hv photonics Review Latest Achievements in Polymer Optical Fiber 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 photonic crystal 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 wavelength; 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 laser 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 DiameterDiameter fiber (POF) Main structures.Main Feature feature Fiber type Transversal section Diameter Main feature Small-diameterFiberSmall-diameter type step-index step- Transversal section 100~200Diameter100~200µm μ m EasyMainEasy coupling coupling feature FiberSmall-diameter type step- Transversal section Diameter100~200