hv photonics

Review Latest Achievements in Polymer Gratings: Fabrication and Applications

Rui Min 1,* , Beatriz Ortega 1 and Carlos Marques 2

1 ITEAM Research Institute, Universitat Politècnica de València, Camino de Vera, s/n 46022 Valencia, Spain; [email protected] 2 I3N& Physics Department, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal; [email protected] * Correspondence: [email protected]; Tel.: +34-96-387-0000

 Received: 18 February 2019; Accepted: 26 March 2019; Published: 29 March 2019 

Abstract: Grating devices in polymer optical fibers (POFs) have attracted huge interest for many potential applications in recent years. This paper presents the state of the art regarding the fabrication of different types of POF gratings, such as uniform, phase-shifted, tilted, chirped, and long period gratings, and explores potential application scenarios, such as biosensing and optical communications.

Keywords: polymer optical fibers; gratings devices; optical filters; fiber sensing

1. Introduction Polymer optical fibers (POFs) show attractive characteristics when compared with silica fibers, such as low Young’s modulus, high failure strain, high flexibility, and bio-compatibility [1]. Different kinds of plastic material with unique advantages can be used for POF fabrication besides polymethyl methacrylate (PMMA), which is the most common material with a low cost [2]. Examples of these materials are low water absorption cyclic olefin copolymers (TOPAS) [3], high glass transition temperature cyclic-olefin polymer (ZEONEX) [4], excellent clarity and impact strength engineering plastic (Polycarbonate) [5], biodegradable and biocompatible poly(D,L-lactic acid) (PDLLA) [6], and low-loss cyclic transparent amorphous fluoropolymers (CYTOP) [7]. In addition to fiber materials, several types of POFs are available in the current market with different core sizes and structures, such as small-diameter step-index (SI) POF, small-diameter microstructure POF, commercial grade-index (GI) POF, and commercial large-diameter PMMA POF, as shown in Table1. Extended reviews about the potential applications of these fibers can be found in the literature as follows. Koike et al. [8] reviewed the status of POFs and achievements on GI POF as a promising candidate for the next generation of optical fibers. Zubia et al. [9] reviewed the most significant features of POFs, including the main types, manufacturing, and potential applications in 2001. Polishuk [10] gave a view about potential large-core POF for low-bit-rate and short distance applications. Micro-structured polymer optical fibers (mPOF) were inspired by fibers, first invented by Knight et al. [11] in 1996; Argyros et al. [12] proposed new potential applications for these POF structures, and also showed their easier single-mode performance compared with step-index POFs, due to the fabrication process. Due to increasing numbers of emerging services, such as intelligent home systems, visible light communications, etc., in indoor networks, the increase of high-capacity demand shifts from long-haul systems to short-range communication links. Indeed, POFs are good candidates for short-range communications [13–21], with low weight and high transmission capacity. Accordingly, Professor Koike and his research group demonstrated data transmission over POF in 1995 [22], where they reported 2.5 Gb/s 100 m data transmission using GI POF at 650 nm ; more recently, in 2016, they developed 120 Gb/s GI POF, as well as the ballpoint pen interconnection technology

Photonics 2019, 6, 36; doi:10.3390/photonics6020036 www.mdpi.com/journal/photonics Photonics 2019, 6, 36 2 of 18

Photonics 2019, 19, x 2 of 18 Photonics 2019, 19, x 2 of 18 for uncompressedPhotonics 2019, 19, x 4K/8K video transmission [23]. Recent achievements in POF-based transmission2 of 18 Photonicsuncompressed 2019, 19, x 4K/8K video transmission [23]. Recent achievements in POF-based transmission2 of 18 networksuncompressed have led 4K/8K to promising video transmission applications [23]. in future Recent home achievements networks, in such POF-based as Pinzon transmission et al. [24], uncompressednetworks have 4K/8Kled to promisingvideo transmission applications [23]. in Recefuturent homeachievements networks, in such POF-based as Pinzon transmission et al. [24], whereuncompressednetworks a five-channel have 4K/8Kled visibleto promisingvideo wavelength-division transmission applications [23]. in Recefuture multiplexingnt homeachievements networks, (WDM) in such systemPOF-based as Pinzon was transmission transmitted et al. [24], networkswhere a five-channel have led to visible promising wavelength-division applications in futuremultiplexing home networks,(WDM) system such wasas Pinzon transmitted et al. over[24], overnetworkswhere 50-m a step five-channel have index led to POF visible promising links wavelength-division with applications a 2 Gb/s, in bidirectional, futuremultiplexing home networks,(WDM) real-time system such link. wasas Moreover, Pinzon transmitted et al. Osahon over[24], where50-m step a five-channel index POF visiblelinks with wavelength-division a 2 Gb/s, bidirectional, multiplexing real-time (WDM) link. Moreover,system was Osahon transmitted et al. over [25] et al.where50-m [25] step has a five-channel demonstratedindex POF visiblelinks gigabit-per-second with wavelength-division a 2 Gb/s, bidirectional, transmission multiplexing real-time over (WDM) a short-rangelink. Moreover,system step-indexwas Osahon transmitted POFet al. usingover [25] 50-mhas demonstrated step index POF gigabit-per-second links with a 2 Gb/s, transmission bidirectional, over real-time a short-range link. Moreover, step-index Osahon POF et using al. [25] a a multilevel50-mhas demonstrated step pulse index amplitude POF gigabit-per-second links modulation with a 2 Gb/s, (PAM-M)transmission bidirectional, scheme, over real-timebased a short-range on link. a Moreover, step-index diode (LD)Osahon POF as theet using al. optical [25] a hasmultilevel demonstrated pulse amplitude gigabit-per-second modulation transmission(PAM-M) scheme, over baseda short-range on a laser step-index diode (LD) POFas the using optical a source,hasmultilevel whichdemonstrated pulse also amplitude shows gigabit-per-second successful modulation simultaneous transmission(PAM-M) scheme, transmission over baseda short-range on of a 16-QAM laser step-index diode 40 (LD) MHz POFas bandwidththe using optical a multilevelsource, which pulse also amplitude shows successfulmodulation simultaneous (PAM-M) scheme, transmission based on of a 16-QAMlaser diode 40 (LD) MHz as bandwidththe optical wirelessmultilevelsource, local which pulse area also networkamplitude shows (WLAN) successfulmodulation and simultaneous (PAM-M) 64 quadrature scheme, transmission amplitude based on of a modulation 16-QAMlaser diode 40 (LD) (QAM)MHz as bandwidththe long optical term source,wireless which local areaalso networkshows successful (WLAN) andsimultaneous 64 quadrature transmission amplitude of modulation 16-QAM 40 (QAM) MHz bandwidth long term evolutionsource,wireless (LTE-A) which local areaalso bands. networkshows Among successful (WLAN) recent andsimultaneous achievements, 64 quadrature transmission Forni amplitude et al. of [26 modulation 16-QAM] must be 40 highlighted,(QAM) MHz bandwidth long due term to wirelessevolution local (LTE-A) area bands.network Among (WLAN) recent and achievements, 64 quadrature Forni amplitude et al. [26] modulation must be highlighted,(QAM) long due term to theirwirelessevolution simultaneous local (LTE-A) area transmission bands.network Among (WLAN) of an recent IEEE and achievements, 64 02.11n quadrature 16-QAM Forni amplitude 40 et MHz al. [26] modulation bandwidth must be highlighted,(QAM) WLAN, long 964 due QAMterm to evolutiontheir simultaneous (LTE-A) bands.transmission Among of recent an IEEE achievements, 02.11n 16-QAM Forni 40 et MHzal. [26] bandwidth must be highlighted, WLAN, 964 due QAM to evolutiontheir simultaneous (LTE-A) bands.transmission Among of recent an IEEE achievements, 02.11n 16-QAM Forni 40 et MHzal. [26] bandwidth must be highlighted, WLAN, 964 due QAM to LTE-AtheirLTE-A bands, simultaneous bands, and and 1.7 1.7 Gb/stransmission Gb/s 4-PAM 4-PAM of baseband baseband an IEEE 02.11n signalssignals 16-QAM over over 50 50 m 40 m of MHz of 1-mm 1-mm bandwidth core core diameter diameter WLAN, GI POF GI964 POF forQAM in- for theirLTE-A simultaneous bands, and 1.7 transmission Gb/s 4-PAM of baseband an IEEE 02.11nsignals 16-QAM over 50 m 40 of MHz 1-mm bandwidth core diameter WLAN, GI POF 964 forQAM in- in-homeLTE-Ahome networks. networks. bands, and 1.7 Gb/s 4-PAM baseband signals over 50 m of 1-mm core diameter GI POF for in- LTE-Ahome networks. bands, and 1.7 Gb/s 4-PAM baseband signals over 50 m of 1-mm core diameter GI POF for in- home networks. home networks. Table 1. Typical polymer optical fiber (POF) structures. Table 1. Typical polymer optical fiber (POF) structures. Table 1. Typical polymer optical fiber (POF) structures. Table 1. Typical polymer optical fiber (POF) structures. Fiber TypeFiber type Table Transversal1. Typical polymer Section section optical Diameter Diameter fiber (POF) Main structures. Main Feature feature Fiber type Transversal section Diameter Main feature Small-diameterFiberSmall-diameter type step-index step- Transversal section 100~200 Diameter100~200µm μ m Easy MainEasy coupling coupling feature FiberSmall-diameter type step- Transversal section Diameter100~200 μ m MainEasy coupling feature (SI) POFSmall-diameterindex step- 100~200 μm PotentialEasyPotential single-mode coupling single-mode performance Small-diameterindex step- 100~200 μm EasyPotential coupling single-mode index(SI) POF Potentialperformance single-mode index(SI) POF Potentialperformance single-mode (SI) POF performance (SI) POF performance

