sensors Review

Recent Developments in Fiber Optics Humidity Sensors Joaquin Ascorbe 1, *, Jesus M. Corres 1,2 , Francisco J. Arregui 1,2 and Ignacio R. Matias 1,2 1 2

*

Department of Electrical and Electronic Engineering, Public University of Navarra, Pamplona 31006, Spain; [email protected] (J.M.C.); [email protected] (F.J.A.); [email protected] (I.R.M.) Institute of Smart Cities, Public University of Navarra, Pamplona 31006, Spain Correspondence: [email protected]; Tel.: +34-948-169-260

Academic Editor: Gary R. Pickrell Received: 27 February 2017; Accepted: 6 April 2017; Published: 19 April 2017

Abstract: A wide range of applications such as health, human comfort, agriculture, food processing and storage, and electronic manufacturing, among others, require fast and accurate measurement of humidity. Sensors based on optical fibers present several advantages over electronic sensors and great research efforts have been made in recent years in this field. The present paper reports the current trends of optical fiber humidity sensors. The evolution of optical structures developed towards humidity sensing, as well as the novel materials used for this purpose, will be analyzed. Well-known optical structures, such as long-period fiber gratings or fiber Bragg gratings, are still being studied towards an enhancement of their sensitivity. Sensors based on lossy mode resonances constitute a platform that combines high sensitivity with low complexity, both in terms of their fabrication process and the equipment required. Novel structures, such as resonators, are being studied in order to improve the resolution of humidity sensors. Moreover, recent research on polymer optical fibers suggests that the sensitivity of this kind of sensor has not yet reached its limit. Therefore, there is still room for improvement in terms of sensitivity and resolution. Keywords: optical fiber humidity sensors; fiber Bragg gratings; long period fiber grating; photonic crystal fiber; tapered optical fiber; modal interferometer; Fabry-Pérot interferometers; resonators; lossy mode resonances

1. Introduction Humidity has an important influence on several industrial processes such as electronic, food or pharmaceutical manufacturing, food storage, etc. All these processes, which can be affected by humidity, require continuous monitoring of air humidity. In addition, proper humidity levels can be critical to the quality of the product and having the right humidity level can contribute to diminishing energy consumption [1]. Optical fiber humidity sensors (OFHSs) offer several advantages over electronic humidity sensors such as miniature design, durability, the possibility of working on flammable environments and at higher temperature and pressure ranges, and, most important, their electromagnetic immunity. Therefore, they can withstand the kind of harsh and demanding conditions found in industrial processes. However, there are some facts that have prevented OFHSs from being a common commercial product. The fabrication of optical fiber sensors is not yet a sufficiently repeatable process to become a serial fabrication product. Some optical structures have a certain degree of uncertainty inherent to the fabrication process. However, the main inconvenience is related to the cost of optical equipment. Halogen white light sources and standard optical fibers can be found on the market at affordable prices, but spectrometers, optical spectrum analyzers (OSA), and other optical equipment have relegated OFHSs to specific applications where there is no other option, as detailed below. Sensors 2017, 17, 893; doi:10.3390/s17040893

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Great research efforts have been made in recent years and results have been generated for various applications. The accurate measurement of relative humidity (RH) is critical in some industries, such as the semiconductor industry, where the performance of fabricated devices is dependent on the humidity [2], as well as in the electronics industry. Recently, another application for these OFHSs has been found high-energy physics (HEP), in experiments performed at the European Organization for Sensorsin 2017, 17, 893 2 of 23 Nuclear Research (CERN) [3,4]. Great research efforts have been made in recent years and results have been generated for More application fields can be found, some of them related to structural health monitoring, various applications. The accurate measurement of relative humidity (RH) is critical in some which is an important case where relative humidity (RH) sensors can find application [5–8]. industries, such as the semiconductor industry, where the performance of fabricated devices is OFHSs offer the possibility of monitoring engineering using, another for instance, fiber Bragg dependent on the humidity [2], as well civil as in the electronics structures, industry. Recently, application gratingsfor(FBG) [6,7], or even obtaining distributed measurements for large structures these OFHSs has been found in high-energy physics (HEP), in experiments performed atcombining the European Organization for Nuclear (OTDR) Research (CERN) [3,4]. Optical Time Domain Reflectometry with chemically sensitive water swellable polymers fields be found, some of them related structural health monitoring, (hydrogels) More [9]. application In addition, thecan FBG sensors mentioned abovetohave the potential to act as road which is an important case where relative humidity (RH) sensors can find application [5–8]. OFHSs parameter sensors (humidity, ice, temperature, etc.) [7]. Another application found for distributed offer the possibility of monitoring civil engineering structures, using, for instance, fiber Bragg measurement for tunnel leakage detectiondistributed [10]. The measurements possibility of for making distributed measurements gratingsis(FBG) [6,7], or even obtaining large structures combining is one ofOptical the strongest advantages of OFHS [11,12]. Another sector where this kind of sensor Time Domain Reflectometry (OTDR) with chemically sensitive water swellable polymersis useful (hydrogels) [9]. addition, FBG sensors humidity mentioned control above have the potential to act as road is food processing andInstorage [6].the Furthermore, is essential for the correct storage of (humidity, ice, temperature, valuableparameter artwork,sensors such as in museums or archivesetc.) [2].[7]. Another application found for distributed measurement for tunnel leakage detection [10]. The possibility of making distributed Apart from theisaforementioned applications, continuous RH measurement and control is measurements is one of the strongest advantages of OFHS [11,12]. Another sector where this kind of important for human comfort such as in air-conditioning monitoring and achieving controlled hygienic sensor is useful is food processing and storage [6]. Furthermore, humidity control is essential for the conditions [6].storage In addition, OFHSs have found applications in[2]. clinical treatment due to the need correct of valuable artwork, such as innew museums or archives for humidification of inspired gases in critical respiratory care [13,14]. Apart from the aforementioned applications, continuous RH measurement and control is human comfort as intrends air-conditioning and achieving controlled Theimportant present for paper reports thesuch current of opticalmonitoring fiber humidity sensors. Novel optical hygienic conditions [6]. Instudied addition,materials, OFHSs have found new applications in clinical treatment dueatodetailed structures, as well as recently will be analyzed and commented on. Then, the need for humidification of inspired gases in critical respiratory care [13,14]. analysis of OFHSs based on lossy mode resonances will be made. This kind of sensor occupies a The present paper reports the current trends of optical fiber humidity sensors. Novel optical unique structures, position alongside optical fiber sensors due to its relatively simple fabrication process and as well as recently studied materials, will be analyzed and commented on. Then, a high sensitivity to the surrounding medium refractive index (SMRI). Finally, concluding detailed analysis of OFHSs based on lossy mode resonances will be made. some This kind of sensorremarks about the sensing performance all these sensors be expounded. occupies a unique position of alongside optical fiberwill sensors due to its relatively simple fabrication process and high sensitivity to the surrounding medium refractive index (SMRI). Finally, some

2. Recent Trends in Optical Fiber Humidity Sensors concluding remarks about the sensing performance of all these sensors will be expounded. Here, a brief review of the most recently developed OFHSs will be presented. The OFHS have 2. Recent Trends in Optical Fiber Humidity Sensors been classified according to their working principle. The first group includes OFHS based on the Here, a brief review of the most recently developed OFHSs will be presented. The OFHS have optical absorption of materials, were theprinciple. first kind offirst OFHS developed. The next group been classified according towhich their working The group includes OFHS based on theincludes OFHS based fiber Bragg gratings which (FBG) were and long-period gratings (LPFG). Another possibility opticalon absorption of materials, the first kindfiber of OFHS developed. The next group includes OFHS based on fiber (FBG) and long-period Anotherkind of for the development of OFHS is Bragg basedgratings on interference, which canfiber be gratings divided(LPFG). into several possibilitysuch for the of Sagnac, OFHS is based on interference, which can and be divided several interferometers asdevelopment Fabry-Pérot, Mach-Zehnder, Michelson, modalinto interferometers. kind of interferometers such as Fabry-Pérot, Sagnac, Mach-Zehnder, Michelson, and modal The next category includes OFHS based on micro-tapers, micro-ring, and micro-knot resonators interferometers. The next category includes OFHS based on micro-tapers, micro-ring, and (MKR), micro-knot as well as resonators other sensors based on whispering galleries modes (WGM). Finally, OFHS based on (MKR), as well as other sensors based on whispering galleries modes (WGM). electromagnetic resonances, specifically lossy mode resonances (LMRs), be discussed. A scheme Finally, OFHS based on electromagnetic resonances, specifically lossy mode will resonances (LMRs), will of the proposed classification shown in Figure 1. is shown in Figure 1. be discussed. A scheme ofisthe proposed classification

Figure 1. Classification of optical fiber humidity sensors.

Figure 1. Classification of optical fiber humidity sensors.

