- Email: [email protected]
Strain and temperature sensor using a combination of polymer and silica fibre Bragg gratings
Optics Communications 219 (2003) 139–142 www.elsevier.com/locate/optcom
Strain and temperature sensor using a combination of polymer and silica fibre Bragg gratings H.B. Liu a,*, H.Y. Liu a, G.D. Peng a, P.L. Chu b,1 b
a Optical Communications Group, University of New South Wales, Sydney 2052, Australia Optoelectronics Research Centre, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
Received 24 October 2002; received in revised form 5 February 2003; accepted 3 March 2003
Abstract We show that a sensor scheme consisting of a combination of a polymer fibre Bragg grating and a silica fibre Bragg grating gives large discrimination against temperature and strain. It provides large sensitivity and dynamic range for sensing temperature and strain changes simultaneously and independently. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.81.Pa Keywords: Sensor; Fibre Bragg grating; Strain; Temperature; Polymer fibre grating; Silica fibre grating
1. Introduction Fibre Bragg gratings have been receiving increasing applications in optical sensing. Their Bragg wavelength shift is proportional to the temperature or strain experienced by the gratings. Polymer fibre gratings are many times more sensitive to these influences than silica fibre gratings. However, a fibre Bragg grating sensor must be able to discriminate against the two factors, i.e., temperature and strain. A single grating cannot achieve this purpose. A number of solutions have been proposed. These *
Corresponding author. Tel.: +612-9385-4014; fax: +6129385-5993. E-mail addresses: [email protected] (H.B. Liu), [email protected] (P.L. Chu). 1 Tel.: +852-2788-9132; fax: +852-2784-4674.
include the combination of fibre Bragg grating and long period grating [1], the use of two fibre Bragg gratings of different core diameters [2], the superposition of a polarization-rocking filter onto a fibre Bragg grating [3], the combination of a fibre Bragg grating and an EDFA [4], the use of long period gratings [5], and a single Bragg grating straddling over the junction of two fibres [6]. All these methods rely on the use of two different silica fibre gratings. However, these sensor methods mentioned above have some drawbacks. For example the long period grating method utilises the multiple resonance bands in a single long period grating to measure strain and temperature simultaneously. Nevertheless, it is difficult to accurately measure small shifts in the resonant wavelength in long period grating by using broad bandwidth spectrum. The splicing joint method complicates the sensor head design and
0030-4018/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0030-4018(03)01313-0
140
H.B. Liu et al. / Optics Communications 219 (2003) 139–142
might limit the maximum strain, which can be applied to sensor head. At the same time, all of these methods involve complex gratings, special made fibres or other optical elements. These specials may add up the system cost. The purpose of the present paper is to show that a combination of a simple polymer optical fibre grating and a silica fibre grating can also function as a sensor that discriminates against temperature and strain. It has been shown that a polymer fibre grating can be tuned mechanically over 74 nm [7] and thermally over 20 nm [8]. It is therefore obvious that a sensor consisting the combination of a simple polymer grating and a silica grating would be more sensitive and cost effective than those of sensors constructed purely of silica gratings. In this paper, we show this is the case.
Fig. 1. Strain response of polymer fibre grating.
2. Strain and temperature response of a polymer fibre Bragg grating The strain and temperature responses of a silica fibre Bragg grating are well known [9] and do not need to be repeated here. However, for polymer fibre Bragg grating, this is not the case because it is a new development. These responses will therefore be described here first. The polymer fibre reported here is made of PMMA and has a core diameter of 6 lm and outer diameter of 125 lm. The refractive index difference between the core and the cladding is 0.0086. The fibre is single-moded in the 1550 nm window. The Bragg grating in the fibre was written with an irradiation beam of wavelength 325 nm. The grating has a Bragg wavelength of 1523.1 nm with a maximum reflectivity of 80% and a spectral width less than 0.5 nm. Fig. 1 shows the strain response of this grating when it was stretched and then released. It is clear that the response is linear and there is no hysteresis effect. The Bragg wavelength shift over a strain range of 10.71 milli-strain is more than 15 nm. Generally, polymer fibre can stand up to 3% strain without yielding. This is compared with the maximum Bragg wavelength shift in silica fibre grating of 2 nm due to tensile strain. The strain response curve in Fig. 1 can be represented mathematically by: kB ¼ 1522:9ð1 þ 0:9736eÞ:
ð1Þ
Fig. 2. Temperature response of polymer fibre grating.
