A miniature high-sensitivity, high-temperature fiber sensor with an interferometer based on a bare small-core-diameter dispersion compensation fiber (DCF) is reported. The sensing head is a single-mode-fiber (SMF) DCF configuration formed by a 4 mm long bare DCF with one end connected to the SMF by a fusion splicing technique and the other end cleaved. Due to the large mode index difference and high thermo-optic coefficient induced by two dominative interference modes, a miniature high-temperature fiber sensor with a high sensitivity of 68.6 pm/°C is obtained by monitoring the wavelength shift of the interference spectrum. This type of sensor has the features of small size, high sensitivity, high stability, simple structure, and low cost

Dong, Bo

Wei, Li

Zhou, Da-Peng

English

Wilfrid Laurier University

Wilfrid Laurier University Scholars Commons @ Laurier Physics and Computer Science Faculty Publications Physics and Computer Science 11-2009 Miniature High-Sensitivity High-Temperature Fiber Sensor with a Dispersion Compensation Fiber-Based Interferometer Bo Dong Wilfrid Laurier University Li Wei Wilfrid Laurier University, lwei@wlu.ca Da-Peng Zhou University of Waterloo Follow this and additional works at: https://scholars.wlu.ca/phys_faculty Recommended Citation Dong, Bo; Wei, Li; and Zhou, Da-Peng, "Miniature High-Sensitivity High-Temperature Fiber Sensor with a Dispersion Compensation Fiber-Based Interferometer" (2009). Physics and Computer Science Faculty Publications. 7. https://scholars.wlu.ca/phys_faculty/7 This Article is brought to you for free and open access by the Physics and Computer Science at Scholars Commons @ Laurier. It has been accepted for inclusion in Physics and Computer Science Faculty Publications by an authorized administrator of Scholars Commons @ Laurier. For more information, please contact scholarscommons@wlu.ca. Miniature high-sensitivity high-temperature fibersensor with a dispersion compensationfiber-based interferometerBo Dong,1,* Li Wei,1,2 and Da-Peng Zhou21Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo,Ontario N2L 3C5, Canada2Department of Physics and Astronomy, Guelph-Waterloo Physics Institute, University of Waterloo,Waterloo, Ontario N2L 3G1, Canada*Corresponding author: dbo1978@163.comReceived 13 August 2009; revised 25 October 2009; accepted 30 October 2009;posted 2 November 2009 (Doc. ID 115664); published 12 November 2009A miniature high-sensitivity, high-temperature fiber sensor with an interferometer based on a baresmall-core-diameter dispersion compensation fiber (DCF) is reported. The sensing head is a single-mode-fiber (SMF) DCF configuration formed by a 4mm long bare DCF with one end connected tothe SMF by a fusion splicing technique and the other end cleaved. Due to the large mode index differenceand high thermo-optic coefficient induced by two dominative interference modes, a miniature high-temperature fiber sensor with a high sensitivity of 68:6pm=°C is obtained by monitoring the wavelengthshift of the interference spectrum. This type of sensor has the features of small size, high sensitivity, highstability, simple structure, and low cost. © 2009 Optical Society of AmericaOCIS codes: 060.2310, 060.2370.1. IntroductionTo meet the increasing demands of modern industry,different types of fiber temperature sensor based onfiber gratings, High-birefringence (Hi-Bi) fiber inter-ferometers, multimode fiber (MMF) interferometer,etc., have been developed. Among them, the use offiber Bragg gratings [1–3] is particularly attractivefor their small size, wavelength encoding, and wave-length multiplexing features, but the temperaturesensitivity of fiber Bragg gratings is limited(∼10pm=°C). The use of long-period gratings [4] pro-vides a high sensitivity to temperature, but long-period gratings easily suffer from the bend effect.Moreover, the fabrication technique for fiber Bragggratings and