Microstructure POF 100~250 μm Easy to obtain single- MicrostructureMicrostructure POF POF 100~250100~250µm μm Easy Easy to obtain to single-modeobtain single- Microstructure POF 100~250 μm Easymode performanceto obtain single- Microstructure POF 100~250 μm performance Easymode performanceto obtain single- Can bemodeCan made be performance made with awith single a single material modeCan be performance made with a single Canmaterial be made with a single Canmaterial be made with a single material

material Commercial grade- 490 μm Low loss Commercial grade- 490 μm Low loss CommercialCommercialindex grade-index(GI) POF grade- 490490µ mμm LowLowLarge loss loss bandwidth (GI) POFCommercialindex (GI) POF grade- 490 μm LargeLowLarge bandwidth loss bandwidth index (GI) POF Large bandwidth index (GI) POF Large bandwidth

Commercial large- 1 mm Low price Commercial large- 1 mm Low price Commercialdiameter polymethyl large- 1 mm LowEasy pricehandling CommercialCommercialdiameter large-diameter polymethyl large- 11 mm mm Low LowEasy price pricehandling polymethyldiametermethacrylate methacrylate polymethyl EasyEasy handling handling diametermethacrylate polymethyl Easy handling (PMMA)methacrylate(PMMA) POF POF methacrylate(PMMA) POF (PMMA) POF (PMMA) POF

Sensing is also an emerging area of POF applications under research, as stated in the extensive Sensing is also an emerging area of POF applications under research, as stated in the extensive literature.Sensing Peters is also et anal. emerging[27] presented area of a POFreview applications work focused under on research, strain andas stated temperature in the extensive sensing literature.Sensing Peters is also et anal. emerging[27] presented area of a POFreview applications work focused under on research, strain andas stated temperature in the extensive sensing literature.applications, Peters based et on al. POFs [27] usingpresented different a review solution works. POF-based focused on sensing strain techniques and temperature can be classified sensing literature.applications,Sensing isPeters also based anet on emergingal. POFs [27] usingpresented area different of POFa review solution applications works. POF-based focused under on research,sensing strain techniques and as stated temperature can in the be classified extensive sensing applications,on intensity modulationbased on POFs sensing, using differentBrillouin solution scattering,s. POF-based and wavelength sensing sensitivitytechniques gratingcan be classified devices. literature.applications,on intensity Peters modulationbased et al. on [ 27POFs] sensing, presented using differentBrillouin a review solution scattering, works. POF-based focused and wavelength on sensing strain sensitivitytechniques and temperature gratingcan be classified devices. sensing onBilro intensity et al. [28] modulation presented sensing, a review Brillouin paper aboutscattering, intens andity wavelengthmodulation sensitivitysensing, where grating all devices.sensors applications,onBilro intensity et al. based [28] modulation onpresented POFs usingsensing, a review different Brillouin paper solutions. aboutscattering, intens POF-based andity wavelengthmodulation sensing techniquessensitivitysensing, where grating can beall classified devices.sensors Bilroinclude et aal. light [28] source, presented an optical a review fiber, paper and an about opti calintens spectrumity modulation analyzer orsensing, a photodetector where all when sensors the on intensityBilroinclude et aal. modulationlight [28] source, presented an sensing, optical a review Brillouinfiber, paper and scattering,an about optical intens spectrum anditywavelength modulation analyzer or sensitivitysensing, a photodetector where grating all when sensors devices. the includeaccuracy a islight not source, so critical, an optical such as fiber, structural and an health optical monitoring spectrum analyzer[27,29], deformation or a photodetector monitoring when [30], the Bilroincludeaccuracy et al. [28 a islight] presentednot source, so critical, aan review optical such paperas fiber, structuralabout and an intensityhealth optical monitoring spectrum modulation analyzer[27,29], sensing, deformation or a where photodetector all monitoring sensors when include [30], the accuracymedical instrument is not so critical, [31–34], such environment as structural monitoring health monitoring [35,36], mechanical [27,29], deformation measurement monitoring [37–39], [30],and a lightaccuracymedical source, instrument is an not optical so critical, [31–34], fiber, such and environment as an structural optical monitoring spectrum health monitoring analyzer [35,36], mechanical or [27,29], a photodetector deformation measurement when monitoring [37–39], the accuracy [30],and medicalchemical instrument detection [40,41]. [31–34], The environment second technique, monitoring based [35,36], on Brillouin mechanical scattering measurement in POFs [26], [37–39], is mainly and is notmedicalchemical so critical, instrument detection such as[40,41]. [31–34], structural The environment seco healthnd technique, monitoring monitoring based [ 27[35,36], on,29 Brillouin], deformationmechanical scattering measurement monitoring in POFs [26], [[37–39],30 ],is medicalmainly and chemicalfocused towardsdetection distributed [40,41]. The sensing, second technique,such as structural based on Brillouinhealth mo scatteringnitoring inby POFs POF [26],strain is mainlysensor instrumentchemicalfocused [31towards detection–34], environment distributed [40,41]. The sensing, monitoringsecond technique,such [ 35as,36 structural based], mechanical on Brillouinhealth measurement mo scatteringnitoring inby [37 POFs POF–39], [26],strain and is chemical mainlysensor focusedtechnology towards [42], according distributed to thesensing, fundamental such as propertiesstructural ofhealth Brillouin monitoring scattering by in POF POF strain as reviewed sensor detectionfocusedtechnology [40 towards,41 [42],]. The according distributed second technique,to thesensing, fundamental basedsuch as on propertiesstructural Brillouin scatteringofhealth Brillouin mo innitoring scattering POFs [by26 in ],POF isPOF mainly strain as reviewed focusedsensor technologyby Mizuno et[42], al. according[43]. to the fundamental properties of Brillouin scattering in POF as reviewed towardstechnologyby Mizuno distributed et[42], al. sensing,according[43]. such to the as structuralfundamental health properties monitoring of Brillouin by POF scattering strain sensor in POF technology as reviewed [42], by MizunoThe last et technique,al. [43]. based on wavelength sensitivity grating devices, is the most promising area by MizunoThe last et technique,al. [43]. based on wavelength sensitivity grating devices, is the most promising area accordingfor POFThe to sensing.last the technique, fundamental In contrast based propertieswith on wavelengththe applications of Brillouin sensitivity of grating scattering grating devices indevices, POF in silica asis the reviewed fiber, most which promising by Mizunohave beenarea et for POFThe sensing.last technique, In contrast based with on wavelengththe applications sensitivity of grating grating devices devices, in silica is the fiber, most which promising have beenarea al. [43forunder]. POF intense sensing. research In contrast for withmore the than applications 30 years of[44], grating the first devices POF in Bragg silica fiber,grating which (POFBG) have beenwas forunder POF intense sensing. research In contrast for withmore the than applications 30 years of[44], grating the first devices POF in Bragg silica fiber,grating which (POFBG) have beenwas under intense research for more than 30 years [44], the first POF Bragg grating (POFBG) was under intense research for more than 30 years [44], the first POF Bragg grating (POFBG) was

