Simultaneous strain and temperature fiber
grating laser sensor based on radio-frequency
measurement
Yan-Nan Tan,1,2 Yang Zhang,1 Long Jin,2 and Bai-Ou Guan2,*
1
PolyU-DUT Joint Research Center for Photonics, Dalian University of Technology, Dalian 116024, China
2
Institute of Photonics Technology, Jinan University, Guangzhou 510632, China
*tguanbo@jnu.edu.cn
Abstract: We propose and experimentally demonstrate a novel
simultaneous strain and temperature fiber optic sensor. The sensing head is
formed by two concatenated ultra-short distributed Bragg reflector lasers
that operate in single longitude mode with two polarization modes. The total
length of the sensing head is only 18 mm. The two lasers generate two
polarization mode beat notes in the radio-frequency range which show
different frequency response to strain and temperature. Simultaneous strain
and temperature measurement can be achieved by radio-frequency
measurement. This approach has distinctive advantages of ease of
interrogation and avoidance of expensive wavelength detection.
©2011 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (060.2840) Heterodyne; (060.3510) Lasers, fiber;
(060.3735) Fiber Bragg gratings.
References and links
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S. W. James, M. L. Dockney, and R. P. Tatam, “Simultaneous independent temperature and strain measurement
using in-fibre Bragg grating sensors,” Electron. Lett. 32(12), 1133–1134 (1996).
P. M. Cavaleiro, F. M. Araújo, L. A. Ferreira, J. L. Santos, and F. Farahi, “Simultaneous measurement of strain
and temperature using Bragg gratings written in germanosilicate and boron-codoped germanosilicate fibers,”
IEEE Photon. Technol. Lett. 11(12), 1635–1637 (1999).
H. B. Liu, H. Y. Liu, G. D. Peng, and P. L. Chu, “Strain and temperature sensor using a combination of polymer
and silica fibre Bragg gratings,” Opt. Commun. 219(1-6), 139–142 (2003).
M. G. Xu, J. L. Archambault, L. Reekie, and J. P. Dakin, “Discrimination between strain and temperature effects
using dual-wavelength fibre grating sensors,” Electron. Lett. 30(13), 1085–1087 (1994).
M. Sudo, M. Nakai, K. Himeno, S. Suzaki, A. Wada, and R. Yamauchi, “Simultaneous measurement of
temperature and strain using PANDA fiber grating,” in Proc. 12th International Conference Optical Fibre
Sensors, pp. 170–173, Williamsburg, Virginia, USA, October 28–31, 1997.
B. O. Guan, H. Y. Tam, X. M. Tao, and X. Y. Dong, “Simultaneous strain and temperature measurement using a
superstructure fiber Bragg grating,” IEEE Photon. Technol. Lett. 12(6), 675–677 (2000).
H. F. Lima, P. F. Antunes, J. D. L. Pinto, and R. N. Nogueira, “Simultaneous Measurement of Strain and
Temperature With a Single Fiber Bragg Grating Written in a Tapered Optical Fiber,” IEEE Sens. J. 10(2), 269–
273 (2010).
B. O. Guan, H. Y. Tam, S. L. Ho, W. H. Chung, and X. Y. Dong, “Simultaneous strain and temperature
measurement using a single fibre Bragg grating,” Electron. Lett. 36(12), 1018–1019 (2000).
H. J. Patrick, G. M. Williams, A. D. Kersey, J. R. Pedrazzani, and A. M. Vengsarkar, “Hybrid fiber Bragg
grating/long period fiber grating sensor for strain/temperature discrimination,” IEEE Photon. Technol. Lett. 8(9),
1223–1225 (1996).
T. Lui, G. F. Fernando, L. Zhang, I. Bennion, Y. J. Rao, and D. A. Jackson, “Simultaneous strain and
temperature measurement using a combined fibre Bragg grating/extrinsic Fabry-Perot sensor,” in Proc. 12th
International Conference Optical Fibre Sensors, pp. 40–43, Williamsburg, Virginia, USA, October 28–31, 1997.
D. P. Zhou, L. Wei, W. K. Liu, Y. Liu, and J. W. Y. Lit, “Simultaneous measurement for strain and temperature
using fiber Bragg gratings and multimode fibers,” Appl. Opt. 47(10), 1668–1672 (2008).
B. Dong, J. Z. Hao, C. Y. Liaw, B. Lin, and S. C. Tjin, “Simultaneous strain and temperature measurement using
a compact photonic crystal fiber inter-modal interferometer and a fiber Bragg grating,” Appl. Opt. 49(32), 6232–
6235 (2010).