long-period gratings is complicated,which results in an expensive cost. Hi-Bi fiber inter-ferometer temperature sensors [5] have the advan-tages of easy fabrication and high sensitivity, butthey need long Hi-Bi fibers and polarization control-ler or polarizers, which is impractical for distributedsensing. Recently MMF interference temperaturesensors have attracted much attention for theiradvantages of low-cost and high-temperature opera-tion. A conventional type of sensor [6] using step-index MMF with a large core diameter has lowtemperature sensitivity (∼15pm=°C) and needs along section of MMF, about 50mm; although the tem-perature sensitivity was improved to 58:5pm=°C byusing a large-core-diameter graded-index MMF [7],that required a 1:8m long graded-index MMF, andhigh-temperature operation was not demonstrated.The above reported sensors with long MMFs arenot practical for real application for two main rea-sons. First, the long MMF is easily influenced by am-bient disturbances; second, for high-temperatureapplication, the above sensing head has to adoptthe bare fiber in order to avoid fiber coating burning,which induces undesirable strain to the sensing0003-6935/09/336466-04$15.00/0© 2009 Optical Society of America6466 APPLIED OPTICS / Vol. 48, No. 33 / 20 November 2009head. Obviously, it is difficult to fabricate a bare fibersensing head by using a long section of MMF.Miniature fiber sensors are highly desirable inhigh-temperature sensing. Although miniaturehigh-temperature fiber sensors based on Fabry–Perot (FP) interference [8–10] were demonstrated,they needed complicated fabrication process suchas etching [8] and laser micromachining [9] to forma single FP cavity, or cascading three sections of fi-bers [a photonic crystal fiber, a hollow optical fiberand a single-mode fiber (SMF)] to form a hybridFP cavity [10], and the length of the FP sensors con-structed with fiber is generally more than 1 cm. Onthe other hand, a miniature high-temperature fibersensor based on modal interference was fabricatedwith a 1:2 cm length by cascading two MMFs andthree SMFs [11]; however, the structure of the sen-sing head is not simple, and the fabrication processesis not easy.In this paper, we present a miniature high-sensitivity, high-temperature fiber sensor with abare small-core dispersion compensation fiber(DCF)-based interferometer. Within the C band,the DCF can act as an MMF. The sensing headhas an SMF-DCF configuration formed with a4mm long bare DCF with one end being spliced toa section of SMF and the other end being cleaved.To the best of our knowledge, the proposed 4mm longfiber sensor is the shortest fiber-based sensor to thisday. Within the temperature measurement range of25 °C–600 °C, a high sensitivity of 68:6pm=°C is ob-tained by monitoring the wavelength shift of the in-terference spectrum. Moreover, our proposed sensorcan be made 22 or 84 times smaller than the two sen-sors in [6,7], considering that all the sensors have thesame free spectral range (FSR). This is because thesmall-core-diameter DCF has amuch larger mode in-dex difference between the two interference modesthan does the conventional step-index or graded-index MMF. Such a unique property allows us tobuild ultracompact sensing heads, which are moresuitable for distributed sensing, as well as biomedi-cal or chemical sensing applications. This type of sen-sor has the features of small size, high sensitivity,high stability, simple structure, and low cost.2. Experimental Setup and PrincipleFigure 1 shows the schematic diagram of the experi-mental setup. The temperature is controlled by atube furnace (Thermolyne type 21100). The tempera-ture of the fiber sensor was measured by a type-Kthermocouple, which was connected to a digital ther-mometer. The sensor head was attached to the ther-mocouple, and both of them were inserted into thetube furnace. A C-band broadband source (BBS) withan