Photonics 2019, 6, 36 3 of 18

The last technique, based on wavelength sensitivity grating devices, is the most promising area for POF sensing. In contrast with the applications of grating devices in silica fiber, which have been under intense research for more than 30 years [44], the first POF Bragg grating (POFBG) was demonstrated by Peng’s group in 1999 [45]. Since then, the literature has presented a large number of papers reporting significant achievements, such as Dobb et al.’s [46] report in 2005 on the first FBG in few mode and endlessly single mode mPOFs, using a continuous wave laser at 325 nm wavelength; the first (UV) inscription of Long Period Grating in POFs published by Saez et al. [47] in 2010; or the first FBG in multimode POF at 827 nm wavelength, published by Johnson et al. [48] in 2010. The main drawback of PMMA fibers is water absorption, which can be reduced by using other polymer materials. An example is TOPAS, a cyclic olefin copolymer with moisture absorption that is at least 30 times lower than that of PMMA. In this sense, Wu et al. reported the first experimental results about a humidity-insensitive POF Fiber Bragg Grating [49]. Moreover, Woyessa et al. [4] demonstrated the very first low, endlessly single-mode and humidity-insensitive mPOF made of ZEONEX-grade 480R with a glass transition temperature of 138 ◦C, and the first FBG was inscribed in ZEONEX mPOF at a low-attenuation 850 nm region. Polycarbonate is another material, first introduced by Fujitsu in 1989 [50], which exhibits excellent transparency and impact strength. Fasano et al. [5] reported the first experimental demonstration of an endlessly single-mode polycarbonate mPOF, and also inscribed the first FBG in polycarbonate mPOF. Last but not least, cyclic transparent fluoropolymers (CYTOP) exhibit excellent transmission characteristics from the visible to near-infrared spectrum, due to the optical absorption wavelength shift to 7.7~10 µm, which exceeds the transmission limit of other polymer materials, such as high-loss PMMA, induced by the high overtone vibration of C–H at 3~3.5 µm. This, therefore, allows Bragg gratings to be inscribed in CYTOP fibers by using a femto-second laser [51–56] and phase mask method [57–59]. As happens in silica fibers, special grating devices are attractive for a variety of applications. Birefringent gratings have been employed to change the polarization state of light at different [60,61], and laser micromachining has been used for obtaining POF gratings with different microstructures in the cladding, in order to enhance their humidity response time [62] with a high strain sensitivity [63]. A Fabry–Perot cavity based on POF gratings was fabricated by Dobb et al. [64], phase-shifted (PS) FBGs in POF were obtained by uniform UV exposure of the central part of 1-cm-long gratings [64], and tilted FBGs were reported by Hu et al. in 2014 [65] in step-index POFs, among others included in [66]. Finally, chirped FBGs were fabricated by using a chirped phase mask in 2017 [67]. Previous literature includes extensive reviews published by Webb [1], Luo et al. [66], Canning [68], Marques et al. [69,70], Broadway et al. [71], Berghmans et al. [72], and Nogueira et al. [73], focusing on the uniform POF gratings fabrication and applications. Due to the emerging POF applications in communications and sensing, intense work has been done in recent years towards obtaining a short, flexible, and reliable fabrication process for gratings in these fibers. After discussing historical records and motivation in the introduction section of this paper, we report below a review work about the latest results on POF grating device fabrication and applications, structured as follows. Recent results on POF gratings fabrication using different UV and fibers will be reported in Section2, and Section3 will focus on recently identified potential applications of different POF grating devices. Finally, Section4 summarizes the main conclusions and outlines the most promising POF gratings research lines, to be followed within the short- and mid-term.

2. Fabrication of Polymer Optical Fiber Gratings POF Bragg gratings (POFBGs) are typically fabricated by direct writing [74], Sagnac interferometry [75], and phase mask [1] techniques, although the preferred POFBG fabrication technique is the latter one, due to easy implementation in spite of the limited flexibility. Direct writing provides flexibility in terms of structure and wavelength, but the femtosecond laser system is required for FBG irradiation, and the resolution imposes limitations on the achievable low wavelengths. Photonics 2019, 6, 36 4 of 18

PhotonicsLong period 2019, 19 gratings, x are usually obtained by direct writing [75,76], heat imprinting, and amplitude4 of 18 masks [77,78]. InIn this this section, section, we we present present the the recent recent achievements achievements in in the the optimization optimization of of the the fabrication fabrication process, process, asas well well as as gratings gratings inscription inscription in in different different ty typespes of of doped doped and and undoped undoped POFs, POFs, and and illustrate illustrate the the flexibilityflexibility of of the the fabrication fabrication procedures. procedures.

2.1.2.1. Optimization Optimization of of the the Fabrication Fabrication Process Process GratingsGratings are are fabricated fabricated in in fibers, fibers, due due to to the the photosensitivity photosensitivity of of polymer polymer material material under under UV, UV, whichwhich leads leads to to the fiber fiber changechange afterafter UVUV absorption.absorption. Indeed, Indeed, photosensitivity photosensitivity stronglystrongly depends depends on on the the wavelength wavelength and and the the inscription inscription mechanism, mechanism, as was as was completely completely detailed detailed by Luoby Luoet al. et [66]. al. [Since66]. Sincethe Bell the laboratories Bell laboratories discovered discovered this around this around 1970s [79], 1970s the [ 79absorption], the absorption of 325 nm of wavelength325 nm wavelength UV radiation UV radiationwas identified was identifiedas the preferred as the technique preferred techniquefor refractive for index refractive change index in PMMAchange material. in PMMA Depending material. on the Depending power and on wavele the powerngth of and the absorbed wavelength light, of several the absorbed mechanisms light, occurseveral in mechanismscombined reactions, occur in combinedsuch as photo reactions, polymerization, such as photo photo polymerization, degradation, photoand cross-linking degradation, changeand cross-linking between the change polymer between chains. theYu et polymer al. [80] reported chains. Yurefractive et al. [index80] reported modification refractive by photo- index isomerization,modification by and photo-isomerization, also showed that and polymer also showed fiber thatcores polymer doped fiber with cores dopants doped (i.e. with trans-4- dopants stilbenemethanol,(i.e. trans-4-stilbenemethanol, or TS) exhibit ordifferent TS) exhibit refracti differentve index refractive changes indexunder changesUV radiation. under Saez UV radiation.et al. [81] reportedSaez et al.that [81 the] reported photosensitivity that the of photosensitivity undoped PMMA of undopedmPOF can PMMA be increased mPOF by can straining be increased the fiber by duringstraining photo the fiberinscription, during which photo is inscription, evidence of which photodegradation is evidence of and photodegradation paved the way and for pavedundoped the PMMAway for POF undoped grating PMMA irradiatio POFn gratingunder 325 irradiation nm wavelength. under 325 nm wavelength. TheThe stability stability of of POF POF gratings gratings is a is critical a critical issue issue mentioned mentioned by several by several papers papers [67,82–85]. [67,82 Pre-–85]. annealedPre-annealed POFs POFs allow allow the fabrication the fabrication of more of stable more short-term stable short-term performance performance gratings gratings at both greater at both straingreater and strain higher and temperatures higher temperatures [85]. However, [85]. However, both non-annealed both non-annealed and annealed and annealedTS-doped TS-doped polymer opticalpolymer fibers optical were fibers studied were studied in terms in termsof FBG of FBGstability, stability, and and the the post-inscription post-inscription thermal thermal annealing annealing processprocess was was necessary necessary to to produce produce stable stable gratings gratings [83]. [83]. POF POF grating grating fabrication fabrication has has been been reported reported usingusing different different lasers, lasers, such such as as an an 800 800 nm nm Ti:sapphire Ti:sapphire fs fs laser laser system system [74,86], [74,86], a a 532 532 nm nm Nd:YVO4 Nd:YVO4 laser laser systemsystem [77,87], [77,87], a a 387 387 nm nm Ti:sapphire Ti:sapphire fs fs laser laser system system [88], [88], a a 355 355 nm nm Nd:YAG Nd:YAG laser laser [89], [89], a a 325 325 nm nm optical optical parametricparametric oscillator oscillator (OPO) (OPO) pulsed pulsed laser, laser, a a dye dye laser, laser, an an He–Cd He–Cd laser laser [81,90–95], [81,90–95], and and a a 248 248 nm nm KrF KrF excimerexcimer laser systemsystem [73[73,96].,96]. Although Although 325 325 nm nm was was the the first first irradiation irradiation wavelength wavelength reported reported by Peng’s by Peng’sgroup group [45], and [45], initially and initially 248 nm 248 wavelength nm wavelength was not was considered not considered suitable suitable for polymer for polymer fiber Braggfiber Bragggrating grating writing, writing, due to due high to absorption, high absorption, the first th successfule first successful Bragg gratingBragg grating inscription inscription in 30 seconds in 30 secondsusing low using flow low and flow a repetition and a repetition rate at 248rate nm at 248 UV nm light UV opened light opened a new fielda new of field interest of interest for grating for gratingirradiation irradiation [96]. Since [96]. then, Since the researchthen, the work research on gratings work inscriptionon gratings using inscription 248 nm wavelengthusing 248 nm has wavelengthbeen continuously has been growing. continuously A typical growing. POF A FBG typical irradiation POF FBG system irradiation is shown system in Figure is shown1, where in Figure the 1,pulse where power the andpulse repetition power rateand can repetition be optimized rate [ca97n,98 be] inoptimized order to shorten [97,98] the in fabricationorder to shorten time, as willthe fabricationbe shown below.time, as will be shown below.

