Optical Fiber Based Temperature Sensors: A Review

1. Introduction

The reliable temperature monitoring plays a key role in the metallurgical industry, aerospace field, nuclear energy production, and medical applications [1,2]. In the metallurgical industry, a real-time monitoring of the internal temperature of high-temperature boilers is keys to measure combustion efficiency and the safety prevention [3,4]. On other hand, temperature monitoring inside the turbines and combustion chambers of an aero-engine or aircraft can help extending its service life [5,6,7]. Recently, the appearance of COVID-19 gain huge interest in temperature monitoring instruments in medical sector for continuous monitoring temperature of individuals at public places [8,9]. Extremely harsh environments with high pressures, high temperatures, and strong electromagnetic radiation present a challenge to convectional temperature sensors.

According to the accuracy, detection and installation techniques, high-temperature monitoring technology can be categorized in two categories (i) contact measurement [10], and (ii) non-contact measurement [11]. Thermocouple sensors made of expensive metals are generally utilized for former type of temperature sensing due to their ease of operation, mature preparation approach, and wide operating range [12]. However, the thermocouple sensors suffer several limitations such as short life, poor corrosion resistance, low accuracy, and susceptibility to electromagnetic interferences. The main concerns of thermocouple devices are its metal or alloys used during the fabrication can be easily oxidized and damaged at high temperatures leading to a shorter service life and sensing accuracy [13]. Infrared thermography is a type of non-contact temperature sensing technology, which is designed to avoid direct contact between the sensing equipment and high-temperature environment to provide a non-destructive sensing performance [14]. Unfortunately, the radiation temperature monitoring technology is suitable only for the surface measurements, i.e., explosion flame, and cannot detect the temperature of the internal structure of the closure device.

Compared to conventional sensing technology, optical fiber based sensors have gain huge interest owing to their excellent properties such as low cost, compact size, able to multiplexing and immune to electromagnetic interferences [15]. Till date, several approach have already been implemented for temperature sensing including photonic crystal fiber (PCF) [16], standard fiber (such as single-mode fiber (SMF) and multi-mode fiber (MMF)) [17], grating based fibers (e.g., fiber Bragg grating (FBG), long period grating (LPG), and tilted FBG (TFBG)) [18]. The commonly employed high-temperature sensing optical fibers are mainly includes silica and crystal fibers. Theoretically, the maximum temperature which a temperature sensor can detect based on the fiber materials rather than that of it’s the sensing mechanism. Usually, the silica-fiber-based temperature sensors are limited to operating within 1000 °C due to the diffusion of germanium dopant [1]. Additionally, temperature sensors based on pure silicon fibers such as PCF and microstructure fiber can operate at 1300 °C which is closer to the melting point of silicon, whereas, the temperature sensors based on single-crystal fibers can operate stably below 1900 °C. Figure 1 illustrates the summary of all types of optical fiber temperature sensors.

In this article we have reviewed several optical fiber based temperature sensors reported in past decades, including their design, fabrication, sensing materials, and performance. This review is organized in following manner: Section 2 provide a brief survey over microstructure optical fiber along with the advancement in the technology; Section 3 cover the standard single mode fiber (SMF) and multi-mode fibers (MMF) and their application in temperature sensing; Section 4 comprises grating based fibers and their role in temperature sensing. Finally, Section 5 provides the brief on future prospects and challenges. Finally the overall conclusion is drawn in Section 6.

2. Microstructure Optical Fiber (MOF):

The last decade has shown the enormous interest in optical fibers used for sensing and long-distance operation applications. The optical fiber in sensors applied as a sensing element to monitor surrounding parameters like strain, pressure, vibration, temperature, etc. . Numerous types of fiber-optic temperature sensors, including as Microstructured Fiber, fibre Bragg gratings (FBGs), long-period gratings (LPGs), Rayleigh scattering, Brillouin scattering, and Raman scattering, have been created for the purpose of monitoring the health of social infrastructures and industrial equipment [19,20,21,22,23,24]. Unlike standard optical fibres, photonic crystal fibres (PCFs) are made up of small air holes that are spaced at regular intervals, allowing for greater design freedom and unique optical properties [25,26]. Based on post-processing techniques, the micro air holes can also be filled with liquid, gas, or even solid materials, greatly changing the PCF guiding characteristics and allowing the fabrication of a variety of functional fibre devices such as electrically controlled optical switches [27,28,29], all-optical modulators [30], temperature-controlled tunable optical filters [31] photonic bandgap fibre polarimeters [32,33], and PCF sensors [34]. In this section, the current status of research on microstructured fiber based temperature sensors is discussed.

Micro optical devices have been quickly developed in recent years as a result of technological advancements in micro-fiber manufacturing [35,36], particularly in the field of dynamic temperature monitoring [37]. A number of significant efforts have been made to improve the sensitivity and reliability of the proposed optical fiber-based temperature sensors, including micron scale processing of couplers and ring resonators [38,39], selective liquid infiltration of high birefringence photonic crystal fibres (HBPCFs) [40,41] and filled photonic crystal fibres [42,43]. Most temperature sensors based on classic fibre constructions with silica backgrounds have temperature sensitivity less than 0.3 nm/°C [44], which is primarily constrained by the modest thermo-optical coefficient difference between the fibre core and cladding. To circumvent these constraints, Qian et al suggested a photonic crystal fibre (PCF)-based temperature sensor that reached an extrahigh sensitivity of 6.6 nm/°C by selectively infiltrating a thermosensitive liquid, such as alcohol, in the air holes to increase the thermo-optic coefficient [45]. In this experiment, the fiber loop mirror of alcohol filled highly birefringence PCF is placed in a temperature controlled container for investigating the temperature characteristics of the proposed sensing device. Figure 2 shows the experimental setup for the temperature measurement and inset of the Figure 2 shows the cross section of the highly birefringence PCF. Dora et al [28,46] suggested a temperature sensitive device based on nematic liquid crystal filled photonic crystal fibre, with a temperature sensitivity of 3.90 nm/°C.

