sensors Review

Fabrication of Polymer Optical Fibre (POF) Gratings Yanhua Luo 1,2 , Binbin Yan 3 , Qijin Zhang 4 , Gang-Ding Peng 1, *, Jianxiang Wen 5 and Jianzhong Zhang 6 1 2 3 4 5 6

*

Photonics & Optical Communications, School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, NSW 2052, Australia; [email protected] State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201600, China State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China; [email protected] CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China; [email protected] Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University, Shanghai 200072, China; [email protected] Key Lab of In-fiber Integrated Optics, Ministry of Education, Harbin Engineering University, Harbin 150001, China; [email protected] Correspondence: [email protected]; Tel.: +61-2-9385-4014

Academic Editors: Christophe Caucheteur and Tuan Guo Received: 6 January 2017; Accepted: 28 February 2017; Published: 4 March 2017

Abstract: Gratings inscribed in polymer optical fibre (POF) have attracted remarkable interest for many potential applications due to their distinctive properties. This paper overviews the current state of fabrication of POF gratings since their first demonstration in 1999. In particular we summarize and discuss POF materials, POF photosensitivity, techniques and issues of fabricating POF gratings, as well as various types of POF gratings. Keywords: polymer optical fibre (POF); POF gratings; photosensitivity; grating fabrication; phase mask; point-by-point; sensor

1. Introduction With the continuing development of material and fabrication technologies over the last three decades, the transmission attenuation of POFs has been greatly decreased. POFs are advantageous for home networks as well as storage interconnections [1]. These developments have been well described in review articles [1–7]. There are also some review papers covering conventional POFs and doped POFs for communications [8,9]. Besides conventional POFs, material properties, fabrication and applications of microstructured POFs (mPOFs) have also been reviewed [10–13]. In general POF has several distinctive advantages over silica fibre for sensing applications [14]. POF sensors have been discussed as a special class of fibre optic sensors included in fibre sensor reviews by Bartlett [15], Grattan [15] and Zubia [3]. Recently reviews specific on POF sensors have been presented by Peters [16], Bilro [17], and Granville [18]. Thematic reviews on the smart textiles, structural health monitoring (SHM), and aircraft SHM applications of POF sensors have also been described by Zhang [19], Kuang [20] and Zubia [21], respectively. Among all different types of POF sensors, those based on fibre gratings written in POFs have attracted great attention since they were first demonstrated in 1999, due to their quite distinctive properties such as higher sensitivity and larger dynamic range compared to their silica counterparts [22]. A number of earlier reviews on POF gratings and their sensing applications had appeared in reports by Peng [23], Canning [24], Kuang [20], Argyros [12,13], Peters [16], Bilro [17], Zubia [21], Granville [18] and Farrell [25]. The difficulties and Sensors 2017, 17, 511; doi:10.3390/s17030511

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challenges encountered during the photo-inscription of gratings in mPOFs have also been reviewed by Berghmans [26]. Recently, more specific reviews on POF grating-based optical sensors have been given by Webb [27,28]. Webb summarized POF gratings’ properties, photosensitivity, sensitivities and their sensing applications. New advances in POF Bragg gratings regarding the produce of smooth POFs end faces with high quality and the increase of quality in the production of FBGs have also been summarized by Bilro [29]. These previous reviews regarding POF gratings are mainly focused on the properties of POF gratings as well as their sensing applications. However, there hasn’t been any systematical review on the fabrication of POF gratings. Through the intense work of nearly two decades, a variety of POF grating techniques and applications have been developed. Therefore, it will be useful to summarize the work conducted on POF gratings during the last 18 years. The review of their fabrication will be appreciated by the community and is also a good starting point for researchers who are new to the field. This paper discusses published work, work being performed at present, as well as prospective future work in POF gratings and their applications. In Section 2, we briefly introduce the general aspects of POF gratings, usual POF materials and their photosensitivity and photosensitive POFs. In Section 3, we describe and discuss techniques and issues of fabricating POF gratings. Different types of POF gratings developed so far have been summarized in Section 4. 2. POF Gratings Optical fibre Bragg gratings (FBGs) have a periodic (or quasi-periodic) modulation of the refractive index along the fibre core. The FBG preferentially reflects light with a wavelength, λB , which is determined by the Bragg condition [30]: λB = 2neff Λ

(1)

where neff is the effective index of optically guided mode and Λ is the period of the modulation. For sensing applications, gratings are very useful because any change of strain or temperature applied to them changes both the period and index of the grating, resulting in a shift of λB . The periodic structure is normally produced by exploiting the intrinsic sensitivity of the core material to inscription light (λw ) and exposing the fibre to a periodic intensity pattern produced by interfering two light beams. The amplitude of index modulation is very closely related to the photosensitivity of POF materials. Thus a proper writing wavelength is to be selected according to the photosensitivity of materials. In the regard we will discuss POF materials and their photosensitivity, photosensitive POFs, as well as inscription and operating wavelength of POF gratings in the following subsections. 2.1. POF Materials And Their Photosensitivity The material information and photosensitivity of various reported POFs have been summarized and listed in Table S1. Poly(methyl methacrylate) (PMMA), polycarbonates (PC), polystyrene (PS), cyclic olefin copolymer (COC produced by TOPAS, Frankfurt, Germany) and amorphous fluoropolymer (CYTOP produced by Asahi Glass, Tokyo, Japan) are the most popular optical polymers used for the POF fabrication. Their chemical structures are listed in Table 1. The material properties, like refractive index (n), glass transition temperature (Tg ) and melting temperature (Tm ), thermal expansion coefficient (α), thermo-optic coefficient (dn/dT), stress-optic coefficient and moisture absorption are also listed in Table 1. The n will determine λB when Λ is fixed. Tg and Tm will determine both the processing and maximum operation temperatures.

