A micro S-shaped optical fiber temperature sensor based on dislocation fiber splice

We fabricated a simple, compact, and stable temperature sensor based on an S-shaped dislocated optical fiber. The dislocation optical fiber has two splice points, and we obtained the optimal parameters based on the theory and our experiment, such as the dislocation amount and length of the dislocation optical fiber. According to the relationship between the temperature and the peak wavelength shift, the temperature of the environment can be obtained. Then, we made this fiber a micro bending as S-shape between the two dislocation points, and the S-shaped micro bending part could release stress with the change in temperature and reduce the effect of stress on the temperature measurement. This structure could solve the problem of sensor distortion caused by the cross response of temperature and stress. We measured the S-shaped dislocation fiber sensor and the dislocation fiber without S-shape under the same environment and conditions, and the S-shaped dislocation fiber had the advantages of the stable reliability and good linearity.


Introduction
Optical fiber sensors have the advantages of inexpensiveness, compactness, low weight, and immunity to electromagnetic interference, which results in a great demand for fiber sensors in the sensing applications. In all kinds of optical fiber sensors, temperature and stress sensors are the first to be developed [1]. Now, many optical methods and typical structures are used to research of temperature and stress fiber sensors, and the main methods include using Rayleigh scattering [2], Raman scattering [3], reflection [4], interference [5], evanescent field [6], and so on, and measuring optical signal intensity, phase, polarization state changes with the temperature or stress variation [7][8]. The mainly used optical structure has the Fabry-Perot (F-P) cavity [9], fiber grating [10], fiber grating array [11], and fiber ring resonator [12], and the applied fiber has a single-mode and multimode, photonic crystal fiber, microstructured fiber, and birefringent fiber. In above methods, the fiber Mach-Zehnder interference (MZI) has the advantages of simple structure, low cost, high sensitivity, and stability. Recently, some special structures were also proposed to fabricate the MZI, such as micromachining the fiber by femtosecond laser [13], splicing a section of single-mode noncircular twin-core fiber [14], splicing special double-cladding fiber [15] between standard single-mode fibers (SMFs), or splicing the fiber with dislocation [16]. However, fiber MZI-based sensors exhibited the cross-sensitivity between temperature and stress. It is also one of the key problems in the practical application of optical fiber sensors.
In our work, we demonstrated a simple, compact, and stable temperature MZI fiber sensor based on an S-shaped dislocated optical fiber. This sensor was fabricated by means of splicing a section of one standard SMF with two standard SMFs. And two splice points have a micro dislocation. Then this fiber was given a micro bending as S-shape, and the S-shaped micro bending part could release stress with the change in temperature. Several advantages are involved in the proposed sensor, such as high stability and linearity.

