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Modal Interferometer Using Three-Core Fiber for Simultaneous Measurement Strain and Temperature Volume 8, Number 4, August 2016 Tao Geng Jiang He WenLei Yang XuDong Chen MaoWei An CuiTing Sun XiRen Jin Libo Yuan

DOI: 10.1109/JPHOT.2016.2585339 1943-0655 Ó 2016 IEEE

IEEE Photonics Journal

Measurement of Strain and Temperature

Modal Interferometer Using Three-Core Fiber for Simultaneous Measurement Strain and Temperature Tao Geng, Jiang He, WenLei Yang, XuDong Chen, MaoWei An, CuiTing Sun, XiRen Jin, and Libo Yuan Key Laboratory of In-Fiber Integrated Optics, Ministry Education of China, Harbin Engineering University, Harbin 150001, China DOI: 10.1109/JPHOT.2016.2585339 1943-0655 Ó 2016 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received April 10, 2016; revised June 17, 2016; accepted June 22, 2016. Date of current version July 13, 2016. This work was supported in part by the National Natural Science Foundation of China under Grant 41174161, Grant 61377084, Grant 61227013, Grant 61275087, Grant 11204047, and Grant 61307104; by the 111 project under Grant B13015; by the Fundamental Research Funds for the Central Universities; by the Harbin Engineering University; and by the ScienceTechnology Creative Foundation for Young Scientists of Harbin City under Grant 2015RQQXJ087 and Grant 2015RAQXJ055. Corresponding author: W. Yang (e-mail: [email protected]).

Abstract: A structure made of tapered three-core fiber and CO2 laser notch long-period fiber grating is proposed in this paper. By adjusting the taper waist diameter of a threecore fiber, this structure is made suitable for simultaneous strain and temperature measurements. The effectiveness of the proposed structure in realizing simultaneous measurement sensitivity is verified by measuring the sensitivities of the two peaks at the variation of the wavelength and the depth difference. Experimental results indicate that the wavelength sensitivities of the two peaks are
2:96 nm m"
1 , 42.9 pm °C−1 and
1:52 nm m"
1 , 47.4 pm °C−1 for strain and temperature, respectively, and the depth sensitivities of two peaks can reach 2.5 dB m"
1 , 0.007 dB °C−1 and
2:8 dB m"
1 , −0.013 dB°C−1. It can therefore be concluded that the proposed structure can be realized by adjusting the taper waist diameter of a three-core fiber for simultaneous strain and temperature measurements. Index Terms: Tapered three-core fiber, long-period fiber grating, simultaneous measurement, strain sensor, temperature sensor.

1. Introduction Simultaneous measurement of strain and temperature is vital for a road or dam construction, as well as the development of advanced materials. Some progress has been made in this area in recent years. Long period fiber grating (LPFG) has been widely used to fabricate fiber sensors since it had been originated by Vengsakar et al. [1] for the first time in 1996, and most of them are applied to simultaneous measurement. For example, our group researchers have put forward two kinds of dual-parameter sensors, including LPFG pair and phase-shifted LPFG achieved by filament heating method [2], [3]. LPFGs written by high-frequency CO2 laser pulses are extensively used in fabrication of optical sensors from various sensing systems because of their high electromagnetic immunity, electrical isolation, low insertion loss, low back reflection, and compactness [4]–[7], but they have low sensitivity of strain and temperature because of their stable structure. It is a way to solve

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IEEE Photonics Journal

Measurement of Strain and Temperature

Fig. 1. Cross section of a three-core fiber under LED exposure.

this problem that LPFGs are combined with some special structures, including LPFGs with a high-birefringence fiber loop mirror [8], and LPFGs with a polarization-maintaining fiber in a loop mirror [9], and the produced sensors can be applied to simultaneous measurement. Great efforts to expand the mode field of LPFG have been made to get the suitable modal interferes, including LPFG inscribed on multimode fiber or tapered fiber [10], [11], and LPFGs connected with hollow-core photonic band gap fiber based on connect with hollow-core photonic band gap fiber [12]. The fiber tapering method is one of the best ways to expand the mode field. Our research group has studied the power coupling ratio of tapered multi-core fiber [13], [14]. In this paper, the tapered three-core fiber (TCF) is used to connect with LPFG. The tapered three-core fiber (TCF) can split light, which provide a possibility to obtain the most suitable light ratio of TCF by adjusting the taper waist diameter. Energy is directly injected into the cladding of LPFG in order to lead to the modal interference and stimulate new resonant peaks which are essential for the simultaneous measurement [15], [16].

