Simultaneous measurement of strain and temperature using a fiber Bragg grating and a thermochromic material

Simultaneous measurement of strain and temperature usinga fiber Bragg grating and a thermochromic material

C. Fernandez-Valdivielso, I.R. Matıas*, F.J. ArreguiDepartamento de Ingenierıa Electrica y Electronica, Universidad Publica de Navarra, E-31006 Pamplona, Spain

Received 18 January 2002; received in revised form 20 May 2002; accepted 5 June 2002

Abstract

A new scheme for simultaneous measurement of temperature and strain using fiber Bragg grating (FBG) and a thermochromic material is

presented. It consists of a dual system with two different sensing elements included in the same fiber: a fiber Bragg grating to measure the

strain and a thermochromic effect based optical fiber sensor to measure the temperature variations and to compensate the cross sensitivity of

the fiber Bragg grating with temperature. Experimental results of the optical dual system are shown and discussed.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Fiber optic sensor; Thermochromic; Strain; Temperature; Fiber Bragg grating (FBG)

1. Introduction

Optical fiber sensor technology is currently generating

considerable interest, particularly with respect to its use in

civil engineering applications or in the biosensors field.

These optical sensors, which are superior in durability,

corrosion resistance, water proof and explosion proof, pro-

vide numerous advantages over conventional electrically

based sensors such as their small size, their low weight or

their immunity to electromagnetic interference. From sev-

eral years ago, fiber Bragg gratings (FBG) are one of the

most used devices in this technology, especially to measure

strain and deformations in many environments such as smart

structures or composite materials in bridges or dams [1–6].

In these applications, the FBG measures the total deforma-

tion including strain due to forces applied to the structures as

well as thermal expansion. On the other hand, this cross

sensitivity of the FBG with temperature may become a

problem in applications that require the discrimination of

the strain and temperature in the sensing point instead of

monitoring the total deformation. In order to overcome

this inconvenience and to measure both parameters (strain

and temperature) at the same time and location, several

techniques have been presented before [7]: some of them

combining the use of a pair of FBG [8,9], others use fiber

polarization-rocking filter [10], interferometrically interro-

gated techniques [11,12] or long period gratings [13,14].

In this work, a new method to measure simultaneously

strain and temperature is presented using two different sensing

elements in the same fiber: a fiber Bragg grating and a

thermochromic compound. The total strain is measured

through the variations in the central wavelength of the FBG

and the temperature is measured with the change in the optical

power reflected by the thermochromic material; besides, this

information could be also used to compensate the temperature

cross sensitivity of the FBG. With this novel technique both

parameters can be measured in the same place with the same

fiber. Also, neither special or complex interrogation and

correction schemes have to be used. Furthermore, this method

allows the use of any commercial FBG avoiding the need of

defining either some geometric parameters or the thermal

sensitivity of the FBG prior to its fabrication [15].

2. Sensing mechanism of the dual sensor

In order to measure the total strain and the temperature

using the same fiber, two different sensing effects and

elements are used, the FBG to measure the deformation

and the thermochromic effect to measure the temperature.

Fig. 1 shows the scheme used to collect the strain and

temperature information.

The system consists of a broadband ‘white light’ source

launched to the combined FBG and thermochromic effect

Sensors and Actuators A 101 (2002) 107–116

* Corresponding author. Tel.: þ34-948-169-288; fax: þ34-948-169-720.

E-mail address: [email protected] (I.R. Matıas).

0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 1 8 8 - 7

based sensor through a 50:50 optical fiber coupler, the

spectral response of these two combined elements is mon-

itored in an optical spectrum analyzer. As can be seen in

Fig. 1, the FBG reflects optical power with the maximum

reflection coefficient centered at the Bragg wavelength. This

wavelength will change due to variations in temperature or

strain, and the measurement of this change can be used to

give a reading of the strain and temperature variations at the

physical location of the dual sensor. Besides, the FBG also

behaves as a stop-band filter, so optical wavelengths outside

of the Bragg grating window are transmitted with negligible

attenuation and reach the temperature optical fiber sensor.

