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Real-time gas sensors, which use chemiresistive metal oxide (MO) semiconductors, have become more important in both research and industry. Fiber optic metal oxide (MO) semiconductor sensors have so increased the utility and demand for optical sensors in a variety of military, industrial, and social applications. Fiber optic sensors’ inherent benefits of lightweight, compact size, and low attenuation were actively leveraged to overcome their primary disadvantage of expensive cost. With the growing need for quicker, more precise, and simpler gas sensing, metal oxide semiconductor gas sensors are focusing on new and novel materials at room temperature. The realization that materials with coexisting magnetic and ferroelectric orders offer up effective ways to alter magnetism using electric fields has drawn scientists from diverse areas together to research multiferroics for gas sensing applications in recent years. The chapter shall encompass a brief summary of the underlying physics related to fiber optic gas sensors and parameters involved in gas sensing, the significance of the fascinating class of metal oxide materials, and an outline of spin frustrated multiferroics for possible applications and its potential possibilities for progress in the future.
Centre for Materials Engineering and Regenerative Medicine, Bharath Institute of Higher Education and Research, India
Rajesh Kumar Rajagopal
School of Nanoscience and Technology, IIT Kharagpur, India
Gopalakrishnan Chandrasekaran*
Department of Physics and Nanotechnology, SRM IST, India
*Address all correspondence to: subhha2010@gmail.com and cgknano@gmail.com
1. Introduction
The air we breathe contains a variety of chemical compounds, some of which are beneficial and others that are harmful. Toxic and hazardous gas emissions have become a source of worry in recent years [1]. As a result, there has been an increase in demand for gas detection and monitoring. Generally, a gas sensor should be designed to accomplish two critical functions: receptor and transducer. The capacity to detect certain gas species (via interactions such as adsorption, chemical, or electrochemical reaction) is the function of a receptor, while the ability to convert gas identification into a sensing signal is the function of a transducer.
Gas sensor research is designed to reinforce two most important functions: the receptor function, which is generally connected to selectivity, and the transducer function, which is directly related to the sensitivity of a gas sensor [2]. To fulfill the two purposes, different sensing techniques are applied, resulting in distinct sensor approaches such as catalytic gas sensors [3], electrochemical gas sensors [4], optical gas sensors [5], thermal conductivity gas sensors [6], and acoustic gas sensors [7, 8]. Over the last few decades, Optical gas sensors have been projected as a good choice considering their benefits compared to other available sensing technologies, as they are immune to electromagnetic interferences [9], do not require any electric power to work, and the possibility of multiplexation, ability to perform in remote areas and under harsh environmental conditions [10].
Commercial applications exist for gas detection using optical fibers. Owing to its advantages and good productivity over non-fiber sensors, such as metal oxide semiconductor (MOS) and spectroscopic approaches, optical fiber gas sensors are still being extensively examined. Between 2017 and 2023, the global optical sensing market is expected to grow at a CAGR of 15.47 percent, from USD 1.13 billion in 2016 to USD 3.47 billion by 2023 [11]. The physical qualities of optical sensors that make them suited for sensing in difficult settings, as well as continuous technological advancements in optical sensors, are major drivers of market growth. Table 1 displays a comparison of the commercially available chemical sensors employed for the detection of Volatile organic compounds (VOCs). The manufacturer’s primary goal is to develop a sensor with the descent range of detection and the lowest response time possible. The choice of the sensor basically depends upon the type of gas that has to be detected, concentration range, if the sensor is intended to be stationary or portable, detect the presence of other gases that could possibly damage the measuring device. A few commercially available sensors are Electrochemical (Amperometric) Sensors [12], Metal oxide semiconducting sensors [13, 14, 15], Nondispersive Infrared Sensors (NDIR) [16, 17], Photoionization Sensor (PID) [18, 19, 20], Thermal Sensor (Pellistor) [21, 22], Fiber optic sensors [23, 24, 25, 26, 27, 28].