Due to its strong thermal resistance and immunity to electromagnetic noise and corrosion, fibre sensors are likely the only option for sensing at temperatures above 800 °C. In 2013, Chen et al. described for the first time a true distributed pressure sensing at room and very high temperature sensing by utilising a pressure-sensitive fibres. It is based on an optical frequency domain reflectometry measurement of Rayleigh scattering in a fibre. This technique has been used to provide a distributed sensing solution for measurements of temperature, axial strain, and transverse stress. Air-hole microstructural fibres are employed to broaden its applicability for pressure sensing at high temperatures. Temperatures ranging from 24 °C to 800 °C are used to detect pressure distributions of up to 13.8 MPa (2000 psi) [47]. Another method for measuring temperature is to utilise Fabry-Pérot interferometers (FPI). Because of its sensitivity, extremely tiny size, easy design, and versatility in modifying sensitivity and dynamic range, FP interferometers are a common sensor arrangement. A microstructure temperature sensor that can be wavelength and frequency encoded at the same time presented by the Li et al. [48]. In this work, the sensor system has the potential for over 1000 multiplexing capacity via a single fibre with the hybrid frequency-division-multiplexing/wavelength-division-multiplexing (FDM/WDM) method. The temperature sensitivity of roughly 7.91 pm/°C is established experimentally using quasi-distributed measurement of temperature change. Tan et al. presented a work which shows how to make an intrinsic FPI sensor for high temperature detection with high fringe contrast by splicing a short piece of microstructured fibre to an single mode fiber [49]. The two strands of the microstructured fiber are spliced together using the conventional arc-discharge process. The interference spectrum's fringe contrast can approach 20 dB. There is theoretical study and thermal testing up to 1000°C. Monitoring the dip wavelength shift yields a temperature sensitivity of 17.7 pm/°C. This suggested sensor is small and has a significant potential for high temperature sensing applications using simply splicing and cleaving techniques.

To metallize a fibre surface in a typical fiber-based sensor, first strip the fibre jacket and then physically or chemically remove the fibre cladding, nearly to the core, to permit evanescent coupling with a plasmon mode. The associated operations directly raise the overall fabrication cost and jeopardise device dependability. The rapid creation of a novel form of PCF with an array of air holes along the propagation direction has piqued the curiosity of researchers worldwide in recent years. Many studies sought to insert metallic layers or rods into PCF and were successful in containing the plasmonic modes within the fibre [50,51,52]. Metallization of PCFs can be accomplished using stack-and-draw [53] or by pumping molten metal into the hollow channels of silica glass PCFs [52]. Metal layers can be deposited in the side of the PCF using either a high pressure chemical vapour deposition (CVD) approach [53] or a wet chemistry deposition process employed in the manufacture of metal-covered hollow waveguides [54]. In 2013, Favero et al experimentally demonstrate a PCF with a specially doped germanium core as shown in Figure 3a [55]. PCF with germanium core sandwiched between two single mode fibers to form an interferometer for the temperature measurement. The experimental set for the temperature detection is shown in Figure 3b. One end of the fiber was deposited with the gold film in order to work in the reflection mode. Proposed device shows very high temperature dependence with the sensitivity of 78 pm/°C.

By inserting a silver nanowire into the holes of a six hole PCF, a temperature sensor was designed based on the surface plasmon resonance effect [56]. Cross sectional area of the proposed sensing device is shown in Figure 4a and different air holes coated with silver nanowires and filled with the high thermos-optic coefficient liquid, a mixture of ethanol and chloroform is shown in Figure 4b. As a sensing medium, a liquid combination (ethanol and chloroform) with a high thermo-optic coefficient is injected into the PCF holes. The filled silver nanowires can sustain resonance peaks, and the peak will move when temperature changes cause changes in the mixture's refractive indices. The temperature change may be observed by monitoring the peak shift. Because the refractive index of the filled mixture is near to that of the PCF material, the resonance peak is particularly temperature sensitive. Our numerical results show that a temperature sensitivity of up to 4 nm/K is possible.

A sensing device with improve sensitivity based on the microstructured optical fiber and worked on the surface plasmon resonance principle was further demonstrated by the same group in 2014 [57]. The MOF's air holes are coated with a silver metal and filled with mixture of ethanol and chloroform which have a very high thermo-optic coefficient. The usage of all six fibre openings, as well as their relatively large size, should make silver coating and liquid mixture filling easier. Temperature fluctuations will cause changes in the coupling efficiency between the core-guided mode and the plasmonic mode, resulting in various loss spectra being recorded. The liquid mixture's refractive index is near to that of the MOF material, which improves coupling efficiency and sensitivity. Numerical results depict the temperature sensitivity of 5.6 nm/K and that the most sensitive range of the sensor can be modified by varying the volume ratios of ethanol and chloroform. Apart from these structures, different temperature sensing devices with different configuration of the MOF are demonstrated [58,59,60,61]. In 2015, Scarcia et al. conducts an accurate theoretical analysis of the viability of a unique fibre optic temperature sensor designed as a standalone device. It is built on a trio of integrated/spliced microstructured optical fibres (MOFs). The first incorporates an appropriate cascade of LPGs into the core. The other two sensor parts are a single mode intermediate MOF and a ytterbium doped MOF laser. The three MOFs are made to be readily spliced together as shown in Figure 5. The entire design is carried out using ad hoc computer code created specifically for this purpose. A low-cost pump diode laser at 980 nm wavelength and a commercial optical power detector are all that is required for a comprehensive temperature monitoring setup. The simulated sensitivity S = 315.1 µW/°C and operation range ΔT = 100°C are adequate for real-world applications [62]. In [63], two all-fiber loop mirrors employing a clover microstructured fibre for simultaneous temperature and strain monitoring are demonstrated. The design of the fibre allowed for the observation of various interferences caused by the microstructured fibre core section. Different sensitivity to temperature and strain were obtained, and both physical characteristics may be discriminated using a matrix technique. For temperature and strain, resolutions of ±2 °C and ±11 με for the first structure and ±2.3 °C and ±18 με for the second were obtained.

