Bend-insensitive long period fiber grating-based high temperature sensor

Optical Fiber Technology xxx (2014) xxx–xxx

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Optical Fiber Technology www.elsevier.com/locate/yofte

Bend-insensitive long period fiber grating-based high temperature sensor Zhiyong Bai, Weigang Zhang ⇑, Shecheng Gao, Hao Zhang, Li Wang, Fang Liu Key Laboratory of Optical Information Science and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 22 January 2014 Revised 26 June 2014 Available online xxxx Keywords: Long period fiber grating Bend-insensitive High temperature sensor Optical fiber sensor

a b s t r a c t A novel bend-insensitive long period fiber grating (LPFG) is presented and applied in high temperature measurement. This LPFG is formed by periodically arranging three micro-core-offsets fabricated by employing the cleaving–splicing method in the standard single mode fiber. As bend is applied onto the LPFG along the most bend-insensitive orientation, the resonant wavelength sensitivity of the transmission spectrum is 0.0097 nm/m1 for a curvature range of 0–3.5 m1, which is about three orders lower than those of the conventional gratings. For other orientations of the LPFG, the bend sensitivities are also one or two orders lower than those of conventional ones. This better bend characteristic could be used to improve the sensing performances of the LPFG, such as improving the measurement accuracy and resolving cross sensitivity issue between bend and other parameters. Moreover, since the mode coupling of the LPFG results from the fiber geometry deformation which indicates a better thermal stability, the LPFG could be used for high temperature measurement occasions. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Optical fiber gratings (OFGs) containing fiber Bragg gratings (FBGs) [1] and long period fiber grating (LPFGs) [2] have been widely used in the field of optical sensing and communicating, owing to their intrinsic merits, such as small size, light weight, electromagnetic immunity, and good compatibility with optical fiber. In the past few years, the applications of OFGs have been investigated in some harsh environments [3,4], such as the power generation system, tunnel fire alarms, and oil exploration. In these applications, an extended temperature measurement range is required, and the thermal stability of the spectra characteristics is one of the most important performance indicators. However, the conventional OFGs written by UV-laser exposure are easy to erase at high temperature before a complex annealing process [3,5]. Recently, high-temperature resistant FBGs based on regenerated gratings or written by femtosecond laser pulses have been reported [3,6,7], and their spectra could be stable at the temperature of 1000 °C. The LPFGs written by CO2 laser or arc discharge in standard single mode fiber present good thermal stability [8,9], and could be survived at temperature of above 1000 °C. Moreover, the structural LPFGs whose mode coupling mainly results from the fiber geometrical deformation have attracted

⇑ Corresponding author.

much research interests as well [10,11], and they possess very short grating length, could normally operate under higher temperature and have become a better candidate for the high temperature measurement application. Actually, the high temperature systems often involve the mechanical vibration and turbulences, and moreover, high temperature could also result in the deformation of the substrate material, both of which could induce the micro-bend of the OFGs, and as a result, to affect the temperature measurement accuracy. Therefore, a method to resolve the cross sensitivity between the bend and temperature is needed. Many solutions have been proposed for discriminating the two different parameters induced wavelength shifts [12–15], such as using reference grating [13] and dual-parameter measurement [14,15]. These methods generally need to add one or more other fiber elements to the sensor structures, and a more complex interrogation process is normally required. A simple method is using a bend-insensitive OFG to test the temperature. In fact, the FBGs written in the standard single mode fiber is almost insensitive to bend, due to the core-to-core mode coupling characteristic [16]. Furthermore, various approaches to obtain the bendinsensitive LPFGs have been reported in the past few years, including employing the optimal orientation of the asymmetric LPFGs [17], fabricating special fiber-based LPFGs [18], and utilizing ultrashort LPFGs [4]. Comparing with the FBGs, the LPFGs present a higher sensitivity for ambient perturbations because of the core-to-cladding mode

E-mail address: [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.yofte.2014.09.004 1068-5200/Ó 2014 Elsevier Inc. All rights reserved.

