Fiber Optic Sensors for Gas Detection: An Overview on Spin Frustrated Multiferroics

*Subha Krishna Rao, Rajesh Kumar Rajagopal and Gopalakrishnan Chandrasekaran*

## **Abstract**

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.

**Keywords:** fiber optic sensors, Multiferroics, spin frustrated, Bi2Fe4O9, YMnO3, Ni3V2O8

## **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


**Table 1.**

*Varous types of chemical sensors for measuring volatile organic compounds that are commercially available.*

*Fiber Optic Sensors for Gas Detection: An Overview on Spin Frustrated Multiferroics DOI: http://dx.doi.org/10.5772/intechopen.106863*

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 [13–15], Nondispersive Infrared Sensors (NDIR) [16, 17], Photoionization Sensor (PID) [18–20], Thermal Sensor (Pellistor) [21, 22], Fiber optic sensors [23–28].

#### **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 [36–38].

#### *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

**Figure 1.**

*(a) Basic components of optical fiber; and (b) principle of operation in fiber optics.*

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].

Generally the number of modes and the refractive index are used to divide optical fiber into two categories.
