial resolution of the above system are being investigated. In this paper, using a light source of power 10 mW and 400 ns pulse widths; a spatial resolution of 25 and 20 m is observed for a pseudorandom coded BOTDR in the presence and absence of CRN respectively. Similarly, in case of conventional BOTDR and coherent BOTDR system in the presence of CRN, the spatial resolutions observed are 40 and 35 m respectively. Numerical simulation results indicate that the pseudorandom coded BOTDR is a better candidate for design of BDTS in terms of spatial resolution as compared to the other two schemes as discussed above.

Photonic crystals (PhC) are materials which present periodic variations of the dielectric constant over distances of the same order of the light wavelength. Their optical properties are highly dependent on construction details such as dielectric constants and sizes of their different constituents. It is possible to turn PhC structures into optical sensors by making some of their structural characteristics responsive to the desired mesurand. These sensors are small, compact, compatible with electronic integration in some cases, and may present some other advantages like high sensitivity and selectivity. In recent years the development of PhC sensors has experienced a substantial increase due to their performance and to the increasing demand of sensing applications such as instrumentation, healthcare, environment security, food quality and industrial control. In this paper we present an overview of PhC sensors focused on their physical working principles. It covers a description of PhC structures, their interaction with radiation, the general strategies to make them responsive and, finally, a selection of sensor proposal of a variety of mesurands.

The principle of operation of optical fibre long period grating (LPG) sensors is described. In particular, the chapter explores the use of LPGs as a chemical sensing platform, discussing the fabrication of LPGs and the various approaches that have been employed to modify the cladding of the LPG and thus sensitise the LPG. Examples of the practical application of LPG chemical sensors are provided.

This chapter is focused in the different optical structures and materials that have been used for the development of optical fiber magnetic field sensors and optical fiber current transducers. First of all, this chapter starts with a short introduction of the main fields of application of these sensors and where they are used, and how materials can be classified with regard to their magnetic behavior. Then, a brief review of different ways of measuring magnetic fields will be made. It will be also introduced the main magneto-optical effects. A brief summary of the different options that have been developed by now will be made, showing the evolution of this research field. Then, a deeper look at the most used structures, the most common materials and the devices having greater sensitivity and resolution will be made.

In this chapter a review of sensing applications working at terahertz (THz) frequencies is performed. Firstly, an introductory section putting in context the THz regime and highlighting particularities and potential applications at this frequency band is outlined. Then, a comprehensive examination of sensing solutions following different technologies investigated during the last two decades is presented. Special attention is given to the fibre optics solutions and free-space approaches based on metasurfaces. Finally, plasmonic sensing platforms and waveguide solutions are discussed as well.

Multimode interference (MMI) devices have attracted a great deal of interest due to their simplicity of fabrication. The MMI device is ready for testing after splicing a section of multimode fiber (MMF) between two single-mode fiber (SMF). In this chapter we provide an overview of the fundamentals behind the formation of self-images in MMI fiber devices, as well as the basic mechanisms for tuning their operational wavelength which is related with their application for sensing applications. The sensitivity enhancement of these MMI fiber sensors is also investigated by reducing the diameter of the MMF via wet chemical etching, as well as coating the MMF with a high refractive index overlay. The MMI fiber sensors are applied to the quality control of gasolines and in particular the real time monitoring of gasohol, mixtures of gasoline and ethanol, which is critical for the proper operation of flexible-fuel vehicles (FFV). The results demonstrate that MMI fiber sensors are well suited for such applications, as well as other applications were the binary mixture of liquids has to be controlled or monitored.

We present a review of the fundamentals and applications of fiber optic sensors based on multicore coupled structures. The fundamentals of these coupled structures are approached in general for arbitrary distributions of N cores on the foundations of coupled mode theory. The principle of operation of fiber optic sensors using this type of architectures is illustrated via numerical simulations of the simplest coupled structure—the two-core fiber. Illustrative experimental results using fiber optic sensors based on two- and seven-core multicore fibers are shown for a number of applications including temperature, curvature, and refractive index sensing. The main aspects of the performance of multicore fiber sensors are highlighted throughout this chapter and their characteristics, especially their sensitivity, are compared to those of other existing fiber sensing architectures such as fiber Bragg gratings, long period gratings, and photonic crystal fibers, among others.