Characterization of Encapsulating Materials for Fiber Bragg Grating-Based Temperature Sensors

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Characterization of EncapsulatingMaterials for Fiber Bragg Grating-BasedTemperature SensorsVenkata Reddy Mamidia, Srimannarayana Kaminenia, L. N. Sai PrasadRavinuthalaa, Venkatapparao Thumua & Vengal Rao Pachavaa

a Department of Physics, National Institute of Technology, Warangal,IndiaPublished online: 19 Aug 2014.

To cite this article: Venkata Reddy Mamidi, Srimannarayana Kamineni, L. N. Sai Prasad Ravinuthala,Venkatapparao Thumu & Vengal Rao Pachava (2014) Characterization of Encapsulating Materials forFiber Bragg Grating-Based Temperature Sensors, Fiber and Integrated Optics, 33:4, 325-335, DOI:10.1080/01468030.2014.932472

To link to this article: http://dx.doi.org/10.1080/01468030.2014.932472

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Fiber and Integrated Optics, 33:325–335, 2014

Copyright © Taylor & Francis Group, LLC

ISSN: 0146-8030 print/1096-4681 online

DOI: 10.1080/01468030.2014.932472

Characterization of Encapsulating Materials for

Fiber Bragg Grating-Based Temperature Sensors

VENKATA REDDY MAMIDI,1

SRIMANNARAYANA KAMINENI,1

L. N. SAI PRASAD RAVINUTHALA,1

VENKATAPPARAO THUMU,1 and

VENGAL RAO PACHAVA 1

1Department of Physics, National Institute of Technology, Warangal, India

Abstract An experimental study has been carried out for the characterization of

encapsulating materials for fiber Bragg grating-based temperature sensors to preventthe formation of micro-cracks and devitrification on the fiber surface at elevated

temperatures by making use of a rigid probe. The developed sensor probe wasconfigured by encapsulating a type 1 fiber Bragg grating with an aluminum nitride

tube and is used to measure temperatures from 20ıC to 500ıC. The adapted encap-sulation technique validated the sensor to achieve linearity of 99.979%, sensitivity of

14.03 ˙ 0.02 pm/ıC, and good repeatability; its practical use in harsh environmentsis predicted.

Keywords fiber Bragg grating, fiber-optic sensors, temperature sensor probe

1. Introduction

Temperature measurement currently encompasses a wide variety of needs and applications

in scientific fields, namely aerospace, metallurgical and civil engineering, solar panels,

nuclear power, shipping, petroleum, and thermal power industries [1]. To meet these

requirements, it has been necessary to develop a large number of sensors and devices.

In addition, conventional sensors, such as thermocouples, are point sensors and cannot

provide distributed data without utilizing a networked plurality of sensors. To obtain

accurate three-dimensional spatial information regarding the temperature distribution

with real-time monitoring, fiber Bragg gratings (FBGs) have been adapted as intelligent

elements for temperature sensing [2].

FBGs are compact intrinsic sensing elements that are relatively inexpensive to pro-

duce. They are obtained by creating periodic variations in the refractive index of the

core of an optical fiber. These periodic variations are created from exposure to an intense

UV interference pattern from a laser source. The periodic structure acts as a Bragg

reflector of a particular wavelength. When the Bragg condition is satisfied, reflections

Received 2 May 2014; accepted 4 June 2014.Address correspondence to Venkata Reddy Mamidi, Department of Physics, National Institute

of Technology, Warangal-506004, India. E-mail: mamidivenkatreddy@gmail.comColor versions ofone or more of the figures in the article can be found online at www.tandfonline.

com/ufio.

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326 V. R. Mamidi et al.

from each successive period will be in phase, whereas the light that does not satisfy

the Bragg condition passes through the FBG. The Bragg condition is expressed as

�B D 2neff ƒ, where �B is the Bragg wavelength of the FBG, neff is the effective index

of refraction of the fiber core at the free-space center wavelength, and ƒ is the grating

period. Even a minute change in the periodic structure due to external perturbation will

cause a considerable wavelength shift. This wavelength shift itself would be the measure

of external perturbation. Sensing technologies based on FBGs have several inherent

advantages that make them attractive for a wide range of scientific and industrial sensing

applications. They are typically small in size, highly sensitive, fast in response, immune

to electromagnetic interference, resistant to harsh environments, and have a capability of

multiplexing to perform distributed sensing [3, 4].