For these wavelengths, the reflected optical power at the

temperature optical fiber sensor will change with variations

in temperature. In this way, this reflected optical power again

passes through the FBG, but this time following the opposite

direction with respect to the incident light. As the spectrum

reflected in the thermochromic material matches the trans-

mission window of the FBG, the optical power transmitted at

the Bragg wavelength is negligibly altered by the presence

of the grating. Therefore, by measuring the reflected optical

power within this unaltered transmission window of the

FBG, it is possible to measure the variations in the intensity

corresponding to changes in temperature, as indicated ear-

lier. This reading of temperature can be used to discriminate

the reading from the FBG in order to obtain the real value of

the measured total strain without the interference due to the

cross sensitivity with temperature of the FBG.

Temperature may be measured by monitoring the

reflected optical power, and the total strain may be inde-

pendently measured by detecting the central wavelength in

nanometers of the FBG spectrum. Once measured both

parameters, the strain measurement from the FBG can be

corrected using the thermochromic sensor optical power

value in order to discriminate its cross sensitivity with

temperature. Because the measurement of wavelength and

intensity are dissimilar, reading of both parameters in this

system allows cross-referencing.

3. Study of the Bragg grating sensor

3.1. Theory

The sensing mechanism of a FBG relies on the shift of its

Bragg wavelength (lB) due to changes of the effective index of

the Bragg grating (neff) as well as changes on its spatial period

(L). Both changes can be caused by applied strain or tem-

perature variations [16]. This fractional change is given by:

DlB

lB

¼ Dneff

neff

þ DLL

(1)

The relative magnitudes of the two changes depend on the

type of perturbation to which the grating is subjected. For

most applications, the effect due to change in effective index

(Dneff/neff) is the dominating mechanism. An axial strain (e) in

Fig. 1. Scheme of the operation mechanism of the optical dual system.

108 C. Fernandez-Valdivielso et al. / Sensors and Actuators A 101 (2002) 107–116

the grating changes the grating period and the effective index

and results in a shift on the Bragg wavelength, given by:

1

lB

DlB

De¼ 1

LDLDe

þ 1

neff

Dneff

De(2)

The first term on the right side of Eq. (2) is unity, while the

second term has its origin in the photoelastic effect. An axial

strain in the fiber serves to change the refractive index of

both the core and the cladding, and this variation causes a

change in the effective index of the glass. Thus, the varia-

tions in neff and L due to strain have contrasting effects on

the Bragg peak, obtaining that the fractional change in the

Bragg wavelength due to axial strain is about 73% of the

applied strain [16]. On the other hand, the sensitivity due to

temperature can be seen as [17]:

1

lB

DlB

DT¼ 1

LDLDT

þ 1

neff

Dneff

DT(3)

The fractional change of wavelength due to variations in a

temperature range from 0 to 85 8C can be approximated to

about 0.016 nm/8C, depending on the used FBG [18].

3.2. Experimental characterization

The sensor to measure the strain was fabricated using a

1,557.716 nm centered FBG purchased from 3M, with a

reflectivity at the Bragg wavelength larger than 99.7% and a

FWHM 3 dB bandwidth of 0.46 nm. The employed optical

fiber was a step index single-mode fiber of 9.4/125 mm of

core and cladding diameter, respectively.

In order to characterize the FBG several tests were done

using an EDFA as a broadband optical source with a stability of

0.01 dBm and an Optical Spectrum Analyzer HP7091B with a

resolution of 0.01 dBm to monitor the optical power reflected

from FBG. First of all, a simple study of its spectral response

was done obtaining the results shown in Fig. 2; where can be

seen that the FBG wavelength is centered at 1,557.716 nm.

Once the spectral response of the FBG was studied and its

Bragg wavelength located, in order to characterize the beha-

vior of the FBG versus strain and temperature and to check the

cross sensitivity between both of them, two tests were done.

The strain experiment consisted of exposing the FBG under

strain variations fixing one end and straining the other one with

the help of a micrometer displacement. These tests were done

increasing and decreasing the applied strain to the FBG.

