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Abstract
We demonstrate a Fabry-Perot fiber cavity for strain measurement with highly improved sensitivity. The cavity is fabricated by the fusion splicing of two etched single-mode fibers with a core offset in the X axis or both X and Y axes. It is found that the strain sensitivity can be increased from 2.39 to 7.75pm/με by adopting offset splicing. Such a Fabry-Perot cavity device is compact in size, simple in structure, easy in fabrication and of low cost, and has high potential in strain sensing.
© 2017 Optical Society of America
1. Introduction
Optical fiber sensors have many advantages such as free of electromagnetic interference, corrosion resistance, good electrical insulation and high sensitivity. In recent years, optical fiber sensors have been widely used in petroleum, chemical, transportation, energy, metallurgy, medicine, military, food, and nuclear industries and many other fields. Compared with other optical fiber sensors, Fabry-Perot interferometer (FPI) fiber sensor is simple in structure, small in size and suitable for mass production [1–14]. Many optical fiber FPI based sensors have been developed for a range of applications, such as strain [1–4,7,10–12], refractive index (RI) [9] and pressure measurements [14].
An efficient way of constructing optical fiber FPI strain sensor is to create an air-cavity inside the optical fiber. A few methods have been proposed to fabricate such an air-cavity, such as the use of silica hollow tube [13] or photonic crystal fiber (PCF) [4], or via direct femtosecond laser micromachining [8-9]. However, these methods may involve with complicated fabrication process or the materials used are rather expensive. Moreover, the typical strain sensitivity achieved is not high.
In this paper, we demonstrate a new way to fabricate FP cavity by fusion splicing two etched single mode fibers (SMFs) with core offset for strain measurement with highly improved sensitivity. As only SMF and fusion splicer are used, the device is of low-cost and simple in fabrication. Compared with FP cavity without core offset, the strain sensitivity of our proposed device can be increased from 2.39 to 7.75 pm/με, representing a large strain sensitivity improvement of 5.36 pm/με.
The FP cavity is formed by fusion splicing of two etched SMFs with core offset, as shown in Fig. 1. Firstly, SMFs are etched in hydrofluoric (HF) acid with concentration of 30% for 25 minutes in order to create a deep hole at the fiber end. Secondly, to place the two etched SMFs on the fusion splicer (Fujikura 80s) in a line as shown in Fig. 1(a), and use the function of motor drive to move one of the SMFs in X-axis or Y-axis for a short distance to create a core offset as shown in Fig. 1(b). Thirdly, the parameters of the fusion splicer are selected as: discharging time and power are 500 ms and 30 bit, respectively. Finally, fusion splicing of two etched SMFs to create an inner air FP cavity device as shown in Fig. 1(c).
Fig. 1 Schematic diagrams of fabrication process of optical fiber FP cavity with core offset positioning the etched SMFs; (b) creating an offset; (c) forming an inner air FP cavity.
2. Device fabrication
The FP cavity is formed by fusion splicing of two etched SMFs with core offset, as shown in Fig. 1. Firstly, SMFs are etched in hydrofluoric (HF) acid with concentration of 30% for 25 minutes in order to create a deep hole at the fiber end. Secondly, to place the two etched SMFs on the fusion splicer (Fujikura 80s) in a line as shown in Fig. 1(a), and use the function of motor drive to move one of the SMFs in X-axis or Y-axis for a short distance to create a core offset as shown in Fig. 1(b). Thirdly, the parameters of the fusion splicer are selected as: discharging time and power are 500 ms and 30 bit, respectively. Finally, fusion splicing of two etched SMFs to create an inner air FP cavity device as shown in Fig. 1(c).
There are two ways to introduce the core offset. One is just to introduce the offset on the X axis, as shown in Fig. 2(a), where O is the core offset in X axis. The other is to create core offsets in both X and Y axes, as demonstrated in Fig. 2(b), where O1, O2 are core offsets in X and Y axis, respectively.
3. Operation principle
The incident light beam propagating in the fiber core is reflected on the two interfaces of the air-cavity respectively, as shown in Fig. 3, where L is the cavity length, O1 and O2 represent the core offset on X and Y axis, respectively. The two reflected light beams recombine in the fiber core at the first air-cavity interface, and form an FP interferometer. The output light intensity of the FP interferometer can be written as
where I1 and I2 are the intensities of the two reflected light beams, φ is the phase difference between the two reflected light beams, given bywhere n is the refractive index of the cavity medium, λ is the wavelength of light beam and φ0 is the initial phase and L is the cavity length.The wavelength of the interference dip wavelength λdip can be expressed by
where m is an integer. The dip wavelength shift is given bywhere ΔL is the change of the cavity length due to strain. Since the refractive index of the medium inside does not change with strain, the strain can be measured by monitoring the wavelength shift.4. Experiment and discussion
Figure 4 shows the Schematic diagram of experimental setup. The incident light beam comes from a broadband light source (BBS) with the wavelength range from 1450 to 1650nm, and arrives at the input port of a circulator. The other two ports of the circulator are linked with the fiber sensor head and an optical spectrum analyzer (OSA) (YOKOGAWA 6390), used to record the reflection spectrum with the resolution of 0.02 nm.
