Spectra profile expansion of Bragg wavelength on nano-particle embedded fiber-Bragg-grating

Available online at www.sciencedirect.com

Sensors and Actuators A 141 (2008) 334–338

Spectra profile expansion of Bragg wavelength on nano-particle embedded fiber-Bragg-grating Pham Van Hoi a,b,∗ , Pham Thanh Binh a , Pham Tran Tuan Anh b , Ha Xuan Vinh a , Chu Thi Thu Ha a , Nguyen Thu Trang a a

National Key Laboratory for Electronic Materials and Device, Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau giay District, Hanoi, Viet Nam b Faculty of Physics Engineering and Nano Technology, College of Technology, Vietnam National University in Hanoi, 144 Xuan Thuy Road, Cau giay District, Hanoi, Viet Nam Received 9 November 2006; received in revised form 4 October 2007; accepted 4 October 2007 Available online 22 October 2007

Abstract This article presents the results of a detailed study on the effects of spectra profile expansion of Bragg wavelength on the performance of the nanoparticle embedded fiber-Bragg-grating (nano-EFBG) for sensing applications. The fiber-Bragg- grating (FBG) was coated by CdSe-nano-particle layers with various thicknesses (600–2000 nm) and bonded on substrates of epoxy or epoxy/Teflon with a large thermal expansion coefficient. With this embedding method, a variation of the line-width expansion of Bragg wavelength with cooling down FBG has been controlled. The nano-EFBG morphology was investigated by FE-SEM and the nano-EFBG sensors are studied in ambient from 77 K (liquid nitrogen) to 393 K. The expansion of spectral profile, which caused by transverse loading from nano-particle/epoxy layers, can be changed in the range of 0.1–1.3 nm between before and after cooling down. This result is for the strain-temperature sensors, but has the potential application in the FBG dispersion compensation devices and many other measurands. © 2007 Elsevier B.V. All rights reserved. Keywords: Nano-particles; Embedded fiber-Bragg-grating; Fiber optic sensors

1. Introduction Wavelength tuning of fiber-Bragg-gratings (FBGs) by lateral or transverse load, temperature and/or vibration is attractive for optical sensing [1–6]. The wavelength response of the FBG upon temperature, lateral and transverse load is highly dependent on the surrounding media, its configuration and the contact conditions. As we know, by fixing FBG on a substrate with a large thermal expansion coefficient, the sensitivity of temperature FBG sensor can be enhanced to 1.5–15 times that of pure FBG [5,7–9]. But, Reid and Ozcan [10] demonstrated that an FBG embedded in composite material at 4.2–300 K showed the same temperature dependence as that of non-embedded FBG sensors, because the composite materials had small thermal expansion ∗

Corresponding author at: National Key Laboratory for Electronic Materials and Device, Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau giay District, Hanoi, Viet Nam. Tel.: +84 4 8360586; fax: +84 4 8360705. E-mail address: [email protected] (P.V. Hoi). 0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.10.034

coefficients. Suresh and Tjin had been developed the embedded FBG with two layers from carbon composite and deformable materials for shear force sensors with a linear variation of the wavelength shift [11]. Therefore, selection of substrate materials is especially important for embedded FBG sensors [14]. For EFBG temperature sensors, when the contact surface between embedded material and glass fiber is homogeneous and smooth, there is no significant change in the profile of both spectra, before and after cooling down [8]. But in practice, contact surface between substrate and FBG has micron-size roughness. From this non-homogeneous contact surface, the transverse load would be changed from point to point along the FBG, when sensor is cooling down, and it is caused a significant change in the line-width of Bragg wavelength between before and after cooling down [12]. In this paper, we propose use of nano-particles thin film of CdSe as a controlled-roughness layer on FBG and it is embedded in epoxy/Teflon with various thickness and configuration, for a fiber optic sensor. We examined several types of embedded FBG sensors in the large temperature range (from 77 K to 393 K).

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The line-width expansion of Bragg wavelength, which depended upon thickness of nano-particle coated layer and bonded material substrate, has been studied and discussed. We compared the experimental spectra expansion of Bragg wavelength caused by transverse load with a theoretical one. 2. Experimental procedure In our work, the FBG was written by holographic method using a KrF-Excimer laser (248 nm) and Talbot interferometer. The optical fiber was a commercially photosensitive germanosilicate single-mode fiber. An FBG has one or multi-grating (maximum five grating at different Bragg wavelengths) into one fiber. The Bragg wavelength at room temperature was in the range of 1530–1550 nm (the spacing between each Bragg wavelength was of 5 nm for FBG-array), a reflectivity was of 75–90% and a full-width-half-maximum (FWHM) bandwidth was of 0.15–0.30 nm. The length of FBG was of 15 mm. The broad-band light source was an amplified spontaneous emission (ASE) from erbium-doped fiber amplifier (EDFA). The spectral measurement was performed with a reflection scheme using 1550 nm fiber optic circulator. The reflection spectrum was observed with an Optical Spectrum Analyzer (Advantest Q8384), which has the spectra resolution of 0.01 nm. The shift of Bragg wavelength induced by the change of temperature is as [8]: λB = [(1 − pe )α + ξ]T λB