amplified spontaneous emission spectrum cen-tered at 1550nm was used as the light source. Theinterference spectra were monitored by an opticalspectrum analyzer (OSA, Agilent 86142B).The sensing head, as shown in the inset of Fig. 1, isfabricated by connecting a 4mm long bare DCF to astandard communications SMF, which serves as aninterferometer based on intermode interference.The DCF has a core/cladding diameter of 1:9=115:7 μm, a large dispersion parameter of −270ps=nm=km at 1550nm, and a cutoff wavelength of1663nm. Within the C band, the DCF can act asan MMF. The free end of the DCF was cleaved asa mirror to reflect the light back to the DCF. Theemitted light of the BBS propagates to the end sur-face of the sensing head through a 3dB coupler, andthen the reflected light propagates to the OSAthrough the coupler again. The fundamental modethat propagates along the SMF will couple into theDCF and excite different interference modes at theSMF-DCF splice point. Reflected back to the DCFby the cleaved end surface of the DCF, these modeswill interfere and recouple into the SMF at the DCF-SMF splice point.Figure 2 shows the measured reflectivity spectrumof the interferometer and the transmission powerspectrum of the applied BBS. The interference canbe approximately considered to be produced by twodominant modes, which indicates that other weakmodes carry few powers and slightly modulate theinterference pattern. The phase difference betweenthe two dominant modes after propagating throughthe DCF can be written as φm ¼ 4πBL=λ, whereFig. 1. (Color online) Schematic diagram of the experimentalsetup (inset, structure of the sensing head).Fig. 2. Interference spectrum of the DCF interferometer.20 November 2009 / Vol. 48, No. 33 / APPLIED OPTICS 6467B ¼ n1 − n2 is the mode index difference of the twodominant modes, and n1, n2 are the mode indicesof the two modes respectively; λ is the signal wave-length in vacuum and L is the length of the DCF.The interference condition of the two dominantmodes can be given byφm ¼4πBLλ ¼ N2π ðN is an integerÞ: ð1ÞWhen ambient temperature changes, the mode indexdifference B and the fiber length L will changeaccordingly, and thus the interference spectrum willhave a wavelength shift Δλ. According to Eq. (1) therelative wavelength shift Δλ=λ caused by a tempera-ture change ΔT can be expressed asΔλλ ≈ ðαþ ξÞΔT; ð2Þwhere α ¼ dL=LdT is the thermal expansion coeffi-cient of the DCF material, and ξ ¼ dB=BdT is thethermo-optic coefficient induced by the two interfer-ence modes in the DCF material. The FSR of the in-terfering spectrum can be approximately given byFSR ¼ λ22BL: ð3ÞIt can be seen that the FSR is inversely proportionalto both the mode index difference B and the length ofthe DCF L. According to Eq. (3) and Fig. 2, the modeindex difference of our proposed DCF interferencesensor is about 2:1 × 10−2. However, the mode indexdifference of the high-temperature 105=125 μmMMFinterference sensor [6] is about 9:6 × 10−14, and thatof the 100=140 μm MMF temperature sensor [7] isonly about 2:5 × 10−4. The mode index difference ofour proposed interference sensor is about 22 and84 times larger than that of the two temperature sen-sors in [6,7]. In other words, our proposed sensor canbe made by 22 or 84 times smaller than the two sen-sors above, considering that all the sensors have thesame FSR. On the other hand, sensor resolution is avery important parameter and is dependent on wave-length measurement errors that are determined bythe resolution of OSA and the bandwidth of the re-sonance peak. For a fixed value of L, a large valueof B gives rise to a small FSR and consequently asmall bandwidth of the resonance peak, which willbe beneficial to the sensor resolution. Therefore,the proposed 1:9=115:7 μm DCF interference sensorwith a large mode index difference can be miniatur-ized readily with a high-temperature resolution. Theparameters