586 IEEE SENSORS JOURNAL, VOL. 7, NO. 4, APRIL 2007

High-Temperature Resistance Fiber Bragg GratingTemperature Sensor Fabrication

Bowei Zhang and Mojtaba Kahrizi

Abstract—Fiber Bragg grating (FBG) temperature sensor andsensor arrays were applied widespread particularly in harsh en-vironments. Although FBGs are often referring to permanent re-fractive index modulation in the fiber core, exposure to high-tem-perature environments usually results in the bleach of the refrac-tive index modulation. The maximum temperature reported forthe conventional FBG temperature sensor is around 600 C dueto its weak bonds of germanium and oxygen. In this paper, wereport design and development of a novel high-temperature re-sistance FBG temperature sensor, based on the hydrogen-loadedgermanium-doped FBG. The refractive index modulation in theFBG is induced by the molecular water. The results of our exper-iments have shown that the stability of the device is substantiallyincreased at high temperature range. Due to the high bonds en-ergy of hydroxyl and the low diffusivity of the molecular water, thethermal testing results of this temperature sensor show the thermalstability of hydrogen-loaded FBG can be increased by using an-nealing treatment; moreover, the highest erasing temperature forthe device could reach to 1100 C or more. The reflectivity of thisnew FBG depends on the concentration of Si–OH and indirectly re-lated to the reflectivity of hydrogen-loaded FBG. Furthermore, theexperimental results have provided a better understanding of theformation of the hydrogen-loaded FBGs and the chemical trans-fers at elevated temperatures in the fiber core.

Index Terms—Fiber Bragg grating (FBG) temperature sensors,high-temperature resistance FBGs, molecular-water inducedFBGs.

I. INTRODUCTION

EXPERTS point out the most serious environmental chal-lenge we have to face in the coming decade and century

is global warming. Burning coal results in more CO emissionsthan any other method of generating electricity, yet we continueto rely on coal for more than half of our electricity generation.The efficiency of current fossil fueled steam power plant perfor-mance is limited by a number of factors, especially the lack ofsensors in various components that could be used for real-timelocal condition monitoring and closed-loop control.

Fiber Bragg grating (FBG) temperature sensors are an idealintelligent distributed temperature sensor for real-time moni-toring of temperature. The principle of the FBG temperature

Manuscript received March 14, 2006; revised September 20, 2006; acceptedOctober 5, 2006. This work was supported by the Natural Science and Engi-neering Research Council of Canada and the Faculty of Engineering and Com-puter Science at Concordia University.

The authors are with the Electrical and Computer Engineering Depart-ment, Concordia University, Montréal, QC H3G 1M8, Canada (e-mail:z_bowei@ece.concordia.ca; mojtaba@ece.concordia.ca). The associate editorcoordinating the review of this manuscript and approving it for publication wasDr. Kailash Thakur.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2007.891941

sensor is based on the measurement of the reflected Bragg wave-length. FBGs are compact intrinsic sensing elements, which arerelatively inexpensive to produce, easy to multiplex, and appli-cable to a range of physical measurands [1]. An FBG is formedby a periodic change in the refractive index caused in a fibercore by exposure to an ultraviolet (UV) laser beam [2].

The FBG temperature sensor reflects one particular wave-length and transmits all others; moreover, the reflected wave-length can vary with the temperature of the sensor, thus, FBGtemperature sensors have been widely used in applications inmonitoring temperature. One of the advantages of FBG temper-ature sensor is that several of these sensors can be multiplexedin a series along with a single optical fiber so that a single in-strument can simultaneously monitor many individual sensors[3]. Additionally, the FBG temperature sensor also has someother advantages such as resistance to electromagnetic interfer-ence, tiny volume, and light weight; however, it has also someserious disadvantages. The FBG exhibits poor stability withinthe high-temperature environment and the grating can be com-pletely erased at temperatures around 700 C [4]. Because ofabove mentioned reasons, nowadays, the FBG-based tempera-ture sensors are usually used under 200 C due to decay of theFBG’s reflectivity, especially the decay of the hydrogen-loadedFBG’s reflectivity [5].

In this paper, we report the high-temperature resistanceFBGs, which are fabricated using hydrogen-loaded conven-tional FBG, to meet our principal goal, which is to developa simple temperature sensor stabilizing at high-temperatureenvironment.

II. MOLECULAR WATER INDUCED FBG

High-pressure low-temperature hydrogenation of germa-nium-doped fibers can significantly enhance photosensitivitywithin optical fibers and produce strong FBGs with 248- or193-nm laser [6]. One explanation for this phenomenon can bededuced from the concentration of germanium–oxygen-defi-cient center (GODC) [7], [8] and drawing-induced defect (DID,a trapped hole with an oxygen vacancy) [8], [9] inside opticalfiber.