The fiber sensor head device sample is placed on a transition stage to measure its strain response. One end of the device is fixed, and the range of the applied axial strain is from 0 to 1400 με with a step of 280 με. The experimental results obtained for the FP cavity device without core offset are shown in Fig. 5, and the strain sensitivity achieved is ~2.39pm/με.
Fig. 5 The reflection spectrum evolution with strain, and the dip wavelength shift versus strain for the device sample without core offset; the inset is the microscope images of the sample without core offset.
Figure 6 shows the reflection spectrum evolution under different strains and the output dip wavelength versus strain for the device samples with offset. The insets show the microscope images of the corresponding device samples. It can be seen from the figure that a good linear relationship between the dip wavelength shift and strain exist for all the five samples, and their strain sensitivities obtained are 3.59, 4.32, 4.14, 3.97 and 2.78pm/με, respectively, for the samples with core offsets of ~1.6, ~2.1, ~3.0, ~3.8 and ~4.1μm respectively. And the insertion loss becomes bigger when the offset is larger. However, all the device samples with offsets in X axis are more sensitive to strain than the sample without offset. This is likely due to that, a relatively large cavity length change is produced for the cavity without offset. From Eq. (4), the wavelength shift induced is also relatively large for the cavity with offset.
Fig. 6 The reflection spectrum evolution with strain and dip wavelength shift versus strain of the device samples with core offset of (a) 1.6μm; (b) 2.1μm; (c) 3.0μm; (d) 3.8μm; and (e) 4.1μm, in X axis; the insets are the microscope images of the corresponding samples.
The characteristics of the five samples are listed in Table 1.
Table 1. The parameters of the device samples with offset in X axis
It can be seen from Table 1 that when the offset distance is ~2.1 μm, the maximum sensitivity of 4.32 pm/με can be achieved. However, reducing or increasing the offset distance leads to the reduction of the strain sensitivity. Thus, an optimum core offset value does exist.
Figure 7 displays the reflection spectrum evolution under different strains and the output dip wavelength versus strain for the five device samples with core offset in both X and Y axes. The corresponding microscope images of the device samples with core offsets in both X and Y axes are shown in the insets. A good linear relationship between the dip wavelength shift and applied strain is found for all the five samples. The strain sensitivities obtained are 3.57, 3.68, 5.21, 7.75 and 6.22 pm/με, respectively, for the samples with core offsets of (9.5μm, 10.2μm), (11.2μm, 12.8μm), (12.2μm, 14.1μm), (13.8μm, 15.1μm), and (15.8μm, 17.1μm), respectively. All the strain sensitivity values obtained are larger than 2.39pm/με, thus the core offset in both X and Y axes can also improve the strain sensitivity of the FP cavity. Overall, the device samples with core offset in two directions are more sensitive to strain than that with offset in only one direction.
Fig. 7 The reflection spectrum evolution with strain and dip wavelength shift versus strain of three device samples with core offsets on both X and Y axes (a) (9.5μm, 10.2μm); (b) (11.2μm, 12.8μm); (c) (12.2μm, 14.1μm); (d) (13.8μm, 15.1μm); and (e) (15.8, 17.1); the insets are the microscope images of the corresponding device samples.
Table 2 summarizes the parameters of the device samples with core offset in both X and Y axes.
Table 2. The parameters of the device samples with offset in both X and Y axes
Similar to the results obtained for the device samples with core offset in only X axis, It can be seen from Table 2 that the optimum core offset values (13.8μm, 15.1μm) exist for the device samples with core offsets in both X and Y axes, and at such optimum values, the maximum sensitivity of 7.75 pm/με is obtained.
It should be notice that cavity length may also affect the strain sensitivity and the smaller cavity length gives the higher strain sensitivity [3]. However, the offset value is found to be more critical. For instance, in Table 1, although sample S5 has smallest cavity length, it still has the lowest strain sensitivity due to its core offset value which is too large.
The temperature responses of the FP device samples of different types are displayed in Fig. 8. The largest temperature sensitivity for a device sample without offset and with a cavity length of ~24μm is 11.6μm/°C, as shown in Fig. 8(a). However, for a device sample with single direction offset of 1.6μm, and similar cavity length of 25.6μm, the largest temperature sensitivity becomes only 1.2 pm/°C.
Fig. 8 The output dip wavelength versus temperature for device sample of (a) without offset of (b) single direction offset of ~25.6μm.
This means that the temperature sensitivity of the FP cavity device can be largely decreased by adopting core offset, and this effectively reduces the temperature cross sensitivity of our proposed device for strain sensing.
5. Conclusion
In conclusion, we have demonstrated FP cavity device for strain measurement, which is formed by fusion splicing of two etched SMFs with core offset. We proposed two different ways of introducing offsets. One is just X axis offset and another is offset on both X and Y axes. The fabricated FP cavity has enhanced the strain sensitivity from 2.39 up to 7.75pm/με. As only conventional SMFs and fusion splicer are used, the device has the advantage of simple in fabrication, compact in size and low in cost.
Funding
National Natural Science Foundation of China (61661166009).
References and links
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