2. Fabrication and Principle As shown in Fig. 1, the distribution of three cores is approximately an isosceles triangle and the base angle is 9°. The cores have the same refractive index but slightly different diameters. All the three cores support the fundamental mode only, and the pitch of side cores is 42.48 m and 43.30 m from the center core. The three cores are so far apart that they can be considered as three independent paths. The light beam is launched into core 2, and no obvious coupling occurs in cores 1 and 3. As shown in Fig. 2, a micro-taper is developed by tapering TCF using filament heating method. When the taper waist diameter is bigger than the critical value, the light energy exist in the core 2 only, and there is no energy in core 1 and 3 [17]. Coupling occurs only when the taper waist diameter is less than critical value that has been studied by researcher [18]. The tapering of TCF function to expand the fiber field and to provide some light in the cladding of LPFG so that the LPFG located next to the non-adiabatic taper can couple a number of cladding modes to the fiber core. As shown in Fig. 3, the light first transmits in the single mode fiber, and then enter the core 2 of TCF. There is no coupling among the three cores because of the safety distance between them. Strong coupling can be produced using fiber tapering technique in the tapered region. Consequently, the beam splitting occurs. When the light enters the tapered region, the light coming from core 2 with enter cores 1 and 3 to change the power transforms from core mode to multiple cladding modes. As shown in Fig. 3(b), the fiber is connected with Super-continuum Light Source (BLS) with a spectrum range of 600–1700 nm at one side, and with Optical Spectrum Analyzer (OSA) on the other side. The incident light is divided into three parts, which transmits independently in TCF, when it passes through the tapered region. There is a series of linearly polarized modes stimulated from the fundamental mode at the TCF-LPFG interface

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IEEE Photonics Journal

Measurement of Strain and Temperature

Fig. 2. Tapered TCF. (a) Light transmission load. (b) Tapered region. (c) Light energy ratio after tapering.

Fig. 3. Structural parameters. (a) Proposed structure. (b) Experimental facility.

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IEEE Photonics Journal

Measurement of Strain and Temperature

Fig. 4. Spectrum obtained for different taper waist diameters. (a) 40 m. (b) 33 m. (c) 20 m.

because of the mode field mismatch [19], [20]. Those modes enter the cladding of LPFG to stimulate the cladding mode and change the resonance peak of LPFG [21]. As shown in Fig. 4, the transmission spectrum moved down in data processing to help the observation and analysis of the original spectrum, the red line moves down by 10 dB while the blue line moves down by 20 dB. The intermodal interfere produces a new resonance peak (peak A) on the left side of the original resonance peak and leads to the movement of the original resonant peak to the right side when the taper waist diameter is 33 m. Thus there is enough energy in the cladding of LPFG, and energy in each core has its effect on the proportion of these two peaks. The proportion of peak A can be changed by changing the taper waist diameter only. If the tapered waist diameter is bigger than 33 m (for example 40 m), the new peak is too small to be used for measurement. If the taper waist diameter is smaller than 33 m (for example 20 m), the strong model Interference results in a tanglesome spectrum, which is not suitable for measurement, and the loss increases as the taper waist diameter decreases.