FigureFigure 1. Typical 1. Typical POF POF FBG FBG irradiation irradiation system; system; imageimage adapted adapted from from [58 ].[58].

2.2. Different Polymer Materials Due to the low photosensitivity of pure PMMA fiber [93], doped POFs have attracted researchers´ interest [80,98,99]. Peng’s group employed a step-index, multimode PMMA fiber with an organic dye doped core [100] for grating irradiation under a 325 nm UV beam, and obtained seven

Photonics 2019, 6, 36 5 of 18

2.2. Different Polymer Materials Due to the low photosensitivity of pure PMMA fiber [93], doped POFs have attracted researchers’ Photonicsinterest 2019 [80, 9819,, 99x ]. Peng’s group employed a step-index, multimode PMMA fiber with an organic5 of 18 dye doped core [100] for grating irradiation under a 325 nm UV beam, and obtained seven peaks in peaksthe reflected in the spectrum.reflected spectrum. The same The group same fabricated group fabricated another fiber another with fiber low-concentration with low-concentration ethyl and ethyla benzyl and methacrylate a benzyl methacrylate doped core, doped which core, led which to obtaining led to obtaining a −28 dB a transmission–28 dB transmission FBG with FBG 85 with min 85exposure min exposure [90]. Tam’s [90]. group Tam’s investigated group investigated a step-index a step-index PMMA POFPMMA with POF a TS with (1% a w.t.) TS (1% and diphenylw.t.) and diphenylsulfide (DPS) sulfide (5% (DPS) mole) (5% doped mole) core doped (diameter core (diameter of 8.2 µm), of as 8.2 well μm), as as pure well PMMA as pure cladding PMMA (diameter cladding (diameterof 150 µm). of Both 150 dopants μm). Both were dopants used to increase were used the refractive to increase index the and refractive enhance photosensitivityindex and enhance [80], photosensitivityand they obtained [80], one and FBG they of obtained about − 10one dB FBG transmission of about –10 after dB transmission 10 min. Saez after et al. 10 [ 93min.] reported Saez et al.a highly [93] reported photosensitive a highly mPOF photosensitive using Benzyl mPOF dimethyl using ketalBenzyl (BDK dimethyl )as a dopant ketal (BDK in the )as core, a dopant and − in23 the dB core,transmission and –23 wasdB transmission achieved after was 13 min,achieved since after the lack 13 mi ofn, extra since dopants the lack required of extra to dopants compensate required for the to compensateindex reduction for the allows index for reduction shorter times allows compared for shorter with times BDK-doped compared step-index with BDK-doped fiber [99 ].step-index Recently, fiberHu et [99]. al. [ 101Recently,] improved Hu et the al. fiber [101] drawing improved technology the fiber withdrawing selected technology center-hole with BDK selected doping center-hole in mPOF, BDKfor a rapidlydoping growingin mPOF, process for a rapidly with 83% growing reflectivity process in 40 with s. Tam’s 83% groupreflectivity investigated in 40 s. aTam’s new dopant group investigatedmaterial, diphenyl a new dopant disulphide, material, which diphenyl enables disulphide, a fast and positive which enables refractive a fast index and changepositive with refractive a low indexultraviolet change dose, with and a low leads ultraviolet to Bragg dose, gratings and fabrication leads to Bragg after gratings just 7 ms fabrication under 325 after nm-wavelength just 7 ms under UV 325signal nm-wavelength irradiation [102 UV]. signal irradiation [102]. f Pospori et al. [84] [84] and Pereira et al. [103] [103] fabricatedabricated FBGs in BDK-doped POF using 248 nm and 266 nm wavelength irradiation, respectively, and obtainedobtained strong POFBGs with a single short laser pulse (15 and 8 ns of duration), duration), as shown in Figure 22,, whichwhich isis eveneven compatiblecompatible withwith thethe fiberfiber drawingdrawing process. BraggBragg gratingsgratings inscription inscription in in the the 850 850 nm nm spectral spectral region region was was also also reported reported by using by using step-index step- indexPMMA PMMA POF irradiation, POF irradiation, using a using 248 nm a 248 krypton nm kr fluorideypton fluoride (KrF) excimer (KrF) laserexcimer system, laser which system, only which took only0.4 seconds took 0.4 with seconds 100 Hz with pulse 100 repetition,Hz pulse repetition, as shown inas Figureshown3 .in In Figure this fiber, 3. In the this cladding fiber, the material cladding is purematerial PMMA, is pure while PMMA, the core while is PMMAthe core dopedis PMMA with do TSped (1% with w.t.) TS and (1% diphenyl w.t.) and sulfide diphenyl (DPS) sulfide (5% (DPS) mole) [98]. (5%to enhance mole) to the enhance photosensitivity the photosensitivity and increase and the increase refractive the index refractive [98]. index

Figure 2. Transmission and reflection spectrum of Fiber Bragg Grating in BDK-doped micro-structured -d polymerFigure 2. optical Transmission fiber (mPOF) and withreflection one single spectrum 266 nm of wavelength Fiber Bragg pulse; Grating image in adapted BDK oped from [micro-103]. structured polymer optical fiber (mPOF) with one single 266 nm wavelength pulse; image adaptedMarques from et al. [103]. [104] reported the inscription of gratings in POFs made of different materials (TOPAS, ZEONEX, and Polycarbonate) under 248 nm wavelength, and compared it with the same fiberMarques irradiation et underal. [104] 325 reported nm, resulting the inscription in a reduction of gratings of the in irradiation POFs made time of bydifferent at least materials 16 times (Table(TOPAS,2) and ZEONEX, better stability and Polycarbonate) when 248 nm under wavelength 248 nm is wavelength, employed. and compared it with the same fiber irradiation under 325 nm, resulting in a reduction of the irradiation time by at least 16 times (Table 2) and better stability when 248 nm wavelength is employed.

Photonics 2019, 19, x 6 of 18

Photonics 2019, 6, 36 6 of 18 Photonics 2019, 19, x 6 of 18

Figure 3. Reflected spectrum of a FBG in a trans-4-stilbenemethanol (TS)-doped, step-index POF, inscribed using a 100 Hz repetition rate and pulse energy of 0.50 ± 0.02 mJ in 0.4 Figure 3. Reflected spectrum of a FBG in a trans-4-stilbenemethanol (TS)-doped, step-index POF, seconds (40 pulses); image adapted from [98]. Figureinscribed 3. usingReflected a 100 spectrum Hz repetition of a rate FBG and in pulsea trans-4- energystilbenemethanol of 0.50 ± 0.02 mJ (TS)-doped, in 0.4 seconds step-index (40 pulses); POF,image inscribed adapted from using [98 ].a 100 Hz repetition rate and pulse energy of 0.50 ± 0.02 mJ in 0.4 seconds (40Table pulses); 2. POFBG image inscription adapted fromwith [98].pulsed 248 nm KrF laser system [104]. Table 2. POFBG inscription with pulsed 248 nm KrF laser system [104]. Inscription FWHM band Optimal POFs Table 2. POFBGInscriptiontime inscription (seconds) Time with (nm)pulsedFWHM 248 nmReflection(dB) KrF laser Band systemOptimal[104].Energy(mJ) Energy POFs PMMA mPOF (seconds)Inscription 25 FWHM(nm) 0.4 (dB)Reflection 32 band (mJ)Optimal 6.0 POFs TopasPMMA 8007 mPOF mPOF time (seconds) 25 (nm) 0.60.4 (dB) 32 31 Energy(mJ) 6.0 5.5 Topas 8007 mPOF 25 0.6 31 5.5 PMMATopas 5013 mPOF mPOF 2520 0.40.6 3223 6.0 Topas 5013 mPOF 20 0.6 23 6.0 Topas step-index POF 11 0.8 31 5.0 TopasTopas 8007 step-index mPOF POF 2511 0.60.8 31 31 5.0 5.5 TopasZeonexZeonex 5013 480R 480R mPOF mPOF 2015 0.60.7 28 2328 3.5 6.03.5 TopasPolycarbonatePolycarbonate step-index mPOF POF 1114 0.80.6 23 3123 3.0 5.03.0 Zeonex 480R mPOF 15 0.7 28 3.5 PolycarbonateMinMin et et al. al. [58] [58 obtained]mPOF obtained the the first first 600 14 600nm nmwavelength wavelength 0.6 grating grating with commercial with 23 commercial CYTOP 3.0CYTOP POF, using POF, ausing 248 nm a 248 KrF nm laserKrF system laser system with a withrepetition a repetition of 40 Hz of and 40 Hz average and average pulse energy pulse energyof ~0.60 of mJ ~0.60 during mJ ~60during min,Min ~60 aset al.shown min, [58] as obtainedin shown Figure in the 4, Figure with first 4potential600, with nm potentialwavelength applications applications grating in the with visible in the commercial visiblerange. range. CYTOP POF, using a 248 nm KrF laser system with a repetition of 40 Hz and average pulse energy of ~0.60 mJ during ~60 min, as shown in Figure 4, with potential applications in the visible range.