The strain sensitivity of the polymer fibre Bragg grating is found to be 1.48 pm/le. This is nearly 1.3 times larger than the value for the silica counterpart, which is 1.15 pm/le at this wavelength. Fig. 2 shows the temperature response of the polymer fibre Bragg grating. Again, the response is linear and there is no hysteresis effect when the grating is heated up or when it is cooled down. Mathematically it can be represented by: kB ¼ 1541ð1 9:44 105 T Þ: ð2Þ It is noted that the slope of this curve is negative. This is in contrast to the temperature response of silica fibre Bragg grating which has a positive slope. 3. Temperature and strain sensor To construct a sensor that discriminate temperature and strain, we adopted the idea in [9] in
H.B. Liu et al. / Optics Communications 219 (2003) 139–142
which two fibre gratings are used as shown in Fig. 3. One is silica fibre Bragg grating and the other is polymer fibre Bragg grating. A broadband optical source is used. In order to sense the temperature change DT and strain change De independently but simultaneously, the Bragg wavelength shift due to each grating will be recorded as Dk1 and Dk2 , respectively, by an optical spectrum analyzer (OSA). These quantities can be expressed in terms of the temperature change DT and strain change De: Dk1 K1T K1e DT ¼ ; ð3Þ Dk2 K2T K2e De where K1T and K2T are the temperature sensitivities of gratings 1 and 2, respectively, while K1e and K2e are the corresponding strain sensitivities. To obtain DT and De, we need to invert Eq. (3) so that: 1 DT K2e K1e Dk1 ¼ : De Dk2 K1T K2e K2T K1e K2T K1T ð4Þ
where the wavelength shifts Dk1 , and Dk2 are in unit of nm while the temperature change DT is in °C and the strain change De in l strain (le). As we know that the matrix inversion technique assumes constant value for K1T , K1e , K2T and K2e . For some special cases, all four parameters can become nonlinear functions of dT and de. Therefore, an accurate measurement is impossible. Nevertheless, it is not the case here. The experiment data prove that nonlinear factor of our polymer fibre Bragg grating (FBG) here is negligible. Hence, when this scheme is used to measure strain and temperature variation under certain reasonable range, it can simultaneously measure strain and temperature accurately. Another advantage of this scheme is the large crosssensitivity does not exist here as the two FBG are written on totally different materials, which have large different strain and temperature responses. Fig. 4(a) shows the temperature variation as a function of the measured wavelength shifts and
It can be seen from Eq. (4) if the two gratings are of the same kind, i.e., either both silica or both polymer, the respective sensitivity parameters would have similar values and this makes the denominator close to zero. As a result, the sensor does not work very well. On the other hand, if one of the gratings is silica and the other one polymer, the parameters are very different. This makes the denominator in Eq. (4) very different from zero and it is then possible to sense both temperature and strain changes more accurately. In fact, we found that for polymer grating, K1T ¼ 149pm=°C, K1e ¼ 1:5 pm=le and for silica grating, K2T ¼ 10:47 pm=°C, and K2e ¼ 1:17 pm=le. We then obtain the following equation operating at the wavelength 1541 nm:
DT De
¼
6:2 7:9
55:1 784
Dk1 ; Dk2
ð5Þ
Fig. 3. Sensor with a combination of polymer and silica fibre gratings.
141
Fig. 4. (a) Temperature change; (b) strain change.
