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Abstract
Temperature crosstalk has always been a critical problem for fiber intermodal sensors. In this work, we have proposed a novel method based on the special temperature response of photosensitive fiber to control the temperature sensitivity of the fiber intermodal sensor. The control of temperature sensitivity has been realized via adjusting the proportion of photosensitive fiber to single-mode fiber in the sensing part. The temperature sensitivity as high as −192 pm/°C, and as low as −2.6 pm/°C can be obtained, satisfying the demand in both research and application. The torsion sensor is taken as an example to illustrate feasibility of this method, showing no evident interference in the measurement of torsion parameters. The proposed method outstrips the conventional one by simple structure, facile manufacture, multiple use and low cost, which brings great promise for further employment in laboratory and industry.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical fiber sensors (OFSs) are an emerging category of sensors with excellent properties including simple structure, high accuracy and strong anti-interference ability, contributing to extensive applications ranging from industrial production and medical health to surveillance security. Among various OFSs, the fiber intermodal sensors (FISs) have attracted considerable interest, on account of compact structure, flexible design, and low cost. The past years has witnessed a strong increase in the use of FISs in measuring important parameters, such as temperature [1–5], refractive index [6–9], torsion [10–13], strain [14], liquid level [15], and biochemistry [16].
The effectiveness of FIS sensing process mainly depends on the ability of accurate distinguishment of target signals and elimination of unexpected signal crosstalk arisen from other physical parameters. A critical challenge in FIS sensing is to suppress temperature interference, since it is impossible to keep the ambient temperature invariable during measurement. Currently, researchers have devoted tremendous efforts to eliminating temperature cross sensitivity. In many related studies, the sensors can simultaneously measure temperature and other physical parameters, and the cross sensitivity caused by temperature can be eliminated by calculating the sensitivity coefficient matrix of the sensors [6–11]. Furthermore, cross-talk caused by temperature can be resolved via introducing a reference channel [16] or a special structure (drop shape) [15] as temperature compensation, at the expense of increasing system complexity. Besides, Shashidharan et al. proposed a temperature insensitive sensor based on the microstructured multicore polymer optical fiber, while the fiber remains temperature-insensitive merely at a specific wavelength (700 nm) and the polymer optical fiber is difficult to couple with commercial optical fiber [17]. Moreover, special fibers with unique structures can achieve temperature insensitivity, such as elliptical-core few-mode fibers [12], photonic crystal fibers [18–21], and dual-side hole fibers [13]. In this case, the influence of temperature fluctuations can be fundamentally eliminated. However, the aforementioned methods are restricted in industrial application due to complicated manufacturing process and high cost. It is highly required to develop a facile and effective method without additional complex structures or back-end calculations to realize suppression of temperature sensitivity and control of temperature response sensitivity in application scenarios.
In this paper, a novel temperature sensitivity controllable method of the FIS based on photosensitive fiber (PSF) is proposed. Based on the opposite response of single-mode fiber (SMF) and photosensitive fiber to temperature, the temperature sensitivity of the sensor is successfully controlled by adjusting the proportion of PSF to SMF in the sensing part. The temperature sensitivity of the sensor can reach −192 pm/°C at the highest and −2.6 pm/°C at the lowest. The feasibility of this method is verified in the torsion sensor, since it shows no evident interference in the measurement of torsion parameters. It is noteworthy that this method is facile in operation, flexible in use, and low in cost, providing promise for further application.
2. Sensor structure and principle analysis
Taking the all-fiber Mach-Zehnder interferometer (MZI) based on the modal interferometer as an example, the method of controlling the temperature sensitivity of the FISs is introduced. The MZI structure diagram is shown in Fig. 1, the sensor is designed to contain two tapers and one twist. Part of the light is coupled to the cladding of the fiber when the input light propagates to the first taper (red arrow). At this time, some of the cladding modes with different effective refractive indices may be excited (green arrow). And the remaining light still propagates in the core as the basic core mode (blue arrow). Then, the cladding modes are recoupled back into the core after the light propagates into the twist taper. Therefore, the MZI-based sensor structure was successfully designed for experimental testing.