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2.1. 2.1. Optical Optical Absorption Absorption Sensors Sensors These These sensors sensors are are based based on on the the interaction interaction of of the the evanescent evanescent field field with with the the coating coating used used as as the the sensitive material, providing changes of the transmitted optical power along the whole spectrum. sensitive material, providing changes of the transmitted optical power along the whole spectrum. For For this this kind kind of of sensor, sensor, plastic-cladding plastic-cladding silica silica (PCS) (PCS) optical optical fiber fiber or or plastic plastic optical optical fibers fibers (POF) (POF) are are commonly used, although there are other possibilities such as the side-polished optical fiber (D-shape). commonly used, although there are other possibilities such as the side-polished optical fiber Some of these optical fibersoptical present advantages as low fabrication [15], the possibility of (D-shape). Some of these fibers present such advantages such as lowcost fabrication cost [15], the measuring with a simplewith setup, and high reliability, they have disadvantages. The main possibility of measuring a simple setup, and highbut reliability, butsome they have some disadvantages. disadvantages are related to the method of measurement, which only detects changes on the The main disadvantages are related to the method of measurement, which only detects changes on transmitted optical power; these changes might be affected by undesired factors such as fluctuations the transmitted optical power; these changes might be affected by undesired factors such as in the light source. fluctuations in the light source. Most recent Most recent research research has has focused focused on on the the study study of of materials materials that that are are becoming becoming common common in in the the development are being development of of optical optical fiber fiber sensors. sensors. These These materials materials are being studied studied for for their their ability ability to to improve improve certain OFHS, such as response time or sensitivity. The studied materialsmaterials are tungsten certain characteristics characteristicsofof OFHS, such as response time or sensitivity. The studied are disulfide [16], reduced graphene oxide (rGO) [17], and zinc oxide [18]. tungsten disulfide [16], reduced graphene oxide (rGO) [17], and zinc oxide [18]. Tungsten beenbeen studied because the physical adsorption of the water molecules 2 ) has Tungstendisulfide disulfide(WS (WS 2) has studied because the physical adsorption of the water onto the WS layer is accompanied by a moderate degree of charge transfer, enabling fast response 2 molecules onto the WS2 layer is accompanied by a moderate degree of charge transfer, enabling fast times [16].times A D-shape fiber was coated with WS2 , with as canWS be2,seen in be Figure this 2, OFHS has response [16]. Aoptical D-shape optical fiber was coated as can seen2,inand Figure and this shown a sensitivity 0.1213 dB/%RH and a resolution 0.475%RH.ofThe rising time is about s and OFHS has shown aofsensitivity of 0.1213 dB/%RH and aofresolution 0.475%RH. The rising 1time is the recovery time is about 5 s. about 1 s and the recovery time is about 5 s.

Figure 2. 2. (a) tungsten disulfide coating andand (b) (b) SEM image of the Figure (a)D-shape D-shapeoptical opticalfiber fibercoated coatedwith with tungsten disulfide coating SEM image of surface of the coating at a magnification of 31270. Reprinted from [16] with permission from OSA the surface of the coating at a magnification of 31270. Reprinted from [16] with permission from Publishing. OSA Publishing.

The sensor developed with rGO has focused on achieving high sensitivity for high humidity The sensor developed with rGO has focused on achieving high sensitivity for high humidity values. It displays a linear response in the 70%–95%RH range, a sensitivity of 0.31 dB/%RH, and its values. It displays a linear response in the 70%–95%RH range, a sensitivity of 0.31 dB/%RH, and its response speed is faster than 0.13%RH/s. response speed is faster than 0.13%RH/s. On the other hand, there are certain materials that provide high sensitivity in extremely On the other hand, there are certain materials that provide high sensitivity in extremely low-humidity environments [19], which is very useful in applications such as lithium-ion battery low-humidity environments [19], which is very useful in applications such as lithium-ion battery manufacturing, semiconductor fabrication, archival storage and preservation, and the manufacturing, semiconductor fabrication, archival storage and preservation, and the pharmaceutical pharmaceutical industry. This OFHS was fabricated using a U-bent optical fiber coated with silica industry. This OFHS was fabricated using a U-bent optical fiber coated with silica film by sol–gel film by sol–gel process and doped with methylene blue. For RH ranging from 1.1% to 4.1%, the process and doped with methylene blue. For RH ranging from 1.1% to 4.1%, the sensor shows a sensor shows a linear relationship, with a sensitivity of 0.087 dB/%RH and a limit of detection of linear relationship, with a sensitivity of 0.087 dB/%RH and a limit of detection of 0.062%RH [19]. 0.062%RH [19]. The response and recovery time of the sensor is 20 s–3 min depending on the RH The response and recovery time of the sensor is 20 s–3 min depending on the RH variation. variation. With regard to novel structures, the use of 2-mm hydrogel spheres for humidity detection should With regard to novel structures, the use of 2-mm hydrogel spheres for humidity detection be mentioned [20]. Rather than coating the bare optical fiber core with a coating of even thickness, should be mentioned [20]. Rather than coating the bare optical fiber core with a coating of even the reported sensor exploits hydrogel spheres on the fiber core (see Figure 3). Nevertheless, the sensing thickness, the reported sensor exploits hydrogel spheres on the fiber core (see Figure 3). range of the current sensor is still very narrow, from 70% to 90%, due to the moisture absorption Nevertheless, the sensing range of the current sensor is still very narrow, from 70% to 90%, due to behavior of the hydrogel used. the moisture absorption behavior of the hydrogel used.

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Figure ofoffiber Figure3.3.Photo Photoofofa afabricated fabricatedsensor sensorwith withfive fivehydrogel hydrogelspheres, spheres,attached attachedonona asection section fibercore. core. (a) The whole sensor; (b) a detailed view of five spheres; and (c) an illustration of the light rays (a) The whole sensor; (b) a detailed view of five spheres; and (c) an illustration of the lightthat rays travel through a single hydrogel sphere withwith a diameter of 2ofmm andand a refractive index ofof1.45. that travel through a single hydrogel sphere a diameter 2 mm a refractive index 1.45. Reprinted from [20] with permission from Elsevier. Reprinted from [20] with permission from Elsevier.

2.2. Fiber Bragg Gratings 2.2. Fiber Bragg Gratings A Bragg grating is an optical structure that consists of a periodic perturbation of the refractive A Bragg grating is an optical structure that consists of a periodic perturbation of the refractive index of a waveguide. A FBG is formed by exposure of the core of the optical fiber to an intense index of a waveguide. A FBG is formed by exposure of the core of the optical fiber to an intense optical interference pattern of ultraviolet light [21]. The exposure produces a permanent increase in optical interference pattern of ultraviolet light [21]. The exposure produces a permanent increase the refractive index of the core of the fiber, creating a fixed index modulation according to the in the refractive index of the core of the fiber, creating a fixed index modulation according to the exposure pattern [22]. When light is launched into the optical fiber, a small amount of light is exposure pattern [22]. When light is launched into the optical fiber, a small amount of light is reflected reflected at each periodic refraction change. All the reflected light signals combine coherently into at each periodic refraction change. All the reflected light signals combine coherently into one large one large reflection at a particular wavelength when the grating period is approximately half of the reflection at a particular wavelength when the grating period is approximately half of the input light’s input light’s wavelength [22]. The grating does not affect light propagating at wavelengths different wavelength [22]. The grating does not affect light propagating at wavelengths different from the Bragg from the Bragg wavelength, which satisfies Equation (1): wavelength, which satisfies Equation (1): =2 , (1) λ B = 2nΛ (1) where λB is the Bragg wavelength, n is the effective refractive index of the grating in the fiber core, and  isλBthe grating period. Due to nthe mask dimensions and index the characteristics where is the Bragg wavelength, is the effective refractive of the gratingofinstandard the fiber communications single mode optical fibers (SMF), Bragg wavelength is usually located at the core, and Λ is the grating period. Due to the mask dimensions and the characteristics of standard infrared range. More recent research the use of wavelength micro-structured [23] located plastic optical fibers communications single mode optical involves fibers (SMF), Bragg is usually at the infrared (POFs) or SMF with reduced core diameter to be able to work in the visible-near-infrared range [24].or range. More recent research involves the use of micro-structured [23] plastic optical fibers (POFs) materials are commonly to develop OFHSs using FBGs because of the strain SMFHygroscopic with reduced core diameter to be ableused to work in the visible-near-infrared range [24]. they can apply to the FBG when swelling [25]. A large number of research papers [25–33] have Hygroscopic materials are commonly used to develop OFHSs using FBGs because of the been strain published recently dealing with this optical structure and, despite its low dynamical range they can apply to the FBG when swelling [25]. A large number of research papers [25–33] and have sensitivity, FBGsrecently still attract greatwith interest due tostructure their inherent multiplexing capability and high been published dealing this optical and, despite its low dynamical range and quality [33]. Moreover, etching the cladding of a FBG and coating it with a sensitive layer enhances sensitivity, FBGs still attract great interest due to their inherent multiplexing capability and high the sensitivity of this optical structure [25]. of a FBG and coating it with a sensitive layer enhances the quality [33]. Moreover, etching the cladding Severalofpolymeric materials sensitivity this optical structurehave [25]. been coated onto a FBG and tested for humidity sensing purposes such as polyimide [4,31], di-ureasil or onto poly(methyl methacrylate) Several polymeric materials have been ,[34] coated a FBG and tested for (PMMA) humidity [25,28], sensing among others. The sensitivity of FBGs developed in [4], coated with polyimide by dip-coating, range purposes such as polyimide [4,31], di-ureasil, [34] or poly(methyl methacrylate) (PMMA) [25,28], from 1.4 to 5.6 pm/%RH depending the developed thickness ofinthe coating. the same has among others. The sensitivity of on FBGs [4], coatedHowever, with polyimide bymaterial dip-coating, provided greater sensitivities (13.6 pm/%RH) when coated by another method (in situ range from 1.4 to 5.6 pm/%RH depending on the thickness of the coating. However, the same imidization) According to the results of(13.6 both pm/%RH) papers, thewhen sensitivity increases withmethod the thickness material has[31]. provided greater sensitivities coated by another (in situ ofimidization) the coating.[31]. When compared with polymer-based solutions, the proposed di-ureasil layer shows According to the results of both papers, the sensitivity increases with the thickness an sensitivity of 22.2 pm/%RH [34]. ofenhanced the coating. When compared with polymer-based solutions, the proposed di-ureasil layer shows an Graphene oxide [32] andpm/%RH carbon nanotubes (CNT) [33] have also been tested as the sensitive enhanced sensitivity of 22.2 [34]. layer for FBG-based CNTs, coated onto an[33] etched theas highest sensitivity Graphene oxideRH [32]sensors. and carbon nanotubes (CNT) haveFBG, also provide been tested the sensitive layer found for conventional FBGs OFHS, which is 31 pm/%RH [33]. A representation of this optical for FBG-based RH sensors. CNTs, coated onto an etched FBG, provide the highest sensitivity found for device can be seen in Figure 4. conventional FBGs OFHS, which is 31 pm/%RH [33]. A representation of this optical device can be seen in Figure 4.

