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Thin Solid Films 516 (2

Hydrogen sensor based on side-polished fiber Bragg gratings coatedwith thin palladium film

Chuen-Lin Tien a,⁎, Hong-Wei Chen b, Wen-Fung Liu a, Shou-Shan Jyu a,Shane-Wen Lin b, Yung-Sen Lin c

a Department of Electrical Engineering, Feng Chia University, Taichung, Taiwan, ROCb Graduate Institute of Electrical and Communications Engineering, Feng Chia University, Taiwan, ROC

c Department of Chemical Engineering, Feng Chia University, Taichung, Taiwan, ROC

Available online 13 July 2007

Abstract

Anew type of hydrogen sensor based on a side-polished fiberBragg grating (FBG) coatedwith thin palladium filmwas demonstrated experimentally.The used FBGwith the reflectivity of 90% is fabricated in a hydrogen-loaded single-mode fiber (SMF-28) by using the phasemaskwriting technique of aKrF excimer laser. The experimental results show that proposed sensor can be applied for hydrogen concentration measurements.© 2007 Elsevier B.V. All rights reserved.

Keywords: Hydrogen sensor; Side-polished fiber; Fiber Bragg grating; Palladium film

1. Introduction

The measurement of hydrogen gas by using fiber optics hasrecently attracted a lot of researchers due to its many advan-tages compared to the conventional methods. For instance,hydrogen gas sensors are based on different types of solid-statetechniques and the semiconductor-based hydrogen sensors arefabricated with the combination of electrical methods [1–4].They have primarily been suitable for the detection of hy-drogen at low concentrations. Many of these sensors involveelectrical conductors, limiting their suitability for use inpotentially explosive environments. Hence, optical techniquesseem to be more attractive owing to the lack of sparkingpossibilities. The thin palladium layer is often used for sensinghydrogen material due to that it has strong atomic bondinginteraction with hydrogen gas. Several different types of fiberoptic sensors based on the palladium thin-film to measure thehydrogen concentration have been reported such as M.A.Bulter proposed an interferometric optical fiber sensor and amicromirror approach to detect hydrogen gas [5–7]. Some

⁎ Corresponding author. Tel.: +886 4 24517250x3809; fax: +886 4 24516842.E-mail address: cltien@fcu.edu.tw (C.-L. Tien).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.07.045

optical hydrogen sensors based on Pd-coated fiber Bragggrating have been demonstrated [8–10]. For our knowledge,the side-polished fiber gratings combing with the palladiumthin-film is first proposed in this paper.

Fiber Bragg gratings (FBGs) have been developed to be oneof the important optical passive components with awide range ofapplications [11–13]. Fiber Bragg gratings are fiber optic de-vices characterized by periodic changes of fiber core refractiveindex. Basic principle of operation normally used in an FBGsensing device is to monitor the wavelength shift of returnedBragg signal against any change in physical parameter beingmeasured. The FBG sensors have advantageous features ascompared to conventional sensors. They have small size, lowweight, immunity to electromagnetic interference, and distrib-uted sensing possibilities. We proposed a novel hydrogen sens-ing device by using side-polished FBG coated with a palladiumfilm. The technique of side-polished fiber is used for removingthe fiber side-cladding tomake the core to be exposed for coatinga sensing film to detect the Bragg wavelength shift caused by thefilm-index change due to the action of the film with hydrogen.Side-polished fibers have an interesting structure in which thecladding is laterally polished till small distance to the corethat can increase sensitivity of fiber sensors. The optical FBGpolished into the core exhibits an interesting property. Becauseof the asymmetric structure of the side-polished FBG, an output

Fig. 1. Schematic diagram of a hydrogen sensing device.

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optical power loss for the reflection or transmission spectracould be observed by an optical spectrum analyzer (OSA). Inthis study, we present the experimental results obtained when thesensing device was exposed to various hydrogen concentrationsat a fixed temperature. The proposed sensor containing a side-polished fiber Bragg grating with the Pd coating could be usedfor a hydrogen sensor.

2. Basic sensing principles

A fiber Bragg grating consists of a periodic modulation ofthe refractive index in the core of a single-mode optical fiber.When the Bragg condition is satisfied, the contributions ofreflected light from each grating plane add constructively in thebackward direction to form a back-reflected peak with a centerwavelength defined by the grating parameters. The first-orderBragg condition is given by

kB ¼ 2neffK ð1Þ

where the Bragg wavelength, λB, is the center wavelength of theinput light that will be back-reflected from the Bragg grating,and neff is the effective index of the fiber core and Λ is theperiod of fiber Bragg grating.

Fig. 2. Experiment set-up used

The shift of reflective Bragg wavelength can be derived as

DkBkB

¼ Dneffneff

þ DKK ;

ð2Þ

where ΔλB is the Bragg wavelength's shift, Δneff is the var-iation of the effective index of the fiber core and ΔΛ is theperiod change of fiber Bragg grating. When the sensing deviceis exposed to hydrogen, the grating period of the fiber is in-creased slightly due to the expansion of the palladium layer. Forthis device, the lateral strain in the side-polished fiber gratingwill cause the shift of Bragg wavelength, in which the indexchange (Δneff) will dominate the parameters comparing with thegrating period change (ΔΛ). Therefore, the grating period var-iation (ΔΛ) can be neglected. The shift of Bragg wavelengthΔαB can be calculated by the induced birefringence, and theshift of Bragg wavelength in Eq. (2) can be expressed simply as

DkBi2DneffK: ð3Þ

Eq. (3) can be used to predict the peak shift of FBG under alateral strain. As the fiber cladding is polished to the core, theeffective index of the fiber core will be varied and result in theshift of reflected Bragg wavelength and bandwidth expansion.