Most combustile vapors (methane, iso-butane, propane)
Photoionization Sensor (PID)
Mocon Baseline series, Alphasense Gray wolf
10–20,000
< 3 s
VOC’s with proper ionization potential (isobutylene, aromatic hydrocarbons)
Fiber Optic sensor
Oxsensis Ltd. (UK), RJC Enterprises LLC (US), Silixa Ltd. (UK), OPTEK Technology Inc. (US), Opsens Inc. (Canada), Intelligent Fiber Optic Systems Corporation (US) and Fotech Solutions Limited (UK).
0–500 ppm
< 30 s
Flammable VOCs such as ammonia, ethanol, acetone, methanol, benzene, IPA etc.
Table 1.
Varous types of chemical sensors for measuring volatile organic compounds that are commercially available.
1.1 Fiber optics sensors (FOS)
After the advent of lasers in 1960, researchers became interested in studying the possibilities of optical fiber communication systems for sensing, data communications, and a variety of other applications [29]. As a result, fiber optic communication systems have become the preferred mode of data transfer for gigabits and beyond. This sort of fiber optic communication is used to send data, voice, telemetry, and video over long distances or within local area networks (LANs) [30, 31]. By converting electronic signals into light, this technique employs a light wave to convey data via a fiber. A few outstanding characteristics of this technology being light in weight, possess low attenuation, smaller diameter, long distance signal transmission, transmission security, etc. [32]. The recent improvements in fiber optic technology have had a significant impact on telecommunication technology. Designers combined the productive outcomes of optoelectronic devices with fiber-optic-telecommunication devices to produce fiber optic sensors in the last revolution. Many of the parts used in these devices were originally designed for fiber-optic sensor applications. Fiber optic sensors have surpassed traditional sensors in terms of capability [33]. The development of optical sensors has recently been described and discussed in a number of excellent review studies from various angles. Mahata et al. provided an overview of the use of rare-earth-based MOFs as luminous sensors to identify nitro explosives, cations, anions, small molecules, pH, and temperature [34]. On-chip biological sensors based on optofluidic photonic crystal cavities were reviewed in-depth by Zhao and his colleagues, and the sensing theories and uses for these sensors were covered in detail [35]. Yoon’s group has made significant contributions to fluorescence-based optical sensors and offered a perceptive viewpoint on the advancement of fluorescent chemosensors [36, 37, 38].
1.1.1 Optical fibers—An overview
Optical fibers are light-transmitting waveguides with two primary components: a core made of glass and a cladding composed of a material with a lower refractive index than the core as shown in Figure 1a. The optical fiber is protected against physical damage and scattering losses produced by micro bending by an extra elastic layer as a buffer composed of plastic surrounding the cladding section. The jacket layer is the final layer, and it can be used to identify the fiber type. Because of its purity, quartz glass is used to make the majority of fibers [39]. Total internal reflection occurs at the interface between the core and the cladding in optical fibers as long as the angle of incident light inside the core is greater than the critical angle. In this way, incident light is reflected back into the core and propagated through the fiber (Figure 1b). If the light strikes the interface at a greater angle than the critical angle, it will not pass through the opposite medium. The angle of incidence, as well as the core and cladding refractive indices, are all factors to consider. The amount of light reflected at the interface is determined by the angle of incidence as well as the refractive indices of the core and cladding [40].
Figure 1.
(a) Basic components of optical fiber; and (b) principle of operation in fiber optics.
Generally the number of modes and the refractive index are used to divide optical fiber into two categories.
1.1.1.1 Single mode fiber (based on number of modes)
Only one kind of light ray can pass through a single-mode fiber. This fiber has a tiny core diameter (5 μm) and a large cladding diameter (70um), with a minimal refractive index difference between the core and the cladding. There is no signal deterioration when travelling through the fiber because there is no dispersion. Using a laser diode, light is sent through it [41] (shown in Figure 2a).
Figure 2.
(a) Single mode optical fiber; and (b) multi mode optical fiber.
1.1.1.2 Multi-mode fiber (based on number of modes)
The light ray flowing through multimode fiber can take on a variety of modes. The cladding has a diameter of (40 μm) and the core has a diameter of (70 μm). The difference in relative refractive index is also bigger than in single mode fiber. Due to multimode dispersion, signal quality suffers. Due to signal dispersion and attenuation, it is not ideal for long-distance transmission [42] (shown in Figure 2b).