Results are shown in Figs. 3 (spectral response) and 4 (Bragg

wavelength as a function of strain), where can be seen that the

obtained resolution is 0:59 � 10�3 nm/me.After that, the FBG was put into a climatic chamber (ACS

HYGROS) and submitted under thermal cycling between

20 and 80 8C. Results of this test are shown in Fig. 5 (spectral

response) and 6 (Bragg wavelength as a function of tem-

perature), where can be seen a temperature dependence of

0.011 nm/8C. With these two experiments, the sensitivity of

the FBG with strain and its cross sensitivity with temperature

were deducted, and therefore, these values will be used in the

dual sensor to obtain the total strain and to discriminate the

temperature effect from the reading of the FBG sensor.

4. Study of the temperature sensor

4.1. Theory

The temperature sensor employed in the system follows a

reflection scheme using the Fresnel’s principle by monitor-

ing the optical power reflected in the thermochromic mate-

rial at the end of the fiber. This principle says that the

reflection occurs when the light meets a refractive index

discontinuity [19], in this case between the fiber and the

thermochromic material. Due to the mismatch of both

Fig. 2. Fiber Bragg grating spectral response.

C. Fernandez-Valdivielso et al. / Sensors and Actuators A 101 (2002) 107–116 109

refractive indexes, part of the light beam is transmitted and

part is reflected. Fig. 7 shows a scheme of the basic con-

figuration of the sensor where the thermochromic material

(lophine) is deposited at the end of a standard single mode

communication fiber, and the laser beam illuminates it. The

reflected signal is collected on a photodetector, by means of

a 50:50 coupler. According to Fresnel’s equations, the field

amplitude reflections coefficients at the interface (fiber/

lophine) for the perpendicular and parallel polarization,

respectively, rpp and rpl, can be expressed as [20]:

rpp ¼ n1 cos y1 � n2 cos y2

n1 cos y1 � n2 cos y2

rpl ¼n2 cos y1 � n1 cos y2

n2 cos y1 � n1 cos y2

(4)

where y1 is the angle of the incident light, y2 the angle of the

transmitted light, n1 the refractive index of the optical fiber

(considering the fiber as a non absorbing optical medium)

and n2 is the refractive index of the lophine. As the optical

fiber used for this system is a single mode and step

index type, it can be assumed that the fundamental guided

mode travels along the paraxial path in the fiber, that is

y1 ffi y1 ffi 0� and jrppj ¼ jrplj ¼ r. So, the intensity reflec-

tion coefficient R can be expressed by:

R ¼ jrppj2 ¼ jrplj2 ¼ n1 � n2

n1 þ n2

��

��

2

(5)

As lophine is a thermochromic material it changes its

refractive index with variations of temperature, so the

Fig. 3. Fiber Bragg grating spectral response when strain is applied.

Fig. 4. Bragg wavelength as a function of strain.


optical power reflected will change when the temperature of

the material at the end of the fiber changes.

4.2. Experimental characterization

The temperature optical fiber sensor used in this system has

been described before [21] and its principle of operation is

based on the use of a thermochromic compound as the sensing

element following a reflection mode scheme. The selected

material is 2,4,5-triphenylimidazole, or also called lophine

(C21H16N2), never used before in this kind of sensors to our

knowledge. It is a hydrophobic (it repels water molecules)

organic compound, with thermochromic properties, Fig. 8

shows its molecular formula and its thermal transition when

heating or cooling. This material is deposited at the end of the

fiber using the dip-coating method, that is, the thermochromic

material is heated until reaching its melting point (275 8C),

and then a previously cleaved standard single-mode optical

fiber (with core and cladding diameters of 9.4 and 125 mm,

respectively) is dipped into the melted lophine and pulled off

for getting the organic material deposited at the end of the

fiber by cooling. With this procedure the thickness of this

coating is around 1 mm. This action produces the solidifica-

tion of the material that remains perfectly adhered to the fiber

forming a coating, obtaining a robust system with no special

need of encapsulating the material.

Fig. 5. Fiber Bragg grating spectral response under temperature variations.

Fig. 6. Bragg wavelength as a function of temperature.