of the refractive indices of the core andthe cladding of the DCF are not known for us. Thelarger mode index difference of the DCF interferom-eter is attributed to the special design of the DCF.3. Experimental Results and DiscussionSince the BBS we used is not polarized, the transmis-sion spectrum of the interferometer is polarization in-dependent. The OSA with a resolution of 0:06nm isused to measure the transmission spectrum of the in-terferometer. When the temperature increases, theDCF interference spectrum has a red shift whilethe spectrum almost keeps the same pattern, asshown in Fig. 3. To monitor a broad wavelength shift,we measure the wavelength variation of the point atthe arrow shown in Fig. 3, starting with the wave-length of 1534:28nm at a room temperature of25 °C. Figure 4 shows the measured relationship be-tween the wavelength and the temperature. It can beseen that there is a good linear relationship betweenthe wavelength and temperature, and the correlationcoefficient square of R2 reaches 0.9983. Within thetemperature range of 25 °C–600 °C, the temperaturesensitivity reaches 68:6pm=°C. Compared with thehigh-temperature MMF-based sensors [6], theproposed high-temperature sensor has highersensitivity, which is mainly because of the higherFig. 3. (Color online) Wavelength shifts of the interferencespectrum at different temperatures.Fig. 4. Measured relationship between the wavelength and thetemperature.6468 APPLIED OPTICS / Vol. 48, No. 33 / 20 November 2009thermo-optic coefficient. The thermal expansion coef-ficient is generally lower by 2 orders of magnitudethan the thermo-optic coefficient for the optical fiber;ignoring the thermal expansion coefficient, the ther-mo-optic coefficient difference of the DCF used in ourexperiment is about 4:4 × 10−5, while that of theMMFin [6] is about 9:7 × 10−6. As the FSR of the proposedfiber sensor is much smaller than the bandwidth ofthe available BBS, the length of the sensing headcan be shortened further to obtain ultracompact sen-singheads. To test the influence of the ambient distur-bance on the sensor, we sway the sensing head underseveral fixed temperatures, andwe find that the inter-ference spectrum does not change and has high stabi-lity under a fixed temperature, which is attributed tothe small size of the sensing head. In addition, it canbe seen that a cleaved surface used as a reflectionmir-ror provides low reflection. However, the low reflec-tion does not affect our measurement accuracy,since we measure the wavelength shift of the spec-trum. The reflectivity can be increased by coatingthe DCF end surface with aluminum.4. ConclusionsA miniature high-temperature fiber sensor with abare small-core-diameter DCF-based interferometerhas been proposed and demonstrated experimentally.Experimental results show that theDCF interferencetemperature sensorwith an only 4mm long bareDCFcan be operated with high sensitivity of 68:6pm=°Cwithin the temperature measurement range of25 °C–600 °C. This type of sensor has the featuresof small size, high sensitivity, high stability, simplestructure, and low cost. It is expected to be appliedin smart and distributed fiber temperature sensing.This work is supported by the Natural Science andEngineering Research Council of Canada (NSERC).The authors acknowledge Sumitomo Electric Indus-tries, Ltd. for providing the 1:9 μm=115:7 μm DCF.References1. K. P. Koo and A. D. Kersey, “Bragg grating-based laser sensorssystems with interferometric interrogation and wavelengthdivision multiplexing,” J. Lightwave Technol. 