A cost-efficient P-D fiber structure-based Sagnac loop sensor is proposed and experimentally demonstrated for measuring temperature and liquid refractive index (RI). The P-D structure is fabricated by fusion splicing a section of polarization-maintaining fiber (PMF) to a piece of multimode D-shaped optical fiber (MMDF). Then the P-D structure is built into a Sagnac loop using a 3dB coupler. The temperature and RI characteristics of the sensor are investigated experimentally. The results show that two resonant dips have different spectral responses of temperature and RI, which indicate that the sensor can realize simultaneous temperature and RI measurement. The high sensitivities of −1.804nm/°C and −131.49nm/RIU are achieved. The obtained resolutions of temperature and RI of the proposed sensor can reach 0.01°C and 2.46 × 10−4RIU, respectively. The proposed sensor has the potential application in biological and chemical fields.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Optical fiber sensors (OFS) have been attracted an extensive attention in sensing application due to the advantages of high sensitivity, easy fabrication, low cost, immunity to electromagnetic interference, suitability for applications in harsh environments. The sensing principle of OFS is mainly based on the fact that alterations in the parameter being measured will cause a predictable change in the light transmission characteristics of the fiber . Cross-sensitivity is a common problem for optical fiber sensors. Especially, simultaneous monitoring of temperature and liquid RI are of great importance for many chemical and biological applications . Meaningful efforts have been made to develop an optical fiber-based sensing system for the simultaneous monitoring of multi-parameters, such as strain, temperature, vibration, refractive index (RI) [3–7]. Recently, optical fiber sensors based on the D-shaped structure have been widely used in sensing applications, including RI , temperature , magnetic field , pressure , biochemical properties . Generally, sensing applications of D-shaped optical fiber is based on that the environmental parameters can be measured by detecting the resonant spectrum wavelength shift or the change of the optical power .
In this paper, a P-D fiber structure-based Sagnac sensor is proposed and experimentally demonstrated for simultaneous monitoring temperature and liquid RI. The P-D fiber structure is fabricated by fusion spliced a section of polarization-maintaining fiber (PMF) to a piece of multimode D-shaped optical fiber (MMDF). Then the P-D structure is built into a Sagnac loop. The experimental results show that the two resonant dips of sensor have different wavelength shift in response to the temperature and RI changes, which indicate that the sensor can realize simultaneous temperature and RI measurement. The sensor presents high sensitivities of −1.804nm/°C and −131.49nm/RIU for measurements of temperature and RI, respectively.
2. Experimental setup and principles
The schematic diagram of the multimode D-shaped optical fiber and the P-D fiber structure are shown in Fig. 1. The P-D fiber structure is fabricated by fusion spliced a section of small mode-field Panda type PMF (PM1550-HP) to a piece of MMDF. Then the two ends of P-D fiber structure are fusion spliced to the single mode fiber (SMF) pigtails of the 3dB-coupler to build into a Sagnac loop. The diameters of core and cladding MMF used are 50μm and 125μm, respectively. The total length of MMF used in experiments is 10 cm. The length LD and the depth h of the polished zone are 20mm and 25μm for the core wear-off, respectively. The structure of MMDF used is shown in Fig. 1(a). The diameters of core and cladding PMF are 8.5μm and 125μm, respectively. The PMF used in our experiments with a beat length of 3 mm at 1550 nm has a germanium-doped core and two stress regions located on both sides of the core (see Fig. 1(b)). And the lengths LP of the PMF are 8 cm and 10 cm, respectively.