The temperature optical fiber sensor was built with stan-

dard single-mode communication fiber and characterized

versus temperature changes (from 22 to 80 8C) with the help

of a climatic chamber (ACS HYGROS). The system was

heated and cooled with a constant speed of 1 8C/min show-

ing the response plotted in Figs. 9 (spectral response) and 10

(reflected optical power as a function of temperature). As

can be seen, its sensitivity is 0.065 dB/8C, and it does not

present any sign of hystheresis as the response of the optical

fiber sensor follows the same way increasing or decreasing

the temperature (previous studies of this sensor have demon-

strated that the sensor presents a response time delay of

7.5 s. [21]). Anyway, taking into account the resolution of

the used OSA, the theoretically minimum step of tempera-

ture variation that can be measured by the optical system is

0.153 8C. Besides, the temperature optical fiber sensor was

checked under relative humidity variations (from 5 to 95%)

obtaining a flat response (not shown), that is, as it was

expected and due to the hydrophobic nature of the thermo-

chromic material this temperature sensor has no cross

sensitivity with humidity. Moreover, after at least 10 days

in under water conditions, the sensor has presented the same

response.

Another important aspect of this temperature optical fiber

sensor for its use in a simultaneous measurement of strain

and temperature system is its cross sensitivity with strain;

but as the material is deposited at the end of the fiber, the

strain applied to the FBG does not affect the optical response

of the thermochromic material.

5. Experimental results of the dual sensor opticalfiber system

In order to measure strain and temperature simulta-

neously, the experimental set-up of Fig. 11 was arranged.

With such a system it was possible to monitor both optical

fiber sensors: the FBG to measure the strain (variations of

the Bragg wavelength) and the thermochromic sensor to

measure temperature (changes in the intensity of the optical

Fig. 7. Basic scheme of the configuration for the temperature optical fiber sensor.

Fig. 8. Thermal transition of lophine.


power reflected). An EDFA was used as the broadband

optical source (with a typical optical power at 1557 nm

of 4 mW, �23.98 dBm) and an Optical Spectrum Analyzer

HP7091B monitored the information obtained from the dual

sensor as in the other tests. In order to obtain tempera-

ture variations, a climatic chamber (ACS HYGROS) was

used and several thermal cycles from 20 to 80 8C were

performed.

First of all, the dual system was tested getting the tem-

perature optical fiber sensor under temperature variations

and leaving the FBG sensor at room temperature. Results are

shown in Fig. 12, where can be seen that the Bragg wave-

length get unaltered and all the curves overlap because there

is no strain applied and the temperature is constant. On the

other hand, as it was explained before, the measurement of

the changes in the temperature is obtained outside the Bragg

wavelength window from the variations in the intensity of

the reflected optical power in the thermochromic material of

the temperature optical fiber sensor. This reading is obtained

from all the wavelengths outside of this Bragg wavelength

window. Besides, this value can be also used to discriminate

cross sensitivity with temperature of the FBG sensor.

Once the operating method was checked, the dual sensor

was introduced into the climatic chamber and thermal cycles

from 20 to 80 8C were performed obtaining results shown in

Fig. 13.

Fig. 9. Temperature optical fiber sensor spectral response under variations of temperature.

Fig. 10. Temperature optical fiber sensor response.


As can be seen, the Bragg wavelength suffers a shift due to

the temperature variations, but it can be corrected with the

information obtained from the temperature optical fiber

sensor. On the other hand, as happened before, the optical

power that passes the FBG arrives at the thermochromic

material and is reflected according to the operation of the

temperature sensor, that is the highest temperature the most

optical power reflected, so the intensity of the reflected

optical power changes as a function of the temperature.

This information gives the absolute temperature and can be

use for normalizing the wavelength shift in the FBG due to

its cross sensitivity with temperature using the calibrating

curve (Figs. 5 and 6). Once the temperature shift produced in

the FBG is corrected and reading from the FBG is normal-

ized in the wavelength domain, the measurement of the

strain can be easily read with the shift produced in the Bragg

wavelength (Figs. 3 and 4). The strain does not affect the

optical fiber temperature sensor. Therefore, it is possible to

subtract the real strain and temperature of the system with

this dual sensor. Besides, the longevity of the dual system

has been tested, obtaining after at least 1 month the same

response as at the first test.