Please cite this article in press as: Z. Bai et al., Bend-insensitive long period fiber grating-based high temperature sensor, Opt. Fiber Technol. (2014), http:// dx.doi.org/10.1016/j.yofte.2014.09.004

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coupling principle [2]. Due to the larger pitch, the LPFGs could be achieved by more fabricating methods at a low cost. In this paper, an ultra-short LPFG based on periodical arrangement of three micro-core-offsets (MCOs) along the single mode fiber (SMF, Corning SMF-28e) axis fabricated by using the cleaving-splicing method (CSM) has been proposed and experimentally demonstrated. Due to the ultra-short grating length, a reduced bend sensitivity of the MCO–LPFG has been obtained experimentally, which is over one order lower than those of the conventional LPFGs. Furthermore, due to the asymmetrical mode coupling achieved by the MCO, the bend sensitivity of the MCO–LPFG is orientationdependent, and at the most bend-insensitive orientation of the grating, the bend sensitivity is only 0.0097 nm/m1, which is about three orders lower than those of the conventional LPFGs and the ultra-short MCO–LPFG could be considered as bend-insensitive components. In addition, the main physical mechanism of the mode coupling in MCO–LPFG is based on the fiber-core offset which is one of the fiber structural changes and could survive in the harsh environments. Therefore, the bend-insensitive MCO–LPFG is used in the field of high temperature measurement. The temperature sensitivity is 0.0977 nm/°C and 0.2652 nm/°C for the temperature range of 20–800 °C and 800–1000 °C, respectively. 2. Operation principle In the past few decades, a good variety of LPFG fabrication methods have been proposed and developed. Generally, the laser-based fabrication technologies include UV-laser exposure [2], CO2-laser irradiation [17,18], and femtosecond-laser ablating [10,11] ; the non-laser-based ones involve arc discharge [4], HF etching [19], CSM [20], and so on. The CSM fabrication method is distinctly different from previously reported ones. The fiber is firstly cleaved with a desired length of hundreds of micrometers, and then two fiber end facets are spliced with a purposed MCO. The sketch of the MCO–LPFG fabrication system with fusion splicer exclusive is presented in Fig. 1, which mainly consists of three parts: a fiber holder fixed on a 3-axis translation stage with a readable accuracy of 0.5 lm, a high-precision fiber cleaver (FITEL S3250) with a cleaving accuracy of <0.5° fixed on the fabrication platform, and a pulley used to attach a weight. The three parts is well aligned at first, which is important to ensure the cleaving performance of the fiber end facets and the accuracy of the fabricated grating pitch. The fusion-splicer (FITEL S178A) used in our experiment is set at the built-in ‘‘OFST-OFST’’ mode to fabricate the MCO. In this fusion splicing mode, the ‘‘offset value’’ can be freely set in the range of 0–70 lm, and other parameters are defaulted for SMF. The MCO–LPFG could be achieved according to the following procedure: (1) The SMF with a bare section is straightly stretched by the fiber holder and a 5-g weight, and the bare section is mounted

on the fiber cleaver. (2) The aligned fiber is cleaved, and then the left end of the fiber is put in the fusion splicer without loosening the fiber holder. The other end is directly put in the fusion splicer to splice with the other end with a purpose MCO whose offset direction is marked by a scotch tape flags attached to the fiber. (3) The fiber is held straightly again and then moved a desired length along the fiber axis in z direction with the assistance of high-precision 3-axis translation stage. This desired length is according to the grating pitch design before our experiment. Next MCO will be successively fabricated in the same way at the new position. The MCO fabrication procedure is repeated three times and a strong mode coupling in the grating will occur with an appropriate offset value of fiber cores. During the MCO–LPFG fabrication process, the transmission spectrum is monitored by an optical spectrum analyzer (OSA) in real time with a supercontinuum source (SCS) serving as the light source. The schematic diagram in the dashed circle in Fig. 1 shows the MCO–LPFG with only two grating pitches. The direct advantage owing to the compactness of ultra-short MCO–LPFG is the reduced the bend sensitivity [4,21]. Furthermore, the inset of Fig. 1 shows the cross section of the MCO with the initial indication of the grating orientations, showing an asymmetrically fiber geometry deformation. An asymmetric mode coupling in the MCO–LPFGs has been observed experimentally [20], and as a result, the bend characteristics of the MCO–LPFGs will be dependent on the orientation of the grating. For an appropriate orientation of the MCO–LPFGs, a very low bending sensitivity could be obtained. By considering the above two factors, a bend-insensitive MCO–LPFG could be achieved by the CSM. Additionally, the mode coupling in the MCO–LPFGs results from the fiber geometrical deformation, which could survive in harsh environment, and therefore, the ultra-short MCO–LPFG could be used as a high temperature sensor as well. 3. Experiment results and discussion An ultra-short MCO–LPFG with only three MCOs (two periods) with a pitch of 548 lm is achieved by employing the CSM, and the preset offset value is 3.0 lm in the splicer mode. The corresponding transmission spectrum is shown in Fig. 2, and a spectral fringe contrast of more than 25 dB is achieved. The measured resonant wavelength kres and peak attenuation p are kres = 1298.79 nm and p = 33.02 dB, respectively. The sensing characteristics of bend and high temperature for this grating are measured according to the following procedure.