Sensor fabrication
As shown in Fig. 1(a), an amplified spontaneous emission (ASE) source with a laser wavelength ranging from 1530 nm to 1560 nm, an optical spectrum analyzer (OSA) (Ando AQ6317D2), and an optical circulator were employed to monitor the spectrum of the proposed based sensor, which was fabricated as described below.
Firstly, a section of the standard SMF (Corn G.652) with the core and cladding diameters of 9 µm and 125 μm, respectively was spliced to two standard SMFs, with a 2 μm -10 μm offset between the two fiber cores by use of a commercial fusion splicer machine (Fujikura, FSM-100P+), in which there are two pairs of motors, i.e. the axial and vertical moving motors, with a movement accuracy of 0.01 µm and 0.1 µm, respectively.
As shown in Fig. 1(b), the light transmitting in SMF1 is divided into two parts at the misalignment-spliced joint. A part of light is coupled into the core of SMF2 as a core mode, and the other part of the light is coupled into the cladding of SMF2 as cladding modes. The two parts of the light will be coupled into the core of the lead-in SMF3. The two parts light of SMF2 formed the MZI in the SMF2 fiber, and an MZI-based sensor in one fiber was achieved. Then, as shown in Fig. 1(a), we measured the dislocation fiber MZI-based sensor.   As shown in Fig. 2, the spectra of our sensor have the offsets of 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, and an SMF2 length of 6 mm, respectively. Each sample sensor with a certain core offset exhibits a distinct fringe contrast in the interference patterns due to the core-offset-induced change in the ratio of lights coupled into the core and cladding modes in SMF2. And it can be found from Fig. 2 that the sample sensor with a core offset of 6 µm has the largest fringe contrast of 28.18 dB.
As shown in Fig. 3, the spectra of dislocation fiber MZI-based sensor have SMF2 lengths of 2 mm, 4 mm, 8 mm, 10 mm, and offsets of 6 μm. A clear interference fringes pattern is observed in those spectra. Each sample with different SMF2 lengths exhibits a distinct fringe contrast in the interference patterns. From Fig. 3, the sample sensor with an SMF2 length of 6 mm has the largest fringe contrast.
With the above results, the fringe has the largest fringe contrast when the offset of the sensor is 6 µm and the length of SMF2 is 6 mm. Moreover, we calculate the free spectrum range (FSR) of the interference fringes by where eff m n ∆ is the effective refractive index (RI) difference between the core and mth cladding modes in SMF2, and L is the length of SMF2. The measured FSR of the sensor with the offset 6 µm and the length of SMF2 6 mm is about 4.97 nm.
Based on the above parameters, the sensor with the offset 6 µm and the length of SMF2 6 mm was bended as S-shape and packaged. The part of dislocation fiber SMF1 and the SMF3 was inserted into the capillary steel tubes and fixed with epoxy resin glue. And then, the two capillary steel tubes were fixed on the micro displacement platform, a small displacement was given in the vertical direction, and the dislocation fiber SMF2 part became an S-shaped. As shown in Fig. 4, the packaged optical fiber was packaged with a package housing, protective sleeve, and some heat conducting oil to form a true temperature sensor.

Measurements and discussions
We tested the characteristics of the sensor in the case of temperature changes. The test method is in accordance with Fig. 1(a). The test results are shown in Fig. 5. The temperature range was 20 ℃ -80 ℃, and the step size was 10 ℃. Obviously, we can see from the spectra that in the case of temperature change, the fringe retention and stability are very good. Taking the wave through near the 1545 nm wavelength as an example, the offset of the central wavelength is about 0.5 nm per -10 ℃. So we can obtain the temperature sensitivity s=∆λ/∆T = 0.05 nm/℃ = 50 pm/℃. The results proved that our sensor was mainly temperature sensitive, avoiding the sensitive cross between the temperature and stress. The reasons are as follows. The light propagating at the dislocation point between SMF1 and SMF2 was equally split into two beams in SMF2, which were propagating as the core mode and cladding mode. The two beams coupled into the core of SMF3 at the dislocation point between SMF2 and SMF3. We denote the intensities of the core mode and cladding mode in SMF2 as I co and I cl , and the output intensity of sensor can be expressed as co cl co cl 0 2 +2 cos l n I I I I I π ϕ λ where λ is the light wavelength, L is the length of SMF2, Δn = n co -n cl is the effective RI difference between the two interference arms of MZI, where n co and n cl are the effective RIs of the core mode and the cladding mode, and the φ 0 is the initial phase of the interference. From (2), the interference spectrum reaches the minimum value when the following condition is satisfied: where k is an integer, and λ k is the wavelength of the kth order interference dip. When the temperature changes, the sensor's maximum response is the length of SMF2. The change is due to the length of the interference arm. But, in the dislocation fusion point, the stress effect will lead to a lateral change. Therefore, the change in L with temperature is as i j T S L L L ∆ = ∆ + ∆ (4) where ΔL T is corresponding to the temperature changes, and ΔL S is corresponding to the change in stress. So the interference fringe was a relationship between ΔL T and ΔL S . We made the sensor in S-shape, which was given a stress with slow release in the process of temperature change. It counteracted the stress effect of ΔL S . So, the offset was caused by temperature ΔL, which contains only a ΔL T .

Conclusions
We demonstrated a temperature senor based on an S-shaped dislocated optical fiber. We made this fiber a micro bending as S-shape, and the S-shaped micro bending part could release stress with the change in temperature and reduce the effect of stress on the temperature measurement, and this structure solved the problem of sensor distortion caused by the cross response of temperature and stress. The senor has very good linear, retention, and stability, which can utilize the difference between the lengths of the dislocation points to achieve cascade and multipoint monitoring.