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Table 1. The properties of host materials for POF gratings.

Materials Materials

Materials Materials Materials

Structure Structure Structure Structure Structure

Table Table 1. 1. The The properties properties of of host host materials materials for for POF POF gratings. gratings. Table 1. The properties of host materials for POF gratings. n Tg Tm n n n

T Tgggg T

°C °C °C

◦C

T m Tm m T m °C °C °C

Stress-Optic



dn/dT Stress-Optic Moisture Stress-Optic Moisture Coefficient Stress-Optic Moisture Coefficient Absorption Coefficient Absorption Coefficient Absorption −4 ◦ C−1 −4 ◦ C−1 −12 Pa−1 10 10 10 −4 −1 −12 −1 −4 −1 −12 −1 10 10 wt % −4 °C −1 −12 Pa −1 10 °C 10 Pa wt 10−4 °C−1 10−12 Pa−1 wt % %

α α α

C −1 −4 −4 −1 10 −4 °C −1 10 °C 10−4 °C−1

dn/dT dn/dT dn/dT

α

Moisture Absorption wt %

poly(methyl methacrylate) poly(methyl methacrylate) poly(methyl methacrylate) poly(methyl poly(methyl methacrylate) methacrylate) (PMMA) (PMMA) (PMMA) (PMMA)

1.49 160 [31] 1.49[31] [31] 104 [31] 104 [31] 1.49 160 1.49 [31] [31] 104 104 [31] [31] 160 [31] [31]

0.68 160[31] [31] 0.68 0.68 [31] [31]

−1.05 [31] −1.05 [31] −4.5~−1.5 −1.05 [31] −1.05 [31] up to 2.0 [31] 0.68 [31] −4.5~−1.5 −1.05 [31] [31] −4.5~−1.5 −4.5~−1.5 [31] [31] up to to 2.0 2.0 [31] [31] −1.2 [31] −1.2 [31] up [31] −1.2 −1.2 [31] [31]

polycarbonates (PC) polycarbonates polycarbonates (PC) polycarbonates(PC) (PC)

1.58 267 1.58[31] [31] 170 1.58 [31] 170 [31] [31] 267 [31] [31] 1.58 [31] 170 [31]170 [31] 267 [31]

0.66 267[31] [31] 0.66 [31] 0.66 [31]

−1.07 [31] −1.07 −1.07 [31] [31] −1.1~−1.4 −1.1~−1.40.66 [31] −1.1~−1.4 [31] [31] [31] [31]

68 [31] 68 [31] 68 [31]

polystyrene (PS) polystyrene (PS) polystyrene polystyrene(PS) (PS)

1.59 [31]~90 [31] 240 [31] 1.59 [31] ~90 240 1.59[31] [31] ~90 1.59 [31] ~90 [31] [31] 240 [31] [31]

0.7 0.7 [32] 240[32] [31] 0.7 [32]

−1.3 [33]0.7 [32] −1.3 −1.3 [33] [33]

4.8 [31] −1.3 [33] 4.8 4.8 [31] [31]

0.1–0.3 [31] 0.1–0.3 4.8 [31] 0.1–0.3 [31] [31]

cyclic olefin copolymer cyclic olefin copolymer cyclic olefin cyclic olefincopolymer copolymer (COC, TOPAS) (COC, TOPAS) (COC, TOPAS) (COC, TOPAS)

70–177 190–320 70–177 190–320 1.53 1.53 [31] 1.53[31] [31] 70–177 70–177190–320 [31] 1.53 [31] [31] [31] 1.53 [31] [31] [31] [31] [31]

0.6 [31] [31] 0.6 [31][31] −1.0 −1.0 [31]0.6 [31] 190–320 0.6 −1.0 0.6 [31] [31] −1.0 [31] [31]

4.0 [31] 4.0 [31] −1.0 [31] 4.0 4.0 [31] [31]

0.01 [31] 0.01 [31]4.0 [31] 0.01 0.01 [31] [31]

0.01 [31]

−0.97 [34] [35] −0.97 [34] −0.97 [34] 6.5 [31] 6.5 −0.97 1.15–1.20 [34] 6.5 [35] [35]

Fabrication of Polymer Optical Fibre (POF) Gratings.

Gratings inscribed in polymer optical fibre (POF) have attracted remarkable interest for many potential applications due to their distinctive properti...
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