A wide variety of the MOF based temperature sensors has been developed by using the different configuration and sensing techniques [64,65,66,67,68,69,70]. In [71], use of femtosecond laser ablation to etch fibre Bragg gratings into the core of pure silica suspended-core microstructured optical fibre (SCFs) is presented. Because the concentrated femtosecond laser beam is not greatly deformed when travelling through the fibre cladding, the SCF's basic shape (three big holes) enables for direct writing of FBGs. These can withstand temperatures of up to 1300°C and can be wavelength divided multiplexed. Temperature sensing up to 1100°C was achieved by measuring the interference spectrum's fringe changes as temperature varied [72]. The sensor head in this study is constructed by splicing a single mode fibre (SMF) with a suspended core microstructured optical fiber (SCF), resulting in an SMF-2SCF-SMF configuration since the reflected light travels twice the physical length of the SCF. Because the SCF is a single-material device, extremely high temperatures may be measured, while the sensor head's all-splice simplicity results in low-cost and steady operation.

In [73], a very sensitive temperature sensor based on an all-fiber Sagnac loop interferometer and metal-filled side-hole PCF demonstrated. PCFs with two side holes filled with metal provide a structure that may be adjusted to modify the birefringence of the fibre by expanding the filler metal. To investigate the influence of filler metal on the temperature sensitivity of the fiber-optic temperature sensor, bismuth and indium were utilised. Measurements revealed that the indium-filled side-hole PCF could obtain a very high temperature sensitivity of 9.0 nm/°C. An unique microstructure fibre based on side-polished fibre (SPF) presented in [74] for the temperature sensing application. Polystyrene microspheres were self-assembled onto the side-polished surface of SPF to generate the microstructure fiber's colloidal crystal coating. Three primary troughs were identified in the transmission spectrum due to the Bragg reflection of the colloidal crystals coating and the strong interaction between evanescent field and colloidal crystal. The valley modulation amplitude might reach 12 dB. Furthermore, the transmission valley demonstrated strong temperature sensitivity. Transmission optical power exhibited temperature sensitivity was up to 0.487 dB/°C. A brief description about the different sensing structure and mechanism also reported in [75].

By using the optimal polymer optical fiber for Bragg grating sensing, fabrication and analysis of an infinitely single mode microstructured polymer optical fiber presented in [76]. This fibre is constructed of cyclo-olefin homopolymer Zeonex grade 480R, which has a very high glass transition temperature of 138 °C and is not affected by humidity. The temperature sensitivity of this fiber is 24.01 ± 0.1 pm/°C for both increasing and decreasing temperature. Apart from these sensing devices for temperature measurement, many researcher used MOF for the simultaneously measurements of the two or more physical parameters [77,78,79]. In [80], a linearly chirped bragg grating fibre engraved in a microstructured polymer optical fibre used for the temperature detection during thermal treatments. A KrF laser and uniform phase mask were used to inscribe a CFBG of 10-mm length and 0.98-nm bandwidth in a mPOF fibre. The temperature sensitivity of the CFBG is 191.4 pm/°C. A novel sensing approach based on C-type micro-structured fibre for simultaneous monitoring of seawater temperature and salinity was presented by Zhao et al. [81]. The C-type fibre structure as shown in Figure 6, was created by removing the outer wall of one pore from a six-hole micro-structured fibre, allowing birefringence to enter and breaking the original pattern of degeneracy. Furthermore, a gold layer was applied to the structure's surface to improve sensing sensitivity using the SPR phenomena. Model studies demonstrated that the suggested filling structure might generate two SPR loss valleys with distinct sensitivity to temperature and salinity. Maximum temperature sensitivity of 7.609 nm/°C was obtained for Y polarisation, demonstrating that the designed scheme could not only solve the cross sensitivity problem of two parameters but also achieve high sensitivity.

In recent year, using of microstructure optical fiber in sensing filed with different configuration earned a lot of attention [82,83,84]. In [85], Su et al. proposed a high temperature sensor which is based on the suspended –core microstructure optical fiber (SCMF). The sensor is fabricated by fusing a portion of SCMF between two portions of the multimode fiber (MMF). This kind of configuration made multimode interference by the air cladding modes and the core modes in the SCMF. The transmission spectra of the MMF-SCMF-MMF structure are filtered using the Fast Fourier transform. The wavelength shift of the main spatial frequency is measured when the temperature varies between 50 and 800 degrees Celsius. Sensitivities of 31.6 pm/°C and 51.6 pm/°C are reached in the temperature ranges of 50°C - 450°C and 450°C - 800°C, respectively. By using the Fabry-Perot interferometer, Zhao et al. [86] proposed and experimentally demonstrated composite cavity fiber tip for high temperature sensing. An air cavity and a silica cavity make up the composite cavity. An oval air hole inserted in the SMF serves as the air cavity. A small segment of SMF cascaded to the air cavity forms the silica cavity. The CCFT was effectively encased by a tapered-shaped silica capillary to eliminate the effects of other external physical characteristics (refractive index, humidity, etc.) on the CCFT FPI. The packed CCFT FPIs were used for high temperature sensing in the 100-800 °C range. The silica cavity has a temperature sensitivity of 14.31 pm/°C.

