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Abstract
A novel embedded ultra-long period fiber grating (EULPFG) based on a graded index multimode fiber (GI-MMF) is proposed for temperature measurement. Due to the small RI difference of the modes near the GI-MMF self-imaging point, the resonant peak of transmission spectrum is wavelength-insensitive to refractive index (RI), strain and bending. However, the sensor is sensitive to temperature. The experimental results show that the temperature sensitivity of the EULPFG is 90.77 pm/°C. The sensitivities of other physical parameters are suppressed, and the suppressed sensitivities are at least one order of magnitude less than those of similar sensors. The EULPFG with anti-interference from other parameters is expected to be used in ocean monitoring systems to measure the temperature of the seawater.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the melting of glaciers and the rise of sea level caused by global warming, the living environment of human beings, animals and plants will be seriously threatened. Ocean as a key area for regulating climate change, accurate monitoring of its temperature is crucial. In recent years, due to the development of optical fiber processing technology, optical fiber sensors have been widely studied and applied. In ideal environment, optical fiber temperature sensors based on long period fiber grating (LPFG) [1], fiber Bragg grating (FBG) [2] and Mach–Zehnder interferometer (MZI) [3] have been widely studied and achieved remarkable results. However, in the actual measurement, the measurement of single target parameter is often accompanied by the interference of a various factors. It is a research focus to eliminate the cross-sensitivity between multiple parameters so as to achieve accurate measurement.
In existing studies, simultaneous measurement of multiple parameters is a common method to avoid sensitivity cross-talk. For example, Zhao et al. used femtosecond laser and hydrofluoric acid etching method to write FBG into pure quartz grating [4]. Because the light has dual characteristics of energy and wavelength, the simultaneous measurement of temperature and RI is realized. However, the production cost of the sensor is high and the demodulation is complex. Liu et al. proposed a sensor based on the MZI cascaded with FBG for simultaneous measurement of temperature and strain [5]. Pang et al. realized the simultaneous measurement of temperature and surrounding RI by superimposed coated the LPFG and FBG [6]. These sensing elements are formed by combining multiple sensing units, which have the disadvantages of complex structure and difficult preparation. In 2020, Zhang et al. prepared novel sensors based on the MMF embedded LPFG for simultaneous measurement of double physical parameters [7,8]. These sensors have the advantages of simple structure, low cost and easy preparation. Therefore, the sensor with simple structure and low cross-sensitivity has a good application prospect. In 2019, Yao et al. proposed a strain insensitive temperature sensor based on a few-mode double-concentric-core fiber [9]. The sensitivities of temperature and strain are 52.79 pm/°C and 0.23 pm/µε. In the same year, Song et al. used DNA-Cetyltrimethyl ammonium solid film deposited on micro-tapered fiber to achieve high sensitivity temperature measurement [10]. The elastic film can effectively reduce the influence of strain on the sensor. These sensors have low strain cross-sensitivity (4.36×10−3 °С/µε and 7.78×10−3 °С/µε), but they cannot eliminate the influence of other physical parameters on temperature measurement.
In this paper, a novel sensor consisting of periodic alternating splicing of the SMF and GI-MMF is fabricated and used to accurately measure temperature. The doping of germanium in the core increases the thermal-optic coefficient of fiber. Therefore, the proposed sensor has a good temperature sensitivity. By monitoring shift of resonant wavelength, the temperature sensitivity of 90.77 pm/°C is obtained. The interference of the RI, bending and strain is suppressed to a certain extent, and the corresponding sensitivities are −2.24 nm/RIU, −2.25 nm/m−1 and −5.45×10−2 pm/µε. In particular, the sensitivities of the RI and bending are one order of magnitude lower than that of the general LPFG, and the cross-sensitivity of strain-temperature is −6.00×10−4 °С/µε. The sensor is expected to steadily measure temperature in various environments, such as oceans, buildings and so on.
2. Preparation method and principle of operation
2.1 Preparation method
As shown in Fig. 1(a), the GI-MMF with a fixed length (700 µm) are orderly embedded into the SMF to form the EULPFG. The cladding diameter of the SMF (9/125 µm) and GI-MMF (50/125 µm) is same but the core diameter is different. The combination of a segment GI-MMF with 700 µm and a segment SMF with 1000 µm is defined as a period. The preparation process of the sensor is shown in Fig. 1(b1)-(b3). Two fibers with smooth end face are spliced together by arc discharge. The splicing parameters of the fusion splicer (FSM-87c, Fujikura Co., Ltd.) are as follows: Discharge intensity: +15 bits; Discharge time: 3000 ms. Then, the spliced fiber is fixed by fiber clamp and the upper CCD is used to obtain clear splicing points. The fiber segment with different lengths is obtained by moving the fiber cleaver located on the displacement platform. The schematic diagram of the preparation device is shown in Fig. 1(c). By the microscopic imaging system, the average length error of the SMF and GI-MMF (about +4.49 and +0.55 µm) are obtained. Small splicing error avoids the deterioration of contrast ratio and shift of resonance wavelength. Meanwhile, the sensor has high mechanical strength and stable response to temperature, strain and RI because the external shape of the sensing region is not changed.