(1)

where α = (1/Λ)(␦Λ/␦T) is the linear thermal expansion coefficient, pe the photo-elastic constant (pe ∼ = 0.22 [6]) and ξ = (1/neff ) (␦neff /␦T) is the thermo-optic coefficient of fiber, respectively. The coefficients α and ξ are not linearly depending on temperature in the large range. For germanosilicate fibers, α is so small (0.5 × 10−6 K−1 ) that the effect of the thermal expansion is one order less than that of the thermo-optic refractive index change (ξ ∼ = 10−5 K−1 ). If the effect of thermal expansion is large enough, the temperature sensitivity of the FBG sensor may be proportional to the thermal expansion coefficient. When FBG is embedded into materials, transverse strain may arise that will also shift the period of the grating. In addition, the non-homogeneous of contact surface between FBG and substrate causes the perturbation of transverse loading along the fiber, when the temperature is cooling down. This perturbation of strain on fiber provides expansion of spectral profile of Bragg wavelength. The shift of Bragg wavelength by pressure can be calculated by following formula [13]:   λ 1 − 2ν n2eff + = − (2) (1 − 2ν)(2p12 + p11 ) P λB E 2E where ν is Poisson’s ratio, p11 and p12 are components of strain-optic tensor, and E is modulus of elasticity. The FBGs were assembled as shown in Fig. 1. In our experiment we used colloidal nano-particles of CdSe with size of 6–10 nm for coating FBG, because their linear thermal expansion coefficient of 7.4 × 10−6 K−1 [15] was similar with this one of epoxy layer.

Fig. 1. Schematic of FBG temperature sensors: the FBG coated by nano-particle CdSe with thickness of 600–2000 nm (left) and nano-particle coated FBG is bonded into the epoxy cylinder (d = 3 mm) coated by Teflon cylinder with d = 10.3 mm.

The nano-CdSe thin film with effective thickness from 600 nm to 2000 nm was deposited on fiber by dip-coating method. The epoxy (OCI–USA Inc.) and the large thermal expansion materials such as copper, Teflon (with average linear thermal coefficient of 96.10−4 K−1 [16]) were used as substrate. The configurations of substrate are rectangular and/or cylinder with various sizes. The embedded FBG was inserted into metallic housing and put into various environment temperatures such as Dewar vessel containing liquid nitrogen, ice or boiled water and/or thermal furnace. The use of FBG-array permits to study in detail the change of spectra line-width of reflection light in various temperatures. The measurement was performed in thermal equilibrium during several scanning times of Optical Spectrum Analyzer (more than 10 min). 3. Results and discussions Fig. 2a shows SEM images of micro-bending and microsize roughness of contact surface between glass fiber and epoxy substrate of epoxy/Teflon embedded FBG sensor. The microbending of tens-micron radius provides optical loss of reflection power and the micron-size roughness on contact surface provides the perturbation of transverse strain on fiber that causes change of spectral profile of Bragg wavelength. Fig. 2b demonstrates the SEM image of fiber coated by nano-particle of colloidal CdSe with thickness about 2 ␮m. This nano-particle layer can control the level of homogeneous of contact surface between fiber and embedded materials. Fig. 3 presents the experimental results of wavelength shifts of different materials–substrate embedded FBG sensors at low temperature range (77–360 K). At room temperature range (301–360 K) the Bragg wavelength shift of non-embedded FBG was of 0.648 nm and the average temperature sensitivity corresponds to 11.3 pm K−1 . At low temperature range (77–301 K) the Bragg wavelength shifted by 3.49 nm (from 1539.63 nm to 1536.14 nm). For non-embedded FBG, there is no significant change in the line-width spectra and in reflection peak level before and after cooling down. This result is good suitable to sensitivity value of typical silica FBG in ref. [8]. The temperature sensitivity of epoxy/Teflon embedded FBG is not linear from non-linear change of thermal expansion of Teflon substrate in the range 77–300 K [15]. It is remarkable, that when FBG bonded into epoxy, the Bragg wavelength was slightly shifted to long-wavelength zone (for example the Bragg wavelength shift about 0.2 nm in our case). Fig. 4 shows the experimental wavelength shifts of one FBG bonded into epoxy

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Fig. 3. Wavelength shifts of different material–substrate embedded FBG with temperature.