13, 1243–1249(1995).2. G. A. Ball, W. W. Morey, and P. K. Cheo, “Single- and multi-point fiber-laser sensors,” IEEE Photon. Technol. Lett. 5,267–270 (1993).3. V. Bhatia and A. M. Vengsarkar, “Optical fiber long-periodgrating sensors,” Opt. Lett. 21, 692–694 (1996).4. Y. Liu, B. Liu, X. Feng, W. Zhang, G. Zhou, S. Yuan, G. Kai, andX. Dong, “High-birefringence fiber loop mirrors and theirapplications as sensors,” Appl. Opt. 44, 2382–2390 (2005).5. A. N. Starodumov, L. A. Zenteno, D. Monzon, and E. De L.Rosa, “Fiber Sagnac interferometer temperature sensor,”Appl. Phys. Lett. 70, 19–21 (1997).6. E. Li, X. Wang, and C. Zhang, “Fiber-optic temperature sensorbased on interference of selective higher-order modes,” Appl.Phys. Lett. 89, 091119 (2006).7. Y. Liu and L. Wei, “Low-cost high-sensitivity strain andtemperature sensing using graded-index multimode fibers,”Appl. Opt. 46, 2516–2519 (2007).8. X. Chen, F. Shen, Z. Wang, Z. Huang, and A. Wang, “Micro-air-gap based intrinsic Fabry-Perot interferometric fiber-opticsensor,” Appl. Opt. 45, 7760–7766 (2006).9. T. Wei, Y. Han, H. Tsai, and H. Xiao, “Miniaturized fiber inlineFabry–Perot interferometer fabricated with a femtosecond la-ser,” Opt. Lett. 33, 536–538 (2008).10. H. Y. Choi, K. S. Park, S. J. Park, U. Paek, B. H. Lee, and E. S.Choi, “Miniature fiber-optic high temperature sensor based ona hybrid structured Fabry–Perot interferometer,” Opt. Lett.33, 2455–2457 (2008).11. L. V. Nguyen, D. Hwang, S. Moon, D. S. Moon, and Y. Chung,“High temperature fiber sensor with high sensitivity based oncore diameter mismatch,” Opt. Express 16, 11369–11375(2008).20 November 2009 / Vol. 48, No. 33 / APPLIED OPTICS 6469

Miniature High-Sensitivity High-Temperature Fiber Sensor with a Dispersion Compensation Fiber-Based Interferometer

Scholars Commons

Wilfrid Laurier University 
Scholars Commons @ Laurier 
Physics and Computer Science Faculty 
Publications Physics and Computer Science 
11-2009 
Miniature High-Sensitivity High-Temperature Fiber Sensor with a 
Dispersion Compensation Fiber-Based Interferometer 
Bo Dong 
Wilfrid Laurier University 
Li Wei 
Wilfrid Laurier University, lwei@wlu.ca 
Da-Peng Zhou 
University of Waterloo 
Follow this and additional works at: https://scholars.wlu.ca/phys_faculty 
Recommended Citation 
Dong, Bo; Wei, Li; and Zhou, Da-Peng, "Miniature High-Sensitivity High-Temperature Fiber Sensor with a 
Dispersion Compensation Fiber-Based Interferometer" (2009). Physics and Computer Science Faculty 
Publications. 7. 
https://scholars.wlu.ca/phys_faculty/7 
This Article is brought to you for free and open access by the Physics and Computer Science at Scholars Commons 
@ Laurier. It has been accepted for inclusion in Physics and Computer Science Faculty Publications by an 
authorized administrator of Scholars Commons @ Laurier. For more information, please contact 
scholarscommons@wlu.ca. 
Miniature high-sensitivity high-temperature fiber
sensor with a dispersion compensation
fiber-based interferometer
Bo Dong,1,* Li Wei,1,2 and Da-Peng Zhou2
1Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo,
Ontario N2L 3C5, Canada
2Department of Physics and Astronomy, Guelph-Waterloo Physics Institute, University of Waterloo,
Waterloo, Ontario N2L 3G1, Canada
*Corresponding author: dbo1978@163.com
Received 13 August 2009; revised 25 October 2009; accepted 30 October 2009;
posted 2 November 2009 (Doc. ID 115664); published 12 November 2009
A miniature high-sensitivity, high-temperature fiber sensor with an interferometer based on a bare
small-core-diameter dispersion compensation fiber (DCF) is reported. The sensing head is a single-
mode-fiber (SMF) DCF configuration formed by a 4mm long bare DCF with one end connected to
the SMF by a fusion splicing technique and the other end cleaved. Due to the large mode index difference
and high thermo-optic coefficient induced by two dominative interference modes, a miniature high-
temperature fiber sensor with a high sensitivity of 68:6pm=°C is obtained by monitoring the wavelength
shift of the interference spectrum. This type of sensor has the features of small size, high sensitivity, high
stability, simple structure, and low cost. © 2009 Optical Society of America
OCIS codes: 060.2310, 060.2370.
