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Abstract
This paper reports a torsion sensor based on the multimode interference theory. The sensor is fabricated by sandwiching a section of perfluorinated polymer optical fiber (POF) between two silica single mode fibers to construct a single-mode-multimode-single-mode (SMS) structure. The perfluorinated POF is easily connected to the optical fiber via the precise alignment of ceramic ferrules and ceramic mating sleeve. With the considerable flexibility and deformability of the perfluorinated POF, the proposed sensor is especially suitable for torsion measurement. Experimental results show that a wavelength sensitivity of 106.762 pm/(rad/m) and an intensity sensitivity of 0.165 dBm/(rad/m) are obtained within a large torsion rate of −100∼100 rad/m.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Security monitoring in civil engineering applications, such as bridges, buildings, and many other civil structures, has been an important and indispensable subject. Torsion is one of the most important mechanical parameters for security monitoring [1]. Many torsion sensors have been developed before. Traditional torsion sensors such as optical encoders and magnetic sensors [2] always have a large size, which are hardly compatible with building structures. In recent years, the optical fiber sensor has received significant attention due to its excellent characteristics of compact size, chemical robustness and immunity to electromagnetic interference, etc. [3]. Various torsion sensors based on silica optical fiber have been reported, and well summarized in [4]. However, the silica optical fiber is fragile and must be treated carefully. Besides, the measurement range of the torsion is relatively small.
Polymer optical fiber (POF) is preferable on torsion measurement due to its considerable flexibility and deformability. It can be handled to sustain a large torsion easily. The poly (methyl methacrylate) (PMMA) is the flagship polymer, which is applied and published widely up to now [5]. The Young’s modulus of PMMA POF is nearly two orders of magnitude lower than that of silica optical fiber [6]. This characteristic makes PMMA POF more sensitive to the shear force and can measure a larger torsion range [7]. Torsion effects of PMMA POF based on polarization state [8] and torsional strain effect [9] have been reported. However, the torsion responses of these sensors show poor reliability, which are far from the practical application. Torsion sensitivity of fiber Bragg grating in few mode PMMA POF has been demonstrated, whereas the Bragg wavelength is insensitive when the angle is less than 1080° [10]. So, there is a high demand for the developing torsion sensors with reliable performance. In recent years, single-mode-multimode-single-mode (SMS) structure has been widely used in the design of different optical fiber sensors. The SMS structure is sensitive to physical deformation, and can be applied to torsional measurements. However, the core diameter of PMMA POF is relatively large. To the best of our knowledge, the smallest core diameter of PMMA POF is 120 µm (MITSUBISHI RAYON, CK-5), which is far larger than the core diameter of single mode fiber (SMF). The mismatch of core diameters between the PMMA POF and SMF results in significate transmission loss of the structure. Moreover, the PMMA POF mainly transmits visible light with wavelength about 650 nm, instead of telecom-wavelength light at 1550 nm. In recent years, fluorinated polymers have been applied in POF development with the aim of reducing optical attenuation [11]. The optical attenuation of perfluorinated POF in the near infrared is reduced significantly due to the replacement of the carbon-hydrogen bonds with carbon-fluorine [12]. Furthermore, the perfluorinated POF is also characterized by lower water absorption in comparison with PMMA POF [13]. Most importantly, the core diameter of 50 µm of the perfluorinated POF is commercially available. To date, the sensing characteristics of perfluorinated POF have been investigated in temperature [14], strain [15,16] and pressure [17] refractive index [18] measurement.
In this paper, a SMS structure interferometer using perfluorinated POF is proposed for torsion measurement. A section of perfluorinated POF is sandwiched between two silica SMFs to constitute the SMS structure. This configuration results in three advantageous features: (i) easy fabrication with simple and inexpensive tools, (ii) good compatibility with optical devices operating at telecom-wavelength region and (iii) high mechanical strength. When torsion effect is applied to the multimode interferometer, the transmission spectrum with multimode interferential fringes is investigated in experiment. It is demonstrated that this sensor can measure the torsion with a range of −100∼100 rad/m. Both the wavelength shift and transmission intensity variation of the spectral notch exhibit the sinusoidal relationship with the torsion angle.