3. Experimental Results and Discussions As shown in Fig. 5(a) the wavelengths and depths of two peaks change as the strain changes, the structure has negative strain sensitivity because of negative contribution of the photoelastic effect, many factor affect the peak attenuation sensitivity, including fiber core thermo-optic coefficient, the cladding thermal optical coefficient, the mode field overlap integral sensitivity and the coupling constant, among them, the coupling constant is the decisive factor to describe the peak attenuation changes after the grating is fabricated, both strain and temperature changes. The resonance wavelengths have a blue shift with the increasing strain whereas the depths change in opposite directions. The resonant wavelength of peak A shifts from 1274.6 to 1272.2 nm when strain goes up to 1000 ", and the resonant wavelength of peak B shifts from 1289 to 1278.6 nm. Meanwhile, the depth of peak A increases from −25.59 to −23.15 dB, and the depth of peak B decreases from −29.05 to −31.44 dB. As shown in Fig. 5(b), the wavelength and depth change with the increasing strain. The strain sensitivities of peak A are
3 nm m"
1 and 2.5 dB m"
1 , and the strain sensitivities of peak B are
1:5 nm m"
1 and
2:8 dB m"
1 . The sensitivity of TCF-LPFG is greatly enhanced compared with the sensor made of LPFG [22]. As shown in Fig. 6(a), tests were run with TCF-LPFG on an experimental set up for its temperature sensitivity, the positive thermal expansion and thermo-optic coefficients of the SMF makes the resonant wavelength shifted to longer wavelengths. When the temperature increases from 40 to 140 °C, the resonance wavelength of peak A increases from 1274.6 to 1279.2 nm, and the resonance wavelength of peak B increases from 1289 to 1293.8 nm. Simultaneously, the depth of peak A increases from −25.59 to −24.91 dB and the depth of peak B decreases

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IEEE Photonics Journal

Measurement of Strain and Temperature

Fig. 5. Strain sensitivity of TCF-LPFG. (a) Transmission spectra. (b) Resonant wavelength shift and depth of peaks A and B.

from −29.05 to −30.38 dB. As shown in Fig. 6(b), the temperature sensitivities of peak A are 43 pm °C−1 and 0.007 dB °C−1, and the temperature sensitivities of peak B are 47 pm °C−1 and
0.013 dB °C−1. It can be seen from the characteristics of TCF-LPFG that the strain and temperature can be measured simultaneously by detecting the resonant wavelength of peak A or B, and the amplitude is different for peak A and B. The effect of temperature and strain on TCF-LPFG can be expressed in matrix form as 


l
p

 ¼

1 D



Ke
P
KT
P


Ke
l KT
l




e
T

 (1)

where D ¼ K"
P KT

K"
 KT
P .
 is the shift in resonance wavelength of peak A or B,
P is the difference in the peak depth, and
" and
T represent variation of strain and temperature, respectively. K"
P , KT
P , K"
 , and KT
 are coefficients for the temperature and strain difference in peak depth and wavelength, respectively. The experimental results show that K"
 ¼
3 nm/m", KT
 ¼ 0:043 nm= C, K"
P ¼ 2:5 dB/m", and KT
P ¼ 0:007 dB= C for peak A and K"
 ¼
1:5 nm/m", KT
 ¼ 0:047 nm= C, K"
P ¼
2:8 dB/m", and KT
P ¼
0:013 dB= C for peak B. When the wavelength resolution of OSA is 0.02 nm, and the tolerance for the transmission peak power was 0.03 dB. The temperature and strain resolution of the structure are 1 °C and 8.9 " for peak A, The temperature and strain resolution of the proposed

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Measurement of Strain and Temperature

Fig. 6. Temperature sensitivity of TCF-LPFG. (a) Transmission spectra. (b) Resonant wavelength shift and depth of peaks A and B.

Fig. 7. Strain sensitivities of peak versus different temperatures.

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IEEE Photonics Journal

Measurement of Strain and Temperature TABLE 1

Wavelength shift of two peaks (nm)

TABLE 2 Depth shift of two peaks (dB)

structure are 0.9 °C and 14.9 structure.

" for peak B. Fig. 7 shows that the measuring stability of

4. Conclusion It can be seen from the presentation above that 1) as shown in Table 1, the wavelengths of peaks A and B have a red shift with the increasing temperature, and they have a blue shift with the increasing strain; 2) as shown in Table 2, the depth of peak A reduces with the increasing strain and temperature, while the depth of peak B is change in the opposite direction; and 3) the proposed structure can be realized by adjusting the taper waist diameter of three-core fiber for simultaneous strain and temperature measurement, and the wavelength sensitivities of peak A are
3 nm m"
1 and 43 pm °C−1 for strain and temperature, respectively, and those of peak B
1:5 nm m"
1 and 47 pm °C−1, respectively. The depth sensitivities are 2.5 dB m"
1 , 0.007 dB °C−1 and
2:8 dB m"
1 , −0.0013 dB °C−1 for peaks A and B, respectively. The standard deviations of strain sensitivities of the proposed structure are better than expected.

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