1. Introduction
To meet the increasing demands of modern industry,
different types of fiber temperature sensor based on
fiber gratings, High-birefringence (Hi-Bi) fiber inter-
ferometers, multimode fiber (MMF) interferometer,
etc., have been developed. Among them, the use of
fiber Bragg gratings [1–3] is particularly attractive
for their small size, wavelength encoding, and wave-
length multiplexing features, but the temperature
sensitivity of fiber Bragg gratings is limited
(∼10pm=°C). The use of long-period gratings [4] pro-
vides a high sensitivity to temperature, but long-
period gratings easily suffer from the bend effect.
Moreover, the fabrication technique for fiber Bragg
gratings and long-period gratings is complicated,
which results in an expensive cost. Hi-Bi fiber inter-
ferometer temperature sensors [5] have the advan-
tages of easy fabrication and high sensitivity, but
they need long Hi-Bi fibers and polarization control-
ler or polarizers, which is impractical for distributed
sensing. Recently MMF interference temperature
sensors have attracted much attention for their
advantages of low-cost and high-temperature opera-
tion. A conventional type of sensor [6] using step-
index MMF with a large core diameter has low
temperature sensitivity (∼15pm=°C) and needs a
long section of MMF, about 50mm; although the tem-
perature sensitivity was improved to 58:5pm=°C by
using a large-core-diameter graded-index MMF [7],
that required a 1:8m long graded-index MMF, and
high-temperature operation was not demonstrated.
The above reported sensors with long MMFs are
not practical for real application for two main rea-
sons. First, the long MMF is easily influenced by am-
bient disturbances; second, for high-temperature
application, the above sensing head has to adopt
the bare fiber in order to avoid fiber coating burning,
which induces undesirable strain to the sensing
0003-6935/09/336466-04$15.00/0
© 2009 Optical Society of America
6466 APPLIED OPTICS / Vol. 48, No. 33 / 20 November 2009
head. Obviously, it is difficult to fabricate a bare fiber
sensing head by using a long section of MMF.
Miniature fiber sensors are highly desirable in
high-temperature sensing. Although miniature
high-temperature fiber sensors based on Fabry–
Perot (FP) interference [8–10] were demonstrated,
they needed complicated fabrication process such
as etching [8] and laser micromachining [9] to form
a single FP cavity, or cascading three sections of fi-
bers [a photonic crystal fiber, a hollow optical fiber
and a single-mode fiber (SMF)] to form a hybrid
FP cavity [10], and the length of the FP sensors con-
structed with fiber is generally more than 1 cm. On
the other hand, a miniature high-temperature fiber
sensor based on modal interference was fabricated
with a 1:2 cm length by cascading two MMFs and
three SMFs [11]; however, the structure of the sen-
sing head is not simple, and the fabrication processes
is not easy.
In this paper, we present a miniature high-
sensitivity, high-temperature fiber sensor with a
bare small-core dispersion compensation fiber
(DCF)-based interferometer. Within the C band,
the DCF can act as an MMF. The sensing head
has an SMF-DCF configuration formed with a
4mm long bare DCF with one end being spliced to
a section of SMF and the other end being cleaved.
To the best of our knowledge, the proposed 4mm long
fiber sensor is the shortest fiber-based sensor to this
day. Within the temperature measurement range of
25 °C–600 °C, a high sensitivity of 68:6pm=°C is ob-
tained by monitoring the wavelength shift of the in-
terference spectrum. Moreover, our proposed sensor
can be made 22 or 84 times smaller than the two sen-
sors in [6,7], considering that all the sensors have the
same free spectral range (FSR). This is because the
small-core-diameter DCF has amuch larger mode in-
dex difference between the two interference modes
than does the conventional step-index or graded-
index MMF. Such a unique property allows us to
build ultracompact sensing heads, which are more
suitable for distributed sensing, as well as biomedi-
cal or chemical sensing applications. This type of sen-
sor has the features of small size, high sensitivity,
high stability, simple structure, and low cost.