L. Y. Shao, X. Y. Dong, A. P. Zhang, H. Y. Tam, and S. L. He, “High-resolution strain and temperature sensor
based on distributed Bragg reflector fiber laser,” IEEE Photon. Technol. Lett. 19(20), 1598–1600 (2007).
#148869 - $15.00 USD
(C) 2011 OSA
Received 7 Jun 2011; revised 5 Aug 2011; accepted 22 Aug 2011; published 3 Oct 2011
10 October 2011 / Vol. 19, No. 21 / OPTICS EXPRESS 20650
14. O. Hadeler, E. Rønnekleiv, M. Ibsen, and R. I. Laming, “Polarimetric distributed feedback fiber laser sensor for
simultaneous strain and temperature measurements,” Appl. Opt. 38(10), 1953–1958 (1999).
15. R. I. Crickmore, M. J. Gunning, J. Stefanov, and J. P. Dakin, “Beat frequency measurement system for multiple
dual polarization fiber DFB lasers,” IEEE Sens. J. 3(1), 115–120 (2003).
16. B. O. Guan, Y. N. Tan, and H. Y. Tam, “Dual polarization fiber grating laser hydrophone,” Opt. Express 17(22),
19544–19550 (2009).
17. Y. Zhang, B. O. Guan, and H. Y. Tam, “Ultra-short distributed Bragg reflector fiber laser for sensing
applications,” Opt. Express 17(12), 10050–10055 (2009).
1. Introduction
There has been considerable interest in developing simultaneous strain and temperature fiber
optic sensors. This is not only because cross sensitivity is a key issue for the practical
applications of fiber optic sensors, but also because multi-parameter sensors can reduce the
complexity of the sensing systems in situations requiring multi-parameter and multi-point
measurement. The principle of simultaneous strain and temperature sensors are usually based
on the detection of two physical parameters which have different sensitivities to strain and
temperature. Fiber Bragg gratings have been of great interest in sensing technology in recent
years because of their small size, wavelength-encoding and multiplexing capability. Many
techniques based on fiber Bragg gratings have been reported for simultaneous strain and
temperature measurement. A simple and straightforward approach is to employ two
independent Bragg gratings with the first one subjected to strain and temperature and the
second one isolated from strain. The concept of a sensing head formed by two Bragg gratings
with different strain and temperature response has been explored. Examples include
configurations based on two gratings in different diameter fibers [1], in different dopant fibers
[2], in different base material fibers [3], and operating at different wavelengths [4]. Several
approaches based on a single Bragg grating for simultaneous strain and temperature
measurement was also demonstrated, such as utilization of a single Bragg grating in
birefringent fibers [5], superstructure Bragg grating [6], a single Bragg grating in tapered fiber
[7], and a single Bragg grating straddling over the junction of two fibers [8]. A number of
schemes based on a sensing head formed by Bragg gratings in combination with other fiber
optic devices have also been demonstrated. The configurations include the combination of two
Bragg gratings and a long period grating [9], the combination of a Bragg grating and FabryPerot interferometer [10], the combination of a Bragg grating and Mach-Zehnder
interferometer [11], and the combination of a Bragg grating and photonic crystal fiber based
inter-modal interferometer [12]. Recently, a sensing head formed by a dual-polarization fiber
grating laser was demonstrated [13], where the mean wavelength and polarization mode beat
frequency of the laser were utilized to discriminate strain and temperature. All above
approaches can be divided into two categories. The first category is based on the detection of
two separate wavelengths which have different response to strain and temperature. The second
category is based on the detection of wavelength and intensity (or spectrum bandwidth). The
disadvantage of the second category is, the intensity detection undercuts multiplexing
capability of fiber Bragg grating sensors. For all above simultaneous strain and temperature