2. Sensor fabrication and operation principle
The schematic diagram of the perfluorinated POF-based torsion sensor is shown in Fig. 1(a). A section of perfluorinated POF (Chromis Fiberoptics, GigaPOF-50SR) is sandwiched between the silica SMFs on both ends to constitute a SMS structure. At the first interface, where incident light is injected into the perfluorinated POF through the lead-in SMF, a large number of high-order modes will be excited. The multiple modes propagate along the POF section with different field profiles and different propagation constants. The mode of electric field is sensitive to the torsion. At the second interface, the multiple modes interfere each other and couple into the lead-out SMF. When there is an offset between the lead-in SMF and POF, the multiple modes are distributed asymmetrically along the POF cross-section. Therefore, torsion sensing with orientation-dependence can be realized. The POF is a gradient index fiber with a core diameter of 50 µm, a cladding layer of 10 µm, and an additional polycarbonate jacket to protect the fiber. The core and cladding material is cyclic transparent optical polymer (CYTOP), which is an intensively investigated perfluorinated polymer. The core is doped with perfluorinated molecule, which has low volatility and is stable at elevated temperatures [19].
Fig. 1. (a)Schematic diagram of the perfluorinated POF-based sensor structure, (b)cross-section of the perfluorinated POF with over-cladding layer, (c)connection between POF and silica SMF using ceramic ferrule and ceramic mating sleeve.
The key to fabricate sensors with POF is how to connect the POF with other optical fibers or instrumentations effectively. The standard approach relies on fragile UV cured gel splices. This method is not only time consuming, but also highly dependent on the operator’s experience. Here, an easy approach is proposed to overcome these shortcomings. The end of POF is vertically cleaved by a commercial preheated blade and polished to a smooth face with low connection loss [20]. The end face of a well-polished perfluorinated POF is shown in Fig. 1(b). The end of the POF is fitted with a ceramic ferrule with a bore size of 495 µm. A piece of silica SMF with the end face cleaved is also fitted with a ceramic ferrule with a bore size of 126 µm. Then, the two ceramic ferrules are attached via a ceramic mating sleeve (THORLABS, ADAF1). Both the outer diameter of ceramic ferrule and internal diameter of ceramic mating sleeve are 1.25 mm. The precise and stable alignment of fiber cores with high mechanical strength is guaranteed by the configuration presented in Fig. 1(c). This approach is a fast fabrication method, which is independent of experience. These advantages increase the possibilities for implementation and bring it closer to commercialization. It is worth noting that the aim of polishing of the POF end face is to obtain better experimental results by reducing coupling loss between the POF and SMF. Therefore, it is not required in practical application since the proposed technique is based on the spectral response measurement, instead of intensity measurement.
The refractive index profile of the perfluorinated POF is described as:
3. Experiment and discussion
The schematic diagram of the experimental setup is shown in Fig. 2 which consists of a perfluorinated POF-based SMS interferometer, a Lightwave Measurement System (Keysight, 8164B), a fiber holder and a fiber rotator. The Lightwave Measurement System mainframe is equipped with a Tunable Laser module (81960A) and a Power Sensor module (81635A). The operation wavelength of Tunable Laser module ranges from 1505 nm to 1600 nm with resolution of 5 pm. The maximum output intensity is up to + 13 dBm, which is set to + 10 dBm in the experiment. The Power Sensor module is triggered synchronously with the Tunable Laser module, which enables spectral measurement with wavelength resolution of 5pm. The SMS interferometer spliced between the Tunable Laser module and Power Sensor module is shown in dashed rectangular region. One side of the POF is fixed with the fiber holder and the other side is fixed with the rotator (Optosigma, TAM-4010). Both the holder and rotator are mounted on a 3-Axis Stage. Then slight strain is applied on the POF by moving back a bit of the 3-Axis Stage.