Germanium–oxygen (Ge–O) bonds play an important roleduring the index change process of FBG fabrication. This bondis broken by UV laser and then reacts with hydrogen to pro-duce a hydroxyl and a DID. Since the Ge–O bonds are weakerthan the Si–O bonds (Ge–O–Ge is 4.2 eV, Ge–O–Si is 4.5 eV,and Si–O is 5 eV) [10], the Ge–O bonds can be more readilybroken by low-energy radiation. During the annealing process,one of the weakest bonds among the three bonds of the DID

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ZHANG AND KAHRIZI: HIGH-TEMPERATURE RESISTANCE FBG TEMPERATURE SENSOR FABRICATION 587

Fig. 1. Absorption spectra of hydrogen-loaded FBG near 1.4 �m.

will be broken by annealing energy. Therefore, the thermal en-ergy induced structural change from DID into the GODC isthe principal cause of thermal decay of FBG refractivity. Be-cause the thermal stability of hydrogen-loaded FBG is limitedby the bonding energy of the Si–O bond, the photo-induced re-action of hydrogenated optical fiber can easily produce signifi-cant amount of germanium–hydroxyl (Ge–OH) and silicon–hy-droxyl (Si–OH), respectively, within the writing process usingintense fringe pattern UV laser. However, during the thermal an-nealing process, the Ge–OH and Si–OH have been decomposedby the annealing energy of the FBG, and reproduced the Germa-nium–oxygen lattice and silica, in the meantime; the molecularwater was created from OH inside the germanium-doped op-tical-fiber core. The refractive index modulation of the molec-ular water FBG can be considered to be formatted by the peri-odic change of molecular water inside the fiber. The assumed re-action of the molecular water induced FBG shows in (1) and (2)

– – – – (1)

– – – – (2)

The advantages of this formation for molecular-water FBGcan be described as follows: First, the decomposability ofmolecular water is nearly impossible at the temperaturesfrom the deep frozen to the melting temperature of silica[11] due to its strong bonding of the molecular water. Fromthis point of view, we can figure out the difference betweenthe molecular-water FBG and the conventional FBG—thesurvived temperature of the molecular water FBG is muchhigher than the erasing temperature of conventional hydrogenloaded FBG. Second, the molecular water has low diffusivityof 2 10 cm s [12] inside the silica fiber. Athigh-temperature range, the molecular water will be diffusedfrom higher concentration segments to the lower concentrationparts; however, this diffusion process is extremely slow. Basedon this point, the molecular-water induced FBG has a muchhigher thermal stability compared with conventional hydrogenloaded FBG.

TABLE IPARAMETERS OF THE FBG MEASURED IN THIS PAPER

III. EXPERIMENTAL RESULTS

The hydroxyl (OH) group can be easily determined by theirabsorption in the infrared range. The fundamental absorptionvibration band of OH is located at wavelength 2.7 m, whileits first vibration overtone for Si–OH and Ge—OH absorptionbands are 1.385 and 1.405 m, respectively [13]. In this work,two hydrogen-loaded FBGs, Samples A and B, were tested forthe absorption spectra between 1.34 and 1.44 m in our labora-tory for the absorption of hydroxyl and the formation of molec-ular water. The parameters of FBG samples are described inTable I. The obtained OH and water absorption amplitudes areillustrated in Fig. 1.

The interaction of hydrogen molecules at Ge or Si tetrahedralsites inside the fiber core leads to enhanced permanent UV-in-duced losses, particularly due to the increase in the OH contentin the fiber. In general, the hydrogen molecules react at normalSi–O–Ge sites, resulting in the formation of Si–OH and GODC,both of which contribute to the observed index change.

Reflectivity of hydrogen-loaded germanium-doped FBG de-pends on refractive index modulation in the fiber core and isindirectly related to the amount of the Ge–OH and Si–OH inthe UV exposed area of optical fiber core. Obviously, in Fig. 1,the concentration of Ge–OH was directly related to doping con-centration of Ge inside the fiber core and the distinctness of therefractive index modulation for testing samples was based onthe amount of Si–OH for the strong FBG.

588 IEEE SENSORS JOURNAL, VOL. 7, NO. 4, APRIL 2007

Fig. 2. The thermal behavior of normalized grating reflectivity changes as afunction of annealing temperature and annealing time.

Molecular water is one of the impurities inside optical fibercore. The near-infrared absorption of molecular water has peakabsorption at 1.42 m [13]. However, the spectrum profile ofwater absorption was commonly covered up by the spectrumprofile of Ge–OH absorption and shown in Fig. 1.