sensors, wavelength detection is necessary. However, it is know that, complex and expensive
optical systems are required to achieve accurate wavelength measurement. The high cost of
the wavelength detection unit impedes further applications of fiber Bragg grating sensors. It
will be highly desirable if we can develop a simultaneous strain and temperature sensor which
not only shares the advantages of fiber Bragg grating sensors but also avoids expensive
wavelength detection.
Polarimetric fiber grating laser sensor converts the measurrand into a corresponding
change in the beat frequency between the two polarization modes from the laser [14–16].
Because the beat frequency is in the radio-frequency range, this type of sensor has distinctive
advantages of ease of interrogation and avoidance of expensive wavelength detection that is
required in the passive fiber Bragg grating sensors. In this paper, we present a novel
#148869 - $15.00 USD
(C) 2011 OSA
Received 7 Jun 2011; revised 5 Aug 2011; accepted 22 Aug 2011; published 3 Oct 2011
10 October 2011 / Vol. 19, No. 21 / OPTICS EXPRESS 20651
simultaneous strain and temperature fiber optic sensor based on radio-frequency
measurement. The sensing head is formed by two concatenated ultra-short distributed Bragg
reflector (DBR) lasers. The total length of the sensing head is only 18 mm. Both lasers operate
in robust single longitude mode with two polarization modes. Each laser generates a
polarization mode beat note at radio-frequency range. The two lasers have different beat
frequencies which exhibit different response to strain and temperature. Simultaneous strain
and temperature measurement can be achieved by monitoring the two beat frequencies.
2. Principle
Figure 1 shows the schematic diagram of the proposed simultaneous strain and temperature
sensor. The sensor head is formed by two concatenated DBR fiber lasers with the first one
fabricated in Er-doped fiber and the second one fabricated in Er/Yb co-doped fiber. Typical
DBR fiber lasers are a few cm long, leading to the laser longitude mode spacing much smaller
than the grating reflection bandwidth. As a result, there are multiple modes that meet
conditions for lasing. The dominant mode oscillates first and other modes are suppressed, so
normally the lasers can operate in single longitude mode. However, mode hopping will occur
when the laser is subjected to external perturbations that distort the grating spectrum, such as
temperature or strain gradient or a localized perturbation to the subsection of the Bragg
gratings. This is a key problem limiting the practical applications of DBR fiber lasers. To
address this problem, here ultra-short DBR fiber lasers which have longitude mode spacing
comparable to the grating reflection bandwidth were employed. Because the ultra-short cavity
supports only one longitude mode, it absolutely obviates possibility of mode hopping when
the laser is subjected to any external perturbations.
Fig. 1. Schematic diagram of the proposed simultaneous strain and temperature sensor.
Here the DBR fiber lasers operate in single longitude mode with two polarization states.
When the laser output is monitored with a high speed photodetector, a beat note will be
generated by the two polarization lines. The beat frequency is given by
  cB / n0 0
(1)
where c is the light speed in vacuum, λ0 is the laser wavelength, n0 and B are the average
index and birefringence of the optical fiber, respectively. Typical the beat frequency is in the
range from several hundred MHz to several GHz.
When the DBR laser is subjected to strain or temperature perturbation, the birefringence
will be changed. As a result, the beat frequency will shift and so can be considered as an
effective signal output. The response of the beat frequency to strain and temperature can be
expressed as
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Received 7 Jun 2011; revised 5 Aug 2011; accepted 22 Aug 2011; published 3 Oct 2011
10 October 2011 / Vol. 19, No. 21 / OPTICS EXPRESS 20652
     1  B