1.1.1.3 Step index fiber (based on refractive index)
The refractive index of step-index fibers is discontinuous at the interface between the cladding and the core, and the core is constant. Light rays pass through it as meridional rays that cross the fiber axis at the core-cladding interface on every reflection as shown in Figure 3a.
Figure 3.
(a) Step index optical fiber; and (b) graded index optical fiber.
1.1.1.4 Graded index fiber (based on refractive index)
The core of this fiber has a non-uniform refractive index that declines gradually from the centre to the core-cladding interface. The refractive index of the cladding is uniform. Light rays in the form of skew rays or helical rays propagate through it as shown in Figure 3b. At no point does it cross the fiber axis. The refractive index of the core in a gradient index fiber is higher in the centre and gradually drops as the interface approaches [43, 44].
1.1.2 Design of a basic fiber optic gas sensing system
A gas sensor is a device that measures target gas molecules present in a given environment. When gas molecules encounter the sensor’s solid receptors, a potential difference occurs, that is converted into an electrical signal. The sensor’s gas sensitivity and the selectivity are dependent upon the reaction of the sensing materials with the gases [45]. The sensing materials basically used for all types of gas sensors include polymers, organic monolayers, ceramics, semiconductors, porous nanomaterial and nanostructured materials (nanorods, nanotubes, nanodots). Optical fiber sensors have been projected as a good choice, to considering their benefits compared to other available sensing technologies, as they are immune to electromagnetic interferences [46], do not require any electric power to work, possibility of multiplexation, ability to perform in remote areas and harsh environmental conditions [27]. A block diagram of a common fiber optic gas sensor is shown in Figure 4. A light source, a signal input optical fiber, a signal output optical fiber, and a detector make up a fiber-optic gas sensing system (optionally the system may include other components such as an optical modulator and a demodulator). The essential idea is that the light from the source is transferred to the sensor element via the incident optical fiber. The measured parameters are obtained by modulating the optical properties of light, such as intensity, wavelength, frequency, phase, and polarisation state, in the sensor element and sending them to the optical detector through the outgoing fiber. Intrinsic and extrinsic optical fiber sensors are two types of sensors. The intrinsic optical fiber sensor relies on the optical fiber’s own sensitivity to environmental variables. In an intrinsic sensor, specific regions in the fiber cable perform sensing function, thereby detector is not exposed to the light source. However, in extrinsic sensors, the fiber optic cable is used as a data transmission line, wherein the optical cable carries light from an optical source externally to the sensing region. Most of the fiber optic sensors used in VOC gas detection are intrinsic sensors.
Figure 4.
Block diagram of extrinsic and intrinsic fiber optic gas sensor.
1.1.3 Classification of fiber optic gas sensing (FOS) system
The fiber optic sensing region is primarily divided into four kinds of approaches namely the Fabry-Perot Interferometer (FPI), Michelson Interferometer (MI), Mach-Zehnder Interferometer (MZI), Sagnac Interferometer (SI), based on interferometric techniques [47, 48] as shown in Figure 5. These approaches are known for their diverse geometries, operating principles, and their sensitivity. The Michelson, Mach-Zehnder, and Sagnac interferometers use a two-beam interference principle, whereas the Fabry-Perot interferometer uses a multi-beam interference principle. An interferometer’s basic operation is to split an incident light beam into two portions, the reference beam and the sensing beam, the latter of which is changed by a variable of interest that we want to measure. The two beams are then recombined to form an interference pattern that allows information about the desired variable to be recovered. Other fiber-optic geometries based on the SPR principle, gratings, and the evanescent wave absorption (EWA) principle exist. An overview of various kinds of Fiber optic gas sensors is displayed in Figure 6.
Figure 5.
Fiber optic sensors based on interferometric techniques.
Figure 6.
An overview of various kinds of fiber optic gas sensors.