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Figure 4. 4. Schematic Schematic of of an an optical optical fiber fiber humidity humidity sensor sensor using using fiber fiber Bragg Figure Bragg gratings. gratings.

Concerning the inherent to to FBGs, it has been compensated for the cross crossthermal thermalsensitivity, sensitivity,which whichis is inherent FBGs, it has been compensated in [30] inscribing the grating a High-Birefringent (Hi-Bi) optical fiber. optical Due to the birefringence of for in by [30] by inscribing the on grating on a High-Birefringent (Hi-Bi) fiber. Due to the the fiber, the FBG exhibits reflectivity spectrum dominated by two separated wavelengths, birefringence of the fiber, athe FBG exhibits a reflectivity spectrum dominatedBragg by two separated whose separation depends on the depends temperature. Bragg wavelengths, whose only separation only on the temperature. Finally, recent research has focused on improving the sensitivity sensitivity by using PMMA-based micro-structured polymer optical fiber Bragg gratings (POFBG) This structure structure has been Bragg gratings (POFBG) [27]. [27]. This demonstrated totohave a superior response with very low hysteresis, sensitivity (35 pm/%RH), demonstrated have a superior response with very low improved hysteresis, improved sensitivity and pm/%RH), an increasedand stable if it is annealed at high (90%RH). Some results (35 an operation increasedtemperature stable operation temperature if ithumidity is annealed at high humidity related to this optical device are shown in Figure 5. (90%RH). Some results related to this optical device are shown in Figure 5.

Figure 5. (a) Measured humidity response at 25 ◦ C of PMMA mPOFBG annealed up to 90%RH versus time and humidity; (b) Microscope image of the end facet of PMMA mPOF; (c) Corresponding Figure 5. response (a) Measured response at 25 °C of mPOFBG annealed from up to[27] 90%RH stabilized of thehumidity PMMA mPOFBGs annealed upPMMA to 90% and 10%. Reprinted with versus timefrom andOSA humidity. (b) Microscope image of the end facet of PMMA mPOF. (c) permission Publishing. Corresponding stabilized response of the PMMA mPOFBGs annealed up to 90% and 10%. Reprinted from [27] with permission 2.3. Long-Period Fiber Gratingsfrom OSA Publishing.

Long-period fiber gratings (LPG) consist of a periodic modification of the refractive index of the 2.3. Long-Period Fiber Gratings core of a single-mode optical fiber (SMF). In opposition to FBG, which have a sub-micron period and Long-period fiber gratings (LPG) consist of of a periodic modification of the refractive index of the couple light from the forward-propagating mode the optical fiber to a backward counter-propagating core of a single-mode optical fiber (SMF). In opposition to FBG, which have a sub-micron anda mode, LPGs have a period typically in the range of 100 µm to 1 mm. This provokes period in LPGs couple light from the forward-propagating mode of the optical fiber to a backward coupling of light between the guided core mode and various co-propagating cladding modes [35,36]. counter-propagating mode, LPGsofhave a periodbands typically in the range of transmission 100 µm to 1 mm. This This coupling produces a series attenuation in the optical fiber spectrum, provokes in LPGs a coupling of light between the guided core mode and various co-propagating each one centered at a different resonant wavelength. A portion of the electromagnetic field of cladding modes [35,36]. This coupling produces medium a series of in the optical the cladding modes penetrates the surrounding in attenuation the form ofbands an evanescent wavefiber [37]. transmission spectrum, each one centered at a different resonant wavelength. A portion of the Although LPGs were initially developed as rejection-band filters [38], they also present interesting electromagnetic field of the cladding modes penetrates the surrounding medium in the form of an characteristics for sensing. evanescent Although LPGs initially developed as rejection-band [38], they Severalwave kinds[37]. of materials have beenwere explored as coatings for the development offilters LPG-based RH also present interesting characteristics for sensing. sensors, including polymers [39,40], hydrogels [41], gelatin [42], cobalt-chloride-based materials [43], Several kinds of materials have been explored as coatings for the development of LPG-based and SiO 2 nanospheres [44]. Most recent research has focused on studying the performance of this sensor ◦ C) and RH sensors, including hydrogels(−[41], gelatin [42], cobalt-chloride-based for low values of RH (0.4%polymers to 36%RH)[39,40], and temperature 10 to 20 subjected to radiation [37]. materials [43], and SiO 2 nanospheres [44]. Most recent research has focused on studying the For this purpose, the material chosen for the coating was titanium dioxide and the sensitivity obtained performance of this sensor for low values of RH (0.4% to 36%RH) and temperature (−10 to 20°C) and

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subjected radiation [37]. this purpose, material chosen coating was titanium subjected to to radiation [37]. ForFor this purpose, thethe material chosen forfor thethe coating was titanium was 1.4 nm/%RH at low RH values, which is a high sensitivity for a LPG. Measured spectra of device dioxide and sensitivity obtained was nm/%RH low values, which a high sensitivity dioxide and thethe sensitivity obtained was 1.41.4 nm/%RH at at low RHRH values, which is ais high sensitivity developed in [37] are shown in Figure 6. a LPG. Measured spectra device developed [37] shown Figure forfor a LPG. Measured spectra of of device developed in in [37] areare shown in in Figure 6. 6.

Figure 6.Transmittance Transmittance spectra LPG acquired different RH values. Reprinted from [37] with Figure 6. Transmittance spectra ofof LPG acquired at at different RH values. Reprinted from [37] with Figure 6. spectra of LPG acquired at different RH values. Reprinted from [37] with permission from OSA Publishing. permissionfrom from OSA OSA Publishing. Publishing. permission

A novel method the fabrication of LPGs (schematized Figure 7) been developed A method forfor thefabrication fabrication LPGs (schematized in in Figure hashas been developed A novel novel method for the of of LPGs (schematized in Figure 7) has7)been developed in [45]. in [45]. It combines the fiber side-polishing and fiber etching methods to create air gaps on in [45]. It combines the fiber side-polishing and fiber etching methods to create gaps on thethe It combines the fiber side-polishing and fiber etching methods to create air gaps on theair polished region polished region that reach the core of the optical fiber. After coating this device with calcium polished that reach the core the coating optical this fiber. Afterwith coating thischloride, device with calciuma that reachregion the core of the optical fiber. of After device calcium we obtained chloride, we obtained a sensitivity as high as 1.36 nm/%RH in the range 55%–90%RH. chloride, weasobtained a sensitivity asin high 1.3655%–90%RH. nm/%RH in the range 55%–90%RH. sensitivity high as 1.36 nm/%RH the as range

Figure 7. Fabrication process the gap LPG. Reprinted from [45] with permission from Optical Figure 7. process of the air gap LPG. Reprinted from [45] with permission from Optical Figure 7. Fabrication Fabrication process ofof the airair gap LPG. Reprinted from [45] with permission from Optical Society of Japan. Society of Society ofJapan. Japan.

Another novel approach for the development of OFHS based on LPG was studied in [46]. First, Another Anothernovel novelapproach approachfor forthe thedevelopment developmentof ofOFHS OFHSbased basedon onLPG LPGwas wasstudied studiedin in[46]. [46].First, First, a LPG was coated by layer-by-layer nano-assembly (LbL) method with PAH/PAA. Then,byby aa LPG LPG was wascoated coated layer-by-layer nano-assembly (LbL) method with PAH/PAA. Then, by by layer-by-layer nano-assembly (LbL) method with PAH/PAA. Then, by chemically chemically removing half LPG coating, main attenuation band was split into two different chemically removing half of of thethe LPG thethe main attenuation band into two different removing half of the LPG coating, the coating, main attenuation band was split intowas twosplit different contributions. contributions. The coated LPG contribution remained and a second band appeared because contributions. The coated LPG contribution andappeared a secondbecause band appeared becauseprocess. of of thethe The coated LPG contribution remained and aremained second band of the removing removing process. As was expected, this new band corresponds to the half-uncoated LPG area. removing process.this As new was band expected, this new corresponds LPG to the half-uncoated LPG area. As was expected, corresponds to band the half-uncoated area. When this semi-coated When this semi-coated LPG was also exposed and temperature tests, the two new attenuation When this semi-coated LPG exposed to to RHRH and temperature tests, the two new attenuation LPG was also exposed to RHwas andalso temperature tests, the two new attenuation bands presented different bands presented different behaviors for humidity and temperature. The dual-wavelength-based bands presented differentand behaviors for humidity and temperature. Themeasurement dual-wavelength-based behaviors for humidity temperature. The dual-wavelength-based provided a measurement provided a simultaneous monitoring of and temperature, with sensitivity ratios measurement a simultaneous monitoring ofwith RHRH and temperature, sensitivity ratios of of simultaneousprovided monitoring of RH and temperature, sensitivity ratioswith of 63.23 pm/%RH and 63.23 pm/%RH and 410.66 pm/°C for the attenuation band corresponding to the coated contribution, 63.23 andthe 410.66 pm/°C for thecorresponding attenuation band corresponding to the coated contribution, 410.66pm/%RH pm/◦ C for attenuation band to the coated contribution, and 55.22 pm/%RH and 55.22 pm/%RH and 405.09 pm/°C for the attenuation band corresponding to the uncoated and 55.22 pm/%RH and 405.09 pm/°C for the attenuation band corresponding to the uncoated ◦ and 405.09 pm/ C for the attenuation band corresponding to the uncoated grating. grating. grating.