Fig. 11. Experimental set-up for the optical dual system.

Fig. 12. Optical fiber dual system spectral response under variations of temperature at the location of the temperature optical fiber sensor, getting the FBG

sensor isolated from temperature and strain changes.


6. Conclusion

A new scheme for simultaneous measurement of tempera-

ture and strain using a fiber Bragg grating has been presented.

This dual system uses a FBG as a strain sensor and a

thermochromic effect based optical fiber sensor to measure

the temperature, both sensing elements are implemented in

the same optical fiber. The effect of strain and temperature in

the FBG has been theoretically studied and experimentally

presented and characterized; and on the other hand, the

temperature optical fiber sensor is also presented and cali-

brated. This temperature sensor uses a thermochromic mate-

rial as transducer and measures the absolute temperature in

the dual system; besides it is used to compensate and

discriminate the shift produced by the temperature in the

Bragg wavelength. In this dual system, the FBG is used to

measure the strain once the information has been normalized

with the reading of the temperature sensor. With this tech-

nique, the dual system can measure both parameters in the

same location with the same fiber. The designed system has

been successfully tested in the range from 20 to 80 8C.

Taking into account the resolution of the OSA and the

stability of the EDFA, the total error is 0.02 dBm, which

means that the accuracy of the optical system here presented

is 0.306 8C. In addition, to our knowledge this is the first time

that this configuration (FBG in combination with a thermo-

chromic material) is presented for the simultaneous measur-

ing of strain and temperature.

Acknowledgements

This work has been supported by Spanish CICYT research

Grant no. TIC2001-0877-C02-02, and Gobierno de Navarra

research grants.

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Biographies

C. Fernandez-Valdivielso received his MS degree in electrical and

electronic engineering from the Public University of Navarra (UPNA),

Pamplona, Spain, in 1998. In 1999 he obtained a scholarship from the

Science and Technology Spanish Ministry and he joined the optical fiber

sensor group at the Department of electrical and electronic engineering

of the UPNA. In 1999 he was a visiting PhD student at Heriot-Watt

University (Edinburgh, Scotland), where he was engaged in research

on Fabry-Perot temperature and pressure sensors. His research interest

is in the areas of optical fiber sensors, optical networks and domotic

systems.

I.R. Matıas received his MS degree in electrical and electronic

engineering from the Polytechnic University of Madrid (UPM), Spain,

in 1992. He was involved in optical communications research project

from 1990 to 1992 in the Alcatel SESA Research Center in Madrid. In

1993 he joined the optical fiber group in the photonic Technology

Department, UPM, where he received his PhD degree, specialty in

Optical Fiber Sensors. In 1996 he took up a lectureship at the Public

University of Navarra (Pamplona, Spain). He has co-authored more than

100 of journal and conference papers and three books chapters related to

fiber optics. Presently he is an associate professor of Electronic

Technology at the Public University of Navarra. His research interest

is in the areas of optical fiber sensors, passive optical devices and

systems and optical networks for smart structures and buildings. He is an

IEEE member.

F.J. Arregui received the MS degree in electrical engineering from the

Catholic University of Navarra (San Sebastian, Spain) in 1994 and the PhD

degree from the Public University of Navarra (Pamplona, Spain) in 2000.

He has been a member of the CEIT Research Center (San Sebastian,

Spain) for 2 years and has been involved in different projects with industry

including medical instrumentation, monitoring of high power lines and

communications hardware. Since 1995 he has been working at the Public

University of Navarra (Pamplona, Spain). During 1998 and 2000 he was a

visiting scientist at the Fiber and Electro Optics Research Center, Virginia

Polytechnic Institute and State University (Blacksburg, VA, USA). His

main research interests include optical fiber sensors, sensor materials and

nanostructured materials. He has served as a referee for the journals Opt.

Eng., Sens. Actuators A and IEEE Sens. J. Francisco J. Arregui is an IEEE

and an SPIE member.


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Simultaneous measurement of strain and temperature using a fiber Bragg grating and a thermochromic material