2. Experimental Setup and Principle
Figure 1 shows the schematic diagram of the experi-
mental setup. The temperature is controlled by a
tube furnace (Thermolyne type 21100). The tempera-
ture of the fiber sensor was measured by a type-K
thermocouple, which was connected to a digital ther-
mometer. The sensor head was attached to the ther-
mocouple, and both of them were inserted into the
tube furnace. A C-band broadband source (BBS) with
an amplified spontaneous emission spectrum cen-
tered at 1550nm was used as the light source. The
interference spectra were monitored by an optical
spectrum analyzer (OSA, Agilent 86142B).
The sensing head, as shown in the inset of Fig. 1, is
fabricated by connecting a 4mm long bare DCF to a
standard communications SMF, which serves as an
interferometer based on intermode interference.
The DCF has a core/cladding diameter of 1:9=
115:7 μm, a large dispersion parameter of −270ps=
nm=km at 1550nm, and a cutoff wavelength of
1663nm. Within the C band, the DCF can act as
an MMF. The free end of the DCF was cleaved as
a mirror to reflect the light back to the DCF. The
emitted light of the BBS propagates to the end sur-
face of the sensing head through a 3dB coupler, and
then the reflected light propagates to the OSA
through the coupler again. The fundamental mode
that propagates along the SMF will couple into the
DCF and excite different interference modes at the
SMF-DCF splice point. Reflected back to the DCF
by the cleaved end surface of the DCF, these modes
will interfere and recouple into the SMF at the DCF-
SMF splice point.
Figure 2 shows the measured reflectivity spectrum
of the interferometer and the transmission power
spectrum of the applied BBS. The interference can
be approximately considered to be produced by two
dominant modes, which indicates that other weak
modes carry few powers and slightly modulate the
interference pattern. The phase difference between
the two dominant modes after propagating through
the DCF can be written as φm ¼ 4πBL=λ, where
Fig. 1. (Color online) Schematic diagram of the experimental
setup (inset, structure of the sensing head).
Fig. 2. Interference spectrum of the DCF interferometer.
20 November 2009 / Vol. 48, No. 33 / APPLIED OPTICS 6467
B ¼ n1 − n2 is the mode index difference of the two
dominant modes, and n1, n2 are the mode indices
of the two modes respectively; λ is the signal wave-
length in vacuum and L is the length of the DCF.
The interference condition of the two dominant
modes can be given by
φm ¼
4πBL
λ ¼ N2π ðN is an integerÞ: ð1Þ
When ambient temperature changes, the mode index
difference B and the fiber length L will change
accordingly, and thus the interference spectrum will
have a wavelength shift Δλ. According to Eq. (1) the
relative wavelength shift Δλ=λ caused by a tempera-
ture change ΔT can be expressed as
Δλ
λ ≈ ðαþ ξÞΔT; ð2Þ
where α ¼ dL=LdT is the thermal expansion coeffi-
cient of the DCF material, and ξ ¼ dB=BdT is the
thermo-optic coefficient induced by the two interfer-
ence modes in the DCF material. The FSR of the in-
terfering spectrum can be approximately given by
FSR ¼ λ
2
2BL
: ð3Þ
It can be seen that the FSR is inversely proportional
to both the mode index difference B and the length of
the DCF L. According to Eq. (3) and Fig. 2, the mode
index difference of our proposed DCF interference
sensor is about 2:1 × 10−2. However, the mode index
difference of the high-temperature 105=125 μmMMF
interference sensor [6] is about 9:6 × 10−14, and that
of the 100=140 μm MMF temperature sensor [7] is
only about 2:5 × 10−4. The mode index difference of
our proposed interference sensor is about 22 and
84 times larger than that of the two temperature sen-
sors in [6,7]. In other words, our proposed sensor can
be made by 22 or 84 times smaller than the two sen-
sors above, considering that all the sensors have the
same FSR. On the other hand, sensor resolution is a
very important parameter and is dependent on wave-
length measurement errors that are determined by
the resolution of OSA and the bandwidth of the re-
sonance peak. For a fixed value of L, a large value
of B gives rise to a small FSR and consequently a
small bandwidth of the resonance peak, which will
be beneficial to the sensor resolution. Therefore,
the proposed 1:9=115:7 μm DCF interference sensor
with a large mode index difference can be miniatur-
ized readily with a high-temperature resolution. The
parameters of the refractive indices of the core and
the cladding of the DCF are not known for us. The
larger mode index difference of the DCF interferom-
eter is attributed to the special design of the DCF.