The finite element approach was used to examine a temperature sensor based on a Sagnac interferometer and a liquid crystal-filled microstructured optical fibre [87]. Near the core of the fiber, six submicron liquid crystal columns were added whose refractive index in slightly greater than the silica. In this configuration, the power of the fundamental core modes remained restricted to the silica core, and the mode field properties were heavily impacted by the six submicron liquid crystal columns. The temperature-sensing property of the resultant liquid crystal-filled microstructured optical fibre in the range of 265-325 K was examined using a Sagnac interferometer based on the thermo-optic effect. The temperature sensitivity was 6.23 nm/K with high linearity in the region of 265-295 K. Zhao et al. demonstrated a Sagnac interferometer temperature sensor based on the glycerine filled MOF [88]. Here, the photosensitive substance was a glycerin solution which is filled in a polarization-maintaining photonic crystal fibre (PMPCF). According to the finite element approach, the temperature sensor had the maximum sensitivity when the PMPCF was filled with a 70% mass fraction of glycerine solution. A linear fit was built between the dip wavelength and the temperature in the experiment, with the temperature rising from -25 °C to 85 °C in 5 °C steps. The findings showed that the suggested sensor had the greatest temperature sensitivity of 1.5005 nm/°C within the temperature range of -25 °C to 85 °C.

In another work, the finite element approach has suggested a very sensitive temperature sensor for magnetic fluid (ethanol) utilising a ring-core-based microstructured optical fibre (MOF) with two huge elliptical air holes alongside the ring core [89]. To boost the ring core's high sensitivity responsiveness, highly sensitive magnetic fluid ethanol is introduced. For the broad working wavelength range of 6500-7000 nm and temperature range of 10-60°C and applied magnetic field strength range of 50-200 Oe, the highest sensitivity response of 25,641.025 nm/RIU and 10 nm/°C is achieved. Here, the sensing device simultaneously measured three different physical parameters. In [90], an investigated was carried out for a temperature sensor based on the modes coupling effect in a liquid crystal-filled MOF. A full-vectorial finite-element technique (FEM) with completely matched layer and scattering boundary conditions was used to study the sensing properties. Liquid crystal E7 was developed to be infiltrated into one cladding air hole in PCF to generate a defect core for temperature sensing. As the phase matching requirement is met, the core modes couple to a variety of ordered defect core modes, resulting in a significant rise in the confinement loss spectrum. In an approach, [91] experimentally demonstrated a temperature sensor based on the Yb-doped silica micro-structured optical fibers. In this work, authors describe an unique method for producing Yb-doped silica microstructured optical fibres and its application in high-temperature optical fibre sensing. The Yb-doped silica micro structured optical fibre preforms were made out of Yb-doped silica core rods (0.1 wt% Yb2O3) and holey pure silica claddings created using UV curable tube moulding and digital light processing 3D printing techniques, respectively. The rod-in-tube approach was used to draw silica preforms to Yb-doped silica microstructured optical fibres without causing significant deformation. Doped microstructured fibres were used in high-temperature sensing ranging from 296.15 to 653.15 K. Figure 7a depicts the scanning microscopic image of the fabricated Yb-doped silica microstructured optical fibres and the refractive index profile of this MOF is shown in Figure 7b–d depicts the attenuation spectrum and absorption loss of the proposed MOF based sensing device. The experimental results shows that the proposed sensing device have maximum value of temperature sensitivity and relative temperature sensitivity of 0.0023 K at 348 K and 0.8%/K at 296K respectively.

Theoretical analyses represent a temperature sensor with a high sensitivity based on the MOF Sagnac interferometer [92]. Ethanol is inserted in the air holes of the MOF. Two big air holes are also introduced for enhancing the birefringence and sensitivity of the proposed device. Simulation results shows that the proposed MOF temperature sensor has sensitivity of 14.7 nm/°C in the temperature range varies from 15 to 75 °C. In [93], the silver mirror reaction was used for coating the silver film on the surface of self-made MOF to stimulate the surface plasmon resonance effect, and Polydimethylsiloxane (PDMS) with a high thermal-optical coefficient was coated on the silver film as temperature-sensitive material. The MOF with silver and PDMS films was coupled with multi-mode fiber on both sides to form the temperature sensor. In this sensor system, the energy is coupled into the cladding of the microstructure fiber by multi-mode fiber, and the surface plasmon resonance can be further excited in the MSF. The refractive index of PDMS changes with change in the temperature of the external environment. The proposed sensors exhibits very high sensitivity of 0.83 nm/°C and -0.84 nm/°C during heating and cooling for the temperature range of 35 °C to 95 °C, respectively.