The past decades have seen tremendous growth in the number of FBG-based sensor

systems [5, 6]. The principle of the FBG-based temperature sensor involves the mea-

surement of Bragg wavelength shift with a variation in temperature. Two parameters that

are responsible for the shift of Bragg wavelength with change in temperature are (i) the

change in grating period due to the thermal expansion of fiber and (ii) the change in

refractive index. Thus, the wavelength shift for a temperature change (�T ) could be

written as

��B.t/ D �B.˛ C �/�T; (1)

where ˛ D .1=ƒ/@ƒ=@T / is the thermal expansion coefficient of the fiber, and � D

.1=neff /.@neff =@T / represents the thermo-optic coefficient [7, 8].

When the bare FBG is in direct contact to the sensing environment, the presence of

water vapor, pollutants, etc. affects the sensitivity of the sensor, and it cannot be used

repeatedly for measurement purposes. Due to the fragility of the bare FBGs, it is hard to

use them directly in harsh environments without any protection. Further, the formation

of micro-cracks at elevated temperatures due to an ambient environment penetrates deep

into the core, resulting in scattering and leakage of light from the core; consequently,

there will be a reduction in FBG reflected peak power. Therefore, it is necessary to

develop an encapsulation technique to protect the surface of bare FBGs and to use them

for engineering applications [9, 10].

Trying to encapsulate the fiber in another material is also difficult due to the lack of

knowledge on materials that are capable of withstanding harsh environments and allowing

the fast and linear temperature response of gratings. This is one of the challenges that must

be overcome to make the FBG sensor a reality. Therefore, FBGs must be encapsulated

with a suitable material so that the encapsulating structure protects the fragile bare FBGs

without compromising the optical response of the FBG [11, 12].

This article presents the detailed description of an FBG encapsulation technique,

testing methodology, experimental results, and characteristics of the sensor by making

use of various thermal encapsulating materials. The sensor performance is analyzed with

and without encapsulating it, in terms of wavelength shift and optical power, up to 500ıC.

2. Experimental Details

A polyimide coated type 1 FBG of Bragg wavelength at 1,552.96 nm has undergone

thermal characterization in its bare form, as shown in Figure 1. Light from a broad

band source (BBS; 1,525–1,565 nm) passing through a three-port optical circulator

illuminated the FBG. The reflected peak of the FBG is redirected to an optical spectrum

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FBG Sensor Probe for Temperature Measurement 327

Figure 1. Schematic of experimental setup for temperature study on FBGs.

analyzer (OSA) using port 3 of the circulator to analyze the parameters, wavelength, and

power. Temperature of the bare FBG is increased to 500ıC, and the corresponding Bragg

wavelength shift as well as peak power are recorded for each 1ıC of temperature change

using the OSA.

To characterize the encapsulating materials for the FBG temperature sensor, the

FBG is encapsulated with five capillary tubes made of different materials: silicon car-

bide, borosilicate glass, stainless steel, copper, and ceramic aluminium nitride, one after

another, respectively. Figure 2 shows the FBG sensor probes with the five encapsulating

materials. All tubes are 20 cm in length with a 2–2.5-mm hole diameter, given in Table 1.

Figure 2. FBG sensor probes with five different encapsulating materials.

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328 V. R. Mamidi et al.

Table 1

Thermal and mechanical properties of capillary tubes used and their performances after

encapsulating the FBG

Encapsulated

material

Diameter

inner/outer

(mm)

Thermal

capability

(ıC)

Thermal

conductivity

(W/m-ıC at

25ıC)

Linearity in

wavelength

shift

Response time

for sudden

change of

temperature

80ıC (sec)

Silicon carbide 2.5/7.0 2,730 60 99.85% 150

Borosilicate glass 2.5/7.0 500 1.14 99.92% 230

Stainless steel 2.0/3.2 1,400 16 99.9% 190

Copper 2.0/3.2 600 401 99.76% 50

Aluminum nitride 2.0/3.2 1,700 150 99.98% 90

The FBG is inserted carefully into the tube from one end and is sealed from both sides

using a high-temperature ceramic adhesive. Care has been taken to avoid the fiber contact

to the walls of the capillary tube and is perfectly aligned along the axis. Before inserting

the fiber into the tube, the polyimide buffer coating is removed so that it does not burn

off and contaminate the fiber at elevated temperatures. The fiber is then passed through

ceramic inserts that are fixed on the fiber at various points, as shown in Figure 3. This

is to prevent fiber contact with the side walls and make it pristine from possible sources

of contamination. To avoid the effect of induced strain on the FBG due to thermal

expansion of the capillary tube, it is fixed only at one end and the other end is left

free. After encapsulating with each capillary tube, the FBG sensor probe is subjected to

temperature change at two different rates (16ıC/min and 30ıC/min) within the range of

20ıC–500ıC, and the corresponding Bragg wavelength shift is recorded.

To investigate further the response time of the sensor probe for each encapsulating

material, it is subjected to a sudden change of temperature from 20ıC–100ıC, and the

time-dependent Bragg wavelength is recorded every 10 sec up to 330 sec. All experiments

are repeated six times, and the averaged responses have been taken to overcome random

errors.