Fig. 1. The schematic diagram of (a) the sensing structure; (b) preparation process of the sensing structure; (c) preparation device and microscopic images.
2.2 Principle analysis
The RI distribution of the GI-MMF has a parabolic form. The relationship between the RI distribution and the radius can be described as [11]:
where, ${n_1}$ and ${n_2}$ are the RI of the core and cladding, r is the core radius and a is the distance from the fiber core axis. Figure 2 depicts the light field distribution profile of the step index multimode fiber (SI-MMF) and GI-MMF with the same numerical aperture (NA=0.2) and transmission distance (Z=700 µm). Due to the drastic variation of the RI distribution on the splicing ends of the two fibers, the direct coupling loss between the SMF and SI-MMF is large. The energy in the SI-MMF core is rapidly reduced with the increase of transmission distance. Different from the SI-MMF, the light in the GI-MMF will all return to the fiber core after transmitting a certain distance, which is called the self-imaging phenomenon, as shown in Fig. 2(c).Fig. 2. The diagram of the light field distribution of (a) the SI-MMF; (b) the GI-MMF; (c) the core energy ratio along the transmission distance.
The distance between the self-imaging points of the experimental GI-MMF is 600 µm. In other word, splicing loss and coupling efficiency of the EULPFG are optimized in near 600 µm. However, since there is no phase difference between the modes at the self-imaging point, the grating cannot be formed [12]. Therefore, the length of 700 µm is selected to provide a certain phase difference between the two modes.
The phase matching condition for the formation of the EULPFG is:
where ${\mathrm{\lambda }_{res}}$ is the resonant wavelength, $n_{co}^{eff}$ and $n_{cl}^{eff}(r )$ represent the effective RI of the core and cladding, and $\mathrm{\Lambda }$ is the period length of the EULPFG.The temperature sensitivity of the EULPFG can be expressed as [13]:
In the numerical simulation, Rsoft software is used to simulate the distribution and transmission of light in the fibers. The core/cladding diameters of the SMF and GI-MMF are the same as those used in experiment. The transmission spectra of the EULPFG with different GI-MMF and SMF lengths are shown in Fig. 3(a) and (b). Consistent with the theory, when the 600 µm GI-MMF is embedded in the LPFG, the grating cannot be formed. And the length of the GI-MMF is farther from the self-imaging point, the insertion loss of the resonant peak is larger. With the increase of the SMF length, the contrast ratio of the resonant peak increases first and then decreases and the resonance peaks shift to longer wavelength. The simulated light field distribution and spectrum evolution are plotted in Fig. 4. The insertion of 700 µm GI-MMF causes the light in the core to partly leak into the cladding. However, due to the strong energy near the self-imaging point of the GI-MMF, most of the light still remains in the core, resulting in low insertion loss of the sensor. The distribution of output light field is obtained by electric field scanning, and coupling mode of the model at the resonance wavelength of 1438.0 nm is $L{P_{03}}$. The reliability of the simulation provides a strong theoretical support for the implementation and measurement of the experiment.
Fig. 4. The diagram of (a) light field distribution; (b) simulated spectrum evolution of the EULPFG.
3. Experimental results and discussion
3.1 Experimental results
The experimental spectrum evolution of the EULPFG is shown in Fig. 5. Due to the imperfection of the experimental operation, the transmission spectra between the simulation and the experiment are slightly different. When the phase matching condition is satisfied, a dominant resonance peak appears at 1418.2 nm in the transmission spectrum. The insertion loss and contrast ratio of the resonant peak are −4.33 dB and −34.62 dB, respectively. The high contrast ratio and narrow bandwidth (6.8 nm) ensure accurate measurement of physical parameters through wavelength demodulation.
Figure 6 shows the integrated measuring device, including a thermostat, a super-continuum (SC) source, an optical spectrum analyzer (OSA), two fiber clamps and two controllers. It is used to measure temperature, strain, refractive index (RI) and bending. The sensing structure is placed in the middle and both ends are fixed by clamps. The SC (YSL Photonics, China) light and OSA (AQ6317b, Agilent Tech. Inc.) are connected at both end of the fiber to obtain the change of transmission spectra. The thermostat provides a precise and controlled heat source and an operating platform for the RI measurement. The L-shaped plate above the operating platform and the right fixed end of the fiber are controlled to apply the curvature and strain by two controllers, respectively. In addition, the thermostat is required to be removed during bending measurements to provide a space for bending.