Apart from these sensing techniques, researcher used nonlinear properties of the MOF for the sensing purpose. A unique fiber-optic soliton self-frequency shift (SSFS) temperature sensor was developed, which was built utilising an in-house MOF [94]. SSFS-based sensing was systematically examined using this sensor with variations in average pump wavelength and pump power. The sensing performance of the proposed sensor was assessed experimentally and conceptually, subject to the average pump wavelength and pumps power, by detecting the centre wavelength shift of the 3-dB bandwidth of the soliton with temperature change. Greater sensitivity was achieved when the pump wavelength is longer while the average pump power was fixed. At an average pump power of 300 mW and pump wavelength of 1600 nm, the maximum sensitivity of the proposed sensing device comes to be 1.759 nm/°C. A small fiber-optic temperature sensor is suggested and experimentally proven using refractive index liquid functionalized side-hole microstructured optical fibres (SHMOFs) [95]. The transmission spectrum of the suggested temperature sensor has been examined experimentally and theoretically. Multiple resonance dips in the transmission spectrum are present due to resonance coupling between the fundamental core mode and refractive index liquid rod modes, which may be adjusted by modifying ambient temperature. According to the experimental data, the maximum temperature sensitivity is 13.1 nm/°C over a temperature range of 80-100 °C.

Zhang et al. [96] demonstrated surface plasmon resonance (SPR) based temperature sensor made of silver-coated multi-hole optical fiber. The structure of the proposed sensor is consisting of an input lead-in MMF, a silver-coated multi-hole optical fiber and an output lead-out MMF. The middle silver-coated multi-hole optical fiber (SMHOF) is filled with two types of thermo- sensitive liquid. In the previous reported sensors, the external temperature was monitored by measuring the wavelength shift of single SPR peak. However, the proposed work utilizes the concept of dual SPR dips which ultimately improves the sensitivity and expanded the detection range. Here, in a single optical fiber structure, two distinguish SPR dips at different wavelength which caused by adjusting refractive index of two types of thermo-sensitive liquid moves in opposite direction having some temperature difference are excited. The illustrative view of proposed SMHOF temperature sensor and the cross-section of thermos-sensitive liquid filled SMHOF are shown in Figure 8a,b respectively. A linear relationship between the environmental temperature and the interval of the two SPR dips is demonstrated. The reported SMHOF temperature sensors offer the high sensitivity of 7.72 nm/°C and -7.81 nm/°C in the range of 20-60 °C and -20-20 °C, respectively.

3. Standard Fiber

The last decade has shown the enormous interest in optical fibers used for sensing and long-distance operation applications. The optical fiber in sensors applied as a sensing element to monitor surrounding parameters like strain, pressure, vibration, temperature, etc. [97]. There are several ways of converting temperature signal into optical and then an electrical signal. The most popular used approach is to employ single-mode fiber (SMF), multimode fiber (MMF), tapered and etched fibers as temperature sensor. Other than this, the application of single-mode-multimode-single-mode fiber is being also utilized [98,99]. The integration of Mach-Zehnder interferometer (MZI), Fabry Perot interferometer (FPI) or Bragg grating on optic fiber is widely used for the sensing application [97 1]. The advantage associated with these systems is the capability to monitor temperature at every position and over long distances. However, each method has its own merits and demerits; few of these suffer from high cost, fabrication complexity and sophisticated equipment. In this section, the current status of research on SMF, MMF, tapered and etched fiber as temperature sensor is presented.

Wang et al. presented [100] a temperature sensor having single-mode-no-core-single-mode fiber structure with PDMS coating. In this work, PDMS due to its negative thermos-optic coefficient and large thermal expansion coefficient (TEC) is employed to increase the TEC of no-core optical fiber to finally enhance the temperature sensitivity of the proposed sensor. The study reports the result obtained using both theoretical analysis (MATLAB) and experiment. The schematic and physical diagram of the sensor is shown in in Figure 9.

The theoretical finding is found to be in good agreement with the experimental results and the temperature sensitivity is –260 pm/°C. In addition, the proposed sensor also offers strong resistance to external interference and mechanical damage, simple fabrication. In 2022, Xiaowei et al. [101] reported a sensor having arc-shaped misaligned structure MZI on a SMF.A layer of composite material (UV-curable polymer) is coated over the sensor to improve the temperature sensitivity. Along with temperature sensitivity, the strain sensitivity is also measured which found negligible. The proposed work shows the result from both theoretical analysis and experiment. Compared to previous reported sensors, the proposed temperature sensor shows an improved sensitivity which is -0.953nm/°C. Owing to simple manufacturing and small size, the proposed device may find its application in the domain of biology and chemistry. Similarly, Sun et al. [99] presented a sensor having single-mode fiber spliced with multimode fiber on which a Bragg grating structure has been written. The reported sensor with good reliability, repeatability and accuracy finds its application in the sensing of high temperature and micro-bending. Temperature and micro-bending’s measurement range was defined as 0~900°C and 0~0.453m⁻¹. The temperature sensitivity and maximum bending sensitivity in the corresponding range was found to be 13.4 pm/°C and 23.03 dB/ m⁻¹. Similarly, the accuracy was measured as 0.002~0.005 dB.

In 2019, Huang et al. [102] proposed temperature and refractive index sensor based on multimode-single-mode-multimode fiber structure having graphene-metal hybrid coating over it. The sensitivity was improved by adding the graphene layer above or below of gold coating in fiber. It was found that the refractive index sensitivity of the Graphene-Gold-Au on Ag nanoparticle film is higher than the conventional Au on Ag nanoparticle film, Graphene-Au or Au-Graphene films. The sensitivity of graphene Au-Au@Ag NPs film (1591 nm/RIU) is higher than that of Au@Ag NPs (29 nm/RIU), graphene-Au (1224 nm/RIU), Au (1034 nm/RIU) and Au-graphene film (807 nm/RIU) modified MMF-SMF-MMF structure at the RI of 1.333. Similarly, polydimethylsiloxane (PDMS) has been added to the Graphene-Gold-Au on Ag nanoparticle film for sensing of temperature exhibiting the higher sensitivity of 1.02 nm/°C with good repeatability. The proposed sensor finds its application in protection, environmental, nano-medicine and cleans energy fields. Gao et al. [103] demonstrated an optical fiber based temperature sensor with PDMS/silica hybrid structure which is based on modal interference principle. The hybrid structure is placed between two single mode fibers that act as the sensing unit. Here, PDMS is integrated to enhance the sensitivity which is inserted into the hollow core fiber to ultimately develop PDMS/silica hybrid fiber structure. The illustrative sensor structure is shown in Figure 10. Here, the PDMS filled length, core diameter and air-gap length was taken as 20 mm, 30 um and 1 mm respectively.