Visible interference fringe pattern is observed in the transmission spectrum using the experimental setup. It can be known from Eq. (5) that the interference pattern is related to the POF length. Figure 3 illustrates the transmission spectra of the configurations with different POF lengths of 4, 5, 6, 7 and 8 cm. The offset between the lead-in SMF and POF is 2 µm. Each configuration with different POF lengths exhibits distinct fringe, which contrasts in the interference patterns. Complicated interference spectra are appeared because of the obvious multimode interference effect. With the increment of POF length, a larger fringe contrast and smaller free spectrum range are shown in the interference fringe. As shown in the red dotted frame in Fig. 3, the spectral notch depth is up to −15dBm is obtained with POF length of 8 cm, which is preferable to torsion measurement.
Fig. 3. Transmission spectra of the perfluorinated POF-based torsion sensor with different POF length.
Torsion experiment with torsion angle ranging from −360° to 360° at 5° intervals is performed. The POF length is 8 cm. The torsion sensitive region between the holder and rotator is approximate as long as 6.3 cm, so the corresponding torsion rate is −100∼100 rad/m ($\gamma = \theta /L$). Figures 4(a) and 4(b) shows the spectral notches at 60° intervals recorded with counter-clockwise and clockwise twisting of the rotator respectively. Wavelength shift and variation of fringe contrast due to the torsional strain can be observed in the figures.
Fig. 4. Transmission spectral characteristics of the perfluorinated POF-based torsion sensor under different torsion angles ranging from:(a) −360° to 0° with counter-clockwise rotation, (b)0° to 360° with clockwise rotation.
Ten repeat measurements are conducted under the same experimental conditions. The average wavelength response in the repeat measurements is plotted as the function of torsion rate, as shown in Fig. 5(a). It can be seen that the wavelength shift presents high linearity from −50 rad/m to + 50 rad/m, as shown by red line. The sensitivity is 106.807 pm/(rad/m). The orientation-dependence of the torsion response has been achieved, which is caused by the asymmetrical distribution of the multiple modes over the POF cross-section. The green line presents the nonlinear fitting of a sinusoidal function from −100 rad/m to 100 rad/m. The standard deviation (SD) characterizing the sensor repeatability is indicated by error bar in the figure.
Fig. 5. Average wavelength response of the torsion sensor in ten repeat measurements with different POF lengths:(a) 8 cm, (b) 7 cm, (c) 6 cm.
Further experiments are performed using sensors with different POF lengths for comparation. Figures 5(b) and 5(c) show the wavelength shifts of the sensors with POF lengths of 7 cm and 6 cm. The torsion sensitive regions are 5.3 cm and 4.3 cm respectively. The sensitivities, SDs and determination coefficients are displayed in figures. It is seen that the sensor with longer POF length has higher sensitivity and smaller determination coefficient. The differences are determined by the relative phase differences between the various modes of the POF, which are significantly dependent on the POF length. Furthermore, the longer POF length results in larger SD in Fig. 5. The measured error may relate to the polycarbonate jacket. Since the polycarbonate jacket has not been removed, fiber slippage is likely occur, especially for longer POF.
The intensity responses of the three sensors with POF lengths of 8, 7 and 6 cm are provided in Figs. 6(a), 6(b) and 6(c), respectively. The torsional stress will increase the refractive index of POF, so the intensity of the spectral notch increase with the increment of torsion rate. The green lines are the nonlinear fittings of the sinusoidal functions, and the red lines are the linear fittings in the particular regions. The sensor with longer POF length shows higher sensitivity, large SD and determination coefficient. It should be noted that the POF is less sensitive to torsion as the polycarbonate jacket acts as both a counterforce and a buffer between torsion effect and sensor. The polycarbonate jacket is extremely resistant to removal and control of this process does not yet afford a great degree of precision in the same way that PMMA can be etched down for specialized sensing. The further improvement should be conducted in the near future research.
Fig. 6. Average intensity response of the torsion sensor in ten repeat measurements with different POF lengths:(a) 8 cm, (b) 7 cm, (c) 6 cm.