Fig. 2. SEM images of non-homogenous surface between epoxy and glass fiber in epoxy/Teflon embedded FBG (a) and CdSe-nano-particle coated FBG (b).

cylinder with diameter of 3 mm (Fig. 4a) and the other coated by 2-␮m layer of nano-CdSe and epoxy (Fig. 4b) at 301 K, 277 K and 77 K. Both types of EFBG were coated by Teflon cylinder with diameter of 10.3 mm. The sensitivity of both epoxy/Teflon

substrate sensors is the same value (about 164 pm K−1 at room temperature and average sensitivity corresponds to 71–73 pm K−1 from room temperature to liquid nitrogen temperature). The signal peak level decreased by some decibel (3–5 dB) and it is considered to be micro-bending induced into the coating of the fiber (see Fig. 2). The significant change in the spectral profile of Bragg wavelength of EFBG, when the sensors inserted into liquid nitrogen several times, has been observed. To study of line-width expansion of Bragg wavelength, two groups of EFBG sensors were taken. The epoxy/Teflon embedded FBG with rectangular and cylinder configurations were taken for the first group. The nano-CdSe coated layer/epoxy/Teflon embedded FBGs were in the second one. The nano-CdSe effective thickness of 600 nm, 1000 nm, 1500 nm and 2000 nm was tested. The spectra line-width of EFBG has been compared with that of non-substrate FBG at the same temperature. The spectra linewidth at −10 dB was increased from 0.3 nm for non-substrate FBG to 1.62 nm and to 0.4–0.51 nm for the epoxy/Teflon and

Fig. 4. The expansion of spectral profile of epoxy/Teflon substrate FBG was of 1.62 nm (a) and of CdSe-nano epoxy/Teflon embedded FBG was of 0.51 nm (b) observed after some times cooling down.

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Table 1 Characteristics of various embedded FBG temperature sensors Type of sensors

T0 (K)

λ0 (nm)

Ts (K)

λs (nm)

Wavelength shift (nm)

Line-width at −10 dB (nm)

Average sensitivity (pm K−1 )

Epoxy-embedded FBG Epoxy-copper FBG Epoxy/Teflon FBG (one side) Epoxy/Teflon FBG (two side) Epoxy/Teflon FBG (cylinder) nano-CdSe coated/epoxy/Teflon FBG Non-embedded FBG

301 301 301 301 301 301 301

1529.90 1550.00 1550.00 1550.20 1539.89 1539.81 1539.63

77 77 77 77 77 77 77

1524.32 1544.91 1544.36 1532.78 1523.91 1523.44 1536.14

5.58 5.08 5.64 17.42 15.98 16.37 3.49

1.1 1.2 1.2 1.54 1.62 0.51 0.3

24.91 22.68 25.18 77.78 71.34 73.08 15.58

nano-CdSe coated/epoxy/Teflon sensors, respectively. We can explain this effect by the micron-size space between the fiber and the surrounding materials, which made non-homogeneous of contact surface of FBG. The micron-size roughness on contact surface would provide a perturbation of transverse strain from point-to-point along the fiber, when the embedded FBG was cooling down. This perturbation of transverse strain caused the non-homogeneous change of Bragg periods of the FBG. We used the value of ν, p11 and p12 , E for silica glass fiber from ref. [13], and the neff changed from 1.444 at room temperature to 1.441 at 77 K for calculation of transverse pressure on FBG. The calculation result of P = 10.4–136.9 MPa is good suitable to elasticity condition of strain on silica glass fibers Table 1. 4. Conclusion The nano-embedded FBG temperature sensors for large temperature range (from 77 K to 373 K) have been developed. A nano-EFBG sensors using epoxy/Teflon cylinder configuration achieved the high-sensitivity coefficient of 164 pm K−1 and 73 pm K−1 at room temperature and at large range from 77 K to 373 K, respectively. The expansion of spectra line-width of EFBG sensors at low temperature changed from 0.1 nm to 1.3 nm and it depended upon thickness of nano-particle coated layers. This phenomenon may be explained by a perturbation of transverse strain along the fiber, which caused by micron-size space between fiber and surrounding materials. Acknowledgment This work was supported by the Vietnamese Physics Research Program for 2006–2007. References [1] C.R. Giles, Light wave applications of fiber Bragg gratings, Light wave Technol. 15 (1997) 1391. [2] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, E.J. Friebele, Fiber grating sensors, Light wave Technol. 15 (1997) 1442. [3] S. Ekannellopoulos, V.A. Handrek, A.J. Rogers, Simultaneous strain and temperature sensing with photo generated in-fiber gratings, Opt. Lett. 20 (1995) 333.