The hybrid fiber with different modes experiences the phase delay which is influenced by the external environmental conditions. Owing to the modal interference concept, the resonance wavelength shifted with respect to the variation in temperature. The experimental finding determined the sensor sensitivity to be −384 pm/°C for 25 °C–80 °C of range with the the hollow core fiber of core diameter of 30 um. The proposed temperature sensor can be used to sense the real time temperature in domains of Chemical, biological detection and medical. A similar work has been reported by Cai et al. [104] where they proposed an optical fiber based temperature sensor. The sensor follows the modal interference phenomenon of modes that occur in multimode fiber. The sensor includes the segment of single-mode fiber, hollow core fiber and no-core fiber with PDMS integrated over the cavity of hollow core fiber. The high thermo-optic coefficient and thermal-expansion coefficient of PDMS makes the effective refractive index to change larger with the change in temperature and hence makes a good temperature sensor. Both the experimental and simulation results have been presented where the experimentally obtained results found at par with the results obtained by simulation. A temperature sensitivity of 580.6 pm/°C in the range of 28 °C to 50 °C has been obtained.

In 2017, Peng et al. [105] reported a U-shaped bent single mode fiber based temperature sensor. The U-shaped fiber has a double coating of outer nickel protection coating and inner. A linear relation between temperature and bend loss at a fixed wavelength and bending radius has been presented. The inner coating worked as absorption coating and outer nickel coating worked as a protector of sensor. The proposed sensor offers temperature resolution and bends loss response of 0.5 °C and 0.023 dB/°C. Hence, the proposed sensor can also be used for monitoring temperature in harsh environment. Wildner et al. [106] shows the combination of two materials (glass particles and transparent oil/polymer) with different thermos-optic coefficient into the formation of temperature sensor. Here, the key performance parameters that varies the sensing of temperature such as particle size, filling degree and length are varied. The work demonstrated the freedom to select materials in according with the desired output of sensor. Small (large) differences in thermo-optic coefficient of two materials are selected for wide (small) measuring range. In a similar work, Noor et al. [107] proposed the use of SMF-MMF-SMF following the multimode interference phenomenon as temperature sensor. The bent MMF unit because of its small curvature radius projects a balloon shape. The MMF is coated with acrylate coating which offer higher temperature sensitivity due to its higher thermos-optic coefficient. The work reports both simulated and experimentally obtained results. In a temperature range of 27°C to 31°C, wavelength and intensity based interrogation and temperature sensitivity of -25.1 nW/°C and -2060 pm/°C respectively has been obtained. The work also points the importance of properly selecting the sensor dimension such as length of MMF, core diameter and bending radius to further improve the temperature sensitivity. Hao et al. [108] using SMF core-offset structure developed a low-cost and simple temperature sensor. The schematic diagram of the sensor is shown in Figure 11. With the application of temperature over core-offset structure, the output laser wavelength due to variation in pass-band varies.

The experimental outcome shows the achievement of single-wavelength laser with the application of core-offset structure in the ring cavity. With the pump power of 100Mw, the output laser peak power and SMSR of -8.93 dB and 47 dB respectively has been found. For the temperature range of 30°C- 270°C, temperature sensitivity of 0.0449 nm/°C has been obtained. Compared to the previous reported sensors, the proposed sensor offered higher output power, higher SMSR, large measuring range and lower cost and therefore can find its application in laser sensing domain.

In 2013, Manoj et al. [109] presented a comparative study of temperature sensor developed using single-multi-single mode fiber using step and graded index multimode fibers. Here, GeO₂ with two different doping concentration has been employed in the core of multimode fiber. For the entire wavelength range of 0.7-1.6 μm, the temperature sensitivity of proposed sensor with graded index MMF was found larger (25-285 times) than the step-index MMF. In the same year, Yang et al. [110] liquid-sealed S-tapered fiber based highly sensitive temperature sensor. Here, the thermo-optic effect of liquid and thermal expansion of S-taper fiber makes the temperature sensor more sensitive. The length of the sensor can be varied from 10 mm to 1 mm. A detailed experimental analysis has been presented. The highly sensitive sensor with sensitivity of -1.403 nm/°C was demonstrated. For high temperature monitoring, Tan et al. [111] in the same year reported a partial-reflection-enabled compact Fabry-Perot interferometer (FPI) based sensor. The Fabry-Perot interferometer includes single mode fiber and a small core micro-structured fiber. It follows the basic principle of occuring partial Fresnel reflection at the interface of the two fibers and end surface of the micro-structured fiber. The compact and simple temperature sensor showed the response to high temperature such as 1000°C with the temperature sensitivity of 17.7 pm/°C at 1570 nm. The senor also finds its application in space-oriented high temperature monitoring applications. In 2012, Arun et al. [112] reported the design and fabrication of SMF-based temperature sensor developed using wet-etching method. A dependency of sensitivity on etched fiber diameter was found. The sensor showed an insensitive functioning of an un-etched fiber and 3.8 µW/°C (0.2 µW/°C) as the highest sensitivity for 11.2 µm (25 µm) etched diameter. The sensor outcome was found to be appropriate for analogue signal transmission over optical fiber through thermal modulation. In addition of these reviewed article some other article utilzied these standard fibers to achieve supirior sensing performance as shown in Table 1.

