Fibre heterostructure for simultaneous strain and temperature measurement
10°C, 0 με 30°C, 0 με 10°C, 1556 με 30°C, 1556 με
25 24
22
21 loss, dB
loss, dB
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Q. Wu, Y. Semenova, P. Wang and G. Farrell
peak2
peak1
22 21 20
20 10°C, 0 με 30°C, 0 με 10°C, 1556 με 30°C, 1556 με
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19 1600
1602
1608
1604
1610 l, nm
a
b
a In vicinity of peak1 b In vicinity of peak2
Fig. 2 shows that, at a fixed value of strain, as the temperature increases, the values of peak1 and peak2 increase, but that this increase is different for different values of strain. At a fixed temperature, as the strain increases, the wavelength changes in peak1 and peak2 have different rates of decrease with respect to strain. The detailed wavelength shifts for the two spectral peaks against temperature and strain are shown in Fig. 3, where l1 is the measured central wavelength of peak1 and l2 is the measured central wavelength of peak2.
measured λ1 at T = 10°C; measured λ1 at T = 20°C; measured λ1 at T = 30°C;
1602.5
linear fit linear fit linear fit
measured λ1 at S = 0 me; measured λ1 at S = 889 me; measured λ1 at S = 1556 me;
1602.5
1602.0
1601.5
1601.5
1601.0 peak1
b
1613.0
measured λ2 at T = 10°C; measured λ2 at T = 20°C; measured λ2 at T = 30°C;
1612.5
1613.0 linear fit linear fit linear fit
measured λ2 at S = 0 me; measured λ2 at S = 889 me; measured λ2 at S = 1556 me;
1612.5
1611.5 1611.0
1611.5 1611.0
1610.5
1610.5 peak2 0
peak2
1610.0 400
800 strain, με
1200
1600
10
c
singlemode fibre
singlemode fibre
Fig. 1 Schematic diagram of SMMS fibre structure
Experimental investigations were carried out on an SMMS fibre structure to verify the feasibility of the approach. The singlemode fibre used was SMF28 and the MMF1 and MMF2 fibres were AFS105/125Y and GIF625, respectively. Both the MMF1 and MMF2 sections have a length of 20 mm. Fig. 2 shows the spectral responses for two selected spectral peaks at circa 1601 and 1611 nm identified as peak1 and peak2, respectively, for the SMMS fibre structure at different values of temperature and strain.
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20 25 temperature, °C
30
d
Fig. 3 Relationships between measured and fitted l1 , l2 , strain and temperature a l1 against strain b l1 against temperature c l2 against strain d l2 against temperature
Fig. 3a shows that, as the strain increases, l1 decreases linearly and the slope of this decrease in the range of temperatures from 10 to 308C is approximately the same. Fig. 3b shows that, as temperature increases, l1 increases linearly and the slope of this dependence in the range of strains from 0 to 1556 m1 is again approximately the same. The relationship between l1 , strain and temperature can be expressed as:
l1 = C1 + k1 S + k2 T multimode fibre 2
linear fit linear fit linear fit
1612.0 λ2, nm
λ2, nm
1612.0
1610.0
linear fit linear fit linear fit
1602.0
a
multimode fibre 1
1614
Fig. 2 Spectral responses for the SMMS fibre structure at temperatures 10, 308C and strain 0, 1556 m1
1601.0 peak1
System description and experimental investigation: A schematic diagram of an SMMS fibre structure is shown in Fig. 1. The multimode fibre 1 (MMF1) and multimode fibre 2 (MMF2) in Fig. 1 have different fibre core diameters and refractive index profiles. The light injected into MMF1 from a singlemode fibre will excite multiple modes propagating in the fibre and these multiple modes are then coupled into MMF2. When these modes from MMF2 are coupled into the output singlemode fibre, as a result of interference the transmission spectrum will be wavelength dependent, displaying several spectral peaks. For a conventional SMS structure, with a single multimode fibre section, changes in temperature and strain alter the refractive index of both the core and cladding and the fibre length, resulting in a change in the transmission spectrum, where it is not possible to independently isolate the effect of temperature or strain. For an SMMS structure it is also true that as temperature or strain change, the refractive indices and length of both the fibre core and cladding change for each MMF. These changes to the MMF sections result in a wavelength shift of the spectral response of the SMMS fibre structure. However, unlike an SMS structure, in an SMMS fibre structure, as the temperature-induced refractive index changes and the straininduced length changes for MMF1 and MMF2 are different, the result is that the spectral variation induced by temperature is different from that induced by strain. By utilising the relationship between temperatureinduced spectral variation and strain-induced spectral variation, the temperature and strain can be independently determined.