0 -5

0o

z 180o

SCS

OSA Translation stage

Cleaver

Transmission(dB)

y

270o

90o

x O

-10 -15 -20 -25 1298.79 nm 33.02 dB

-30 -35 1100

Fig. 1. The schematic diagram of MCO–LPFG fabrication system. The dashed circle shows the structure of the MCO–LPFG, and the inset shows the cross-sectional geometry of the MCO.

1200

1300

1400

1500

1600

1700

Wavelength(nm) Fig. 2. The transmission spectrum of the MCO–LPFG with a pitch of 548 lm.

Please cite this article in press as: Z. Bai et al., Bend-insensitive long period fiber grating-based high temperature sensor, Opt. Fiber Technol. (2014), http:// dx.doi.org/10.1016/j.yofte.2014.09.004

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Z. Bai et al. / Optical Fiber Technology xxx (2014) xxx–xxx 1305 -26

Resonant wavelength(nm)

Transmission (dB)

1304

The bend experiment setup is shown in Fig. 3. The SMF with a MCO–LPFG put in a capillary is mounted on two aligned slits and passes through two torsion disks. One of the fiber ends is fixed by the torsion disk A, and the other end goes through disk B and could freely move along z direction to eliminate the effect of strain. The two torsion disks are used to achieve the rotation orientation of the grating and two slits are applied to support the capillary to bend. A metal rod fixed on a moving stage pushes the capillary to induce the MCO–LPFG bending along the direction shown as the arrow in Fig. 3. In our experiment, the length, external and inner radius of the capillary is 10 cm, 600 lm and 280 lm, respectively. The distance between two slits is 9 cm. The bending curvature is calculated by considering the bent capillary as the arc of a circle. The chord length of the arc is 2L, and the moving distance of the metal rod is d, thus the bending curvature is expressed by 2 2d=ðL2 þ d Þ. The MCO–LPFG is measured within a curvature range of 0–3.5 m1 for four grating orientations corresponding to directions of 0°, 90°, 180°, 270°. The transmission spectral evolutions with respect to the bending curvatures are plotted in Fig. 4. Fig. 4(a) shows a blue shift of the resonant wavelength and decay of the peak attenuation as the MCO–LPFG is bent along 0° direction, and the total fluctuations of them is 0.36 nm and 1.12 dB, corresponding to the sensitivity of –0.082 nm/m1 and 0.326 dB/m1, respectively. Fig. 4(b) shows the transmission spectral evolutions against to the bend along 90° direction. As the curvature increases, the resonant wavelength and peak attenuation only change a little, and the total fluctuations of them respectively are 0.08 nm and 0.12 dB, corresponding to the sensitivity of –0.0097 nm/m1 and 0.0297 dB/m1. Fig. 4(c) demonstrates the transmission spectral evolution with respect to the bend curvatures along the bend directions of 180°. The total fluctuations and the sensitivities of the resonant wavelength and peak attenuation are 0.48 nm, 0.53 dB, 0.1269 nm/m1, and 0.1615 dB/m1, respectively. Fig. 4 (d) shows the transmission spectral responses to the bend along 270° directions. The resonant wavelength shift displays two distinct steps: the resonant wavelength shifts towards short wavelength in the curvature range of 0–1.775 m1, and returns to the initial position in the curvature range of 1.775–3.436 m1. The maximum of the resonant wavelength shift is 0.32 nm occurring at the curvature of 1.775 m1, corresponding the bend sensitivity of –0.1490 nm/m1 in 0–1.775 m1 and 0.1877 nm/m1 in 1.775– 3.436 m1. The peak attenuation approximately linearly decays according to a slope of 0.1773 dB/m1 in the curvature range of 0–3.436 m1. To clearly describe the bend characteristic for the four directions, the related parameters are listed in the Table 1. The terms dk, dp, Sk and Sp respectively represent the total variations and sensitivities of resonant wavelength and peak attenuation. From the Table 1, the bend sensitivity of the MCO–LPFG is orientation-dependent. For the most bend-insensitive orientation, the bend sensitivity is about three orders lower than those of the conventional LPFGs, and even at the most bend-sensitive orientation, the bend sensitivity is one or two orders lower than those of the conventional LPFGs. Therefore, when the MCO–LPFG is utilized to measure the ambient perturbations, such as temperature, strain,

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0 0.395 0.987 1.578 2.168 2.755 3.339

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1290

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Peak attenuation(dB)

3.1. Bend sensing characteristics

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peak attenuation(dB)

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Resonant Wavelength(nm)

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(c)

0 -1 0.296m -1 0.592m -1 0.889m -1 1.480m -1 2.070m -1 2.657m -1 3.242m

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1305 0 -1 0.395m -1 0.691m -1 1.086m -1 1.677m -1 2.266m -1 2.852m -1 3.436m

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1301 1300

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Curvature(m ) Fig. 4. Bend responses of the transmission spectrum of the MCO–LPFGs for the orientations of (a) 0°, (b) 90°, (c) 180°, (d) 270°.