4. Grating Based Fiber

Nowadays, the fiber Bragg grating (FBG) are considered as one of the most popular optical components which are widely exploited in optical networking, physical and chemical sensing due to their ease of fabrication, small size, multiplexing feature, and low cost. An FBG comprises a periodic variation of the refractive index (RI) within the core of a single-mode fiber, which satisfies the phase matching condition between the fundamental mode and other modes, either the core mode or the cladding modes or radiation (or leaky) modes [116]. In 1978, Hill et al. fabricated and reported first FBG in germanium-doped core [117]. The grating was made by using laser lithography technique to incorporate the permanent periodic variation of the refractive index in fiber core.

The sensing principle of FBG based fibers can be defined as the grating period, grating length, and the effective refractive index of such fibers that are affected by the variation in the surrounding media [118]. The change in the outer environment leads to the change in its resonance condition; consequently, the variation in resonance wavelength takes place. Based on the property of these gratings, it can be classified into three categories: (1) fiber Bragg grating (FBG), (2) long-period fiber grating (LPFG), and (3) tilted FBG (TFBG). The grating based optical fibers gains huge attention in physical and chemical sensing due to their unique features of immunity to electromagnetic interferences, compact size, highly sensitive, multiplexing capability and in-situ monitoring. Owing to their design of label-free monitoring of surrounding environment the grating-based sensors, such as FBG, LPG, etched FBG, and TFBG, have attracted extensive attention in order to develop physical and chemical sensors. In the grating based fibers, the Bragg wavelength is calculated by the refractive index modulation period (L) and the effective refractive index of the fiber (n_eff), and shown by following relation:

$${\mathit{\lambda }}_{\mathit{B}\mathit{r}\mathit{a}\mathit{g}\mathit{g}}=2{\mathit{n}}_{\mathit{e}\mathit{f}\mathit{f}}\mathit{L}$$

(1)

A change in the refractive index of the fiber is observed, under the variation of the temperature, leading to a linear drift in the value of the Bragg wavelength. Temperature monitoring using grating fibers can be easily achieved by demodulating the Bragg wavelength variation. Compared to other optical fiber sensors, the grating based temperature sensors showed several advantages including good linearity, high stability, multiplexing capability, mass production, etc., which have been used widely in commercialization.

In 2014, Zaynetdinov et al. developed a FBG based temperature sensor operated ranging from 2–400 K. along with a temperature resolution better than 10 mK for temperatures below than 12 K [119]. The sensing head was fabricated from polyimide coated optical fiber. The response was achieved by mounting the section of the fiber on a polytetrafluoroethylene tape, which has a non-negligible coefficient of thermal expansion down to <4 K. The sensors exhibited an excellent stability over multiple trials along with the superior repeatability of temperature cycles. In 2016, Hsiao et al. reported a chromium nitride (CrN) coated FBG sensor for extreme temperature environments [120]. The sensing configuration was fabricated by depositing CrN using physical vapor deposition (PVD) technique. During the sputtering process the pressure of the deposition system was fixed at 6 × 10⁻³ Torr with 70 W power applied to the Cr target. The film was deposited for 15 min to achieve an approximate thickness of 2 μm over the FBG. The sensor was based on wavelength interrogation technique anlayzed by utilizing an optical spectrum analyzer (OSA). The reflection spectra of the fabricated sensor were monitored and compared with bare FBG for temperatures varying from 100 °C to 650 °C. The results exhibited that the CrN-coated FBG sensor gives a 14.0 pm/°C greater temperature sensitivity compared to the bare FBG fiber without CrN.

In 2018, Dong et al. reported the utilization of graphene oxide (GO) as sensing layer integrated over FBG for simultaneous pressure and temperature monitoring [121]. The sensing configuration was based on Fabry–Perot interferometer (FPI). The light reflectors of the fabricated FPI sensor comprise with a thin graphene film coated over end facet of FBG, as shown in Figure 12. The sensor exhibited good sensitivities of 501.4 nm/kPa and 306.2 nm/°C, for pressure and temperature, respectively. The introduction of FBG along with GO can successfully prevent the cross-impact of the FPI sensor. Following the improvement of sensing performance, in same year of 2018, Jasmi et al. also reported an FBG based temperature sensor by utilizing polyurethane-GO nanocomposite as sensing layer [122]. The physical, chemical, and conductivity of PU-GO were observed to be improved after introducing GO in the pristine PU. The thermogravimetric analysis exhibited that the thermal stability of PU improved to 217 °C because of the strong intermolecular interaction with graphene flakes leading to an enhanced in sensing performance. The sensing material was coated over FBG using dip-coating to get uniform thickness, while the sensing performance of the device was evaluated for a wide range of temperature varying from 25 to 60 °C. The sensor exhibited a good linearity along with an improved sensitivity of 6 pm/°C.