After exposing the hydrogen-loaded fibers to UV laser, thehydrogen was diffused out of the fiber by heating the samplesin 350 C for 40 s. Sample A was annealed inside the furnacefrom room temperature (22.5 C) up to 500 C. At each newtemperature point, the tested FBG was treated with isothermalannealing for 10 min and then its OH absorption at wavelength1.385, 1.405, and 1.42 m and its reflectivity at Bragg wave-length were well recorded. Fig. 2 indicated that the thermal be-havior of the FBG reflectivity changes as a function of annealingtemperature and annealing time.

The obtained Ge–OH, Si–OH, and molecular water absorp-tion amplitudes for the annealed Sample A are shown in Fig. 3,which clearly shows the drastic difference of thermal stabilitybetween Ge–OH and Si–OH. The weak bond between Ge andOH was less thermal stabilization than the Si–OH bond. There-fore, the absorption of Ge–OH was decreased sharply; further-more, Ge–OH groups were almost completely decomposed atannealing temperature of 500 C. In conclusion, the thermalstability of hydrogen-loaded FBG is related to the amount ofSi–OH [15]. The results of the normalized refractive index mod-ulation revealed in Figs. 2 and 3 confirmed our hypothesis [15]once again.

In a higher temperature range, thermal decay of the hydrogen-loaded FBG can be described as the process of Si–OH decom-position and is related to the amount of Si–OH as well as the de-fect of Si–OH. The highest decomposition temperature for thehydrogen loaded FBG is around 940 C, which is indicated inFig. 4.

Molecular water is the essential part of the high-temperatureresistance FBG. In theory, the molecular water formation in-side FBG results in the decomposition of the Si–OH and thecomposition from OH group. In our experiments, the molec-ular-water-induced reflectivity was observed starting at temper-atures around 950 C. The molecular-water-induced reflectivity

Fig. 3. Normalized magnitude changes of Si–OH absorption (1.385 �m),Ge–OH absorption (1.405 �m), and water absorption (1.42 �m) versusannealing temperature and annealing time [14].

Fig. 4. Reflectivity of hydrogen-loaded FBG versus the annealing tempera-tures.

of FBG, which is shown in Fig. 4, can be described as two cor-relative steps: the grating grown up (950 C to 1000 C in Fig. 4)and the grating stabilization (isothermal annealing at 1005 C).

High-temperature resistance FBG can be easily fabri-cated using hydrogen-loaded conventional FBG and thermalannealing treatment at around 1000 C. The mechanism of fab-rication is the process of composite molecular water in the FBGarea. To test the characteristics of molecular-water-inducedFBG in thermal annealing, the transmitted spectra of the testedspecimen was measured from room temperature to 1000 C.Fig. 5 reveals the reflectivity of tested FBG in the differentannealing temperature ranges. The molecular-water-inducedFBG has extremely stable reflectivity at the high temperaturescompared with the conventional FBGs.

The shift of the Bragg wavelength with temperature has beenwidely used as an effective factor for temperature sensing ortemperature compensation in other sensors. The temperaturesensitivities of the gratings were determined by observing thechange of Bragg wavelength with temperature. The spectral

ZHANG AND KAHRIZI: HIGH-TEMPERATURE RESISTANCE FBG TEMPERATURE SENSOR FABRICATION 589

Fig. 5. Reflectivity versus temperature for molecular-water-induced FBG fromroom temperature to 1000 C [13].

Fig. 6. Bragg wavelength versus temperature for molecular-water-inducedFBG from room temperature to 1000 C.

response of molecular-water-induced FBG was displayed inFig. 6. As shown in Fig. 6, this variation of the Bragg wave-length shifting with respect to temperature is almost linear athigher temperature range. The sensitivity of the sensor at lowtemperatures is 0.009 nm C and changes to 0.0175 nm C attemperature around 1000 C. These results show the sensitivityof the molecular water FBG is slightly higher than the conven-tional hydrogen-loaded FBG (0.0166 nm C compared with0.015 nm C [16] at temperature around 700 C).

IV. HIGH-REFLECTIVITY HIGH-TEMPERATURE

RESISTANCE FBGS

Molecular-water-induced FBG was designed, fabricated, andtested in our lab. The tested specimen shows this type of FBGtemperature sensor can be used to monitor the temperaturechanges from room temperature to over 1000 C; however, lowreflectivity (around 25% at 1000 C, Fig. 5 [14]) is its weakside.

Fig. 7. Transmitted spectrum for high-reflectivity hydrogen-loaded FBG (mea-sured using AQ6319 optical spectrum analyzer from Ando).