142
H.B. Liu et al. / Optics Communications 219 (2003) 139–142
Fig. 4(b) shows the strain change as a function of the same variables. Both figures indicate that the sensor response is in the form of a plane joined to the origin of the coordinates. The slopes of these planes are determined by the sensitivity parameters of the two gratings. These slopes are larger for the case of a combination of polymer and silica fibre gratings than for the case of silica fibre gratings alone. Furthermore, since the tuning range of polymer grating is much larger than that of silica grating, the dynamic ranges of DT and De are correspondingly much larger than in the case of pure silica gratings. 4. Conclusion We have shown that a sensor consisting of a combination of a polymer fibre Bragg grating and a silica fibre Bragg grating can discriminate the temperature effect and the strain effect with much more sensitivity. The large cross-sensitivity issue has also been successfully solved by this scheme. It is noted that the fusion splicing technique cannot be used to join the polymer fibre and the silica fibre. Instead, we propose the use of an elastomeric connector, which does not
incur a joint loss of more than 0.5 dB provided that the parameters of the two fibres are nearly the same, i.e., similar core and cladding diameters and similar core and cladding refractive indices.
References [1] H.J. Patrick, G.M. Williams, A.D. Kersey, J.R. Pedrazzani, A.M. Vengsarkar, IEEE Photon. Technol. Lett. 8 (9) (1996) 1223. [2] S.W. James, M.L. Dockney, R.P. Tatam, Electron. Lett. 32 (12) (1996) 1133. [3] S.E. Kanellopoulos, V.A. Handerek, A.J. Rogers, Opt. Lett. 20 (3) (1995) 333. [4] J. Jung, H. Nam, J.H. Lee, N. Park, B. Lee, Appl. Opt. 38 (13) (1999) 2749. [5] V. Bhatia, D. Campbell, R.O. Claus, A.M. Vengsarkar, Opt. Lett. 22 (9) (1997) 648. [6] B.O. Guan, H.Y. Tam, S.L. Ho, W.H. Chung, X.Y. Dong, Electron. Lett. 36 (12) (2000) 1018. [7] Z. Xiong, G.D. Peng, B. Wu, P.L. Chu, Electron. Lett. 36 (12) (2000) 135. [8] H.Y. Liu, G.D. Peng, P.L. Chu, Proceedings of International Conference on Polymer Optical Fibres, Boston, 5–8 September, 2000, p. 83. [9] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, E.J. Friebele, J. Lightwave Technol. 15 (8) (1997) 1442.
Strain and temperature sensor using a combination of polymer and silica fibre Bragg gratings H.B. Liu a,*, H.Y. Liu a, G.D. Peng a, P.L. Chu b,1 b
a Optical Communications Group, University of New South Wales, Sydney 2052, Australia Optoelectronics Research Centre, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
Received 24 October 2002; received in revised form 5 February 2003; accepted 3 March 2003
Abstract We show that a sensor scheme consisting of a combination of a polymer fibre Bragg grating and a silica fibre Bragg grating gives large discrimination against temperature and strain. It provides large sensitivity and dynamic range for sensing temperature and strain changes simultaneously and independently. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.81.Pa Keywords: Sensor; Fibre Bragg grating; Strain; Temperature; Polymer fibre grating; Silica fibre grating
1. Introduction Fibre Bragg gratings have been receiving increasing applications in optical sensing. Their Bragg wavelength shift is proportional to the temperature or strain experienced by the gratings. Polymer fibre gratings are many times more sensitive to these influences than silica fibre gratings. However, a fibre Bragg grating sensor must be able to discriminate against the two factors, i.e., temperature and strain. A single grating cannot achieve this purpose. A number of solutions have been proposed. These *
Corresponding author. Tel.: +612-9385-4014; fax: +6129385-5993. E-mail addresses: [email protected] (H.B. Liu), [email protected] (P.L. Chu). 1 Tel.: +852-2788-9132; fax: +852-2784-4674.