The schematic diagram of experimental setup is shown in Fig. 2. The light from a broadband light source (BBS) is launched into a 3dB-coupler. After the 3dB-coupler, the light is divided into two beams. Then the two beams travel at clockwise and anti-clockwise along the proposed P-D fiber structure-based Sagnac loop. At clockwise direction, the light traveling along the input SMF enters the MMF and excites a number of guided modes in the MMF due to the mismatch of the core diameter between the SMF and the MMF. A part of guided modes travels along the core, and the other part guided modes propagate into the external environment when reach the polished zone and re-couple into the fiber core of MMF after passing the polished zone. When the light travels to the PMF, partial core mode will couple into the stress zone of PMF and form cladding mode because of the mismatch of the core diameter between the PMF and the MMF. The cladding mode re-couples into the core mode when the light reach the SMF located downstream of PMF. At anti-clockwise direction, the light travels with the reverse process. Because the polished zone is with a length of 20 mm, so the light leaking out of the D-type multimode fiber will propagate in the free space, which will reduce the intensity when it re-couple into the fiber core of MMF. If the D-type fiber is bent, the re-coupling efficiency will decrease when the light pass through the polished zone into the fiber core of MMF, and the re-coupling efficiency would be seriously affected by the curvature of the D-type fiber [14, 15]. In order to ensure the coupling efficiency, the whole P-D fiber structure must be kept straight during the experiments. In Sagnac loop, the light propagating clockwise and anti-clockwise interferes at a 3dB-coupler output port due to phase difference. The interference spectrum is monitored by optical spectrum analyzer (OSA).
According to the light transmission process and ignoring the loss of fiber Sagnac loop. The transmission intensity I in terms of phase difference can be expressed as :
The wavelength of the resonant dip in interference spectrum meets the phase conditions, and the m is a random integer. Thus, the resonant dip wavelength can be described as :18]:Eq. (4), it can be seen that, surrounding RI variation can be measured by monitoring the wavelength shift of MMDF interference-induced dip and alternation of temperature can be determined by calculating the wavelength shift of P-D fiber structure interference-induced dip with surrounding RI effect compensated. If the thermal expansion and thermal-optic coefficients of fibers are constant, the Eq. (4) can be simplified as:Eq. (5), both external temperature and RI change will result in the wavelength shift of resonant dip. If two resonant dips have different spectral responses to temperature and RI, the temperature and RI cross-sensitivity problem could be solved. In our experiments, two adjacent resonant dips are chosen for simultaneous monitoring the external temperature and effective RI . In addition, based on Eq. (6), the temperature sensitivity is inversely proportional to the PMF and D-type fiber lengths(Lp and LD), the birefringence index Bp of the PMF, and the effective refractive index difference between the core effective RI and the external effective RI of the MMDF if other variables are invariant.
To evaluate the function of the PMF in the sensor structure, the transmission spectra of sensor constructed by independent MMDF and by the P-D fiber structure were respectively measured when the MMDF were immersed completely in NaCl solution with a RI of 1.3376. The obtained transmission spectra are shown in the insets of Fig. 3. It is clear that the transmission spectrum of P-D fiber structure shows a higher extinction ratio than that of independent MMDF-based sensor. In order to analyze the characteristics of the excited cladding modes which contribute to the interference, Fast Fourier transforms are taken to obtain the spatial frequency spectra, as shown in Fig. 3(a) and Fig. 3(b). From the figures, the dominant peak at zero corresponds to the core mode, which indicates that the powers are mainly distributed in the core mode. The multiple minor peaks in each frequency spectrum verify that several cladding modes are excited and participate the interference . The number of cladding modes becomes more when the PMF add to the D-shaped. As a consequence, the transmission spectrum is modulated by interference of PMF.
3. Experimental results and discussion
The schematic diagram of experimental setup is shown in Fig. 2(a). The P-D fiber structure is immersed completely in a container containing NaCl solution, and the container is placed in an incubator (WD2005) with the resolution of 0.1°C to control the temperature. The interference spectra are recorded by the OSA (YOKOGAWA, AQ6370C).
3.1 Temperature sensing
The temperature measurement performance of the proposed P-D fiber structure-based Sagnac interferometer was firstly evaluated. In the experiment, 8cm and 10cm length PMF were successively used, and the temperature of incubator rose from 10°C to 30°C with the temperature interval of 5°C. The spectra are measured by the OSA with the resolution of 0.1nm after thermal equilibrium. The typical transmission spectra shifting with the change of temperature are shown in Fig. 4(a).