Figure 4. Reflected spectrum of the FBG in cyclic transparent amorphous fluoropolymers (CYTOP) Figure 4. Reflected spectrum of the FBG in cyclic transparent amorphous fluoropolymers fiber; image adapted from [58]. (CYTOP) fiber; image adapted from [58]. 2.3. SpecialFigure Grating4. Reflected Devices spectrum of the FBG in cyclic transparent amorphous fluoropolymers 2.3. Special Grating Devices (CYTOP)Recently, fiber; a novel image bandpass adapted transmission from [58]. filter based on PS FBG at telecom wavelength [105] was obtainedRecently, usinga novel the bandpass Moiré method transmission by 325 filter nm Kimmonbased on laserPS FBG system at telecom exposure wavelength for about [105] 20 mins, was 2.3.obtainedas shown Special using in Grating Figure the Devices 5Moiré. A Moir methodé structure by 325 is nm formed Kimmo by superimposingn laser system twoexposure gratings for ofabout equal 20 amplitude mins, as shownbut withRecently, in Figure slightly a novel 5. different A Moirébandpass periods.structure transmission Figureis formed6 filtera showsby based superimposing the on resultPS FBG of twoat two telecom gratings superimposed wavelength of equal pulsesamplitude [105] was on obtainedbuta single-mode, with slightlyusing the BDK-dopeddifferent Moiré periods.method mPOF Figureby by 325 using 6a nm shows Kimmo a 248 the nmn result laser KrF ofsystem laser two emittingsuperimposed exposure an for output pulsesabout pulse 20on mins,a single- power as shownmode,of 2.5 mJ BDK-dopedin energyFigure and5. AmPOF aMoiré 15 nsby structure duration using a is [248106 formed ].nm The KrF by obtained lasesuperimposingr emitting grating showedan two output gratings 0.035 pulse nmof equalpower bandwidth amplitude of 2.5 andmJ but8 dB with in the slightly rejection different band, periods. with a high Figure level 6a of shows flexibility the result and no of needtwo superimposed for strain accuracy. pulses At on the a 850single- nm mode, BDK-doped mPOF by using a 248 nm KrF laser emitting an output pulse power of 2.5 mJ

Photonics 2019, 19, x 7 of 18

Photonicsenergy 2019and, a19 15, x ns duration [106]. The obtained grating showed 0.035 nm bandwidth and 8 dB in7 theof 18 rejection band, with a high level of flexibility and no need for strain accuracy. At the 850 nm Photonics 2019, 6, 36 7 of 18 energywavelength and a region, 15 ns duration PS FBG [106].was also The obtained obtained directly grating duringshowed the 0.035 grating nm bandwidth fabrication, and by placing8 dB in thea rejectionnarrow blocking band, with aperture a high in theleve cel nterof flexibility of the UV and beam no (see need Figure.6b) for strain [107]. accuracy. A high-quality At the 850Bragg nm wavelengthwavelengthgrating structure region,region, was PSPS obtained FBGFBG waswas with also –16.3 obtained dB and directly –13.2 dB during during dips inthe the transmission. grating grating fabrication, fabrication, The only by bydrawback placing placing a anarrowis narrow one narrow blocking blocking line aperture apertureneeding inblocking in the the ce centernter accuracy, of of the which UV beam must (see(see be put FigureFigure.6b) on 6theb) [phase [107].107]. mask. AA high-qualityhigh-quality BraggBragg gratinggrating structure structure was was obtained obtained with with− –16.316.3 dBdB andand− –113.23.2 dB dips inin transmission.transmission. TheThe onlyonly drawbackdrawback isis one one narrow narrow line line needing needing blocking blocking accuracy, accuracy, which which must must be be put put on on the the phase phase mask. mask.

Figure 5. Phase-shifted (PS) FBG fabricated by 325 nm irradiation: (a) uniform FBG (first Figurefabrication 5. Phase-shifted step), (b) (PS)PS FBG FBG (Moiré fabricated structure by 325 based nm irradiation: on two overlapped (a) uniform FBGuniform (first gratings); fabrication step),Figureimage (b adapted)5. PS Phase-shifted FBG from (Moir [105].é structure(PS) FBG based fabricated on two by overlapped 325 nm irradiation: uniform gratings); (a) uniform image FBG adapted (first fromfabrication [105]. step), (b) PS FBG (Moiré structure based on two overlapped uniform gratings); image adapted from [105].

Figure 6. PS-FBG fabrication by 248 nm irradiation: (a) Moiré overlapping method (image adapted fromFigure [106 6.]); PS-FBG (b) a narrow fabrication blocking by in 248 the centernm irradiation: (image adapted (a) Moiré from [overlapping107]). method (image adapted from [106]); (b) a narrow blocking in the center (image adapted from [107]). Although chirped FBGs in POF were proposed for tuning without a wavelength shift Figure 6. PS-FBG fabrication by 248 nm irradiation: (a) Moiré overlapping method (image in 2005Although [108], the chirped first chirped FBGs in FBG POF in were POF wasproposed inscribed for dispersion in 2017 using tuning an KrFwith excimerout a wavelength laser, operating shift atin 248 2005adapted nm, [108], and thefrom a 25-mmfirst [106]); chirped long (b) a chirpedFBG narrow in POF phase blocking was mask, inscribed in the customized center in 2017 (image using for 1550-nm anadapted KrF excimer grating from [107]). laser, inscription operating [67]. Theat 248 laser nm, pulse and ratea 25-mm was 1long Hz (5chirped mJ), and phase only mask, a few customized shots were for employed 1550-nm forgrating the gratinginscription response [67]. Although chirped FBGs in POF were proposed for dispersion tuning without a wavelength shift depictedThe laser in pulse Figure rate7a, withwas 1 a Hz 3.9 nm(5 mJ), bandwidth and only and a few 1.2 nm/cmshots were chirp. employed The chirped for the phase grating mask response method in 2005 [108], the first chirped FBG in POF was inscribed in 2017 using an KrF , operating offersdepicted significant in Figure stability, 7a, with with a high3.9 nm cost bandwidt and no flexibilityh and 1.2 as nm/cm its main chirp. drawbacks. The chirped Since then,phase different mask at 248 nm, and a 25-mm long chirped phase mask, customized for 1550-nm grating inscription [67]. techniquesmethod offers have significant been demonstrated stability, with to be high valid cost for and fabricating no flexibility chirped as its gratings main drawbacks. in POF. Theodosiou Since then, et The laser pulse rate was 1 Hz (5 mJ), and only a few shots were employed for the grating response al.different used the techniques femtosecond have direct been writing demonstrated method toto obtainbe valid chirped for fabricating FBG in commercial chirped gratings CYTOP POFin POF. [52], depicted in Figure 7a, with a 3.9 nm bandwidth and 1.2 nm/cm chirp. The chirped phase mask whichTheodosiou consisted et al of. used 2000 the periods, femtosecond with a direct total lengthwriting of method ~4.5 mm to obtain and a chirped 10-nm bandwidth FBG in commercial (chirp of method offers significant stability, with high cost and no flexibility as its main drawbacks. Since then, ~2.22CYTOP nm/mm), POF [52], as shownwhich consisted in Figure 7ofb. 2000 A femtosecond periods, with laser a total direct length writing of ~4.5 was mm used and for a flexible10-nm different techniques have been demonstrated to be valid for fabricating chirped gratings in POF. chirpedbandwidth grating (chirp writing, of ~2.22 with nm/mm), limitations as shown for low in Figure wavelengths. 7b. A femtosecond However, thelaser first direct tunable writing chirped was Theodosiou et al. used the femtosecond direct writing method to obtain chirped FBG in commercial FBGused was for fabricatedflexible chirped in a tapered, grating BDK-dopedwriting, with mPOF limitations by using for low a uniform wavelengths. phase maskHowever, under the 248 first nm CYTOP POF [52], which consisted of 2000 periods, with a total length of ~4.5 mm and a 10-nm UVtunable [109]. chirped The spectral FBG was reflected fabricated power in a of tapered, a 10 mm BDK-doped grating with mPOF chirp by ofusing ~0.26 a uniform nm/mm phase under mask 1.6% bandwidth (chirp of ~2.22 nm/mm), as shown in Figure 7b. A femtosecond laser direct writing was strainunder is 248 shown nm UV in Figure[109]. The7c, andspectral the tunablereflected properties power of a were 10 mm given grating by thewith strain chirp and of ~0.26 temperature nm/mm used for flexible chirped grating writing, with limitations for low wavelengths. However, the first sensitivity,under 1.6%with strain 0.71 is± shown0.02 pm/ in Figureµε and 7c, 56.7 and pm/ the◦C[ tunable109,110 properties], as shown were in Figure given8 .by However, the strain largely and tunable chirped FBG was fabricated in a tapered, BDK-doped mPOF by using a uniform phase mask chirpedtemperature POFBGs sensitivity, have been with also 0.71 fabricated ± 0.02 bypm/ hotμε water-assisted and 56.7 pm/°C gradient [109,110], thermal as shown annealing, in Figure as shown 8. under 248 nm UV [109]. The spectral reflected power of a 10 mm grating with chirp of ~0.26 nm/mm inHowever, Figure7d largely [ 111], chirped where one POFBGs grating have with been ~1.1 also nm/mm fabricated chirp by washot water-assisted obtained. The gradient simplicity thermal of this under 1.6% strain is shown in Figure 7c, and the tunable properties were given by the strain and method is one of its main advantages, since no special phase mask or additional etching are needed, temperature sensitivity, with 0.71 ± 0.02 pm/με and 56.7 pm/°C [109,110], as shown in Figure 8. and it enables easy control tuning of the central wavelength and chirp characteristics. However, largely chirped POFBGs have been also fabricated by hot water-assisted gradient thermal