The sensing part of the sensor is composed of photosensitive fiber and single-mode fiber. The length of the sensing part of the sensor is L, in which the length of the photosensitive fiber is ${L_p}$ and the length of the single-mode fiber is ${L_s} = L - {L_p}$. The mode field diameters of the PSF (CorActive, 100256-USV-652) and SMF (YOFC, A0035384CF4) used in this paper are 9.9um and 10.8um respectively, and their cladding diameters are both 125um. The cross section diagram of the PSF is shown in Fig. 1. The core refractive index of the PSF is $n_p^{co}$, and the cladding refractive index is $n_p^{cl}$. The expression of light intensity during interference is:
where ${I_{co}}$ and ${I_{cl}}$ are the intensities of light propagating in the core and cladding of the fiber, respectively. And ϕ is the phase difference between the core mode and the cladding mode, expressed as follows: where λ is the wavelength of the laser, $\Delta {n_s} = n_s^{co} - n_s^{cl}$ is the difference between the effective refractive index of the SMF core $n_s^{co}$ and cladding $n_s^{cl}$, $\Delta {n_p} = n_p^{co} - n_p^{cl}$ is the difference between the effective refractive index of the PSF core $n_p^{co}$ and cladding $n_p^{cl}$. When the phase difference ϕ=(2m+1)π, m=0, 1, 2, …, the interference light intensity reaches the minimum value and a dip appears. The wavelength of the dip obtained by expression (2) is: where ${\lambda _m}$ is the central wavelength of the m-th order interference dip. The interference fringe spacing between adjacent interference gaps about is:It can be seen from the expression (3) that the MZI-based sensor can response to external variables through investigating changes in wavelength. When the ambient temperature changes, the length and the effective refractive index of the sensor will change, as follows:
where $\Delta {L_{sT}}$ and $\Delta {L_{pT}}$ are the variables of ${L_s}$ and ${L_p}$, respectively, which are induced by temperature changes. ${\alpha _s}$ and ${\alpha _p}$ are the thermal expansion coefficients of the SMF and PSF, respectively. $\delta \Delta {n_{sT}}$ and $\delta \Delta {n_{pT}}$ are the variables of $\Delta {n_s}$ and $\Delta {n_p}$, respectively. $\zeta _s^{co}$ and $\zeta _s^{cl}$ are the thermo-optic coefficients of the SMF core and cladding, respectively. $\zeta _p^{co}$ and $\zeta _p^{cl}$ are the thermo-optic coefficients of the PSF core and cladding, respectively. And ΔT is the temperature change of the external environment. The wavelength change can be expressed as:Therefore, by substituting Eqs. (5), (6), (7) and (8) into (9), the relationship between wavelength change $\Delta {\lambda _{mT}}$ and temperature change can be written as:
The reason for choosing photosensitive fiber is that the core of the photosensitive fiber is doped with Ge and B. The thermo-optical coefficient of GeO2, B2O3 and SiO2, are 1.9 × 10−5, −1.9 × 10−5 and 1.4 × 10−5 /°C, respectively [22–24]. This will cause the thermo-optical coefficient of the PSF core $\zeta _p^{co}$ to be smaller than that of the cladding $\zeta _p^{cl}$ [25]. As well as, the thermo-optical coefficient of the SMF core $\zeta _s^{co}$ is larger than that of the cladding $\zeta _s^{cl}$. This will cause the SMF and PSF to respond opposite to temperature, so the control of temperature sensitivity can be achieved via adjusting the proportion of PSF to SMF in the sensing part. In theory, the temperature sensitivity of the sensor can be controlled to infinitely small.
When the sensor is subjected to shear strain, it can be seen from expression (3) that the influence of variable L can be ignored, and the effective refractive index of the sensor will change, as follows:
where, $\delta \Delta {n_{s\tau }}$ and $\delta \Delta {n_{p\tau }}$ are the variables of $\Delta {n_s}$ and $\Delta {n_p}$, respectively, which are induced by the birefringence due to the photo-elastic effect and linearly proportional to the twist rate τ. $g_s^{co}$ and $g_s^{cl}$ are the elastic coefficients of the SMF core and cladding, respectively. $g_p^{co}$ and $g_p^{cl}$ are the elastic coefficients of the PSF core and cladding, respectively. The wavelength change can be expressed as:Therefore, by substituting Eqs. (12) and (13) into (14), $\Delta {\lambda _{m\tau }}$ in response to the twist rate τ can be derived as:
Therefore, the wavelength change $\Delta {\lambda _{m\tau }}$ is linearly proportional to the twist rate τ and the MZI sensor can measure torsion.
3. Experiment and result analysis
3.1. Fabrication
The device diagram of fabricating this MZI sensor is illustrated in Fig. 2, a CO2 laser fusion machine (Fujikura, LZM-100) was selected as the manufacturing equipment of the sensor. It is equipped with a high-stable CO2 laser as the heating source, with the highest output power of 30W (900bits) and the standard power of 413 bits. The output spot energy shows a Gaussian distribution, and the half width of the spot energy is about 300 microns after the lens is installed. After passing through the beam splitter mirror, the emitted light of CO2 laser is divided into two lasers with equal energy, which illuminate the optical fiber. The angle between the two lasers is about 170 degrees. The LZM-100 has two translation motors (Z motors), two rotating motors (θ motors), and two cameras to monitor the manufacturing process. Sensor parameter setting and manufacturing process is controlled by a user-defined specific program.