[4] S.W. James, M.L. Dockney, R.P. Tatum, Simultaneous independent temperature and strain measurement using in-fiber Bragg grating sensors, Electron. Lett. 32 (1996) 1133. [5] A. Inoue, M. Shigehara, M. Ito, M. Inai, Y. Hattori, T. Mizunami, Fabrication and application of fiber Bragg grating—a review, Optoelectron. Dev. Technol. 10 (1995) 119. [6] S.C. Tjin, J.Z. Hao, Y.Z. Lam, Y.C. Ho, B.K. Ng, A pressure sensor using fiber Bragg grating, Fiber Integr. Opt. 20 (2001) 59. [7] G. Meltz, W.W. Morey, Bragg grating formation and germanosilicate fiber photosensitivity, Proc. SPIE 1516 (1992) 185. [8] T. Mizunami, H. Tatehata, H. Kawashim, High-sensitivity cryogenic fiberBragg-grating temperature sensors using Teflon substrates, Meas. Sci. Tecnol. 12 (2001) 914. [9] S. Gupta, T. Mizunami, T. Yamao, T. Shimomura, Fiber Bragg grating cryogenic temperature sensors, Appl. Opt. 35 (1996) 5202. [10] M.B. Reid, M. Ozcan, Temperature dependence of fiber optic Bragg gratings at low temperatures, Opt. Eng. 37 (1998) 237. [11] R. Suresh, S.C. Tjin, Effects of dimensional and material parameters and cross-coupling on FBG based shear force sensor, Sens. Actuators A (120) (2005) 26–36. [12] T.Nguyen Thu, H. Do Thanh, H. Pham Van, B. Pham Thanh, V. Ha Xuan, H. Chu Thi Thu, Improvement of temperature sensitivity of fiberBragg-grating sensors using large thermal expansion coefficient material substrates, Proc. of Vietnam National Conf. on Optics and Spectroscopy 2006, in press. [13] O. Frazao, R. Romero, F.M. Araujo, L.A. Ferreira, J.L. Santos, Straintemperature discrimination using a step spectrum profile fiber Bragg grating arrangement, Sens. Actuators A (120) (2005) 490–493. [14] M.G. Xu, L. Reekie, Y.T. Chow, J.P. Dakin, Optical in-fiber grating high pressure sensor, Electron. Lett. 29 (1993) 398–399. [15] V. Kumar, B.S. Sastry, Thermal expansion coefficient of binary semiconductors, J. Crys. Res. Technol. 36 (6) (2001) 565–569. [16] R.K. Kirby, Thermal Expansion of polytetrafluoroethylene (Teflon) from −190◦ to +300 ◦ C, J. Res. Nat. Bureau Standards 57 (2) (1956) 91–94.

Biographies Pham Van Hoi was born in Hanoi, Vietnam, in 1952. He received the PhD degree in optoelectronics from Lebedev Institute of Physics, Russian Academy of Sciences, Moscow, Russia, in 1986. At present time, he is a principal researcher at the State Key Laboratory for Electronic Materials and Devices, Institute of Materials Science, Vietnamese Academy of Science and Technology and a professor at College of Technology, Vietnam National University in Hanoi. His field of interest is in the area of photonic and fiberoptic devices for applications in the communication and sensory. He is the author and coauthor of more than 100 publications, including international journals and conferences. Ha Xuan Vinh was born in Dalat, Vietnam, in 1961. He received the engineering degree in Physics from Dalat University and master degree from Institute of Physics, Vietnamese Academy of Science and Technology, in 1983 and 2003, respectively. He is currently pursuing the PhD degree in fiberoptic communi-

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cation devices in the Institute of Physics, VAST of Vietnam. His main research interests are light amplification and optical resonators. Pham Thanh Binh was born in Phutho, Vietnam, in 1977. He received BS degree from College of Natural Sciences, Vietnam National University, Hanoi and MS degree from ITIMS, Hanoi, Vietnam, in 2000 and 2005, respectively. His research interests are fiberoptic FBGs for applications in the telecom. He is permanent researcher in the State Key Laboratory for Electronic Materials and Devices, Institute of Materials Science, Vietnamese Academy of Science and Technology and is currently pursuing the PhD degree in optical sensor in the Institute of Materials Science, VAST of Vietnam. Chu Thi Thu Ha was born in Hanoi, Vietnam, in 1979. She received B.S. degree from Faculty of Technology, Vietnam National University, Hanoi and

M.S. degree from College of Technology, VNUH, in 2001 and 2005, respectively. Her research interests are fiberoptic communication devices, light amplification and optical resonators in Institute of Materials Science, Vietnamese Academy of Science and Technology. Nguyen Thu Trang was born in Hanoi, Vietnam, in 1980. She received BS degree from Faculty of Technology, Vietnam National University, Hanoi and MS degree from College of Technology, VNUH, in 2002 and 2005, respectively. Her research interests are fiberoptic communication devices and light amplification in Institute of Materials Science, Vietnamese Academy of Science and Technology and is currently pursuing the PhD degree in fiberoptic communication devices in College of Technology, Vietnam National University, Hanoi.