1.1.3.1 Fabry-Perot interferometer (FPI)
Because of its simplicity, sensitivity, and cost-effectiveness, this is the most commonly used interferometric geometry for chemical sensing. The FPI is made up of two parallel reflecting mirrors that are spaced apart. It’s known as an etalon. A single light beam enters the FP cavity and is multiplicatively internally reflected. The superposition of both reflected and transmitted beams at the two parallel surfaces causes interference. The FP cavity can be created inside or at the end of an optical fiber in the case of FOS. Extrinsic FPI and intrinsic FPI are the two types of FPI that exist. An extrinsic FPI uses the reflections from the cavity surfaces to create a cavity at the end of the optical cable. As a result, an extrinsic FPI is defined as a cavity material that is not made up entirely of fiber. The cavity and reflections arise within the fiber itself in the case of an intrinsic FPI [49]. The FPI is easy to build and responds well to external inputs.
1.1.3.2 The Michelson interferometer (MI)
The Michelson interferometer (MI) is one of the most frequent and straightforward interferometers. The beams are separated and recombined using a single beam splitter (coupler). The reference beam is reflected by a fixed mirror in a traditional Michelson interferometer, while the sensing beam is reflected by a moving mirror [50].
1.1.3.3 The Mach-Zehnder interferometer (MZI)
The light beam is split into two pieces that go through the reference and detecting arms, respectively, before being recombined in this arrangement. The optical channels are only crossed once, unlike in the Michelson interferometer. Two photodetectors are installed in each of the interferometer’s outputs. The phase shift caused by variations in the sensing arm caused by temperature, strain, or refractive index changes, among other things, can be quantified using the intensities detected by both detectors. This interferometer can be implemented in integrated optics, which protects the device, reduces its size, and allows it to be used for other purposes like biosensing [51, 52].
1.1.3.4 The Sagnac interferometer (SI)
The input beam is split into two equal-intensity counter-propagating beams that move along a ring route. The Sagnac effect occurs when the Sagnac interferometer is spun, causing a relative phase change between the beams. The interference spectrum is determined by the setup’s angular frequency. Currents, acoustic waves, strain, and temperature can all be monitored using this interferometer [53].
1.1.3.5 Surface plasmon resonance (SPR)
Because of its light guidance inside fiber based on the TIR effect, simple and flexible design, compactness, and remote sensing ability, optical fiber has proven to be a very useful instrument in the SPR sensing system. When light travels through the core of the fiber, some of it will pass through the cladding region. The plasmonic metal surface interacts with the evanescently leaked light, which stimulates the surface’s free electrons. Surface plasmon is created when the evanescent field and free electron surface frequencies are in resonance. This generated wave propagates along the metal-dielectric interface and exhibits both charge motion in the metal (surface plasmon) and electromagnetic wave in the air or dielectric (polariton) [54].
1.1.3.6 Evanescent wave absorption type (EWA)
The leakage/loss of electromagnetic energy at the interface of the core and the cladding medium during a total internal reflection (TIR) event determines the evanescent wave absorption based sensing phenomenon. If the diameters of a fiber are big in comparison to the wavelength of light, this method may be applied. When a propagating beam’s incident angle (θi) is greater than the critical angle (θc), TIR causes the ray to reflect back from the core-cladding contact. A small fraction of the guided wave’s energy enters the cladding medium at each TIR event, generating an electromagnetic field known as a “evanescent wave.” The evanescent wave is greatly affected by variations in refractive index at the core-cladding contact, which provides a foundation for sensors based on evanescent wave absorption as shown in Figure 7 [55, 56].
Figure 7.
Fiber optic gas sensing set-up using evanescent wave absorption based on experiment [55].