3. Results and Discussion

It is evident from the obtained results that throughout the temperature range, the shift

in Bragg wavelength is almost the same in all five cases of FBG encapsulation, shown in

Figure 3. Photograph of FBG encapsulation process.

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FBG Sensor Probe for Temperature Measurement 329

Figure 4. Temperature response of encapsulated FBG at temperature change rate of 16ıC/min.

Figure 4. This means that although the encapsulating materials have different thermal

and mechanical properties, they cause almost the same response in the optical spectrum

of the FBG. They could not be differentiated in providing temperatures onto the FBG at

the rate of temperature change of 16ıC/min.

When the rate of temperature change is increased to 30ıC/min, the temperature-

dependent Bragg wavelength shift corresponding to each encapsulation material could

be differentiated as shown in Figure 5. This shift may be due to difference in their

thermal and mechanical properties, such as conductivity and thickness. However, encap-

sulation with the copper material provided more shift in Bragg wavelength, while in

case of aluminum nitride, silicon carbide, stainless steel, and borosilicate glass, the shift

continuously reduces monotonously.

In general, the thermal conductivity is a measure of the ability of a material to trans-

mit heat in unit time by conduction. It can be analyzed that the thermal conductivity as

well as thickness of a material do influence only the rate of amount of heat transfer across,

but not the amount of heat transfer. Among the five materials used for encapsulation,

stainless steel, copper, and aluminum nitride tubes have the same dimensions, whereas

the other two are different from the above and similar to each other. It is clear from the

response of stainless steel, copper, and aluminum nitride that when there is a high rate

of temperature change, the response of FBG encapsulated with a poor thermal conductor

will get delayed. Apart from the low thermal conductivity, the higher thickness of the tube

as well as the air gap are also responsible for the slow temperature response of the FBG

sensor probe in the case of silicon carbide and borosilicate glass. It is also observed from

Figure 5 that the response of the FBG encapsulated with a copper tube exhibits a bit of

non-linear response. The temperature dependence of its thermal conductivity might be

responsible for this behavior.

The temporal response of an encapsulated FBG subjected to sudden change of

temperature from 20ıC to 100ıC is obtained in all five cases, as shown in Figure 6.

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330 V. R. Mamidi et al.

Figure 5. Temperature response of encapsulated FBG at temperature change rate of 30ıC/min.

Figure 6. Temporal response of encapsulated FBG at sudden temperature change from 20ıC–

100ıC.

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FBG Sensor Probe for Temperature Measurement 331

The materials have provided better FBG response times in the following order:

copper > aluminum nitride > silicon carbide > stainless steel > borosilicate glass:

The thermal conductivity of the materials, the linearity of FBG wavelength shift encap-

sulated with the five materials, and the response time at sudden temperature change of

80ıC are given in Table 1.

The FBG encapsulated with copper is found to be better in response time; however, it

is known from experience that copper is an inappropriate material for FBG encapsulation

due to its non-linear response and temperature limit of 600ıC. On the other hand,

aluminum nitride provided better linearity and offered a faster response compared to the

other three materials. Also, it is a type of ceramic that can withstand temperatures to

1,700ıC. Thus, aluminum nitride is believed to be a more suitable encapsulating material

for FBGs to get a linear and fast response in temperature sensing.

4. Sensor Probe Design and Testing

From the above results, the first priority has been given to aluminum nitride for FBG

encapsulation. The ceramic aluminum nitride capillary tube used to design the sensor

probe is 20 cm in length and 2 and 3.2 mm inner and outer diameters, respectively. Since

the ceramic tube is brittle in nature, it is inserted inside a 3.5-mm-diameter stainless-

steel tube as a safeguard. To avoid the effect of the stainless steel tube on the response

time of the sensor, aspiratory holes are provided on it at the location of FBG. The

FBG inside the probe is fixed at one end and left free at the other end to ensure the

FBG gets no strain impact by the either the ceramic tube or stainless-steel tube. The

Figure 7. Assembly of FBG sensor probe.

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Figure 8. Photograph of experimental setup with FBG sensor probe.

sensor probe is then inserted in a steel rod by making use of a brass holder and is sealed

with high-temperature adhesive. A heat-resistant handle made of nylon is employed for

ease and safe handling from thermal hazards, as shown in Figure 7. An optical fiber

patch card with a fiber connector/physical contact connector makes the sensor probe

portable. The encapsulation not only protects the sensing element, but also makes it easy

for installation.

