A novel hybrid fiber Bragg grating (FBG) for simultaneous measurement temperature and strain is proposed. This sensor is fabricated by inscribing fusion splice joint between two types of fibers using excimer laser and phase mask. Due to the different characteristics of the two fibers, the two resonance peaks have different responses to temperature and strain, and both physical parameters can be demodulated by analyzing the central wavelength shifts. Practical measuring results show that the hybrid FBG proposed in this paper can precisely achieve simultaneous measurement of temperature and strain. This work provides a new scheme for multi-parameter simultaneous sensing.
Fiber Bragg Gratings (FBGs) have received a great attention in recent years due to their advantages of anti-electromagnetic interference, compact architecture, low insertion loss, cost-effectiveness, and so on. So far, FBG-based fiber sensors have been proved to have high precision and stabilization in the measurement of temperature, strain, curvature, twist, and so on [1–4]. However, a FBG is usually sensitive to several different physical parameters simultaneously, so the cross-sensitivity of FBG should be considered in actual application, especially in cases where temperature and strain are involved.
There are some methods to discriminate temperature and strain simultaneously, which are mainly grouped into two categories: temperature compensation  and multi-resonance peaks. The former approach required two independent FBGs, in which one of FBG is used for sensing both strain and temperature while and the other one is only used as a temperature compensation sensor. Although this method is effective, the process of implementation is a bit more complex in actual measurement. At present, the common methods are mainly dependent on the multi-variables which have different sensitivities to the multi-physical parameters. Therefore, the key of these methods is how to create several variables to express different physical parameters. For instance, fabricating FBG cavity  or long period fiber grating (LPFG) pair , cascading FBG with LPFG  or chirped FBG , cascading LPFG with tapered LPFG  or taper Mach-Zehnder interferometer (MZI) , writing more than one FBGs in fiber with various diameter  and using two different gratings . All these mentioned above have the function of separating the temperature and strain responses, but they all consist of two gratings or other complex structures and the preparation processes may be relatively complicated. The transmission spectra of titled FBGs , superstructure  and dual-mode fiber based FBG  include too many resonance peaks which can also realize multi-parameter measurement simultaneously, but all of that sensor will take up too large spectrum range. Some other methods can achieve two peaks in spectrum after a single fabrication cycle such as using substrate flake  or writing FBG on fusion splice [18–20]. Scholars have reported a single grating schemes that FBG was written on the splice joint between two different fibers  or the same fibers [18-19]. The two dips have extremely similar temperature and strain sensitivity when FBG was written on the same fibers, and as a result it would cause errors in demodulation.
In this work, a novel hybrid FBG for simultaneous measurement of temperature and strain is fabricated and experimentally demonstrated. It is inscribed by putting the fusion splice point of two fibers under excimer laser through a 3 cm uniform phase mask. There are two dips in the transmission spectrum and each dip has different responses to temperature and strain. This is the first time that SMF and double cladding fiber (DCF) have been combined for simultaneous measurement of temperature and strain. Due to the difference in effective refractive index, the temperature or strain sensitivity gap between two dips is greater compared with ordinary phase-shifted FBG. For example, in Ref. , the wavelength shift difference between two dips is only 0.03 nm when the fiber in the temperature range from -50 to 150 ℃ (0.15 pm/℃), while the hybrid FBGs fabricated in this paper is about 0.9 pm/℃, which is six-fold higher than prior work. Besides, this hybrid FBG consist of two common fibers: SMF and 10/130 DCF and the insert loss of the hybrid FBG is low. So, the use of DCF achieve a more accurate and cost-efficient measurement of temperature and strain. Experiment results illustrate the superiority of this novel hybrid FBG used as a fiber sensor for simultaneous measurement of temperature and strain. Furthermore, this approach may extend potential application in fiber laser due to the existence of DCF.
2. Fabrication and principles
Commonly, arc discharge is used to fabricate PS-FBG on uniform FBG. This approach can be implemented with fusion splicer by electric shocking the midpoint of FBG. The repeatability error of this method depends, to great extent, on the operating personnel or performance of machine. There is an alternative that inscribing PS-FBG through a phase mask (PM), but this method needs special customized mask. In this work the manufacture process includes three steps. Firstly, a single mode fiber (SMF-28e) and a double cladding fiber (DCF) (manufactured by Nufern, model: LMA-GDF-10/130-M) are cleaved into two parts with good ends respectively. Then the SMF and 10/130 DCF are spliced by fusion splicer (Fujikura: 62S). Finally, the hybrid fusion spliced structure is inscribed by an excimer laser (COMPexPro110, made by Coherent Corporation, using KrF) through a uniform PM and the fusion point is putted in the middle position behind the PM. The length of this PM is about 3 cm with the period of 1069 nm. Due to this PM diffract most of the energy in the first diffraction orders, the resulting grating period is half of the period of PM. The transmission spectrum is interrogated by an optical analyzer (Agilent: 8163B).