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2.4. Modal Interferometers 2.4. Modal Interferometers Fiber optics technology offers many degrees of freedom for the generation of modal Fiber optics technology offers many degrees of freedom for the generation of modal interferometers. Therefore, several structures have been studied, all of them providing advantages interferometers. Therefore, several structures have been studied, all of them providing advantages such as stability, compactness, small size, lightness, etc. [47–49]. The interferometric phase difference such as stability, compactness, small size, lightness, etc. [47–49]. The interferometric phase difference is built up by considering the difference in the effective refractive indices of different fiber modes [50]. is built up by considering the difference in the effective refractive indices of different fiber There are [50]. alternative topological configurations such as Michelson interferometers, modes There are alternative topological configurations such or as Mach-Zehnder Michelson or Mach-Zehnder which can be implemented of splicing different types of fibers in a hybrid structure [51] or interferometers, which canby bemeans implemented by means of splicing different types of fibers in a hybrid bystructure other methods [52]. [51] or by other methods [52]. 2.4.1. Photonic 2.4.1. PhotonicCrystal CrystalFibers Fibers Photonic fibers(PCF) (PCF) be included as Mach-Zehnder interferometers (MZI), Photoniccrystal crystal fibers cancan be included as Mach-Zehnder interferometers (MZI), although although they can also be used as Michelson or Sagnac interferometers [53]. These fibers they can also be used as Michelson or Sagnac interferometers [53]. These fibers are characterized by aare characterized by aofcomplex pattern of microscopic air-holes theruns transverse that runs . all complex pattern microscopic air-holes in the transverse planein that all along plane the fiber [53–55] along theare fiber [53–55] .great PCFsinterest are attracting interest due to the different alternatives to construct PCFs attracting due togreat the different alternatives to construct all-fiber modal all-fiber modal interferometers such asPCF tapered PCF or hybrid structures [53]. key The key element these interferometers such as tapered or hybrid structures [53]. The element in in these interferometers a microscopic region which voids PCF fully collapsed [53]. The interferometers is is a microscopic region in in which thethe voids of of thethe PCF areare fully collapsed [53]. The two two collapsed interfaces between PCFSMF andsegments SMF segments produce the excitation and recombination collapsed interfaces between PCF and produce the excitation and recombination of core ofcladding core andmodes cladding modes [56]. of A the scheme of the opticaland structure and the obtained transmitted and [56]. A scheme optical structure the obtained transmitted spectrum can spectrum can be observed in Figure 8. be observed in Figure 8.

Figure 8. (a): Schematic representation of photonic the photonic crystal interferometer; (2) cross Figure 8. (a): (1) (1) Schematic representation of the crystal fiberfiber interferometer; (2) cross section section of the PCF used to build up the interferometer; (3) micrograph of the PCF-SMF junction of the PCF used to build up the interferometer; (3) micrograph of the PCF-SMF junction showing theregion; collapsed (b) Transmission spectrum of the PCF-I. Reprinted with theshowing collapsed (b) region; Transmission spectrum of the PCF-I. Reprinted from [56] from with [56] permission permission from Elsevier. from Elsevier.

In [57] it is reported that a coating is not needed with these interferometers to obtain a In [57] it is reported that a coating is not needed with these interferometers to obtain a temperature-insensitive OFHS. Coating the PCF with hygroscopic materials such as agarose [58,59], temperature-insensitive OFHS. Coating the PCF with hygroscopic materials such as agarose [58,59], polyvinyl alcohol (PVA) [60], or poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) polyvinyl alcohol (PVA) [60], or poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) [56] [56] improves the sensitivity of these interferometers, reaching 2.35 nm/%RH in the range improves the sensitivity thesematerials interferometers, reaching 2.35 nm/%RH in the range 75%–95%RH. 75%–95%RH. However,ofthese present non-linear behavior, although linearization can be However, materials present non-linear behavior, achievedthese by using a digital processing algorithm [56].although linearization can be achieved by using a digital processing algorithm [56]. 2.4.2. Tapered Optical Fibers 2.4.2. Tapered Optical Fibers As previously mentioned, MZI require two different optical paths for generating interference. Asoptical previously MZI require two different optical paths generating One path ismentioned, the core of the optical fiber, while the other optical pathfor is the cladding, interference. where the One optical path is the core of the optical fiber, while the other optical path is the cladding, wheretothe cladding modes are guided through. For this reason, it is necessary to allow the cladding modes cladding modes are guided through. For this reason, it is necessary to allow the cladding modes to propagate through the cladding to obtain a modal interferometer based on MZI. Several approaches propagate through thefor cladding to obtain a modal interferometer based on MZI. Several approaches have been followed this purpose. have been forwell-known this purpose. Onefollowed of the most structures is the non-adiabatic tapered optical fibers (NATOF). In NATOFs fundamental modestructures is coupled to higher order modes, generate modal One of the the most well-known is the non-adiabatic taperedwhich optical fibers (NATOF). In NATOFs the fundamental mode is coupled to higher order modes, which generate modal

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interference and, In aa tapered tapered SMF, SMF, the Sensors 2017, 17, 893 therefore, 8 ofcentral 23 interference and, therefore, an an oscillatory oscillatory optical optical power power output output [61–63]. [61–63]. In the central region of the taper acts as a multimode fiber and the light is now guided through the cladding of region of the taper acts as a multimode fiber and the light is now guided through the cladding of the the interference and, therefore, an oscillatory optical power In a tapered SMF, thecoupling central fiber, which makes the surrounding surrounding medium play theoutput roleofof[61–63]. thenew new cladding [61]. This coupling fiber, which makes the medium play the role the cladding [61]. This of region of in thethe taper actswaist as a multimode fiber and thesensitive light is now guided through the cladding of the of modes taper makes the taper very to surrounding medium refractive index modes in the taper waist makes the taper very sensitive to surrounding medium refractive index fiber, which makes the surrounding medium play theadding role of the appropriate new cladding [61]. ThisThe coupling of (SMRI) (SMRI) changes, changes, allowing allowing its its use use as as an an OFHS OFHS by by adding an an appropriate coating. coating. The silica silica taper taper modes in the taper waist makes the taper very sensitive to surrounding medium refractive index developed withwith a waist diameter of 3.8 µm, RH sensitivityRH of 97.76 pm/%RH developed inin[64], [64], a waist diameter of has 3.8demonstrated µm, has demonstrated sensitivity of (SMRI) changes, allowing its use as an − OFHS by adding an appropriate coating. The silica taper In a ◦ C, without with a cross thermal sensitivity of only 0.048%RH/ any extra functional coatings. 97.76 pm/%RH with a cross thermal sensitivity of only −0.048%RH/°C, without any extra functional developed in [64], with a waist diameter of 3.8 µm, has demonstrated RH sensitivity of similar way, micro-wires in [65,66] do not requiredo additional coatings for obtaining an coatings. In athe similar way, theutilized micro-wires utilized in [65,66] not require additional coatings for 97.76 pm/%RH with a cross thermal sensitivity of only −0.048%RH/°C, without any extra functional OFHS, providing sensitivities of 114.7pm/%RH in the 30%–90%RH range [65] and 0.14 rad/%RH in obtaining ana OFHS, providing sensitivities of 114.7pm/%RH in require the 30%–90%RH range [65] coatings. In similar way, the micro-wires utilized in [65,66] do not additional coatings for and the 20%–70%RH range [66]. 0.14 rad/%RH in the 20%–70%RH range [66].of 114.7pm/%RH in the 30%–90%RH range [65] and obtaining an OFHS, providing sensitivities Double in-line adiabatically tapered optical Double in-line adiabatically tapered optical fibers fibers (see (see Figure Figure 9) 9) can can also also be be considered considered aa modal modal 0.14 rad/%RH in the 20%–70%RH range [66]. MZI [67]. The first tapered region diffracts the fundamental mode and consequently allows MZI [67]. The first adiabatically tapered region diffracts fundamental and be consequently the Double in-line tapered opticalthe fibers (see Figure mode 9) can also considered aallows modal the cladding modes to become excited. The differences between the effective refractive indices of cladding to become The differences betweenmode the effective refractive allows indicesthe of the the MZI [67].modes The first tapered excited. region diffracts the fundamental and consequently core and in shifting. Increasing the RH affects affects the effective effective RIthe of core and cladding cladding modes result in phase phase shifting. Increasing RH the RI of the the cladding modes tomodes becomeresult excited. The differences between thethe effective refractive indices of cladding modes, with the RI of the core mode remaining unmodified. Based on that working principle, cladding theresult RI ofin the core modeIncreasing remainingthe unmodified. Based on that core and modes, claddingwith modes phase shifting. RH affects the effective RI ofworking the sensitivities of 20 pm/%RH were without any sensitive layer. main is its cladding modes, with the RI of obtained, the corewere mode remaining unmodified. Based ondisadvantage that working principle, sensitivities of 20 pm/%RH obtained, without anyThe sensitive layer. The main high temperature sensitivity. principle, sensitivities of 20 pm/%RH were obtained, without any sensitive layer. The main disadvantage is its high temperature sensitivity. disadvantage is its high temperature sensitivity.

Figure 9.9. Scheme of a double in-line tapered conforming a modal Mach-Zehnder Figure Scheme of a double in-line optical taperedfiber optical fiber to conforming to a modal Figure 9. Scheme of a double in-line tapered optical fiber conforming to a modal Mach-Zehnder interferometer. Mach-Zehnder interferometer. interferometer.