Since the offset between the lead-in SMF and POF leads to the asymmetrical distribution of the multiple modes, the verification experiment of eccentricity is performed. The eccentricity is controlled precisely using a commercial fusion splicer. Then, the lead-in SMF and POF are connected via UV gel coupling. Three sensors with offsets of 0, 2 and 4 µm are fabricated. The POF length is 8 cm. Figure 7(a) shows the wavelength shifts of the three sensors. The wavelengths are drawn on the same graph with an offset. For the sensor with eccentricity of 0, the multiple modes are distributed symmetrically over the POF cross-section. Therefore, the wavelength response is independent of orientation, which is similar to the result in [22]. When the lead-in SMF and POF are connected with an offset, the wavelength shift with orientation-dependence is realized. The sensor with offset of 4 µm shows a high sensitivity than that of 2 µm. The intensity variations of the sensors are shown in Fig. 7(b). The power variation is related to the propagation constants of the eigenmodes, which is independent of orientation. It is seen that intensity variations of the three sensors have the same trend. The sensitivity difference is related to coupling loss caused by different eccentricity.
Fig. 7. (a)Wavelength response of the torsion sensor with different connection offset, (b)intensity response of the torsion sensor with different connection offset.
The temperature characteristics of the proposed sensor are researched from 20− to 55−. The POF length is 8 cm, and the offset is 2 µm. The wavelength shift shown in Fig. 8(a) presents a linear sensitivity of 1.17 nm/°C. The measured intensity variation is about 8.2 dBm with the temperature rising, as shown in Fig. 8(b). For the SMS structure fabricated using pure silica optical fiber, the temperature sensitivity is very low [25]. The ultra-high temperature sensitivity of the proposed sensor originates not from the SMS structure but from the large thermo-optic coefficient and thermal expansion coefficient of CYTOP [24]. The temperature variation affects both the propagation constants and field distributions of the eigenmodes, due to the thermo-optic effect and thermal expansion effect. The change of propagation constants affects the relative phase differences between the various modes, and leads to wavelength shift of interferential fringe. The change of the field distribution affects the fringe contrast of the interference spectrum, and affect the power of the spectral notch. Therefore, both temperature and wavelength are varied with temperature. It should be noted that the temperature responses are also related to the polycarbonate jacket behavior.
Fig. 8. Transmission response of the perfluorinated POF-based torsion sensor at different temperature: (a)wavelength response; (b)intensity response.
From the experimental results, the torsion sensing performance is affected significantly by temperature, where 14°C can account for 100 rad/m for wavelength variations. Therefore, the torsion sensor needs to be temperature compensated before it can be applied in practice. In the above torsion experiments, the ambient temperature is kept stable to minimize the influence of temperature fluctuation and achieve highly accurate torsion measurement.
4. Conclusion
A perfluorinated POF-based sensor is reported for torsion measurement. A section of perfluorinated POF is sandwiched between two silica SMFs. In this way, a typical SMS structure is formed by two spatially-dislocated mode-coupling events that couple multimode with different field confinements. The sensing mechanism is analyzed and experimentally validated. Results indicate the wavelength and intensity reach the maximum torsion sensitivities of 106.762 pm/(rad/m) and 0.165 dBm/(rad/m), respectively. Unfortunately, the sensor has a high temperature sensitivity due to the high thermo-optic coefficient and thermal expansion coefficient of the perfluorinated POF. Therefore, proper calibration and temperature compensation are needed to achieve accurate measurement. With low attenuation at telecom-wavelength region, easy fabrication and high mechanical strength, the perfluorinated POF-based torsion sensor may find many applications in structural health monitoring.
Funding
National Natural Science Foundation of China (61405159, 61675152, 61735011, U1833104); Natural Science Foundation of Tianjin City (16JCQNJC02000); National Basic Research Program of China (973 Program) (2016YFC0401902); Key Laboratory of Opto-electronics Information Technology, Ministry of Education (2018KFKT013).
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