Owing to the refractive index difference between the two types of fiber, there will be two main resonant loss peaks existing in the transmission spectrum while the hybrid structure exposed by excimer laser as shown in Fig. 1. From the spectrum of this structure, the insertion loss is about here are about 2.5 dB slightly larger than the loss of splice joint between two SMFs, it may be due to the slight difference in the core diameter of two fibers.
For this hybrid FBG inscribed by UV laser, the thermo-optical coefficient and elasto-optical coefficient of the two types fiber are different, so the wavelength and amplitude of two dips have different responses to temperature and strain. In actual measurement for the double physical parameter, ΔλA and ΔλB are used to represent the shifts of two peaks, ΔT and Δε are the temperature and strain variation. Therefore, when the temperature and strain are applied on the hybrid structure simultaneously, the matrix form for the response of temperature and strain can be written as formula :
3. Results and discussion
As shown in Fig. 1, the wavelength of resonance peaks A and peak B are about 1547.5 nm and 1550.7 nm with the amplitude of 18 dB and 27 dB respectively. This inconsistency is due to the different refractive index and core diameter which influent the wavelength and amplitude of resonance peak. The system of test device is shown in Fig. 2, the fiber is connected with ASE light source and two fiber holders is fixed on double electric displacement platform (PI M-L01.8A1) respectively.
In order to research the temperature sensitivity of this hybrid FBG, it is heated by a high precision heating equipment in the range of 30 ℃ to 90 ℃ with a increment of 10 ℃. For this hybrid FBG, the thermo-optic coefficient of the two different fiber is unequal, so the temperature sensitivities between two peaks is different. The transmission spectrum under different temperature is shown in Fig. 3(a). The wavelength shifts of two resonant peaks are counted as presented in Fig. 3(b). According to the linear fitting of temperature versus wavelength shift, the calculated sensitivity temperature of peak A and peak B are about 12.3 pm/℃ and 13.2 pm/℃ respectively.
The following experiments are executed to investigate the strain response of this hybrid FBG at the room temperature and the strain response is measured in the range of 0-2000µε. Figure 4(a) displays the transmission spectrum evolution with the increasing of strain. The wavelength of two loss peaks with respect to different strain is presented in Fig. 4(b). The strain sensitivity of peak A and peak B are 0.69 pm/µε and 0.76 pm/µε respectively which obtained by linear fitting. The slight difference result from the different elasto-optic coefficient and the stain sensitivity of this sensor shows a good linearity and repeatability.
From the temperature and strain experiment demonstrated above, it can be observed that simultaneous measurement of temperature and strain can be achieved by using the two loss peaks. Based on the above experiments, KTA=12.3pm/℃, KTB = 13.2 pm/℃, KεA=0.69 pm/µε, KεB=0.76 pm/µε, substituting and these values into matrix (2) mentioned in Section 2, so it can be written as:
It can be seen from matrix (3) that the temperature and strain can be calculated simultaneously by measuring the variations of both wavelengths. The value of D is three times bigger than the value in Ref., so the system error of this paper is lower. The experiment result also proves that DCF can be used in simultaneous measurement. During the measuring processes of temperature and strain sensitivity, the errors include finite resolution of spectrometer, the error of calibration and the standard errors in data analysis. The errors could be reduced by repeating the test process several times and calculating the average value on the basis of error theory.
As shown in Fig. 5, experiments are executed under various temperature and stress levels to prove the feasibility of simultaneous measurement of the two parameters. Figure 5 (a) shows different temperature sensitivity under different strain. Figure 5 (b) shows different strain sensitivity under different temperature. Their standard deviation indicates a good stability during the whole measurement process. In the experiment, optical spectrum analyzer with resolution 1 pm is exploited, so the theoretical precision of temperature and strain can reach 0.8 ℃ and 1.3 µε, respectively. By analyzing all the values mentioned above, the RMSE of the measured values relative to the calibration indicate fluctuations up to ±1.6 ℃ and ±26.7 µε. The comparison of the characteristics between several fiber structures is presented in Table. 1. As can be seen from Table 1, this proposed sensor is simple, accurate and stable.
In summary, a novel hybrid FBG for simultaneous measurement of strain and temperature has been proposed. This hybrid FBG is inscribed on the joint region of two fibers where fusion point is included and has two resonance peaks in transmission spectrum. Experiments are conducted with this hybrid FBG and the experimental results indicated that the two loss peaks have different sensitivities to temperature and strain. The temperature sensitivities of two peaks are 12.3 pm/℃ and 13.2 pm/℃, respectively while the strain sensitivities are 0.69 pm/µε and 0.76 pm/µε. As compared to the PS-FBG mentioned in Ref. , such a hybrid FBG has an advantage over simultaneous measurement. The stability and feasibility of using this hybrid FBG for measuring temperature and strain simultaneously are full demonstrated in the whole test process.
Natural Science Foundation of Hunan Province (2019JJ20023); National Natural Science Foundation of China (11974427).
The authors declare no conflicts of interest.
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