There are four resonant dips can be observed in the spectral range (1350nm∼1650nm). The two dips ranging from 1425nm to 1550nm are selected as the resonant dips to be measured. The free spectral width observed in the interference spectrum is 56nm.
The plots of the dips wavelength shift in response to the temperature change are shown in Fig. 4(b). It is clear that the dips show a blue-shift when the temperature increases. When the length of the PMF is 8cm, the sensitivities of −1.624nm/°C and-1.804 nm/°C are achieved for the dip1(blue line) and dip2(black line) in experiment respectively. According to the measured free spectral width and the sensitivities, the corresponding temperature measuring ranges are 34°C and 31°C, respectively. For comparison, the dips wavelength shift versus the temperature change when the length of the PMF is 10cm is also plotted in Fig. 4(b), and the sensitivities of −1.480nm/°C and −1.454 nm/°C are obtained for the dip1(green line) and dip2(red line). The results indicate that the sensor has higher temperature sensitivity when 8cm length PMF is used, which is consistent with theoretical analysis in Eq. (6). Therefore, the P-D fiber structure with the length PMF of 8cm is used for the following refractive index measurements.
3.2 Refractive index sensing
In order to evaluate the RI measuring performance of the proposed sensor, six concentrations of NaCl solutions with the RI ranging from 1.3376 to 1.3618 (measured by an Abbe refractometer, WYA-2S with measurement accuracy of 10−4 RIU) are employed in the experiments. The measured RI range is limited by the saturation concentrations of NaCl solutions.
The transmission spectra for different refractive RI of NaCl solutions are displayed in Fig. 5(a), and the plots of the two dips wavelength shift in response to the RI change are shown in Fig. 5(b). According to Fig. 5(a), it is obvious that the dips show a blue-shift when the RI increases. Based on Fig. 5(b), the achieved sensitivities of dip1 and dip2 are −81.43nm/RIU and −131.49nm /RIU, respectively.
According to the experimental results above, the two dips have different spectral responses of temperature and RI, which indicate that the sensor have the capability of simultaneous measuring the temperature and RI. On the basis of the present from the Fig. 4 and Fig. 5, the sensitivity coefficients of temperature and RI of the proposed sensor can be listed in the table1.
According to the measured sensitivity coefficients in Table 1, the matrix formula can be turned into:
The condition number of the sensitivity matrix is 359.036, and the calculated cross sensitivity of the RI on the temperature is about 0.015°C/RIU. Therefore, simultaneous temperature and RI measurement can be realized based on Eq. (9). Considering the minimum resolution of 0.02nm of the OSA, the resolutions of temperature and RI of the proposed sensor can reach 0.01°C and 2.46 × 10−4RIU, respectively. It needs to point out that, in order to eliminate measurement errors, a proper install and package procedure without causing extra strain to the sensor is needed because the sensor is also sensitive to strain.
In conclusion, a double-parameters optical fiber sensor based on P-D structure-based Sagnac interferometer has been presented and demonstrated experimentally. The temperature and RI characteristics of the sensor are investigated experimentally. According to the different dips responses of temperature and RI, simultaneous temperature and RI measurement can be realized. The sensitivities of temperature and RI can reach up to −1.804nm/°C and −131.49nm/RIU, respectively. Considering the minimum resolution of 0.02nm of the OSA, the resolutions of temperature and RI of the proposed sensor can reach 0.01°C and 2.46 × 10−4RIU, respectively. The sensor shows the advantages of easy fabrication, high sensitivity and cost-effective. Such sensor can satisfy the requirements of high accuracy temperature and RI measurements at the same time.
National Natural Science Foundation of China (grant nos. 61565002 and61307096), the Guangxi province key research and development program No. AB17129027 and Guangxi province technical innovation guidance program No.AC16380101, and the Innovation Project of Guangxi Graduate Education No. XYCSZ2017083.
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