In the temperature measurement, the sensing structure is horizontally placed on the thermostat, and a thermal cover is pressed on the structure to prevent heat diffusion. The transmission spectrum is recorded every 10 °C from 30 °C to 150 °C. As can be seen from the illustration in Fig. 7, the resonant peak appears obvious red shift with the increase of temperature. By linear fitting between the wavelength and temperature, the temperature sensitivity of the sensor is 90.77 pm/°C, which is higher than that of general sensors [15,16].
Fig. 7. The change of transmission spectra under the different temperatures and the linear fitting between the wavelength and temperature.
Subsequently, in order to verify the influence of other parameters, the sensitivities of the RI, strain and bending are also measured. At the room temperature, the RI sensitivity of −2.24 nm/RIU is experimentally obtained by changing the external RI environment (1.333-1.437) of the sensing structure. The contrast variation of resonance peaks with the RI changes are inserted into Fig. 8(a). When the external RI is 1.437, the resonance peak disappears.
Fig. 8. The variation of resonant peak and linear relationship between (a) the RI and wavelength; (b) the strain and wavelength; (c) the curvature and wavelength.
The fiber clamp at the right end moves a distance of $\Delta L$, and the axial strain of the fiber can be calculated by formula: ${\varepsilon } = \frac{{\Delta L}}{L}$. The distance between the two clamps is L (L=30 cm). The transmission spectrum is recorded at a step of 167 µε from 0 to 1670 µε, as shown in Fig. 8(b). The strain sensitivity is −5.45×10−2 pm/µε, which is two orders of magnitude less than that of the similar sensor [17,18]. The experimental results show that the sensor is almost unaffected by strain.
Finally, the bending sensitivity of the sensor is measured. The fixed drop value ($\Delta d$) of the L-shaped plate is 0.5 mm, and different curvatures can be obtained by calculation [19]. As shown in Fig. 8(c), with the increase of curvature, the transmission spectrum shows slightly blue shift, and the bending sensitivity is −2.25 nm/m−1 in the curvature range of 0-1.247 m−1.
3.2 Discussion
In the EULPFG, the sensitivity of the strain, bending and RI can be expressed as [20,21]:
The experimental results show that the RI, strain and bending sensitivities of the EULPFG are −2.24 nm/RIU, −5.45×10−2 pm/µε and −2.25 nm/m−1, respectively. The sensitivities are one to two orders of magnitude lower than those of the same type of sensors [22–24]. In terms of crosstalk, the strain-temperature cross-sensitivity of the proposed sensor is −6.00×10−4 °С/µε, which is two orders of magnitude lower than that of other LPFG sensor [25]. It indicates that the effect of strain on temperature measurement can be ignored. The cross-sensitivity of the RI and bending is −24.68 °С/RIU and −24.79 °С/m−1, which are more than 30 times smaller than that of the general LPFGs (−818.35 °С/RIU and 855.90 °С/m−1) [26,27]. When the RI and bending change by one unit, the influence on accurate measurement of temperature is huge. However, in seawater thermometry, the RI variation at a fixed depth is small (about 0.02 RIU). Therefore, the EULPFG not only inherits the advantages of optical fiber sensor [28], but also has the ability to suppress multi-parameters interference, which can achieve accurate measurement of seawater temperature.
4. Conclusion
In this paper, a high stability temperature sensor is proposed. The 700 µm GI-MMF is periodically embedded into the SMF, and the EULPFG not only shows excellent temperature sensitivity, but also has inhibition effect on other physical parameters. In addition, the resonant peak with an insertion loss of −4.33 dB and a contrast ratio of −34.62 dB ensures accurate parameter measurement through wavelength demodulation. The temperature sensitivity is 90.77 pm/°C. The advantages of easy preparation, high temperature stability and low cross-sensitivity make the sensor suitable for precise temperature measurements in the ocean.
Funding
Joint Research Fund in Astronomy under cooperative agreement between the National Natural Science Foundation of China (NSFC) and Chinese Academy of Sciences (CAS) (U1831115, U1931206, U2031130, U2031132); National Natural Science Foundation of China (11704086, 61775044); Natural Science Foundation of Heilongjiang Province (ZD2019H003); Project of Shandong Province Higher Educational Youth Innovation Science and Technology Program (2019KJJ011); Fundamental Research Funds for the Central Universities to the Harbin Engineering University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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