The temperature response of the FBG sensor is tested after making it a rigid probe,

as shown in Figure 8. In comparison to the bare FBG, the encapsulated FBG seems to lag

slightly in Bragg wavelength shift but maintains linearity. The resonance peak power has

been stabilized significantly at high temperatures, which is desirable, shown in Figure 9.

The FBG sensor probe has shown fine temperature characteristics with a linearity of

�99.979% and sensitivity of 14:03 ˙ 0:02 pm/ıC. Figure 10 shows the spectral response

of the sensor that is monitored by the OSA.

Figure 9. Temperature response of FBG before and after encapsulation.

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FBG Sensor Probe for Temperature Measurement 333

Figure 10. Temperature response on reflected spectrum of sensor probe over span of 20ıC–500ıC.

Figure 11. Repeatability test results of sensor probe of final design.

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334 V. R. Mamidi et al.

Figure 11 exhibits the two most dissent trials while heating as well as cooling

among the six trials is conducted. The maximum repeatability error is found to be 6ıC,

represented by the inner graph. Hence, the experimental results reveal that the system

works accurately from room temperature to 500ıC and accomplishes good repeatability.

5. Conclusion

A characteristic study was made on encapsulating materials—silicon carbide, borosilicate

glass, stainless steel, copper, and aluminum nitride—for an FBG temperature sensor. From

the experimental results, it is concluded that aluminum nitride is the most suitable material

to encapsulate the FBG for temperature sensing. An indigenous FBG-based temperature

sensor probe is developed and demonstrated with test results. The excellent linearity of

99.979%, sensitivity of 14:03 ˙ 0:02 pm/ıC, and good repeatability obtained by this

encapsulation technique are promising its practical use in areas with high temperatures

and harsh environments.

Acknowledgments

The authors thank Dr. Balaji Srinivasan, Department of Electrical Engineering, IIT

Madras, for sparing FBGs and Mr. Jude Benedict, Fatima Industrial Training Institute,

Kazipet, for providing fabrication facilities.

Funding

The authors acknowledge the fine gesture of Department of Electronics & Information

Technology (DEIT) under the Ministry of Communications and Information Technologies,

New Delhi, for providing financial assistance to carry out this work.

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Biographies

Venkata Reddy Mamidi received his B.Sc. from Osmania University, Hyderabad,

India, in 2008 and his M.Sc. (Tech.) in engineering physics with instrumentation special-

ization from National Institute of Technology, Warangal, India, in 2011. He is currently

pursuing his Ph.D. at the Department of Physics, NIT, Warangal. His research inter-

ests include optical fiber sensors and its applications, electrostatics, and optoelectronic

instrumentation. He is a member of SPIE and OSA.

Srimannarayana Kamineni received his B.Sc. from Andhra University, Visakhap-

atnam, in 1971 and his M.Sc. (Tech.) and Ph.D. in engineering physics in the fields of

photonics and holography for non-destructive testing from Regional Engineering College

(now NIT), Warangal, in 1974 and 1978, respectively. He has been a faculty member

of the Physics Department at National Institute of Technology, Warangal, since 1977

and is presently working as a professor. His research interests are in the areas of fiber-

optic sensors, passive components in telecommunications, optical techniques in precision

measurements, laser metrology, and holography for NDT. He is a member of SPIE

and OSA.

L. N. Sai Prasad Ravinuthala received his B.Sc. from Sri Venkateswara University,

Tirupati, India, in 1976 and his M.Sc. from Andhra University, Visakhapatnam, in

1979. He completed his Ph.D. at Regional Engineering College (now NIT), Warangal,

in 1985. He joined as a faculty member of NIT, Warangal, in 1986 and is presently

working as a professor in the Department of Physics of that institute. His present research

interest includes electronic instrumentation, holographic NDT, FBG sensors, and signal

processing. He is a member of SPIE and ISTE.

Venkatapparao Thumu received his B.Sc. from Osmania University, Hyderabad, in

1983 and his M.Sc. in physics from Kakatiya University, Warangal, in 1987. He obtained

his Ph.D. from Kakatiya University in 1994. He worked as a faculty member at Kakatiya

University from 1994 to 2008. He has been working as an assistant professor at National

Institute of Technology, Warangal, since 2008. His research interest includes highly

correlated electron systems, optoelctronics, fiber-optic sensing, polymer degradation,

polymer blends, and spintronic materials.

Vengal Rao Pachava received his B.Sc. and M.Sc. specializing in electronics and

communications from Osmania University, Hyderabad, India, in 2004 and 2009, respec-

tively. He is currently pursuing his Ph.D. while investigating the use of optical sensing

techniques at the Department of Physics, NIT, Warangal, India. His research interests

include low- and high-pressure sensing using fiber Bragg grating sensors. He is a fellow

of Optical Society of India, a member of SPIE, and a member of OSA.

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