2.4.3. Modal Interferometers Obtained by Different Combinations of Fibers 2.4.3. Obtainedby byDifferent DifferentCombinations Combinations Fibers 2.4.3.Modal Modal Interferometers Interferometers Obtained of of Fibers Another method to obtain a MZI is by splicing a standard SMF to a thin-core optical fiber, as Another Anothermethod method to to obtain obtain aa MZI MZI isisby bysplicing splicing aastandard standard SMF SMF to to aathin-core thin-core optical optical fiber, fiber, as aswas was studied in [29]. A fraction of the light that was being guided through the core of the SMF is was studied in [29]. A fraction of the light that was being guided through the core of the SMF is studied in [29]. A fraction of the light that was being guided through the core of the SMF is guided guided through the cladding of the thin-core fiber. Therefore, light has two different optical paths guided through the cladding of the thin-core fiber. Therefore, lighttwo hasdifferent two different optical through the cladding of the thin-core fiber. Therefore, light has optical pathspaths and the and the interferometer is generated. Furthermore, a FBG was written onto the thin-core section, and the interferometer is generated. Furthermore, a FBG was onto written the thin-core interferometer is generated. Furthermore, a FBG was written theonto thin-core section, section, which was which was also coated by Layer-by-Layer Layer-by-Layernano-assembly nano-assembly (LbL), so simultaneous measurement which was also coated by (LbL), so simultaneous measurement of of also coated by Layer-by-Layer nano-assembly (LbL), so simultaneous measurement of temperature temperature and RH can be be accomplished. accomplished.The Theestimated estimated resolution of the sensor is 0.78%RH. temperature and RH can resolution of the sensor is 0.78%RH. and RH can be accomplished. The estimated resolution of the sensor is 0.78%RH. More approaches have been beenfollowed followedininthe thelast lastfew few years to advance modal interferometry. More approaches have years to advance modal interferometry. More approaches have been followed in the last few years to advance modal interferometry. InIn[51], the MZI is constructed of two waist-enlarged tapers and other explored options [51], the MZI constructed of two waist-enlarged tapers and other explored options are are one one In [51], the MZI is constructed of two waist-enlarged tapers and other explored options are one waist-enlarged followedby byan anoffset offset[68]. [68].These These optical structures provide a good interference waist-enlarged taper followed optical structures provide a good interference waist-enlarged taper followed by an offset [68]. These optical structures provide a good interference pattern and and changes changes on dB/%RH [51]. Recently developed pattern on the the transmitted transmittedoptical opticalpower powerofof0.35 0.35 dB/%RH [51]. Recently developed pattern and changes on the transmitted optical power of 0.35 dB/%RH [51]. Recently developed Michelson interferometers interferometers are with thethe difference of light going Michelson are quite quite similar similartotoprevious previousMZI, MZI, with difference of light going Michelson interferometers are quite similar to previous MZI, with the difference of light going through through each optical path twice, as schematized in Figure 10. Therefore, similar structures were used through each optical path twice, as schematized in Figure 10. Therefore, similar structures were used each optical path twice, as schematized in Figure 10. Therefore, similar structures were used for the forthe thedevelopment development of sensitivities of 135 pm/%RH [69].[69]. for of these these interferometers, interferometers,which whichprovides provides sensitivities of 135 pm/%RH development of these interferometers, which provides sensitivities of 135 pm/%RH [69].

Figure 10. 10. Schematic interferometer-based humidity sensor. Figure Schematic diagram diagramofofthe theproposed proposedMichelson Michelson interferometer-based humidity sensor. Figure 10. from Schematic diagram of the proposed Michelson interferometer-based humidity sensor. Reprinted [69] with permission from Elsevier. Reprinted from [69] with permission from Elsevier. Reprinted from [69] with permission from Elsevier.

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The single mode-multimode-single mode (SMS) fiber structure has found many applications owing to single its unique spectral characteristics [70].(SMS) The physical principlehas of found this interferometer is that The mode-multimode-single mode fiber structure many applications light transmitted through the fundamental [70]. mode of physical the SMFprinciple is coupled several modes into the owing to its unique spectral characteristics The of to this interferometer is that MMF section and re-coupled to the fundamental mode of the SMF at the end of the MMF light transmitted through the fundamental mode of the SMF is coupled to several modes into the segment [70].and A SMS optical coated withmode PVAofprovides a the sensitivity of MMF 90 pm/%RH MMF section re-coupled to fiber the fundamental the SMF at end of the segment [71], [70]. whereas by combining SMS optical taperingofmethod, and a[71], coating consisting of SiO2 A SMS optical fiber coated with PVA structure, provides athe sensitivity 90 pm/%RH whereas by combining nanoparticles, the OFHS developed in [72] shown in Figure 112 nanoparticles, offers a sensitivity of SMS optical structure, the tapering method, and aand coating consisting of SiO the OFHS 584.2 pm/%RH. developed in [72] and shown in Figure 11 offers a sensitivity of 584.2 pm/%RH.

Figure 11. (a) (a) Schematic Schematic diagram diagram of of the the proposed proposed Michelson Michelson interferometer-based interferometer-based humidity Figure 11. humidity sensor. sensor; (b) Transmission Transmission spectral spectral responses responses of of dip dip BB under under different different environmental environmental RH RH levels. levels. Reprinted Reprinted (b) from [72] with permission from from Elsevier. Elsevier.

2.4.4. Complex Coated onto onto Modal Modal Interferometers Interferometers 2.4.4. Complex Refractive Refractive Index Index Materials Materials Coated An interesting study the performance performance of An interesting study is is the of modal modal interferometers, interferometers, such such as as SMS SMS structures, structures, when they are coated with a material having a complex RI. As has been shown, the sensitivity to the when they are coated with a material having a complex RI. As has been shown, the sensitivity external RI can be increased by coating the multimode segment [70,73]. The studies performed to the external RI can be increased by coating the multimode segment [70,73]. The studies performed in [74,75] [74,75] focused focused on on coatings coatings with with materials materials having complex RI and on on the the optimization optimization of the in having aa complex RI and of the coating parameters for achieving the maximum sensitivity. The most important conclusion extracted coating parameters for achieving the maximum sensitivity. The most important conclusion extracted from [74] [74] is is that that there there is is an an optimum optimum thickness thickness of of the the coating coating that that leads leads to to aa higher higher sensitivity sensitivity to to the the from thin-film refractive refractive index, index, thin-film thin-film thickness, thickness, and and surrounding surrounding medium medium refractive refractive index. index. When When the the thin-film thickness of the coating approaches this optimum value, there is an attenuation of the observable thickness of the coating approaches this optimum value, there is an attenuation of the observable interference pattern. pattern.This This is the so-called fading This region fadingisregion by the interference is the so-called fading region.region. This fading caused is by caused the attenuation attenuation bands byhaving the material having a complex RI. These are bands produced by produced the material a complex RI. These attenuation bandsattenuation are obtainedbands at certain obtained at certain thicknesses of the coating and they experience a wavelength shift as the thickness thicknesses of the coating and they experience a wavelength shift as the thickness increases. That is increases. is the reason forpattern the interference pattern and appearing similar the reasonThat for the interference disappearing anddisappearing appearing again. A similaragain. study A has been study has been performed with photonic crystal fibers (PCF) [56], which behave following the same performed with photonic crystal fibers (PCF) [56], which behave following the same trend. Therefore, trend. the fade region indicates thickness obtaining the highest the fadeTherefore, region indicates the optimum thicknessthe for optimum obtaining the highest for sensitivity. sensitivity. 2.5. Fabry-Pérot Cavities 2.5. Fabry-Pérot Cavities One optical phenomenon extensively used for the development of optical fiber sensors and One optical phenomenon used for the development fiber and specifically for OFHSs fabricationextensively is Fabry-Pérot interferometer (FPI). FPIsof areoptical based on thesensors interference specifically for OFHSs fabrication is Fabry-Pérot interferometer (FPI). FPIs are based on caused by multiple reflections of light between two reflecting surfaces. Transmitted beams, beingthe in interference caused by multiple reflections of light between two reflecting surfaces. phase, generate constructive interference, whereas whether the transmitted beams are outTransmitted of phase or beams, being in interference phase, generate constructive interference,towhereas whetherminimum. the transmitted beams not, destructive occurs and this corresponds a transmission Constructive are out of phase or not, destructive interference occurs and this corresponds to a transmission interference corresponds to a high-transmission peak of the etalon. Whether the multiply reflected minimum. Constructive to a high-transmission peak of the etalon. Whether beams are in phase or notinterference depends oncorresponds the wavelength of the light, the angle at which the light travels the multiply reflected beams are in phase or not depends on the wavelength of the light, the angle at through the etalon, the thickness of the etalon, and the refractive index of the material between the which the surfaces light travels the etalon, theonly thickness ofdeveloped the etalon, by and the refractive reflecting [76].through This section focuses on FPIs coating the endindex facet of the materialfiber between optical [77]. the reflecting surfaces [76]. This section focuses only on FPIs developed by coating the end facet FPIs of thethe optical fiber [77]. In such two semi-reflective surfaces are generated by the fiber–coating interface and Incoating-air such FPIs the two semi-reflective are oxides generated bybeen the fiber–coating interface andthis by by the interface. Transparent surfaces conducting have proven a good choice for the coating-air Transparent conducting oxides have been proven a good choiceThe forgood this optical structure,interface. due to their optical constants and especially for humidity sensing purposes. optical structure, dueoxides to their constants and especially forelectrostatic humidity sensing purposes. performance of metal asoptical humidity transducers is due to the attraction betweenThe the

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good performance of metal oxides as humidity transducers is due to the electrostatic attraction oxygen of the water molecule and the cationic side of the metal oxide surface, caused by the polar between the oxygen of the water molecule and the cationic side of the metal oxide surface, caused by nature of the water molecule [78]. the polar nature of the water molecule [78]. Different materials have been studied, such as tin dioxide coated onto the end facet of a SMF [77], Different materials have been studied, such as tin dioxide coated onto the end facet of a which presents a large dynamical range (90 nm), low response time, and a sensitivity of 1.27 nm/%RH. SMF [77], which presents a large dynamical range (90 nm), low response time, and a sensitivity of The optical response of tin dioxide to of RH high linearity low hysteresis, making it a good 1.27 nm/%RH. The optical response tinshows dioxide to RH showsand high linearity and low hysteresis, option foritthe development Experimental for this results opticalfor device are depicted making a good option for of theOFHSs. development of OFHSs.results Experimental this optical device in Figure 12. are depicted in Figure 12.

Figure12. 12.(a) (a)Evolution Evolutionof of the the reflected reflected spectra RH. (c)(c) Reflected Figure spectra for for(b) (b)several severalcycles cyclesofofchanging changing RH. Reflected spectra for different values of RH and (d) wavelength of interference plotted as a function of RH. spectra for different values of RH and (d) wavelength of interference plotted as a function of RH. Reprinted from [77] with permission of Springer International Publishing. Reprinted from [77] with permission of Springer International Publishing.