Firstly, splice a segment of PSF with a length of LP in the middle of the SMF, which fusion diagram is shown as Fig. 3(a). Put the treated uncoated optical fiber into the fixtures of the CO2 laser fusion machine. The distance between the fixtures is about 40 mm. In the case of laser irradiation, rotate and pull the fiber at the same time to form a torsion taper. The customized special functional processing mode of optical fiber is as follows, in which the boldface in brackets is the set value of the program when processing PSF:
Fig. 3. (a) Fusion diagram of PSF and SMF, (b) The microscope diagram of transition region of torsional taper, (c) The microscope diagram of taper waist of torsional taper, (d) The microscope diagram without twisted taper.
Step 1: the two beams of the CO2 laser are focused on the same point of single-mode (photosensitive) fiber with the power value of 393bits (383bits) and continues for 2000 ms. The z-left and z-right motors move to the left at the same time, the moving speed is 0.01um/ms, and the optical fiber is preheated.
Step 2: laser power value is 443 bits (428 bits), the moving speed of the z-left motor and the z-right moto are 0.09 um/ms and 0.01 um/ms, respectively, direction to the left. Keeping the θ-right motor still, θ-left motor rotated at a speed of 0.15°/ms, and continues for 15000 ms. As the moving speed of the z-left motor and z-right motor is different, the result of the fiber being pulled in the taper during the rotation is shown in Figs. 3(b) and 3(c).
The above process shows that the preparation of the torsion taper is completed, and then the next taper is prepared with distance of 17 mm from the torsion taper. The device program is written in the form of repeated pull taper, as shown in Table 1. The same program is used for both PSF and SMF. Use an optical spectrum analyzer (OSA, Yokogawa, AQ6370C) for real-time observation, and pause the LZM-100 after each cycle. Once the transmission spectrum meets the requirements, immediately stop the heating and tapering operations, the result is shown in Fig. 3(d).
Table 1. The program for fabricating the taper without torsion.
3.2 Results and discussion
In order to study the temperature response characteristics of SMF, a single mode fiber MZIA sensor was fabricated. One end of the MZIA sensor is connected to the super-radiation laser diode (SLD, LightPromotech), and the other end is connected to the OSA to record spectra simultaneously. As shown in Fig. 4, the experimental measuring set-up is used to demonstrate the temperature characteristics of the sensor. The sensor is fixed on a heating platform and the temperature was changed once every 10 °C (30∼90 °C). It can be seen from Fig. 5(a), as the temperature continues to increase, the wavelengths at Dip 1521 nm and Dip 1548 nm have a red shift. And the temperature sensitivities of 1521 nm and 1548 nm are 54 pm/°C and 60 pm/°C, respectively, as show in Fig. 6(a). Therefore, MZIB sensor, which sensing part is composed of PSF, was prepared to improve the temperature sensitivity. The temperature characteristics of Dip 1535 nm and Dip 1561 nm were also studied to determine if the proposed MZIB sensor had a favorable effect on the temperature. The results are shown in Fig. 5(b), as the temperature continues to increase, the wavelengths at Dip 1535 nm and Dip 1561 nm have a blue shift. And the temperature sensitivities of 1535 nm and 1561nm are −166 pm/°C and −192 pm/°C, respectively, as show in Fig. 6(b). The temperature sensitivity has been significantly improved. By comparing MZIA sensor and MZIB sensor, it could be seen that SMF (red shift) and PSF (blue shift) have opposite responses to temperature, as show in Figs. 5(a) and 5(b), which is consistent with the theory.
Therefore, the temperature sensitivity of the sensor is possible to be controlled by adjusting the proportion of PSF to SMF in the sensing part of the sensor. To further verify the feasibility of the method. When the length of the PSF is chosen as 0.8cm and 0.3cm, MZIC sensor and MZID sensor are fabricated. The SMF length of the sensing part of the MZIC sensor and MZID sensor are 0.9cm and 1.4cm, respectively. The experiment results of sensor MZIC are shown in Fig. 5(c), the temperature sensitivities of 1508 nm and 1535nm are −85pm/°C and −85 pm/°C, respectively, as show in Fig. 6(c). MZIC sensor is less sensitive to temperature than MZIB sensor, and as the temperature continues to increase, the wavelengths at Dip 1508nm and Dip 1535 nm have a blue shift. Therefore, temperature insensitive sensors can be prepared by reducing the proportion of PSF in the sensing part of the sensor. The results of sensor MZID are shown in Fig. 5(d), the temperature sensitivities of 1541 nm and 1573nm are −2.6pm/°C and 2.7 pm/°C, respectively, as show in Fig. 6(d). The MZID sensor can be considered insensitive to temperature. In order to demonstrate the repeatability and reliable performance of the sensors, more verification experiments have been carried out. Firstly, the MZID sensor was heated via the heating platform and measured at every 10 °C from 30 to 90 °C, and held at 90 °C for 5 h. Hereafter, the MZID sensor was gradually cooled and measured at every 10 °C from 90 to 30 °C, and the same heating and measurement process was repeated. The error bars of MZID sensor are show in Fig. 6(d). During the entire cycle, the MZID sensor exhibited a maximum wavelength deviation of ∼0.03 nm.