3. Experimental Results and Discussion
Since the BBS we used is not polarized, the transmis-
sion spectrum of the interferometer is polarization in-
dependent. The OSA with a resolution of 0:06nm is
used to measure the transmission spectrum of the in-
terferometer. When the temperature increases, the
DCF interference spectrum has a red shift while
the spectrum almost keeps the same pattern, as
shown in Fig. 3. To monitor a broad wavelength shift,
we measure the wavelength variation of the point at
the arrow shown in Fig. 3, starting with the wave-
length of 1534:28nm at a room temperature of
25 °C. Figure 4 shows the measured relationship be-
tween the wavelength and the temperature. It can be
seen that there is a good linear relationship between
the wavelength and temperature, and the correlation
coefficient square of R2 reaches 0.9983. Within the
temperature range of 25 °C–600 °C, the temperature
sensitivity reaches 68:6pm=°C. Compared with the
high-temperature MMF-based sensors [6], the
proposed high-temperature sensor has higher
sensitivity, which is mainly because of the higher
Fig. 3. (Color online) Wavelength shifts of the interference
spectrum at different temperatures.
Fig. 4. Measured relationship between the wavelength and the
temperature.
6468 APPLIED OPTICS / Vol. 48, No. 33 / 20 November 2009
thermo-optic coefficient. The thermal expansion coef-
ficient is generally lower by 2 orders of magnitude
than the thermo-optic coefficient for the optical fiber;
ignoring the thermal expansion coefficient, the ther-
mo-optic coefficient difference of the DCF used in our
experiment is about 4:4 × 10−5, while that of theMMF
in [6] is about 9:7 × 10−6. As the FSR of the proposed
fiber sensor is much smaller than the bandwidth of
the available BBS, the length of the sensing head
can be shortened further to obtain ultracompact sen-
singheads. To test the influence of the ambient distur-
bance on the sensor, we sway the sensing head under
several fixed temperatures, andwe find that the inter-
ference spectrum does not change and has high stabi-
lity under a fixed temperature, which is attributed to
the small size of the sensing head. In addition, it can
be seen that a cleaved surface used as a reflectionmir-
ror provides low reflection. However, the low reflec-
tion does not affect our measurement accuracy,
since we measure the wavelength shift of the spec-
trum. The reflectivity can be increased by coating
the DCF end surface with aluminum.
4. Conclusions
A miniature high-temperature fiber sensor with a
bare small-core-diameter DCF-based interferometer
has been proposed and demonstrated experimentally.
Experimental results show that theDCF interference
temperature sensorwith an only 4mm long bareDCF
can be operated with high sensitivity of 68:6pm=°C
within the temperature measurement range of
25 °C–600 °C. This type of sensor has the features
of small size, high sensitivity, high stability, simple
structure, and low cost. It is expected to be applied
in smart and distributed fiber temperature sensing.
This work is supported by the Natural Science and
Engineering Research Council of Canada (NSERC).
The authors acknowledge Sumitomo Electric Indus-
tries, Ltd. for providing the 1:9 μm=115:7 μm DCF.
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20 November 2009 / Vol. 48, No. 33 / APPLIED OPTICS 6469


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Miniature High-Sensitivity High-Temperature Fiber Sensor with a Dispersion Compensation Fiber-Based Interferometer

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