Other humidity-sensitive materials used in FPIs include semiconductors [77,79], ceramics [80], Other humidity-sensitive materials usedhas in been FPIs studied includeand semiconductors ceramics [80], polymers [81,82], etc. Porous anodic alumina a sensitivity of[77,79], 0.31 nm/%RH was polymers etc. Porous anodic alumina been studied and awhen sensitivity of with 0.31 nm/%RH achieved[81,82], [80]. An important factor that must has be taken into account working porous was achieved An important factor that must be takenofinto account when witha porous materials is [80]. the required time for complete desorption water, which can working range from few materials thetorequired time[80], for complete desorption which can range a few seconds [82] secondsis[82] 22 minutes depending on the sizeofofwater, the pore, among other from factors. A different is on followed FPIamong is generated by a water-swelling material. to 22 minutes [80], approach depending the sizewhen of thethe pore, other factors. Previous FPIs were based on changes inwhen the RIthe of the due toby theawater absorbed into the A different approach is followed FPImaterial is generated water-swelling material. pores [80] oxide [77,79]. Water-swelling theirinto Previous FPIsand/or were onto basedthe onmetal changes in surface the RI of the material due to materials the waterchange absorbed dimensions theyonto are subjected changes of RH.[77,79]. Therefore, the length of the etalon ischange modified the pores [80]when and/or the metaltooxide surface Water-swelling materials their and subsequently the interference condition changes. This is the working principle of FPI dimensions when they are subjected to changes of RH. Therefore, the length of the etalon isthe modified developed in [81], which is developed with a 10-µm Nafion film principle and hasofathe sensitivity of and subsequently the interference condition changes. This is the working FPI developed 3.5 nm/%RH. This is the highest sensitivity found in this review. in [81], which is developed with a 10-µm Nafion film and has a sensitivity of 3.5 nm/%RH. This is the

highest sensitivity found in this review. 2.6. Sagnac Interferometers 2.6. Sagnac Interferometers The most well-known application for Sagnac interferometers is as a gyroscope [83], but they have also been widely studied and applied in other sensor applications [84]. Sagnac interferometers The most well-known application for Sagnac interferometers is as a gyroscope [83], but they use a coherent monochromatic light source. Monochromatic light makes interfering behavior more have also been widely studied and applied in other sensor applications [84]. Sagnac interferometers predictable and coherence is required for phase shift detection [85]. The laser beam is split into two use a coherent monochromatic light source. Monochromatic light makes interfering behavior more different beams forced to follow a single path but in opposite directions. After rounding the enclosed predictable and coherence is required for phase shift detection [85]. The laser beam is split into two area, the two beams are recombined in the splitter and the phase shift becomes an optical power different beams forced to follow a single path but in opposite directions. After rounding the enclosed area, the two beams are recombined in the splitter and the phase shift becomes an optical power output

onto a chemical etched polarization maintaining optical fiber (PMF), which forms one of the arms of the interferometer. The other arm is formed by an unmodified PMF. Therefore, interference will happen owing to the relative-phase difference introduced to the guided modes by the PMFs. The obtained attenuation bands, with attenuations of 20 and 25 dB, have a FWHM of 11.8 and 5.2 nm, respectively. A sensitivity of 111.5 pm/%RH was achieved within the humidity range 20–80%RH Sensors 2017, 17, 893 11 of 23 with a response time of about six seconds. Moreover, the sensitivity to temperature is only 7.2 pm/°C. Greater sensitivities were obtained in [87]. fabrication of this Sagnac variation. Two different approaches have recently beenThe proposed [86]. Both of them use ainterferometer, similar setup represented Figure 13, requires a morelight complex of theanalyzer sensitive(OSA). optical (see Figure 13),in which includes a broadband sourcesetup (BBS)for andthe anfabrication optical spectrum fiber, which is a Hi-Bi optical fiber that consists of an elliptical microfiber. Furthermore, polarization The main difference is related to the use of an additional sensitive layer in one case [86], whereas in controllers needed.principle The finalisdevice sensitivitybetween of 422 pm/%RH with response the other casewere the sensing basedshows on the ainteraction the evanescent field of atimes highof only 60 ms. (Hi-Bi) The ability of measuring RH the without adding anyenvironment. coating enables faster measurements. birefringence optical fiber [87] and humidity of the

Figure Schematic diagram Hi-Bi elliptical microfiber Sagnac interferometer-based RH Figure 13.13. (a)(a) Schematic diagram ofof thethe Hi-Bi elliptical microfiber Sagnac interferometer-based RH sensor; (b)(b) Recorded transmission spectrum of dips different RH values. Reprinted from [87] under sensor. Recorded transmission spectrum of at dips at different RH values. Reprinted from [87] creative license https://creativecommons.org/licenses/by-nc-nd/4.0/. under common creative common license https://creativecommons.org/licenses/by-nc-nd/4.0/.

2.7.Polyvinyl Resonatorsalcohol and Whispering Galleries Modes (PVA) is used as the sensitive material in the first approach [86]. It was coated onto a chemical etched polarization maintaining optical (PMF), which forms one of the of the The optical devices explained in this section arefiber based on microtapers. In the firstarms subsection, interferometer. The other arm is resonator, formed bywhile an unmodified PMF.subsection Therefore,they interference happen the microtapers constitute the in the second are usedwill to couple the owing to the relative-phase difference introduced to the guided modes by the PMFs. The obtained light to the resonator structure. attenuation bands, with attenuations of 20 and 25 dB, have a FWHM of 11.8 and 5.2 nm, respectively. A 2.7.1. sensitivity of 111.5 was achieved within the humidity range 20–80%RH with a response Microloop andpm/%RH Microknot Resonators time of about six seconds. Moreover, the sensitivity to temperature is only 7.2 pm/◦ C. This optical structure presents a desired characteristic for all optical sensors: their high Q factor. Greater sensitivities were obtained in [87]. The fabrication of this Sagnac interferometer, The Q factor is related to the quality of the filter performance and involves higher resolution for a represented in Figure 13, requires a more complex setup for the fabrication of the sensitive optical sensor. Resonators can be fabricated in different ways. fiber, which is a Hi-Bi optical fiber that consists of an elliptical microfiber. Furthermore, polarization In microloop resonators, due to the diameter of the fiber, a large fraction of the guided field is controllers were needed. The final device shows a sensitivity of 422 pm/%RH with response times of left outside the fiber as evanescent waves. Then, these evanescent waves can be self-coupled to the only 60 ms. The ability of measuring RH without adding any coating enables faster measurements. parallel segment of the microfiber and interfere with the light being guided through the loop. A microfiber loop developed using standard SMF [88] achieved a sensitivity of 1.8 pm/%RH 2.7. Resonators andresonator Whispering Galleries Modes without any sensitive coating. Its Free Spectral Range (FSR) is 350 pm and the fringe contrast reaches The optical devices explained in this section are based on microtapers. In the first subsection, 7 dB. the microtapers constitute the resonator, while in the second subsection they are used to couple the A similar structure is the so-called microknot resonator. Two different microknot resonators light to the resonator structure. (MKR) were developed in [89]. One of them was made of silica (standard SMF) and the other one 2.7.1. Microloop and Microknot Resonators This optical structure presents a desired characteristic for all optical sensors: their high Q factor. The Q factor is related to the quality of the filter performance and involves higher resolution for a sensor. Resonators can be fabricated in different ways. In microloop resonators, due to the diameter of the fiber, a large fraction of the guided field is left outside the fiber as evanescent waves. Then, these evanescent waves can be self-coupled to the parallel segment of the microfiber and interfere with the light being guided through the loop. A microfiber loop resonator developed using standard SMF [88] achieved a sensitivity of 1.8 pm/%RH without any sensitive coating. Its Free Spectral Range (FSR) is 350 pm and the fringe contrast reaches 7 dB. A similar structure is the so-called microknot resonator. Two different microknot resonators (MKR) were developed in [89]. One of them was made of silica (standard SMF) and the other one was built using polymethyl methacrylate (PMMA) as the waveguide. The silica MKR (1.2 µm diameter)

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was built using polymethyl methacrylate (PMMA) waveguide. The silica MKR (1.2 was built using polymethyl methacrylate (PMMA) as as thethe waveguide. The silica MKR (1.2 µmµm diameter) had a Q factor of 15,000 and a FSR of 0.22 nm. The PMMA MKR (2.1 µm diameter) showed had a Q factor a FSRand of 0.22 nm.ofThe MKR (2.1 MKR µm diameter) showed a showed Q factor diameter) had aofQ15,000 factorand of 15,000 a FSR 0.22PMMA nm. The PMMA (2.1 µm diameter) a Q factor of 20,000 and a FSR of 0.17 nm. This last device has reached sensitivities of 8.8 pm/%RH of 20,000 and a FSR of 0.17 nm. This last device has reached sensitivities of 8.8 pm/%RH (~8 times a Q factor of 20,000 and a FSR of 0.17 nm. This last device has reached sensitivities of 8.8 pm/%RH (~8 times higher than the silica MKR) with a high resolution of 0.0023 RH. The greater sensitivity higher than the silica withMKR) a highwith resolution 0.0023 RH.ofThe greater found for this (~8 times higher thanMKR) the silica a highofresolution 0.0023 RH.sensitivity The greater sensitivity found for this optical structure is 490 pm/%RH [90] for a microknot developed with polyacrylamide optical structure is 490 pm/%RH [90] for a microknot developed with polyacrylamide (PAM), which is found for this optical structure is 490 pm/%RH [90] for a microknot developed with polyacrylamide (PAM), which is depicted in Figure 14. depicted in Figure 14. (PAM), which is depicted in Figure 14.

Figure 14. (a) Schematic Schematicdiagram diagramofof a PAM microring for humidity sensing; (b) Redshift of Figure a PAM microring humidity sensing. Redshift of resonance Figure 14.14. (a) Schematic diagram of a PAM microring forfor humidity sensing. (b)(b) Redshift of resonance resonance peaks when the microring is exposed to 17%RH, 17.9%RH, and 18.9%RH, respectively. peaks when the microring is exposed to 17%RH, 17.9%RH, and 18.9%RH, respectively. Reprinted peaks when the microring is exposed to 17%RH, 17.9%RH, and 18.9%RH, respectively. Reprinted Reprinted from [90] with permission from OSA Publishing. from with permission from OSA Publishing. from [90][90] with permission from OSA Publishing.