2.1 Spin frustrated Multiferroics: A novel frustrated ordering studied in gas sensors
Spin frustration in ME multiferroics have received enormous attention ever since the concept was introduced in 1977 by Gerard Toulouse. It is an important feature in the field of magnetism and multiferroics, as it stems from the relative arrangement of spins. Interestingly, it is not the strength of ME coupling or the high polarization that makes these materials unique; in fact, such systems have weak coupling and low magnitude of polarization compared to other multiferroics [57, 58, 59, 60]. Still, the reason for its long sought control of electric properties by magnetic fields, lies in the magnetic origin of their ferroelectricity, which is induced by the presence of complex spin structures, characteristic of frustrated magnets [61]. Basically, in most of the magnetic solids, the magnetic moments form a long range ordered ferromagnetic (↑↑ moments) or antiferromagnetic (↑↓ moments) structure when cooled below a certain temperature. However, in “frustrated” magnetic solids, there is no long range ordering at low temperature phase. Instead, they develop high degenerate states with short range spin correlations. Typical examples of geometrically frustrated structures are 2D triangular or tetrahedron non collinear magnetic structures, where the lattice consists of triangular or tetrahedral arrangements of anti-ferromagnetically (AFM) coupled spins as shown in Figure 8. The spin on the third site can be in any one direction, either up (or) down, but then the interaction between this site and the other two sites will be different. As a consequence, the third site will move closer to one site and away from the other, which will break the symmetry and induce ferroelectricity. This kind of lattice geometry creates large degeneracy of ground states within which the system can fluctuate with almost no energy expenditure, even below few milli Kelvin temperatures. A one dimensional anti-ferromagnetic material has ground state in alternating series of spins: up, down, up, down, etc. But, in 2D equilateral triangular AFM lattice, multiple ground states can occur, with three spins, one on each vertex. If each spin can take on only two values (up or down), there are 23 = 8 possible states of the system, six of which are ground states. Two situations, which does not favor ground states are when all three spins are up or are all down. However, in other six states, there will be two favorable interactions and one unfavorable one. This illustrates spin frustration: the inability of the magnetic system to find a single ground state [58, 60].
Figure 8.
Spin frustration in 2D (a) triangular; and (b) tetrahedron magnetic structure.
2.1.1 Factors to consider for sensor interrogation of spin frustrated multiferroic materials
The main factors that are critical in gas detection, particularly when employing multiferroic materials as sensing materials are
Porosity, Surface area and Crystallinity of the material
Sensitivity and Selectivity of the material
Dynamic Response and Stability of the material
Doping or Substitution effect in the material
Temperature and Humidity of the surrounding environment
2.2 Few spin frustrated Multiferroics
This section discusses a few spin frustrated multiferroics that have been studied as gas sensors.
2.2.1 Bi2Fe4O9
The Spin frustrated Mullite type Bi2Fe4O9 (BFO), yielded as an undesired secondary phase from the production of BiFeO3 [61], is being extensively evaluated due to its range of industrial applications in the form of catalyst for oxidation of ammonia, sensors for magnetic field detection, and even as memory storage devices [62, 63, 64, 65, 66]. However, the most common issue experienced in this single-phase multiferroic system is the occurrence of weak magnetic/electric fields at room temperature. The Fe6 atoms that are synchronized in an octahedral fashion, interact ferromagnetically with one another while the Fe4 atoms synchronized tetrahedrally towards the inner axis of the unit cell interact in antiferromagnetic order thus ensuring a spin frustrated configuration with weak magnetic order beyond 264 K and weak ferroelectric ordering below 250 K [67]. As documented in reports, partially doping any transition metal ion or rare-earth (RE) ion into either of the sites of BFO can enhance the gas sensing parameters such as its conductivity, catalytic activity required for oxygen adsorption, electron mobility, chemical stability, and enhanced sensitivity [68, 69, 70, 71]. Given the usage of a magnetic dopant, it is important to understand the nature of magnetism in this sample Bi2Fe4O9 samples exhibited significant changes in magnetic and electrical characteristics, which are necessary for improved photocatalytic and gas sensing applications [68, 70]. Bi2Fe4O9 crystal structure consists of two Fe sites, one consisting of the Fe in the tetrahedral coordination (Fe1) and other Fe in octahedral coordination (Fe2) as shown in Figure 9. Bi2Fe4O9 is widely used for the detection of VOC test gases at laboratory scale owing to their good selectivity, good long-term stability, low synthesis cost and most importantly its ability to sense at room temperature. Recently Subha et al. [55] reported rare earth Nd doped Bi2Fe4O9 (BNFO3) as a gas sensing material using Fiber optic evanescent wave absorption set up to detect VOC’s such as ammonia, ethanol, methanol and acetone at room temperature as shown in Figure 7. The improved sensing ability in the Nd-doped Bi2Fe4O9 sample was seen towards ammonia vapours as shown in Figure 10 with an extremely short response and recovery time of 40 sec and 48 sec, respectively, thereby making them an efficient fiber optic ammonia gas sensor. The same group also investigated room temperature gas sensing ability in Gd doped Bi2Fe4O9 sample, and observed enhanced sensing capability towards ethanol gas at 500 ppm. The clad modified fiber optic gas sensing studies suggested that Gd doping improved ethanol gas sensing ability from 0 to 500 ppm at room temperature compared to host Bi2Fe4O9 with high sensitivity, quick reaction and recovery period of around 38 s and 67 s, respectively. All of these findings highlight the Bi2Fe4O9 with enhanced gas sensing capability suitable for industrial applications [26].