Photonics 2019, 19, x 8 of 18 annealing,Photonics 2019 as, 19 shown, x in Figure 7d [111], where one grating with ~1.1 nm/mm chirp was obtained.8 ofThe 18 simplicity of this method is one of its main advantages, since no special phase mask or additional annealing, as shown in Figure 7d [111], where one grating with ~1.1 nm/mm chirp was obtained. The etching are needed, and it enables easy control tuning of the central wavelength and chirp simplicity of this method is one of its main advantages, since no special phase mask or additional characteristics. etching are needed, and it enables easy control tuning of the central wavelength and chirp Photonics 2019, 6, 36 8 of 18 characteristics.

Figure 7. The reflected spectral power of a chirped POFBG fabricated by different

Figuretechniques: 7. The (a reflected) chirped spectral phase powermask (image of a chirped adapted POFBG from fabricated [67]), (b by) femtosecond different techniques: directly Figure 7. The reflected spectral power of a chirped POFBG fabricated by different (awriting) chirped on phase CYTOP mask POF (image (image adapted from from [65]), [67]), ( (cb) )under femtosecond 1.6 % directlystrain with writing tapering on CYTOP method POF (imagetechniques: from [65 (a]),) chirped (c) under phase 1.6 % strainmask with (image tapering adapted method from (image [67]), adapted (b) femtosecond from [109]), directly and (d) (image adapted from [109]), and (d) thermal annealing (image adapted from [111]). thermalwriting annealing on CYTOP (image POF adapted (image from from [111 ]).[65]), (c) under 1.6 % strain with tapering method (image adapted from [109]), and (d) thermal annealing (image adapted from [111]).

Figure 8. The reflected spectral power of chirped POFBGs (a) under different strain and (b) under Figure 8. The reflected spectral power of chirped POFBGs (a) under different strain and (b) different temperatures; image adapted from [109]. under different temperatures; image adapted from [109]. Figure 8. The reflected spectral power of chirped POFBGs (a) under different strain and (b) Finally, regarding long period gratings in POF, the extensive literature during the last years under different temperatures; image adapted from [109]. showsFinally, different regarding mechanisms long and period methods gratings to be in used POF, for the fabricating extensive them literature in POF during [47,75 ,the76,87 last,95 ,years112]. Recently,shows different Min et mechanisms al. [113] demonstrated and methods a − to20 be dB used transmission for fabricating LPG them in mPOF in POF (see [47,75,76,87,95,112]. Figure9a) using Finally, regarding long period gratings in POF, the extensive literature during the last years theRecently, point-by-point Min et al. method, [113] demonstrated with an slit widtha –20 dB of transmission 0.2 mm; the beam LPG in was mPOF shifted (see 1 Figure mm for 9a) inscribing using the shows different mechanisms and methods to be used for fabricating them in POF [47,75,76,87,95,112]. everypoint-by-point point, and method, 25 steps with were an completed slit width toof obtain0.2 mm; an the LPG beam total was length shifted of 251 mm mm. for Each inscribing inscription every Recently, Min et al. [113] demonstrated a –20 dB transmission LPG in mPOF (see Figure 9a) using the pointpoint, was and irradiated 25 steps were by two completed 15 ns pulses to obtain emitted an byLPG the total UV length laser at of 1 25 Hz mm. frequency Each inscription repetition point rate, point-by-point method, with an slit width of 0.2 mm; the beam was shifted 1 mm for inscribing every andwas therefore, irradiated a by 2 stwo irradiation 15 ns pulses time meansemitted a by significant the UV laser reduction at 1 Hz from frequency the 42 srepetition per point rate, writing and point, and 25 steps were completed to obtain an LPG total length of 25 mm. Each inscription point timetherefore, reported a 2 ins irradiation a previous time work means [95]. a The significant strain sensitivity reduction aboutfrom the−2.3 42± s 0.05per point nm/mstrain writing wastime was irradiated by two 15 ns pulses emitted by the UV laser at 1 Hz frequency repetition rate, and measured for increasing strain, whereas −2.25 ± 0.05 nm/mstrain was measured for decreasing strain, therefore, a 2 s irradiation time means a significant reduction from the 42 s per point writing time due to polymer hysteresis, as shown in Figure9b; this is slightly larger than the value presented in the previous literature, which ranged from −1.40 to −1.44 nm/mstrain for increasing strain and between −1.30 and −1.40 nm/mstrain for decreasing strain [114]. Photonics 2019, 19, x 9 of 18

Photonicsreported 2019 in, 19a ,previous x work [95]. The strain sensitivity about –2.3 ± 0.05 nm/mstrain was measured9 of 18 for increasing strain, whereas –2.25 ± 0.05 nm/mstrain was measured for decreasing strain, due to reportedpolymer inhysteresis, a previous as work shown [95]. in TheFigure strain 9b; sensit this isivity slightly about larger –2.3 ± than0.05 nm/mstrainthe value presented was measured in the forprevious increasing literature, strain, which whereas ranged –2.25 from ± 0.05 –1.40 nm/m to –1strain.44 nm/mstrain was measured for increasing for decreasing strain strain, and between due to polymer–1.30 and hysteresis, –1.40 nm/mstrain as shown for in decreasing Figure 9b; strain this [114].is slightly larger than the value presented in the previous literature, which ranged from –1.40 to –1.44 nm/mstrain for increasing strain and between Photonics 2019, 6, 36 9 of 18 –1.30 and –1.40 nm/mstrain for decreasing strain [114].

Figure 9. (a) Transmission of Long Period Grating, and (b) wavelength change induced by Figure 9. (a) Transmission of Long Period Grating, and (b) wavelength change induced by increasing increasing and decreasing strain; image from [113]. Figureand decreasing 9. (a) Transmission strain; image of from Long [113 Period]. Grating, and (b) wavelength change induced by 3. Applications increasing and decreasing strain; image from [113].

3. ApplicationsDispersion compensation using using chirped chirped POFBG POFBG was was first first proposed in 2005 [107], [107], but the experimental demonstration of a largely tunable dispersive device based on a chirped FBG in mPOF Dispersion compensation using chirped POFBG was first proposed in 2005 [107], but the was published by Min et al. [[115].115]. Figure 10 depicts the reflectedreflected spectral power with a bandwidth experimental demonstration of a largely tunable dispersive device based on a chirped FBG in mPOF from 0.11 0.11 to to 4.86 4.86 nm, nm, as well as well as the as group the group delay delayvariation variation under strain, under which strain, corresponds which corresponds to a tunable to was published by Min et al. [115]. Figure 10 depicts the reflected spectral power with a bandwidth dispersiona tunable dispersionfrom 513.6 from to 11.15 513.6 ps/nm, to 11.15 respectively, ps/nm, respectively, with potential with potentialapplications applications in both inoptical both from 0.11 to 4.86 nm, as well as the group delay variation under strain, which corresponds to a tunable communicationsoptical communications and microwave and microwave photonics photonics systems. systems. dispersion from 513.6 to 11.15 ps/nm, respectively, with potential applications in both optical communications and microwave photonics systems.