2.7.2. Whispering Galleries Modes 2.7.2. Whispering Galleries Modes 2.7.2. Whispering Galleries Modes This kind resonator consists of two optical structures, waveguide and coupler. The This kind resonator consists optical structures, the waveguide and the coupler. This kind ofofof resonator consists of of two optical structures, thethe waveguide and thethe coupler. The dielectric resonator, with circular structures, supports the electromagnetic surface oscillations, The dielectric resonator, with circular structures, supports the electromagnetic surface oscillations, dielectric resonator, with circular structures, supports the electromagnetic surface oscillations, which are evanescently coupled to the waveguide. Total internal reflections from the resonator’s which are evanescently coupled to the waveguide. Total internal reflections from the resonator’s which are evanescently coupled to the waveguide. Total internal reflections from the resonator’s curved surface confine energy light inside resonator, generating some transmission curved surface confine thethe energy of of thethe light inside thethe resonator, generating some transmission curved surface confine the energy of the light inside the resonator, generating some transmission dips. The spectral positions of the transmission dips are strongly dependent on the geometry dips. The spectral positions of the transmission dips are strongly dependent on the geometry of of thethe dips. The spectral positions of the transmission dips are strongly dependent on the geometry of the resonator and the optical properties of the resonator material. They offer high resolution and resonator and of of thethe resonator material. TheyThey offeroffer high high resolution and the ability resonator and the theoptical opticalproperties properties resonator material. resolution and thethe measure really low values. to ability measure really low RHlow values. ability to to measure really RHRH values. A tapered optical fiber was used waveguide and a silica microsphere was used A tapered optical fiber was used as as thethe waveguide and silica microsphere was used as as thethe A tapered optical fiber was used as the waveguide and aa silica microsphere was used as the resonator structure [91,92]. The resonator was coated with SiO 2 nanoparticles by layer-by-layer resonator structure structure [91,92]. [91,92]. The The resonator resonator was was coated coated with with SiO SiO22 nanoparticles nanoparticles by by layer-by-layer layer-by-layer resonator nano-assembly. This OFHS has a resolution of 0.003%RH [91] using the coated WGM microspheres nano-assembly. This OFHS has a resolution of 0.003%RH [91] using the coated WGM microspheres at nano-assembly. This OFHS has a resolution of 0.003%RH [91] using the coated WGM microspheres at low RH (0%–12%RH). The other material tested using a similar structure was agarose, which low RH (0%–12%RH). The other material tested using a similar structure was agarose, which provides at low RH (0%–12%RH). The other material tested using a similar structure was agarose, which provides aof sensitivity pm/%RH [92]; obtained results are shown Figure a sensitivity 518 pm/%RH [92]; the obtained results are shown Figure 15.in provides a sensitivity of of 518518 pm/%RH [92]; thethe obtained results arein shown in Figure 15.15.

Figure 15. (a) Humidity response of the WGM spectrum for an uncoated microsphere resonator; Figure Figure 15. (a) (a) Humidity Humidity response response of of the the WGM WGM spectrum spectrum for for an an uncoated uncoated microsphere microsphere resonator; resonator; spectral shift versus RH for the sensors based on same diameter microsphere coated with (b)(b) spectral shift versus RH sensors based onsame thethe same diameter microsphere coated with spectral shift versus RH forfor thethe sensors based on the diameter microsphere coated with agarose agarose solutions of different concentrations. Reprinted from [92] with permission from OSA agarose of concentrations. different concentrations. Reprinted from [92] with permission from OSA solutionssolutions of different Reprinted from [92] with permission from OSA Publishing. Publishing. Publishing.

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Finally, Finally, aa similar similar structure structure but but with with toroidal toroidal form form was was used used in in [93] [93] and and is shown shown in Figure 16, achieving a sensitivity of 12.98 pm/%RH in the range 0%–12%RH. The silica microtoroid achieving a sensitivity of 12.98 pm/%RH microtoroid was was coated coated with improves the the sensitivity of the by nearly two orders withpoly(N-isopropylacrylamide), poly(N-isopropylacrylamide),which which improves sensitivity ofdevice the device by nearly two of magnitude. The Q-factor was 2 ×was 105 ,2providing high resolution. orders of magnitude. The Q-factor × 105, providing high resolution.

Figure 16. 16. (a) polymer-coated device while on Figure (a) A A representative representativetransmission transmissionspectrum spectrumfor fora athinner thinner polymer-coated device while resonance. TheThe dashed lineline is is a Lorentzian Q of of 2.5 2.5 ×× 10 1055;. on resonance. dashed a Lorentzianfitfittotothe thespectrum, spectrum, which which yields yields aa Q (b)Rendering Renderingof of testing setup. The shows inset the shows the optical taperedfiber optical fiber and microtoroid; the hybrid (b) thethe testing setup. The inset tapered and the hybrid microtoroid. (c) Noise measurement and resonant wavelength shift as a function of relative (c) Noise measurement and resonant wavelength shift as a function of relative humidity changehumidity at 23 ◦ C change at 23 °C for bare silica, thinner, and thicker polymer-film-coated devices. The error bars in are the for bare silica, thinner, and thicker polymer-film-coated devices. The error bars in the measurement measurement are smaller Reprinted than the symbols. from [93] permissionLLC. of AIP Publishing smaller than the symbols. from [93]Reprinted with permission of with AIP Publishing LLC.

3. Optical Fiber Humidity Sensors Based on Lossy Mode Resonances 3. Optical Fiber Humidity Sensors Based on Lossy Mode Resonances The first time dealing with lossy mode resonances (LMR), these were confused with surface The first time dealing with lossy mode resonances (LMR), these were confused with surface plasmon resonance (SPR) [94], but, despite its similarities, important differences can be distinguished plasmon resonance (SPR) [94], but, despite its similarities, important differences can be between the phenomena. What makes them similar is that both are electromagnetic resonances distinguished between the phenomena. What makes them similar is that both are electromagnetic that generate an attenuation band on the transmitted spectrum. However, in SPRs there is energy resonances that generate an attenuation band on the transmitted spectrum. However, in SPRs there transference from light to free electrons of the noble metal, whereas in LMRs the light is coupled with is energy transference from light to free electrons of the noble metal, whereas in LMRs the light is the coating. Another difference is the possibility of being observed on the transverse magnetic (TM) coupled with the coating. Another difference is the possibility of being observed on the transverse and transverse electric (TE) modes, which might simplify the required setup and number of materials magnetic (TM) and transverse electric (TE) modes, which might simplify the required setup and available for LMR generation, which broadens the application of LMR-based sensors. The possibility of number of materials available for LMR generation, which broadens the application of LMR-based generating several attenuation bands at tunable wavelengths is another advantage of LMRs. However, sensors. The possibility of generating several attenuation bands at tunable wavelengths is another one of the most relevant factors that make LMRs a good choice for optical fiber sensors development advantage of LMRs. However, one of the most relevant factors that make LMRs a good choice for is their ability to generate an optical phenomenon that can be detected by the wavelength detection optical fiber sensors development is their ability to generate an optical phenomenon that can be method with the same material that acts as the sensitive layer to the parameter to be measured. detected by the wavelength detection method with the same material that acts as the sensitive layer The structure of a LMR-based device consists of a waveguide, which allows for accessing the to the parameter to be measured. evanescent field, coated with a thin film of the appropriate material. The condition for LMR generation The structure of a LMR-based device consists of a waveguide, which allows for accessing the is that the real part of the thin film permittivity is positive and higher in magnitude than both its own evanescent field, coated with a thin film of the appropriate material. The condition for LMR imaginary part and the real part of the material surrounding the thin film [95]. LMRs are generated generation is that the real part of the thin film permittivity is positive and higher in magnitude than when there is a resonant coupling of light to modes guided in the external coating. Figure 17 shows both its own imaginary part and the real part of the material surrounding the thin film [95]. LMRs the fundamental mode (HE1,1 ) confined in the fiber core except at 1420 nm, where a fraction of the are generated when there is a resonant coupling of light to modes guided in the external coating. power transmitted by the core mode is coupled to the thin film, generating the LMR. It also shows one Figure 17 shows the fundamental mode (HE1,1) confined in the fiber core except at 1420 nm, where a cladding mode (TE1,1 ) that is confined at the cladding at 1690 nm, but is guided through the coating fraction of the power transmitted by the core mode is coupled to the thin film, generating the LMR. It for wavelengths smaller than 1420 nm [96]. also shows one cladding mode (TE1,1) that is confined at the cladding at 1690 nm, but is guided through the coating for wavelengths smaller than 1420 nm [96].

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Figure 17. Electric field intensity inin the transversal section ofof D-shaped optical fiber coated with ITO. Figure 17. Electric field intensity the transversal section D-shaped optical fiber coated with ITO. Figure 17. Electric field intensity in the transversal section of D-shaped optical fiber coated with ITO. The fundamental mode HE 1,1 and the first mode that experiences a transition to guidance in the The fundamentalmode modeHE HE1,1and andthe the first mode that experiences a transition to guidance in the The fundamental first mode that experiences a transition to guidance in the overlay 1,1 overlay 1,11,1 ), ),are for wavelengths. Reprinted from overlay (TE areanalyzed analyzed fordifferent different wavelengths. Reprinted from [96]with withpermission permission from (TE1,1 ),(TE are analyzed for different wavelengths. Reprinted from [96]from with[96] permission from Elsevier. Elsevier. Elsevier.