Figure 9.
Crystal structure of Bi2Fe4O9 and the bond angles and bond lengths of the material [71].
Figure 10.
(a) Sensitivity of BNFO3 towards various VOC gas vapors; (b) response analysis of BNNFO3 sample; and (c) gas sensing mechanism in BNFO3 sample [55].
2.2.2 YMnO3
The usage of YMnO3 perovskite materials in gas sensing properties has recently garnered attention. Such system shows high reactive and stable behaviour over multiple oxidation/reduction states in the experimental cycles. Yttrium and rare-earth manganites in RMnO3 oxide stabilises in two structural phases. The hexagonal phase (space group P63cm, Z = 6) forms with R = Ho, Er, Tm, Yb, Lu, or Y, which have a small ionic radius whereas the orthorhombic phase (space group Pnma, Z = 4) forms with R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, or Dy, which possess a larger ionic radius. The orthorhombic phase exhibits ferromagnetic ordering, whereas the hexagonal phase shows both ferromagnetic and ferroelectric ordering [72, 73, 74]. Hexagonal YMnO3 oxide, with an orthorhombic crystal structure (Figure 11) is an excellent material for non-volatile memory and metal-ferroelectric-semiconductor (MFS) devices, because of their coupled magnetic and ferroelectric behaviour [76, 77, 78, 79]. Such electromagnetic multiferroics, which exhibit simultaneous ferroelectricity and magnetism, can be exploited in both electrical and magnetic applications. To date, many reports have focused on YMnO3 based materials because of their unique dielectric, ferroelectric, and magnetic properties [75, 80, 81, 82, 83].
Figure 11.
The orthorhombic crystal structure of YMnO3, showing the magnetic element Mn [75].
Recently, gas sensing properties of YMnO3 was tested on LPG, H2, CO and H2S gases and among the gases it showed maximum sensitivity to H2S gas when compared to other gases. The gas sensor fabrication is shown in Figure 12a wherein spin frustrated YMnO3 of 50 μm thickness is coated inside a cylindrical tube of 8 mm length and 2 mm diameter. Nichrome wire was fixed inside the tube to heat the chamber and to carry out the temperature dependent sensitivity measurement. Chromel alumel thermocouple was used for monitoring the temperature of the tube [78]. Voltage drop in with the application of 10 V was used for sensing the gas. The sensing property of YMnO3 was measured in the presence gases such as CO, H2, H2S and LPG at a concentration of 500 ppm. Figure 12b shows the dynamic sensing properties as a function of operating temperature for the YMnO3 different gases. The response of the sensor increases with temperature up to 100°C, and then it decreases with temperature. The YMnO3 sensor element shows a 96% response for an operating temperature of 100°C for H2S gas. Other reducing gases such as CO, H2, and LPG show comparatively low responses [78].
Figure 12.
(a) Schematic diagram of YMnO3 sensor element; and (b) sensor response towards 500 ppm of reducing gases as a function of operating temperature [78].
2.2.3 Ni3V2O8
Ni3V2O8 is an important system of spin frustrated magnet at low temperatures it undergoes a series of competing magnetic ordering. Ni3V2O8 has orthorhombic crystal structure with space group Cmca, which is characterized by triangular lattices and short range antiferromagnetic interactions [84]. In Ni3V2O8, the magnetic lattice is made up of magnetic ion Ni2+ (S = 1, d8), based on an anisotropic Kagomé lattice. Non-magnetic VO4 tetrahedra layer separates the magnetic layers. Due to this separation the geometric frustration is reduced and this allows long-range magnetic ordering in the material. Ni2+ has two kind of positions that is “spine” and “cross tie” correspondingly. The deviation from the ideal Kagomé geometry introduces many new interactions that relieve the frustration of underlying Kagomé antiferromagnet interacting. The structure of Ni3V2O8 is shown in Figure 13.