include the combination of fibre Bragg grating and long period grating [1], the use of two fibre Bragg gratings of different core diameters [2], the superposition of a polarization-rocking filter onto a fibre Bragg grating [3], the combination of a fibre Bragg grating and an EDFA [4], the use of long period gratings [5], and a single Bragg grating straddling over the junction of two fibres [6]. All these methods rely on the use of two different silica fibre gratings. However, these sensor methods mentioned above have some drawbacks. For example the long period grating method utilises the multiple resonance bands in a single long period grating to measure strain and temperature simultaneously. Nevertheless, it is difficult to accurately measure small shifts in the resonant wavelength in long period grating by using broad bandwidth spectrum. The splicing joint method complicates the sensor head design and
0030-4018/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0030-4018(03)01313-0
140
H.B. Liu et al. / Optics Communications 219 (2003) 139–142
might limit the maximum strain, which can be applied to sensor head. At the same time, all of these methods involve complex gratings, special made fibres or other optical elements. These specials may add up the system cost. The purpose of the present paper is to show that a combination of a simple polymer optical fibre grating and a silica fibre grating can also function as a sensor that discriminates against temperature and strain. It has been shown that a polymer fibre grating can be tuned mechanically over 74 nm [7] and thermally over 20 nm [8]. It is therefore obvious that a sensor consisting the combination of a simple polymer grating and a silica grating would be more sensitive and cost effective than those of sensors constructed purely of silica gratings. In this paper, we show this is the case.
Fig. 1. Strain response of polymer fibre grating.
2. Strain and temperature response of a polymer fibre Bragg grating The strain and temperature responses of a silica fibre Bragg grating are well known [9] and do not need to be repeated here. However, for polymer fibre Bragg grating, this is not the case because it is a new development. These responses will therefore be described here first. The polymer fibre reported here is made of PMMA and has a core diameter of 6 lm and outer diameter of 125 lm. The refractive index difference between the core and the cladding is 0.0086. The fibre is single-moded in the 1550 nm window. The Bragg grating in the fibre was written with an irradiation beam of wavelength 325 nm. The grating has a Bragg wavelength of 1523.1 nm with a maximum reflectivity of 80% and a spectral width less than 0.5 nm. Fig. 1 shows the strain response of this grating when it was stretched and then released. It is clear that the response is linear and there is no hysteresis effect. The Bragg wavelength shift over a strain range of 10.71 milli-strain is more than 15 nm. Generally, polymer fibre can stand up to 3% strain without yielding. This is compared with the maximum Bragg wavelength shift in silica fibre grating of 2 nm due to tensile strain. The strain response curve in Fig. 1 can be represented mathematically by: kB ¼ 1522:9ð1 þ 0:9736eÞ:
ð1Þ
Fig. 2. Temperature response of polymer fibre grating.
The strain sensitivity of the polymer fibre Bragg grating is found to be 1.48 pm/le. This is nearly 1.3 times larger than the value for the silica counterpart, which is 1.15 pm/le at this wavelength. Fig. 2 shows the temperature response of the polymer fibre Bragg grating. Again, the response is linear and there is no hysteresis effect when the grating is heated up or when it is cooled down. Mathematically it can be represented by: kB ¼ 1541ð1 9:44 105 T Þ: ð2Þ It is noted that the slope of this curve is negative. This is in contrast to the temperature response of silica fibre Bragg grating which has a positive slope. 3. Temperature and strain sensor To construct a sensor that discriminate temperature and strain, we adopted the idea in [9] in
H.B. Liu et al. / Optics Communications 219 (2003) 139–142
which two fibre gratings are used as shown in Fig. 3. One is silica fibre Bragg grating and the other is polymer fibre Bragg grating. A broadband optical source is used. In order to sense the temperature change DT and strain change De independently but simultaneously, the Bragg wavelength shift due to each grating will be recorded as Dk1 and Dk2 , respectively, by an optical spectrum analyzer (OSA). These quantities can be expressed in terms of the temperature change DT and strain change De: Dk1 K1T K1e DT ¼ ; ð3Þ Dk2 K2T K2e De where K1T and K2T are the temperature sensitivities of gratings 1 and 2, respectively, while K1e and K2e are the corresponding strain sensitivities. To obtain DT and De, we need to invert Eq. (3) so that: 1 DT K2e K1e Dk1 ¼ : De Dk2 K1T K2e K2T K1e K2T K1T ð4Þ