Figure 10.10.Tunable Tunable chirped chirped FBG FBG (a) reflected(a) reflected spectral spectral power power vs strain vs andstrain (b) and group (b) delay group vs delay strain;

vsimage strain; adapted image from adapted [115]. from [115]. Figure 10. Tunable chirped FBG (a) reflected spectral power vs strain and (b) group delay vsAmong strain; theimage POF adapted applications, from [115]. due to polymer characteristics mentioned in the Introduction section of this paper, strain sensing is the most popular application. However, strain sensing under variableAmong temperature the POF andapplications, humidity due conditions to polymer is always characteristics an issue formentioned POF sensing. in the Min Introduction et al. [116] sectiondemonstrated of this paper, that the strain effective sensing bandwidth is the most of popular the tunable application. chirped POFBGHowever, is highlystrain sensing dependent under on the strain, and remains practically constant with temperature and humidity changes, which can be used in combination with wavelength measurement, as shown in Figure 11, to develop strain sensors under temperature- and humidity-variable environments. The strain sensitivity is 9.02 ± 0.02 pm/µε, which is higher than previous results [109] due to larger etching [92]. Photonics 2019, 19, x 10 of 18 Photonics 2019, 19, x 10 of 18 variable temperature and humidity conditions is always an issue for POF sensing. Min et al. [116] variabledemonstrated temperature that the and effective humidity bandwidth conditions of the is tu alwaysnable chirpedan issue POFBG for POF is sensing.highly dependent Min et al. on [116] the demonstratedstrain, and remains that the practically effective constantbandwidth with of tempthe tueraturenable chirped and humidity POFBG changes, is highly which dependent can be on used the strain,in combination and remains with practically wavelength constant measurement, with temp aserature shown and in humidityFigure 11, changes, to develop which strain can besensors used inunder combination temperature- with and wavelength humidity-variable measurement, environments. as shown The in strainFigure sensitivity 11, to develop is 9.02 strain ± 0.02 sensorspm/με, underwhich temperature-is higher than and previous humidity-variable results [109] dueenvironments. to larger etching The strain [92]. sensitivity is 9.02 ± 0.02 pm/με, Photonics 2019, 6, 36 10 of 18 which is higher than previous results [109] due to larger etching [92].

Figure 11. (a) Reflected spectral power vs strain, (b) wavelength shift vs strain, and (c) Figure 11. (a) Reflected spectral power vs strain, (b) wavelength shift vs strain, and (c) bandwidth vs bandwidth vs strain; image adapted from [116]. strain;Figure image 11. (a adapted) Reflected from [spectral116]. power vs strain, (b) wavelength shift vs strain, and (c) bandwidth vs strain; image adapted from [116]. OneOne ofof thethe recentrecent challengeschallenges forfor fiberfiber opticoptic temperaturetemperature sensingsensing isis foundfound inin biomedicalbiomedical applications.applications.One of Anthe essentialrecent challenges feature ofof thesethesefor fiber systemssystems optic isis thetemperaturethe possibilitypossibility sensing ofof detectingdetecti is ngfound temperaturetemperature in biomedical spatialspatial distributions,applications.distributions, An alsoalso essential knownknown asfeatureas thermalthermal of these mapsmaps systems [[117].117]. AA is linearlylinearly the possibility chirpedchirped of POFBGPOFBG detecti hashasng temperature beenbeen demonstrateddemonstrated spatial asdistributions,as aa semi-distributedsemi-distributed also known temperaturetemperature as thermal sensorsensor maps capablecapable [117]. A ofof linearly monitoringmonitoring chirped thethe POFBG temperaturetemperature has been profileprofile demonstrated alongalong thethe gratingasgrating a semi-distributed length for for minimally minimally temperature invasive invasive sensor scenarios scenarios capable [118]. [118 of ].Asmonitoring As shown shown in the inFigure Figuretemperature 12, 12 the, thechirped profile chirped POFBG along POFBG hasthe hasgratingbeen been placed length placed close for close minimally to a to radiofrequency a radiofrequency invasive scenarios applicator, applicator, [118]. withAs with shown a atip tip ininserted insertedFigure 12,in in thesitu situ chirped of the target—thePOFBGtarget—the has applicatorbeenapplicator placed connectedconnected close to totoa radiofrequency thethe RadioRadio FrequencyFrequency applicator, generator.genera withtor. Thea tip reflectionreflection inserted spectrumin situ of wasthe detecteddetectedtarget—the byby OpticalapplicatorOptical BackscatterBackscatter connected ReflectometerReflectometer to the Radio (LUNAFrequency OBROBR genera 4600),4600),tor. andand The thethe reflection temperaturetemperature spectrum gradientgradient was waswas detected estimatedestimated by withOpticalwith the Backscatter Gaussian model. model.Reflectometer High High sensitivity sensitivity (LUNA ofOBR ofchir chirped4600),ped POFBG and POFBG the supports temperature supports the detection the gradient detection of was spatially of estimated spatially non- non-uniformwithuniform the Gaussiantemperature temperature model. by means High by means sensitivityof spectral of spectral ofreconstr chir reconstruction,peduction, POFBG which supports which indicates indicatesthe thatdetection chirped that of chirped spatiallyFBG in FBG mPOF non- in mPOFuniformcan provide can temperature provide significant significant by advantages means advantages of spectral for thermal forreconstr thermal detectinguction, detecting inwhich bio-medical in indicates bio-medical applications. that chirped applications. FBG in mPOF can provide significant advantages for thermal detecting in bio-medical applications.

FigureFigure 12. 12.(a ) Schematic(a) Schematic of thermal of ablationthermal and ablation (b) measurement and (b) ofmeasurement a Gaussian temperature of a Gaussian gradient by using a chirped POFBG; image adapted from [118]. Figuretemperature 12. ( gradienta) Schematic by using of athermal chirped ablationPOFBG; imageand ( badapted) measurement from [118]. of a Gaussian Furthermore,temperature gradient chirped POFBGsby using fabricateda chirped withPOFBG; chirped image phase adapted mask from have [118]. been also demonstrated to provide accurate distributed pressure sensing [67]. The grating was subjected to pressure at various spatial position points of the grating, as shown in Figure 13, and the reflected spectral power showed a wavelength dip related to the pressed region, which can be used for different sensing applications. Photonics 2019, 19, x 11 of 18 Photonics 2019, 19, x 11 of 18 Furthermore, chirped POFBGs fabricated with chirped phase mask have been also demonstrated to provideFurthermore, accurate chirped distributed POFBGs pressure fabricated sensing with [67]. chirped The phase grating mask was have subjected been also to demonstrated pressure at variousto provide spatial accurate position distributed points of the pressure grating, sensing as shown [67]. in FigureThe grating 13, and was the reflectedsubjected spectral to pressure power at showedvarious a spatial wavelength position dip points related of the to grating,the pressed as shown region, in whichFigure 13,can and be usedthe reflected for different spectral sensing power applications.showed a wavelength dip related to the pressed region, which can be used for different sensing Photonicsapplications.2019, 6 , 36 11 of 18

Figure 13. (a) Setup for applying pressure and (b) measured spectrum under point pressure; Figure 13. (a) Setup for applying pressure and (b) measured spectrum under point pressure; imageFigure adapted 13. (a) Setup from for[67]. applying pressure and (b) measured spectrum under point pressure; image adapted from [67]. image adapted from [67]. AnotherAnother interesting interesting application application is is provided provided by by health health equipment equipment for for the the dynamic dynamic monitoring monitoring of of gait.gait. An AnAnother array array of interesting of five five FBGs FBGs application inscribed inscribed in inis CYTOP CYTOPprovided fiberfib byer [health[119]119] waswas equipment embeddedembedded for in inthe aa cork dynamiccork insole, insole, monitoring as as shown shown inof inFiguregait. Figure An 14 array14.. The The of advantages fiveadvantages FBGs of inscribed POFof POF (e.g., in(e.g., higherCYTOP high flexibility fiber erflexibility [119] and was robustness) and embedded robustness) enabled in a enabledcork monitoring insole, monitoring as patients shown patientswithin Figure higher with 14. bodyhigher The mass advantages body compared mass ofcompared withPOF similar(e.g., with high systems simier larflexibility based systems on and silicabased robustness) fiber, on silica with aenabledfiber, mean with sensitivity monitoring a mean of sensitivity~8.14patients PM/kPa, with of ~8.14 higher which PM/kPa, body is almost whichmass four compared is almost times higher four with times thansimi larhigher silica systems FBGs than based(~2.51silica FBGson pm/kPa). silica (~2.51 fiber, pm/kPa). with a mean sensitivity of ~8.14 PM/kPa, which is almost four times higher than silica FBGs (~2.51 pm/kPa).