The parameters of the above four sensors were measured, and the results are shown in Table 2. It could be seen that the temperature sensitivity of the sensor can be controlled by adjusting the proportion of PSF to SMF in the sensing part of the sensor, because of the special temperature characteristics of the photosensitive fiber. The temperature sensitivity of the sensor can reach as high as −192 pm/°C, and as low as −2.6 pm/°C in the experiment. If the proportion of PSF to SMF is optimized further, a temperature insensitivity sensor can be realized. Comparison of sensor performance between sensors in this work and other reported sensors is show in Table 3. Compared with other reported sensors, the temperature sensitivity of the proposed sensors can be controlled by adjusting the proportion of PSF to SMF in the sensing part. In addition, the proposed sensors not only display excellent performance in temperature sensing [1–5], but also exhibit indices superior to the temperature insensitivity sensors [13,18–20].
Table 2. The parameters of the MZIA, MZIB, MZIC, and MZID sensors.
Table 3. Comparison between sensors in this work and other reported sensors.
3.3 Application of the method
In order to verify the feasibility of this method in practical applications, the torsions of the sensors were measured. The torsional characteristics of the sensors were proved by the experimental measurement setup shown in Fig. 2. The sensor is hold on the fixtures of the CO2 laser fusion machine and the distance between two fixtures is L=200 mm. Thus, the twist measurement can be finished by fixing the right motor and rotating the left motor. In this case, the twist angle is measured every π/6 within the range of -π∼π (counterclockwise (-) and clockwise (+) direction). The torsion characteristics of MZIA, MZIB and MZID sensors were studied. It can be seen from Fig. 7, the twist sensitivity of Dip 1521 nm and Dip 1548 nm of the MZIA sensor are 0.287 and 0.348 nm/(rad/m), respectively. The twist sensitivity of Dip 1535 nm and Dip 1561 nm of the MZIB sensor are 0.276 and 0.327nm/(rad/m), respectively, as shown in Fig. 8. And the twist sensitivity of Dip 1541 nm and Dip 1573 nm of the MZID sensor are 0.346 and 0.435nm/(rad/m), respectively, as shown in Fig. 9. It can be seen that whether MZIA, MZIB or MZID sensors, the wavelength shift is approximately linearly proportional to the twist rate, and the change of wavelength has opposite responses to counterclockwise and clockwise twists, corresponding to blue shift and red shift, respectively. Therefore, the sensors can determine the direction of the torsion. With a good twist sensitivity consistency in the counterclockwise and clockwise direction, the sensor can be better applied to torsion measurement. In short, it has been confirmed that adjusting the proportion of the PSF to SMF in the sensing part of the sensor has no evident interference in the measurement of torsion parameters. The slight difference in measurement data is due to individual differences in the sensor.
Fig. 7. (a)Transmission spectra of MZIA at different twist rate, (b) Twist characteristics measurement of Dip A1, (c) Twist characteristics measurement of Dip A2.
Fig. 8. (a)Transmission spectra of MZIB at different twist rate, (b) Twist characteristics measurement of Dip B1, (c) Twist characteristics measurement of Dip B2.
Fig. 9. (a)Transmission spectra of MZID at different twist rate, (b) Twist characteristics measurement of Dip D1, (c) Twist characteristics measurement of Dip D2.
4. Conclusion
A temperature sensitivity control method based on the special temperature characteristics of the photosensitive fiber is proposed. The control of temperature sensitivity can be achieved via adjusting the proportion of PSF to SMF in the sensing part. The temperature sensitivity can reach as high as −192 pm/°C, and as low as −2.6 pm/°C. It is noteworthy that the proposed method is facile, requiring no complicated manufacturing processes or special working conditions. Due to the limitation of the single sensor structure, this method is proved to eliminate the cross influence of temperature merely in the torsion measurement process. Nevertheless, taking into account the generality and versatility of its principle, this method could be extended to more FISs to measure more parameters including strain, humidity and refractive index and so on.
Funding
National Natural Science Foundation of China (61605001); National Key Research and Development Program of China (2016YFC0301900, 2016YFC0301901); Natural Science Foundation of Anhui Province (1408085QF135); Provincial Foundation for Excellent Young Talents of Colleges and Universities of Anhui Province; .
Disclosures
The authors declare no conflicts of interest.
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