1612
l, nm
λ1, nm
Introduction: A singlemode-multimode-singlemode (SMS) fibre structure has the advantages of low cost and simple fabrication and also can be used as a sensor to measure both temperature and strain [1 –7]. However, the cross-sensitivity to both temperature and strain of the SMS fibre structure causes similar problems to those experienced with FBGs. For FBGs it is necessary to utilise complex techniques to discriminate between the two influences when the FBG sensor is used in an environment with variable temperature and strain [8– 10]. In our previous work to overcome the cross-sensitivity of an SMS structure we proposed the use of a bent SMS fibre structure [7]. However, the use of a bent SMS structure to sense strain is mechanically difficult to implement and to date we believe that a means to utilise a straight SMS structure to sense strain and temperature simultaneously has not been published. In this Letter, we propose for the first time a novel structure that employs two hetero-core multimode fibre sections, a so-called singlemode- multimode1- multimode2- singlemode (SMMS) fibre structure, which can measure both strain and temperature simultaneously.
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18 1598
λ1, nm
Reported is an experimental study of a novel fibre heterostructure that utilises multimode interference employing two multimode fibre sections to allow simultaneous measurement of both strain and temperature. By measuring two peak wavelengths, strain and temperature can be determined with a demonstrated sensitivity to strain of 0.35 pm/m1 in the strain range 0 –1556 m1 and a temperature sensitivity of 11.16 pm/8C in the temperature range 10 –308C.
(1)
where C1 is a constant, k1 ¼ 20.3525 + 0.0239 pm/m1 and k2 ¼ 33.9 + 1.9 pm/8C. Fig. 3c shows that as strain increases, l2 decreases linearly; however, the slope is now more strongly related to temperature. Fig. 3d shows that, as temperature increases, l2 increases linearly but the slope is dependent on the applied strain. The relationship between l2 , strain and temperature can be expressed as:
l2 = C2 + k3 S + k4 T
(2)
where C2 is a constant and k3 and k4 are the temperature and strainrelated coefficients, respectively. The calculated k3 and k4 are plotted in Fig. 4.
ELECTRONICS LETTERS 9th June 2011 Vol. 47 No. 12
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Q. Wu, Y. Semenova, P. Wang and G. Farrell (Photonics Research Centre, School of Electronic and Communications Engineering, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland)
calculated K4 linear fit of K4
25 K3 = –0.793 – 0.0113 T k4, pm / °C
k3, pm/me
–0.95 –1.00 calculated K3 linear fit of K3
–1.05
20 K4 = 26.523 – 0.00987 S
E-mail:
[email protected]
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–1.10
References
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20 25 temperature, °C
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a
0
400
800 strain, με
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Fig. 4 Calculated and fitted k3 and k4 a k3 b k4
Fig. 4 shows that k3 ¼ 20.793 2 0.0113T pm/m1 and k4 ¼ 26.523 2 0.00987S pm/8C. Equation (2) hence can be rewritten as
l2 = C2 − (0.793 + 0.0113T )S + (26.523 − 0.00987S)T = C2 − 0.793S + 26.523T − 0.0212ST
(3)
Since the wavelengths of the two spectral peaks are independently measurable, by solving (1) and (3), both strain and temperature can also be determined independently. Conclusion: We have reported for the first time an experimental study of an SMMS fibre sensor for simultaneous independent measurement of both strain and temperature. The sensor has an experimentally demonstrated strain sensitivity of 0.35 pm/m1 and a temperature sensitivity of 11.16 pm/8C. Acknowledgments: Q. Wu is funded by Science Foundation Ireland under grant no. 07/SK/I1200. P. Wang is funded by the Irish Research Council for Science, Engineering and Technology, and co-funded by the Marie-Curie Actions under FP7. # The Institution of Engineering and Technology 2011 7 April 2011 doi: 10.1049/el.2011.0974 One or more of the Figures in this Letter are available in colour online.
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ELECTRONICS LETTERS 9th June 2011 Vol. 47 No. 12