Fig. 8. Absorption spectra of high-reflectivity hydrogen-loaded FBG near1.4 �m.

As we explained earlier, the main point of molecular-water-induced FBG is the concentration of molecular water insidea fiber core; the molecular water is recomposed from the hy-droxyl, which is produced by the decomposition of Ge–OH andSi–OH. From this point of view, we truly believe that the re-flectivity of molecular-water-induced FBG in high-temperatureenvironment is related to the amount of Si–OH or indirectly re-lated to the reflectivity of hydrogen-loaded FBG.

In order to verify our theory, a high-reflectivity molecular-water-induced FBG was also developed in our lab by usingstrong-reflectivity hydrogen-loaded FBG. The spectrum of thehigh-reflectivity hydrogen-loaded FBG and the OH absorptionspectrum for this FBG are shown in Figs. 7 and 8, respectively.

The reflectivity of molecular-water-induced FBG is con-tributed from the production of molecular water inside the fibercore at high temperatures. In order to emphasize the thermalbehavior of molecular-water-induced FBG fabrication in the

590 IEEE SENSORS JOURNAL, VOL. 7, NO. 4, APRIL 2007

Fig. 9. The thermal behavior of grating reflectivity changes during molecular-water formation in high-reflectivity hydrogen-loaded FBG 200 C to over 1100 C.

Fig. 10. The thermal behavior of grating reflectivity is depending on the con-centration of the Si–OH inside fiber core.

high-temperature range, we displayed the recorded data onlyfrom 200 C to over 1100 C, which was shown in Fig. 9.

By comparing Figs. 4 and 9, at 200 C, the reflectivity ofhigh-reflectivity FBG was slightly (circa 4 dB) higher than theconventional FBG: As the annealing temperature was increasedto 700 C, the reduction of the reflectivity for both fabricationprocesses of molecular-water-induced FBG display the samedecay characteristics. Once the annealing temperature was in-creased to 850 C, a strong reflectivity increment (circa 6 dB)of the high-reflectivity molecular-water-induced FBG fabrica-tion was observed.

The erasing point of the initial hydrogen-loaded FBG wasobserved at 940 C for conventional hydrogen-loaded FBGin Fig. 4, however, this point was absent for the high-reflec-tivity molecular-water-induced FBG fabrication in Fig. 9.

The highest testing temperature for high-reflectivity molec-ular-water-induced FBG is above 1100 C. The reflectivity ofhigh-reflectivity high-temperature resistance FBG remained8.3% after 2 h at 1100 C isothermal annealing process. Fig. 10shows the difference of thermal reflectivity profiles between thehydrogen-loaded conventional FBG and high-reflectivity FBGin high-temperature resistance FBG development.

In summary, as expected, the thermal stability and reflectivityof molecular-water-induced FBG depend on the concentrationof Si–OH inside fiber core and was approved in the high-reflec-tivity molecular-water-induced FBG development.

V. CONCLUSION

A high-temperature (from room to over 1100 C) FBG sensorbased on the hydrogen-loaded FBG is designed, developed, andtested.

In this high-temperature resistance FBG, its refractive indexis formed by a periodic modulation of molecular water insidethe fiber core. The induced molecular-water-induced FBGswere produced through hydrogen-loaded conventional fiberwith 248 nm UV laser as well as thermal processing technique.The recorded spectra shows the tested FBGs are reasonablystable at high temperatures. The spectral response of testedFBG does not differ from hydrogen-loaded FBGs.

According to the results of the thermal stability, the thermalability of molecular-water-induced FBG is dependent on theconcentration of Si–OH in the affected FBG area and is alsorelated to the reflectivity of hydrogen-loaded FBG.

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Bowei Zhang received the B.S. degree in physics from Jinan University, China,in 1985 and the M.A.Sc. degree in electrical and computer engineering fromConcordia University, Montreal, QC, Canada, in 2004.

He is currently working towards the Ph.D. degree at Concordia University. Heis a Research Assistant in the Electrical and Computer Engineering Department,Concordia University. His research interests are in optical fiber sensors, opticalcommunications, and micro- and nano-fabrications, in particular with devicesrelated to optical communications.

Mojtaba Kahrizi received the Ph.D. degree in applied solid state physics fromConcordia University, Montreal, QC, Canada.

After spending five years in STFX University in Nova Scotia, Canada, as Re-search Associate and Assistant professor, in 1991, he came back to the Electricaland Computer Engineering Department, Concordia University. Established theMicro Devices and Microfabrication laboratories in that department. In June2001, he joined the department as an Associate Professor, he is involved withteaching and researches focused on fundamental issues related to micro- andnanofabrications, in particular with devices related to optical communicationsand health related issues.