In 2019, Cheng et al introduced strontium titanate (SrTiO₃) thin film coated over FBG using pulsed laser deposition for temperature sensing [123]. SrTiO₃ is an ferroelectric thin films with an typical perovskite like structures with remarkable optical, dielectric, and photoelectric properties. For deposition the FBG was immersed in the acetone solution for 5 min to remove the buffer coating, which was later coated with 271.5 nm thick SrTiO₃ film on the cladding. In order to study the thermal sensing performance of the FBG, a concatenated optical path, two optical paths based on a Michelson interferometer (MI) and one based on Sagnac interferometer (SI), were fabricated using SrTiO₃ coated FBG and another bare FBG. The sensing performance of the device was investigated for a wide range of temperature varying from 40 °C to 150 °C. The sensitivity for each sensing configuration was observed to be 9.0 pm/°C, 10.5 pm/°C and 11.4 pm/°C, for two MI and one SI, respectively. Later in 2020, Gao et al. reported an dual-parameter fiber sensor consist of a few-mode fiber and FBG for simultaneous detection of strain and temperature variation [124]. The sensing configuration was fabricated using 6.5 cm length of few mode fiber offset spliced with single-mode fiber (SMF), as shown in Figure 13. The temperature and strain were measured simultaneously due to the different sensitivities of the obtained spectrum dips. The fabricated sensor exhibited a good temperature sensitivities of -34.3 pm/°C and 10.7 pm/°C, while the strain sensitivities of -2 pm/με and 0.67 pm/με, respectively. The fabricated device showed good potential in fields of the dual-parameter measurement due to its simple configuration, cost effectiveness and compact structure.

Later in 2021, Chen et al. reported an optical fiber sensor based on a fiber surface waveguide and Bragg grating for a simultaneous monitoring of refractive index and temperature variations [125]. The device consists of two fiber Bragg gratings fabricated by a femtosecond laser, one of which is situated in the fiber core for temperature sensing; the other is located in the fiber surface waveguide for both temperature and RI measurements. The fabricated device exhibited a good RI and temperature sensitivities of 10.3 nm/RIU and 9.94 pm/°C, respectively. Later in 2022, Esposito et al. reported a miniaturized metallic package for FBG sensors to improve the temperature sensitivity and eliminate the cross-sensitivities to mechanical effects of impulsive forces or strain [126]. The packaging of device required the encapsulation of FBG into a steel tube, which was later placed inside another larger steel tube, where the fiber was kept loose to prevent any mechanical effect transferring to the grating. The fabricated device exhibited a linear temperature response in the range varying from 5 °C to 50 °C along with a high sensitivity of 28.9 pm/ °C, which was nearly three times higher than the standard FBG. In addition to sensing performance the device also shows a rapid response time of 5 s when tested under temperature of 20 °C. In addition of these reported FBG based sensors, some other sensor also listed in Table 2 which shows an excellent performance and competitive response.

5. Future Prospects and Challenges:

With the advancement of micro- and nano-technology and the development of fibres with unique optical characteristics, there is little question that designs and fabrications of innovative micro-structured fibre optic sensors will continue to be a thriving research topic. Opportunities and challenges are coexisting simultaneously in the work to be done on micro-structured fibre optic sensors in the future. It includes ability to measure multiple parameters simultaneously and with selectivity: Although several micro-structured fibre optic sensors with dual-parameter measuring capabilities have been proposed to date, efforts should be made to reduce measurement error and expand dual-parameter measurement to three-parameter or even more parameter characterizations because crosstalk effect is typically brought on by more than two parameters in many applications. Even greater sensitivity also requires for the MOF sensors with exceptionally high accuracies for more exact measurements. Sensing in severe conditions is require including those with high pressure, strong radiation, and extremely low or high temperatures. An intelligent MOF sensor should have functionalities that can be changed and adjusted in response to various target samples and circumstances. A more adaptable and potent sensor architecture would be desirable. The robustness of the microscopic structures is the key to the sensing performance and longevity of the device, therefore a better packaging without sacrificing sensitivity will be the future goal. Also, there is need of simplified and easy fabrication process for the MOF sensor. One main aspects of the MOF based sensor is to control the attenuation loss which is more important for the practical application.

Real time monitoring of temparatures in patients in inpatient floor and ICU with small devices that can monitor the fluctuations in real time same as telemetry can be extremely useful in patients with pneumonia and or sepsis and any other infections especially those who are intubated and unable to speak in critical care units. It will make more efficient detection of hospital acquired infections and will be helpful for quicker reaction to tackle possible infection in patients. In a normal clinical setting, patient has to inform the nurse that they have a fever or the nurse has to keep checking the temperature of the patient at regular intervals. With a real-time temparature monitoring device it would be easier and faster to catch a new infection and nip it in the bud before it goes to form a major infection or abscess and maybe useful in ultimately reducing a lot of incidences of sepsis.

Temperature monitoring is many times overlooked in critical care when compared to other factors such as pulse oximetry and telemetry but it is a important factor, a biological alarm that signals infection or some other abnormality in the patient. So this technology can be extremely useful in not only detecting cases of Covid or flu during pandemics for screening large number of people easily but it can also provide a means to monitor critically ill patients and give the doctors a head start in treating the infection before it become worse. This can revolutionise not just Screening but also patient monitoring.

6. Conclusion

In this review article, the role of various types of optical fibers and their sensing approach along with the sensor design, sensing material, working principle, and sensing performance including selectivity, sensitivity, and linearity are discussed and presented in detail. This paper provides a comprehensive reviews over the development of optical fiber based temperature sensors in past decades. Each sensing configuration comprises different structures and sensing material to achieve high sensitivity which is listed and discussed for each sensor. The new idea and approaches to improve the sensing performance of such devices are also suggested in future prospects section. Today, the increasing growth in the fields of optics, electronics, materials science, and data analysis has opened up a new path for the study which can be expanded to include these principles for high precision and accuracy.