where the wavelength shifts Dk1 , and Dk2 are in unit of nm while the temperature change DT is in °C and the strain change De in l strain (le). As we know that the matrix inversion technique assumes constant value for K1T , K1e , K2T and K2e . For some special cases, all four parameters can become nonlinear functions of dT and de. Therefore, an accurate measurement is impossible. Nevertheless, it is not the case here. The experiment data prove that nonlinear factor of our polymer fibre Bragg grating (FBG) here is negligible. Hence, when this scheme is used to measure strain and temperature variation under certain reasonable range, it can simultaneously measure strain and temperature accurately. Another advantage of this scheme is the large crosssensitivity does not exist here as the two FBG are written on totally different materials, which have large different strain and temperature responses. Fig. 4(a) shows the temperature variation as a function of the measured wavelength shifts and
It can be seen from Eq. (4) if the two gratings are of the same kind, i.e., either both silica or both polymer, the respective sensitivity parameters would have similar values and this makes the denominator close to zero. As a result, the sensor does not work very well. On the other hand, if one of the gratings is silica and the other one polymer, the parameters are very different. This makes the denominator in Eq. (4) very different from zero and it is then possible to sense both temperature and strain changes more accurately. In fact, we found that for polymer grating, K1T ¼ 149pm=°C, K1e ¼ 1:5 pm=le and for silica grating, K2T ¼ 10:47 pm=°C, and K2e ¼ 1:17 pm=le. We then obtain the following equation operating at the wavelength 1541 nm:
DT De
¼
6:2 7:9
55:1 784
Dk1 ; Dk2
ð5Þ
Fig. 3. Sensor with a combination of polymer and silica fibre gratings.
141
Fig. 4. (a) Temperature change; (b) strain change.
142
H.B. Liu et al. / Optics Communications 219 (2003) 139–142
Fig. 4(b) shows the strain change as a function of the same variables. Both figures indicate that the sensor response is in the form of a plane joined to the origin of the coordinates. The slopes of these planes are determined by the sensitivity parameters of the two gratings. These slopes are larger for the case of a combination of polymer and silica fibre gratings than for the case of silica fibre gratings alone. Furthermore, since the tuning range of polymer grating is much larger than that of silica grating, the dynamic ranges of DT and De are correspondingly much larger than in the case of pure silica gratings. 4. Conclusion We have shown that a sensor consisting of a combination of a polymer fibre Bragg grating and a silica fibre Bragg grating can discriminate the temperature effect and the strain effect with much more sensitivity. The large cross-sensitivity issue has also been successfully solved by this scheme. It is noted that the fusion splicing technique cannot be used to join the polymer fibre and the silica fibre. Instead, we propose the use of an elastomeric connector, which does not
incur a joint loss of more than 0.5 dB provided that the parameters of the two fibres are nearly the same, i.e., similar core and cladding diameters and similar core and cladding refractive indices.
References [1] H.J. Patrick, G.M. Williams, A.D. Kersey, J.R. Pedrazzani, A.M. Vengsarkar, IEEE Photon. Technol. Lett. 8 (9) (1996) 1223. [2] S.W. James, M.L. Dockney, R.P. Tatam, Electron. Lett. 32 (12) (1996) 1133. [3] S.E. Kanellopoulos, V.A. Handerek, A.J. Rogers, Opt. Lett. 20 (3) (1995) 333. [4] J. Jung, H. Nam, J.H. Lee, N. Park, B. Lee, Appl. Opt. 38 (13) (1999) 2749. [5] V. Bhatia, D. Campbell, R.O. Claus, A.M. Vengsarkar, Opt. Lett. 22 (9) (1997) 648. [6] B.O. Guan, H.Y. Tam, S.L. Ho, W.H. Chung, X.Y. Dong, Electron. Lett. 36 (12) (2000) 1018. [7] Z. Xiong, G.D. Peng, B. Wu, P.L. Chu, Electron. Lett. 36 (12) (2000) 135. [8] H.Y. Liu, G.D. Peng, P.L. Chu, Proceedings of International Conference on Polymer Optical Fibres, Boston, 5–8 September, 2000, p. 83. [9] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, E.J. Friebele, J. Lightwave Technol. 15 (8) (1997) 1442.