When the FBG is coated with Pd thin-film, the stress as-sociated with the hydrogen absorption in the film can bemeasured by monitoring the reflection or transmission spectra ofthe FBG. As the hydrogen is absorbed by the Pd film, it expandsbecause hydrogen absorption converts Pd to PdHx which has alower density and larger volume [8]. The expansion of thehydride will stretch the fiber gratings in radial direction andchange its effective index. Frommeasuring the amount of Braggwavelength shift by using an OSA, the hydrogen gasconcentrations can be determined. The resolution of the OSAis set to 0.01 nm, in order to obtain experimental results of highquality.

3. Experimental results and discussion

For the fiber grating fabrication, a standard single-modeoptical fiber (SMF-28) is used with typical parameters: the core

to measure hydrogen gas.

Fig. 3. Reflection spectra of FBG before and after side-polished. Fig. 5. The curve of wavelength shift versus hydrogen concentration.

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diameter of 10 μm, the cladding diameter of 125 μm, the corerefractive index of 1.4647, and the grating period of 0.534 μm.In our experiment, the used FBG is written in a hydrogen-loaded single-mode fiber by using the phase mask writingtechnique of a KrF excimer laser (248 nm). The length of FBGis 20 mm with the reflectivity of 90%. For fiber polishing it wasglued in silicon V groove with the set-up similar to the onedescribed in Ref. [14]. The length of the side-polishedinteracting section Is about 12 mm and the optical losses re-sulting polishing are negligible. The fiber–optic sensing regionwith the diameter to be polished down to 62.9 μm can bedetermined by an optical microscope. The proposed sensingdevice is schematically illustrated in Fig. 1. It is composed ofthe thin palladium film which was deposited by DC sputteringtechnology and fiber Bragg gratings for detecting the hydrogengas. Thin palladium film was deposited on a side-polishedsurface of the single-mode optical fiber in high vacuum. A Pdthin-film with thickness was about 20 nm and the Pd filmabsorbs hydrogen which causing mechanical expansion involume. The microstructure of thin Pd film coated on the side-polished single-mode fiber could be analyzed by the scanningelectron microscopy.

Fig. 4. The comparison of Pd-coated FBG reflection spectra between with andwithout exposing in hydrogen concentrations.

The experiment set-up used to measure the hydrogen gas atatmospheric pressure and room temperature is shown in Fig. 2. Asensing device was put into the hydrogen gas chamber with thepressure of 1.38 MPa in different hydrogen concentrations. Aglass tube was used as a gas cell wherein a homogeneousmixtureof hydrogen and nitrogen (used as a carrier gas) was flowing.Both hydrogen and nitrogen flow rates were individually con-trolled by using high-precision gas valves and flow meters. Forthe experimental measurements, the grating reflection spectrawere firstly recorded for a fixed hydrogen concentration byusing a broadband light source, and then the reflected spectrawere measured in different hydrogen concentrations. Thebroadband light through a 3-dB coupler was coupled into thesensing device for measuring the reflected light using an opticalspectrum analyzer (OSA). As the core of fiber grating section ispolished, the modal effective index changes and the reflectedspectrum with bandwidth variation are illustrated in Fig. 3. Thespectrum shift of the Bragg wavelength was measured andrecorded which includes a comparison of the reflection spectraof Pd-coated FBG with and without to be exposed in a hydrogengas, as shown in Fig. 4. The sensing mechanism of this device isbased on the Pd-coated FBG to absorb hydrogen and to cause thechange of the effective refractive index of fiber Bragg grating.The curve for hydrogen concentrations versus the Bragg wave-length shift is shown in Fig. 5. This hydrogen sensor shows alinear sensitivity for the concentration of less than 40%, but itbecomes nonlinear phenomenon in the concentration of above50% due to the interaction saturation between the palladium andhydrogen molecules. In the experiments, the length of the sens-ing head was 12 mm. By increasing the length of the sensinghead, the sensing sensitivity should be improved. In future work,we expect to improve this sensor to have the property of highersensitivity.

4. Conclusions

We report a simple, side-polished FBG-based sensor thatprovides a linear measuring characteristic in the range of lowhydrogen concentrations. The interacting length, the diameter ofside-polished fiber and the Pd film thickness are related to the

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sensor sensitivity and response time respectively and can becontrolled independently. Although many hydrogen sensorsusing palladium metal to trap hydrogen, finding new materialsfor trapping hydrogen is still a key point to develop an idealhydrogen sensor. A further study of the proposed sensing devicefabrication will be prepared by different deposition methods andcan be applied for different hydrogen concentration and tem-perature measurements. We expect that further improvements ofthis sensor will lead to even higher sensitivity. This includesfurther reducing the side-polished fiber diameter, the interactionlength and response time, which could be tailored during thefabrication process.

Acknowledgements

The authors thank financial support of the National Sci-ence Council of the Republic of China, Taiwan, under contractsNSC95-2221-E-035-106 and NSC 95-2815-C-035-009-E.

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