1 B


 1  pe    
      T ,

 B 

 B T

(2)
where pe, α, and ξ are the strain-optic coefficient, thermal expansion coefficient, and thermooptic coefficient of the optical fiber.
Because of different dopant and slightly different structure-parameters, the DBR lasers in
Er-doped fiber and Er/Yb DBR fiber exhibit different beat-frequency-response to strain and
temperature. When temperature and strain change simultaneously, using the Eq. (2), we can
get the matrix
   Er   kEr ,
k Er ,T   
 
(3)


    K  .
k
k

T

 T 
   Er /Yb   Er /Yb,
Er / Yb ,T  
The coefficient matrix K can be defined by separately measuring the strain and
temperature responses of the polarization beat frequency of the two lasers. Then the strain and
temperature can be determined simultaneously by measuring the beat frequencies of the Erdoped fiber laser and the Er/Yb co-doped fiber laser.
3. Experiment and results
The ultra-short DBR fiber lasers were fabricated by directly inscribing two wavelengthmatched Bragg gratings in active fibers using the setup described in [17]. A 193 nm excimer
laser and phase mask method were used. Because the 193 nm UV light induces index change
by two-photon excitation process, it does not require hydrogen loading to photosensitize the
fiber. This avoids the laser efficiency degradation due to hydrogen-induced loss at pump
wavelength and excited-state lifetime reduction of Er3+ ions. The DBR laser in Er/Yb codoped fiber consisted of 2.2-mm-long low reflectivity grating, 4-mm-long high reflectivity
grating, and 2 mm grating spacing. The total length of the Er/Yb co-doped fiber laser was 8.2
mm. The DBR laser in Er-doped fiber consisted of two 3-mm-long gratings and 2-mm grating
spacing. The entire length of the Er-doped DBR fiber laser was only 8 mm. The two DBR
lasers were concatenated in a single fiber as the sensing head. The total length of the sensing
head was only 18 mm. The 980 nm pump light was launched into the laser array from the Erdoped fiber laser side through a wavelength division multiplexer (WDM). The backward laser
output was launched into a high speed photodetector (PD) through a polarization controller
(PC) and an in-line polarizer. A radio-frequency spectrum analyzer was used to monitor the
beat notes of the lasers.
Figure 2 shows the output spectrum of the laser array with pump power setting to 187
mW. The Er-doped fiber laser operated around 1536.12 nm with signal-to-noise ratio of ~55
dB. The Er/Yb co-doped fiber laser operated around 1539 nm with signal-to-noise ratio of ~60
dB. Figure 3 shows the beat note spectrum of the laser array measured with the radiofrequency spectrum analyzer. The Er-doped fiber laser generated a beat note at 2.664 GHz
with signal-to-noise ratio better than 50 dB. The Er/Yb co-doped fiber lasers generated a beat
note at 1.336 GHz with signal-to-noise ratio better than 60 dB. The beat frequency of the Erdoped fiber laser is much higher than that of the Er/Yb co-doped fiber. This denotes that the
Er-doped fiber has much higher birefringence than the Er/Yb co-doped fiber. In spite of the
Er-doped fiber laser in the front, the Er/Yb co-coped fiber laser had higher laser output and
stronger beat note. This is because the Yb ions have strong absorption at 980 nm and transfer
their energy to the Er ions with high efficiency, significantly increased the laser efficiency.
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Received 7 Jun 2011; revised 5 Aug 2011; accepted 22 Aug 2011; published 3 Oct 2011
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Fig. 2. Optical output spectrum of the concatenated DBR fiber lasers.
Fig. 3. Beat note spectrum of the concatenated DBR fiber lasers.
The strain response was investigated by bonding both sides of the sensing head onto two
translation stages with epoxy. While the sensing head was stretched with the translation stage,
the laser beat frequencies were monitored with the radio-frequency spectrum analyzer. The
environment temperature was kept at 15 °C during the strain response measurement. Applied
strain was calculated from the elongation of the stretched fiber divided by the original length.
During the strain response characterization, environment temperature was kept constant.
Figure 4 shows measured sensor response to strain in the range from 0 to 1200 με. It is clear
that the beat frequencies increase with strain, and the strain sensitivity of Er-doped fiber laser
is higher than the Er/Yb co-doped fiber laser. The strain coefficients of the Er-doped fiber
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10 October 2011 / Vol. 19, No. 21 / OPTICS EXPRESS 20654
laser and Er/Yb co-doped fiber laser were estimated, using linear regression fits, as kEr,ε = 8.75
± 0.104 KHz/με(R2 = 0.9986), and kEr/Yb,ε = 6.42 ± 0.068 KHz/με (R2 = 0.9989), respectively.
Fig. 4. Strain response of the proposed simultaneous strain and temperature sensor.
Fig. 5. Temperature response of the proposed simultaneous strain and temperature sensor.
The temperature response was investigated by putting the sensing head into a tube oven. A
thermocouple was placed near the sensing head for measurement of the temperature. The
sensing head was kept unstrained. Figure 5 shows the measured beat frequency shifts as
functions of temperature in the range from 15 °C to 100 °C. As shown in Fig. 5, the beat
frequencies decrease with temperature, and the temperature sensitivity of the Er/Yb co-doped
fiber laser is higher than the Er-doped fiber laser. The temperature coefficients of the Erdoped fiber laser and Er/Yb co-doped fiber laser were estimated, using linear regression fits,
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Received 7 Jun 2011; revised 5 Aug 2011; accepted 22 Aug 2011; published 3 Oct 2011
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as kEε,T = 678 ± 5.52 KHz/ °C (R2 = 0.9995), and kEr/Yb,T = 1142 ± 4.11 KHz/°C (R2 =
0.9999), respectively.
By taking the inverse matrix of K and the measured coefficients in (3), we can get the
matrix
 