Slit d

SCS Torsion disk A

L

OSA Torsion disk B

Fig. 3. The schematic experimental setup for the bend test.

Please cite this article in press as: Z. Bai et al., Bend-insensitive long period fiber grating-based high temperature sensor, Opt. Fiber Technol. (2014), http:// dx.doi.org/10.1016/j.yofte.2014.09.004

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Z. Bai et al. / Optical Fiber Technology xxx (2014) xxx–xxx

Table 1 The bend-dependent spectral response of the MCO–LPFG.

0° 90° 180° 270°

dk (nm)

dp (dB)

Sk (nm/m1)

Sp (dB/m1)

0.36 0.08 0.48 0.32

1.12 0.12 0.53 0.63

–0.0820 –0.0097 0.1269 –0.1490/0.1877

0.3259 0.0297 0.1615 0.1773

0

(a)

-5

Transmission(dB)

-10

o

20 C o 116 C o 200 C o 300 C o 400 C o 500 C o 600 C o 700 C o 800 C o 900 C o 1000 C

-15 -20 -25 -30 -35 1250

1300

1350

1400

1450

1500

1550

1600

4. Conclusion A novel bend-insensitive MCO–LPFG has been proposed and experimentally demonstrated. For the most bend-insensitive orientation, the transmission spectrum of the MCO–LPFG is almost bend-insensitive for the range of 0–3.5 m1. This ideal bend feature could be exploited to resolve the cross sensitivity between bend and other parameters. And moreover, the mode coupling mechanism of the MCO–LPFG depends on the fiber geometrical deformation, which indicates a better thermal stability of the proposed grating. Thus, the MCO–LPFG could be employed for an extend range of temperature measurement from room temperature to 1000 °C.

1650

Wavelength(nm) 1440

-10

(b)

Linear fitting of λres

1400

Experiment data of p Linear fitting of p

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1360

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1340

Acknowledgments

-15

-30

Peak loss (dB)

Experiment data of λres

1420

Resonant wavelength (nm)

respectively 0.0977 nm/°C and 0.0123 dB/°C for the temperature range of 20–800 °C, and 0.2652 nm/°C and 0.0199 dB/°C for a temperature range of 800–1000 °C. At 1000 °C, the peak loss is still exceeding 12 dB, which indicates that the MCO–LPFG could survive in a high temperature environment. Thus, the MCO–LPFG could be potentially applied for an extended range of temperature measurement from 20 °C to 1000 °C. Due to the proper fabricating parameters, the MCO have a good mechanical strength. No mechanical damage is observed in the bending test. But after the high temperature test, the optical fiber becomes fragile, and thus a good package for the MCO–LPFG is needed.

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This work was jointly supported by the National Natural Science Foundation under Grant Nos. 11274181, 10974100, 10674075, 11004110, 11274182, 11104149, the Doctoral Scientific Fund Project of the Ministry of Education under Grant No. 20120 031110033, the Tianjin Key Program of Application Foundations and Future Technology Research Project under Grant No. 10JCZDJC24300, and the Fundamental Research Funds for the Central Universities, China. References

o

Temperature ( C) Fig. 5. High temperature responses of the transmission spectrum of MCO–LPFG. (a) Temperature-dependent transmission spectral evolutions of the MCO–LPFG. (b) Resonant wavelength shifts and peak attenuation variations for different temperatures.

refractive index and so on, the bend-induced cross sensitivity could be approximately avoided by paralleling the most bend-insensitive orientation and deformation direction of the substrate used to fix the grating, which is very important in the practical application of the fiber grating. 3.2. High temperature sensing characteristics After the bend measurement, the MCO–LPFG is put in a high temperature furnace and heated from 20 °C to 1000 °C with an temperature increment of 100 °C. For each temperature point, the transmission spectrum is recorded after 30 min for spectral stabilization. The transmission spectral evolutions for different temperatures are shown in Fig. 5(a), exhibiting a red shift of resonant wavelength and decrease of spectral fringe contrast. The resonant wavelength shifts and peak attenuation variations with respect to temperature are linearly fitted and depicted in Fig. 5(b). The sensitivities of the resonant wavelength and peak attenuation are

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