FigureFigure 14. 14. ((a) FootFoot plantar plantar area area designation designation and and sensing sensing points points and (b )and FBG (b position) FBG position in the cork in insole; the corkimageFigure insole; from 14. (image [120a) Foot]. from plantar [120]. area designation and sensing points and (b) FBG position in the cork insole; image from [120]. SurfaceSurface plasmon plasmon resonance (SPR)(SPR) is is an an accurate accura andte and reliable reliable technique technique for determining for determining the change the changeof densitySurface of density at theplasmon interfaceat the interfaceresonance between between a(SPR) dielectric isa dielecan medium accuratric mediumte and and a metal,reliable and a which metal, technique has which attracted for has determining attracted huge attention huge the attentionforchange biochemical offor density biochemical sensing at the ininterfacesensing microfluidic in between microfluidic systems. a dielec systems. Hutric et medium al. Hu [120 et] and al. reported [120] a metal, reported the which first excitationthe has first attracted excitation of SPR huge at ofnear-infraredattention SPR at fornear-infrared biochemical telecom wavelength telecom sensing wavelength in with microfluidic gold-coated with systems. gold-coated POF Hu tilted et FBGs.POFal. [120] tilted They reported FBGs. show the theThey first transmission show excitation the ◦ transmissiondipsof SPR corresponding at near-infrared dips corresponding to the telecom orthogonally to wavelength the orthogon polarized withally modes, gold-coated polarized coupled modes, POF in atilted 50-nmcoupled FBGs. gold-coated in aThey 50-nm show 6 gold-tilted the coatedFBGtransmission immersed 6° tilted dips FBG in acorresponding solutionimmersed of in refractive ato solution the orthogon index of refr 1.408.allyactive Thepolarized index transmission 1.408. modes, The spectrum coupled transmission whenin a 50-nm thespectrum grating gold- wheniscoated immersed the 6° grating tilted in different isFBG immersed immersed calibrated in different in liquidsa solution calibrated covering of refr liquids aactive large covering index SRI (sounding 1.408. a large The refractiveSRI transmission (sounding index) refractive spectrum range of −2 index)2.5when× range 10the gratingRIU of 2.5 (refractive is× 10immersed−2 RIU index (refractive in unit). different The index refractometriccalibrated unit). The liquids refractometric sensitivity covering was a sensitivitylarge measured SRI (sounding upwas to measured ~550 refractive nm/RIU, up towithindex) ~550 a nm/RIU,wavelengthrange of 2.5with shift× 10a wavelength−2 as RIU a function (refractive shift ofthe asindex a SRI function unit). value, The of which therefractometric SRI is suitable value, forwhichsensitivity insitu is suitable operation. was measured for in situ up operation.to ~550Finally, nm/RIU, piezoelectric with a wavelength transducers shift of ultrasounds as a function are of widely the SRI used value, in thewhich biomedical is suitable area, forbut in situ the mainoperation. drawback is their sensitivity to electromagnetic fields. Optical fibers are promising for replacing

piezo electric transducers, with the benefit of electromagnetic interference immunity and with good

sensor sensitivity and size for ultrasonic detection. POF shows better robustness and sensitivity to pressure than silica fiber, as required by ultrasound detectors. Broadway et al. [121] presented the first ultrasonic detection at 5, 10, and 15 MHz by using a tilted FBG in commercial CYTOP fibers, as shown in Figure 15, which paves the way towards its eventual applications. Photonics 2019, 19, x 12 of 18

Finally, piezoelectric transducers of ultrasounds are widely used in the biomedical area, but the main drawback is their sensitivity to electromagnetic fields. Optical fibers are promising for replacing piezo electric transducers, with the benefit of electromagnetic interference immunity and with good sensor sensitivity and size for ultrasonic detection. POF shows better robustness and sensitivity to pressure than silica fiber, as required by ultrasound detectors. Broadway et al. [121] presented the Photonicsfirst ultrasonic2019, 6, 36 detection at 5, 10, and 15 MHz by using a tilted FBG in commercial CYTOP fibers,12 of as 18 shown in Figure 15, which paves the way towards its eventual applications.

FigureFigure 15. 15. UltrasonicUltrasonic detectiondetection setup. setup. 4. Conclusion and Outlook 4. Conclusion and Outlook Significant progress has been obtained during recent years in POF grating devices fabrication Significant progress has been obtained during recent years in POF grating devices fabrication and applications, in order to allow fast fabrication of POF grating devices under 248 nm and 266 nm and applications, in order to allow fast fabrication of POF grating devices under 248 nm and 266 nm wavelength UV, such as one short UV pulse (15 ns) for chirped POFBG fabrication. Besides the benefit wavelength UV, such as one short UV pulse (15 ns) for chirped POFBG fabrication. Besides the benefit of potential grating fabrication in the drawing tower, special grating structures also take advantage of of potential grating fabrication in the drawing tower, special grating structures also take advantage the short irradiation time to reduce the stability requirements in the fabrication setup. of the short irradiation time to reduce the stability requirements in the fabrication setup. Besides humidity-, temperature-, and strain-sensitive devices as the main applications of uniform Besides humidity-, temperature-, and strain-sensitive devices as the main applications of POF FBG, special grating devices open new perspectives. As the main relevant examples, this paper uniform POF FBG, special grating devices open new perspectives. As the main relevant examples, reviews chirped POFBGs for high-resolution thermal detection in the biomedical area, which show this paper reviews chirped POFBGs for high-resolution thermal detection in the biomedical area, higher sensitivity and bio-compatibility than silica ones. In addition, tilted POF FBG for SPR sensing which show higher sensitivity and bio-compatibility than silica ones. In addition, tilted POF FBG for and tilted FBGs for high-resolution acoustic detection are promising for in situ operation. Furthermore, SPR sensing and tilted FBGs for high-resolution acoustic detection are promising for in situ operation. optical communication applications, such as dispersion compensation and slicing broadband sources Furthermore, optical communication applications, such as dispersion compensation and slicing for WDM systems, among others, will benefit from the POF advantages in the implementation of broadband sources for WDM systems, among others, will benefit from the POF advantages in the future indoor networks. implementation of future indoor networks. To conclude, POF gratings show attractive performance from the sensing area to the short-range To conclude, POF gratings show attractive performance from the sensing area to the short-range optical communication area. However, as we know, most of the POFs for grating devices fabrication are optical communication area. However, as we know, most of the POFs for grating devices fabrication homemade, which still need more time to make this technology mature for potential real applications; are homemade, which still need more time to make this technology mature for potential real from this perspective, grating devices in commercial CYTOP POF are promising for real applications. applications; from this perspective, grating devices in commercial CYTOP POF are promising for real More progress is required to achieve strong and stable devices in reduced-loss and low-price POFs. applications. More progress is required to achieve strong and stable devices in reduced-loss and low- Authorprice POFs. Contributions: Writing—original draft preparation, R.M., B.O., and C.M.; writing—review and editing, B.O. and C.M. Author Contributions: Writing—original draft preparation, R.M, B.O., and C.M; writing—review Funding:and editing,This B.O research and C.M.; was funded by Research Excellence Award Programme GVA PROMETEO 2017/103 and Fundação para a Ciência e a Tecnologia, I.P. (FCT) through the program UID/CTM/50025/2019, by the NationalFunding: Funds This throughresearch the was Fundaç fundedão para by aResearch Ciência e aExcellence Tecnologia Award / Minist éProgrammerio da Educaç GVAão e Ci PROMETEOência, and by the European Regional Development Fund under the PT2020 Partnership Agreement. This work is also funded by national2017/103 funds and (OE), Fundação through FCT,para in thea scopeCiência of the e frameworka Tecnologia, contract foreseenI.P. (FCT) in the through numbers 4,the 5, andprogram 6 of the articleUID/CTM/50025/2019, 23, of the Decree-Law by 57/2016,the National of August Funds 29, throug changedh bythe Law Fundação 57/2017, para of July a Ciência 19, the Science e a Tecnologia Foundation / ofMinistério Heilongjiang da ProvinceEducação of Chinae Ciência, (F2018026) and andby Universitythe European Nursing Regional Program Development for Young Scholars Fund with under Creative the Talents in Heilongjiang Province (NPYSCT-2018012). PT2020 Partnership Agreement. This work is also funded by national funds (OE), through FCT, in Acknowledgments:the scope of the frameworkThe authors contract acknowledge foreseen the in supported the numbers by FCT, 4, 5, Research and 6 of Excellencethe article Award 23, of the Programme Decree- GVA PROMETEO 2017/103, and the Science Foundation of Heilongjiang Province of China. Law 57/2016, of August 29, changed by Law 57/2017, of July 19, the Science Foundation of ConflictsHeilongjiang of Interest: ProvinceThe of authors China declare (F2018026) no conflict and Univ of interest.ersity Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province（UNPYSCT-2018012） References

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