8.75
1
 T   5639.7 678
 

 6.42      Er 

,
 1142     Er /Yb 
(4)
where the units of δε, δT, δ(ΔνEr), and δ(ΔνEr/Yb) are με, °C, KHz and KHz, respectively. One
can thus employ the coefficient matrix above to simultaneously determine strain and
temperature by measuring the two fiber lasers’ beat frequency shifts of the sensor. In our
experiments, the sensor was interrogated with a RF spectrum analyzer (Anritsu MS2661C)
with resolution of 10 kHz, which denotes resolutions of 1.56 με and 0.015 °C for strain and
temperature measurement, respectively.
4. Conclusion
We reported a novel fiber-optic sensor for simultaneous strain and temperature measurement
based on radio-frequency detection. The sensor head was formed by two concatenated ultrashort DBR fiber lasers with the first one fabricated in Er-doped fiber and the second one
fabricated in Er/Yb co-doped fiber. The total length of the sensing head was only 18 mm. The
two lasers generate two polarization mode beat notes at radio-frequency range, which show
different frequency response to strain and temperature. Simultaneous strain and temperature
measurement can be achieved by monitoring the two beat frequencies. The distinctive
advantages of the proposed simultaneous strain and temperature sensor are ease of
interrogation and avoidance of expensive wavelength detection. Other advantages include
absolute frequency encoding and capability to multiplex a number of sensors on a single fiber
by use of frequency division multiplexing technique.
Acknowledgments
This work was supported by the Key Project of National Natural Science Foundation of China
(60736039) and the Fundamental Research Funds for the Central Universities (21609102).
#148869 - $15.00 USD
(C) 2011 OSA
Received 7 Jun 2011; revised 5 Aug 2011; accepted 22 Aug 2011; published 3 Oct 2011
10 October 2011 / Vol. 19, No. 21 / OPTICS EXPRESS 20656