Figure 13.
(a) Shows the crystal structure of Ni3V2O8; and (b) Kagoma stair case showing only the Ni atoms.
Recently, Ni3V2O8 is used as important materials for gas sensing application [85, 86, 87]. Ni3V2O8 showed the high sensitivity to Ammonia gas. To control the amount NH3 that is sent to the air, a powerful closed loop control system based on the NH3 is required and the schematic of the automobile based sensors are given in Figure 14a. The sensor showed the optimal behaviour at 650°C beyond this temperature the behaviour decreases (Figure 14b). The sensor 90% response and recovery times of the sensor to 500 ppm NH3 were approximately 2 and 10 s, respectively [87].
Figure 14.
(a) Schematic view of exhaust gases after treatment system; and (b) response transients curve for NH3 in the range of 50–500 ppm at 650°C [87].
3. Applications and challenging aspects of fiber optic sensors
Fiber optic sensors (FOS) have been employed in a variety of harsh environmental applications where other sensor types typically fail, due to its unique characteristics such as electromagnetic immunity, intrinsic safety, chemical and heat resistance, and exceptionally small size. Some innovative FOS solutions have been used in high-temperature, high-pressure, and possibly explosive environments, such as oil and gas wells [88], pipelines [89], and in turbines testing and engine testing [90, 91, 92]. It is well recognized that fiber optic sensors play a vital role in many applications. Fiber optic sensors are expected to be utilised to improve the efficacy and cost-effectiveness of many electronic goods, given the vast variety of benefits that fiber optic sensing offers in multiple industries. The usage of fiber optic sensors in environmental monitoring is quadrupling, which is critical for guaranteeing adequate food and water supplies, identifying potential airborne contaminants, and safeguarding structures from corrosion. Water safety, agriculture, transportation, smart structure protection, and biomedical monitoring are all projected to see an increase in environmental monitoring. Further, the future prospects of fiber optic sensors towards various technological aspects are represented in Figure 15, wherein this technology would be employed in smart city initiatives, created specifically for tough and challenging atmosphere. Despite its interesting solutions in environmental monitoring, Clinical environment is still facing challenges in some ways, particularly if FOS is poorly constructed. In a surgical room with a critically ill patient, for example, the instrumentation must be as simple as possible so that the medical personnel may focus on the emergency rather than how the sensor should be attached or the system configured. With a “plug and play” philosophy, it should be as simple as it is with other existing electrical devices. Most practitioners do not yet have the background associated with FOS technologies, and they should not only gain complete confidence in this new technology that is slowly gaining traction in their environment, but also be completely at ease with the potential addition of new steps to their daily medical procedures. In practise, it signifies that an efficacious incorporation of FOS technology is highly reliant on a thorough understanding of medical applications and related clinical measures so as to benefit more from the optical sensing technology.
Figure 15.
Future prospects of fiber optic sensors towards various technological aspects.
Despite the multiple challenges and impediments to sensor deployment in smart technology, R&D breakthroughs would result in the widespread availability of low-cost and precise sensors for monitoring water, soil nutrition, temperature, and humidity. Fiber optic sensor technology will continue to grow slowly and steadily over the next few years. Researchers in the field of photonics will continue to be fascinated by fiber optic sensors. They’re looking forward to developing new technologies and seeing what these sensors can do for the sensing and instrumentation industries. We recognise that this study may not be exhaustive in all categories, but it is an attempt to provide readers with an overview and the most straightforward way when developing and researching Fiber optic gas sensors.
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Written By
Subha Krishna Rao, Rajesh Kumar Rajagopal and Gopalakrishnan Chandrasekaran
Submitted: 05 July 2022Reviewed: 29 July 2022Published: 04 November 2022
Open access peer-reviewed chapter
