DSpace at VNU: Spectra profile expansion of Bragg wavelength on nano-particle embedded fiber-Bragg-grating

Sensors and Actuators A 141 2008 334–338Spectra profile expansion of Bragg wavelength on nano-particle embedded fiber-Bragg-grating Pham Van Hoia,b,∗, Pham Thanh Binha, Pham Tran Tuan An

Sensors and Actuators A 141 (2008) 334–338

Spectra profile expansion of Bragg wavelength on nano-particle embedded fiber-Bragg-grating Pham Van Hoia,b,∗, Pham Thanh Binha, Pham Tran Tuan Anhb,

aNational 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

bFaculty 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 nano-particle embedded fiber-Bragg-grating (nano-EFBG) for sensing applications The fiber-Bragg- grating (FBG) was coated by CdSe-nano-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

condi-tions 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:hoipv@ims.vast.ac.vn (P.V Hoi).

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 materi-als 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 sur-face 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 embed-ded 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) 0924-4247/$ – see front matter © 2007 Elsevier B.V All rights reserved.

doi: 10.1016/j.sna.2007.10.034

The line-width expansion of Bragg wavelength, which depended

upon thickness of nano-particle coated layer and bonded

mate-rial 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

germanosil-icate 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

emis-sion (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

whereα = (1/Λ)(␦Λ/␦T) is the linear thermal expansion

coeffi-cient, pethe photo-elastic constant (pe∼= 0.22[6]) andξ = (1/neff)

(␦neff/␦T) is the thermo-optic coefficient of fiber, respectively.

The coefficients α and ξ are not linearly depending on

tem-perature in the large range For germanosilicate fibers,α is so

small (0.5× 10−6K−1) that the effect of the thermal

expan-sion is one order less than that of the thermo-optic refractive

index change (ξ ∼= 10−5K−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

sub-strate 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

λB =



−1− 2ν E +n2eff

2E(1− 2ν)(2p12+ p11)



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 inFig 1 In our experiment we used

colloidal nano-particles of CdSe with size of 6–10 nm for

coat-ing FBG, because their linear thermal expansion coefficient of

7.4× 10−6K−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 96.10−4K−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 var-ious 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 bending and micro-size roughness of contact surface between glass fiber and epoxy substrate of epoxy/Teflon embedded FBG sensor The micro-bending of tens-micron radius provides optical loss of reflection power and the micron-size roughness on contact surface pro-vides the perturbation of transverse strain on fiber that causes change of spectral profile of Bragg wavelength.Fig 2b demon-strates the SEM image of fiber coated by nano-particle of col-loidal 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 experimen-tal 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 aver-age 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 4shows the experimental wavelength shifts of one FBG bonded into epoxy

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

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

substrate sensors is the same value (about 164 pm K−1 at room temperature and average sensitivity corresponds to 71–73 pm K−1from room temperature to liquid nitrogen tem-perature) 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 (seeFig 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 embed-ded 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 line-width 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.

Table 1

Characteristics of various embedded FBG temperature sensors

shift (nm)

Line-width at

−10 dB (nm)

Average sensitivity (pm K −1)

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

con-tact 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

tempera-ture to 1.441 at 77 K for calculation of transverse pressure on

FBG The calculation result ofP = 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

tem-perature 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−1at 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

trans-verse 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

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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 Sci-ences, 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 pro-fessor 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

engineer-ing 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-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, respec-tively 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 com-munication devices in College of Technology, Vietnam National University, Hanoi.