Several materials,including including metaloxides oxides suchas as ITO[97], [97], SnO22[98,99], [98,99], In2O [100],TiO TiO [95] or 22O 33 [100], 22 [95] Several 3 [100], 2 [95] oror Severalmaterials, materials, includingmetal metal oxidessuch such asITO ITO [97],SnO SnO2 [98,99],InIn O TiO polymers such as poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) [101], have been polymers polymerssuch suchasaspoly(allylamine poly(allylaminehydrochloride) hydrochloride)(PAH) (PAH)and andpoly(acrylic poly(acrylicacid) acid)(PAA) (PAA)[101], [101],have have tested andand checked forfor LMR generation. The large amount of of available materials materials enablesthe the been beentested tested andchecked checked forLMR LMRgeneration. generation.The Thelarge largeamount amount ofavailable available materialsenables enables the development of optical fiber sensors for a wide range of applications [63,102–105]. Besides the research development developmentofofoptical opticalfiber fibersensors sensorsfor fora awide widerange rangeofofapplications applications[63,102–105]. [63,102–105].Besides Besidesthe the on the appropriate materials, LMR supporting structures have evolved and they have been studied on research researchon onthe theappropriate appropriatematerials, materials,LMR LMRsupporting supportingstructures structureshave haveevolved evolvedand andthey theyhave havebeen been a wide range of optical of fibers. studied studiedon ona awide widerange range ofoptical opticalfibers. fibers. The first LMR-based deviceswere were developedononplastic-clad plastic-clad silica(PCS) (PCS) optical fiber (200µm µm The first LMR-based devices The first LMR-based devices weredeveloped developed on plastic-cladsilica silica (PCS)optical opticalfiber fiber(200 (200 µm diameter). Then, by chemically removing the cladding theevanescent evanescent fieldbecomes becomes accessible, diameter). diameter).Then, Then,by bychemically chemicallyremoving removingthe thecladding claddingthe the evanescentfield field becomesaccessible, accessible, which is one of the requirements for LMR generation. ITO was the first material tested andwas was which whichisisone oneofofthe therequirements requirementsfor forLMR LMRgeneration. generation.ITO ITOwas wasthe thefirst firstmaterial materialtested testedand and was coated by dip-coating [102], but other metallic oxides have been studied, such as tin dioxide [106]. coated coatedby bydip-coating dip-coating[102], [102],but butother othermetallic metallicoxides oxideshave havebeen beenstudied, studied,such suchasastin tindioxide dioxide[106]. [106]. The resultsobtained obtained bythis this combination(LMR (LMR generatedbybytintindioxide dioxide andPCS) PCS) show a sensitivity The Theresults results obtainedby by thiscombination combination (LMRgenerated generated by tin dioxideand and PCS)show showa asensitivity sensitivity of0.10.1 nm/RH% in range the range from 20% to 80%RH [106]. an Adding an layer, external layer, which is ofof 0.1nm/RH% nm/RH%ininthe the rangefrom from20% 20%toto80%RH 80%RH[106]. [106].Adding Adding anexternal external layer,which whichisissensitive sensitive sensitive to humidity, the sensitivity device of the device improves, reaching values around 1 nm/%RH [97]. toto humidity, humidity,the thesensitivity sensitivityofofthe the deviceimproves, improves,reaching reachingvalues valuesaround around1 1nm/%RH nm/%RH[97]. [97]. Characterization of this last device is shown in Figure 18. Characterization Characterizationofofthis thislast lastdevice deviceisisshown shownininFigure Figure18. 18.

Figure18. 18. Spectral ofofthe (a)(a) non-tuned and and (b) tuned sensors for 20%for and 90%RH. Figure response (b) 90%RH. Figure 18.Spectral Spectralresponse response ofthe the (a)non-tuned non-tuned and (b)tuned tunedsensors sensors for20% 20%and andDynamical 90%RH. response of the sensors to changes in the RH of the external medium (c) non-tuned sensor and Dynamical response of the sensors to changes in the RH of the external medium (c) non-tuned Dynamical response of the sensors to changes in the RH of the external medium (c) non-tunedsensor sensor (d)(d) tuned sensor (non-tuned sensor (20 PAH/PAA bilayers) and tuned PAH/PAA bilayers)). and sensor (non-tuned sensor (20 bilayers) and tuned sensor (100 and (d)tuned tuned sensor (non-tuned sensor (20PAH/PAA PAH/PAA bilayers) andsensor tuned(100 sensor (100PAH/PAA PAH/PAA Reprinted from [97]from with permission from Elsevier. bilayers)). Reprinted from bilayers)). Reprinted from[97] [97]with withpermission permission fromElsevier. Elsevier.

AAmethod methodtotodecrease decreasethe theresonance resonancewidth widthand andincrease increasethe thesensitivity sensitivityofofLMR-based LMR-basedsensors sensorsisis currently currentlybeing beingstudied studied[96,99,107]. [96,99,107].Side Sidepolished polishedoptical opticalfibers fibersenable enabledistinguishing distinguishingbetween betweenthe the TM TMand andthe theTE TEmodes modesofofthe theLMR LMR[108], [108],obtaining obtaininga anarrower narrowerattenuation attenuationband. band.Moreover, Moreover,working working

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A method to decrease the resonance width and increase the sensitivity of LMR-based sensors is Sensors 2017, 17, 893 15 of 23 currently being studied [96,99,107]. Side polished optical fibers enable distinguishing between the TM and thethe TE modes of the LMR [108], obtaining a narrower attenuation band. Moreover, working with with fundamental mode provides better sensitivity than working with several modes the fundamental mode provides better sensitivity than working with several modes [96,108,109]. [96,108,109]. Therefore, improves thethe previously obtained sensitivities. Different approaches can Therefore,using usinga SMF a SMF improves previously obtained sensitivities. Different approaches becan followed whenwhen working with SMF using side polished opticaloptical fibers,fibers, tapered opticaloptical fibers, be followed working with such SMF as such as using side polished tapered orfibers, cladding SMF (CE-SMF). The first two methods havemethods been used for been the development of or etched cladding etched SMF (CE-SMF). The first two have used for the several kinds ofofsensors The latter has been recently studied towards humidity sensing [99]. development several[63,101]. kinds of sensors [63,101]. The latter has been recently studied towards Etching the sensing cladding[99]. of a Etching SMF with (HF)with reduces the cladding allowing humidity thehydrofluoric cladding ofacid a SMF hydrofluoric aciddiameter, (HF) reduces the the interaction with the evanescent field. cladding diameter, allowing the interaction with the evanescent field. Twodifferent differentmaterials materialshave havebeen beentested tested with with CE-SMF CE-SMF structure, structure, indium Two indium oxide oxide [100] [100] and and tin tin oxide[99]. [99].Both Both transparent conductive oxides meet requirements LMR generation. oxide transparent conductive oxides meet thethe requirements for for LMR generation. TheThe real real part of the refractive of tin oxide is greater, slightlyincreasing greater, increasing the sensitivity the final ofpart the refractive index ofindex tin oxide is slightly the sensitivity of the finalofdevice [99]. device [99]. Theofwavelength of the LMR shifts more nm forfrom RH 20% changes from 20%working to 90% The wavelength the LMR shifts more than 30 nm forthan RH 30 changes to 90% when when working withThe indium The LMR generated byatin oxide hasofa1.9 sensitivity of 1.9 with indium oxide. LMRoxide. generated by tin oxide has sensitivity nm/%RH fornm/%RH the same for the rangeand of RH. Static and dynamic characterization sensor plotted 19. range of same RH. Static dynamic characterization of this sensorofisthis plotted in is Figure 19.inInFigure addition, In addition, this optical structure has ancross unnoticeable cross thermal sensitivity. linearity and this optical structure has an unnoticeable thermal sensitivity. High linearity High and low hysteresis low hysteresis are otherofcharacteristics of this kind of sensor. are other characteristics this kind of sensor.

Figure19.19. (a) Optical spectrum of coated the SnO 2 coated CE-SMF for different RH values; Figure (a) Optical spectrum of the SnO CE-SMF for different RH values; (b) characterization 2 (b) characterization of the LMR wavelength as a function of RH for different (c) RH LMR of the LMR wavelength as a function of RH for different temperatures; (c) LMR temperatures; wavelength and as wavelength and RH as a function of time for different temperatures. Reprinted from [99] a function of time for different temperatures. Reprinted from [99] with permission from Elsevier.with permission from Elsevier.

An in [110] [110] by by coating coating aa SMS SMS Aninteresting interestingcomparison comparison between between SMS SMS and and LMR LMR was was developed developed in and a PCS (200 µm diameter) with a combination of titanium (IV) oxide nanoparticles (TiO ) and and a PCS (200 µm diameter) with a combination of titanium (IV) oxide nanoparticles (TiO22) and poly(sodium (PSS). When comparing the sensitivity to relative humidityhumidity changes, poly(sodium4-styrenesulfonate) 4-styrenesulfonate) (PSS). When comparing the sensitivity to relative the LMR-based device shows a tenfold improvement compared to the SMS-based device. Therefore, changes, the LMR-based device shows a tenfold improvement compared to the SMS-based device.it isTherefore, proved that better to useitan if high sensitivity is high needed. However, the SMS it itisisproved that is LMR-based better to usedevice an LMR-based device if sensitivity is needed. structure provides narrower attenuation bands than LMR generated onto PCS fibers. In any case, However, the SMS structure provides narrower attenuation bands than LMR generated onto PCS coating the SMS structure with the thin film provides better sensitivity than the naked SMS. fibers. In any case, coating the SMS structure with the thin film provides better sensitivity than the naked SMS.

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Table 1. Summary of the most relevant results found for the interval 2010–2016. Reference

Year

Method

Sensing Material

Range

Sensitivity/Resolution

Response Time

Comments

0.123 dB/%RH & 0.475 %RH 0.31 dB/%RH

1–5 s 0.13%RH/s

-

13.6pm/%RH 31 pm/%RH & 0.03%RH 35 pm/%RH

22–29 min 9.7–39.4 min

In situ imidization Annealed 90%RH

1.4 nm/%RH at low RH 1.36 nm/%RH 63 pm/%RH & 411pm/◦ C

-

Radiation&Ta