Optical fiber sensors are widely used in physical parameter detection, such as refractive index (RI), temperature, pressure, strain, humidity, and bending sensing[
Chinese Optics Letters, Volume. 15, Issue 2, 020603(2017)
Temperature-insensitive refractive index sensor based on Mach–Zehnder interferometer with two microcavities
We propose a temperature-insensitive refractive index (RI) fiber sensor based on a Mach–Zehnder interferometer. The sensor with high sensitivity and a robust structure is fabricated by splicing a short photonic crystal fiber (PCF) between two single-mode fibers, where two microcavities are formed at both junctions because of the collapse of the PCF air holes. The microcavity with a larger equatorial dimension can excite higher-order cladding modes, so the sensor presents a high RI sensitivity, which can reach 244.16 nm/RIU in the RI range of 1.333–1.3778. Meanwhile it has a low temperature sensitivity of 0.005 nm/°C in the range of 33°C–360°C.
Optical fiber sensors are widely used in physical parameter detection, such as refractive index (RI), temperature, pressure, strain, humidity, and bending sensing[
In this study, we proposed an easily fabricated and robust RI sensor with high sensitivity, which consists of a section of a PCF and two microcavities. The micrcavities embedded in the optical fiber were fabricated by splicing a piece of the PCF between two sections of SMFs. Higher-order cladding modes can be excited by the first microcavity with a large equatorial dimension, and coupled from the SMF core to the PCF cladding by the second microcavity. The experiment results show that the sensitivity of the sensor can reach 244.16 nm/RIU in the RI range of 1.333–1.3778, and the RI response has good linearity and high repeatability.
The proposed MZI for the RI measurement was fabricated by using the fusion technique. The schematic diagram of the sensor is shown in Fig.
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Figure 1.Schematic diagram of the microcavities-based MZI.
In the fabrication, different discharging locations and splicing parameters can result in different splicing phenomena. When discharging at the side of the SMF about 30 μm away from the junction, no microcavity was formed. However, when discharging at the side of the PCF about 30 μm away from the junction, a microcavity was formed. Because of the high arc discharge, the PCF air holes around the splicing point collapsed entirely, and the air in the air holes was pressed to the splicing point, thus, a microcavity was fabricated at the junction of the SMF and PCF. The microscopic image of the PCF cross-section is shown in Fig.
Figure 2.Microscopic image of (a) the PCF cross-section, (b) microcavity 1 of sensor 2, (c) microcavity 1 of sensor 3, (d) microcavity 1 of sensor 4, and (e) microcavity and collapse area.
In this Letter, we fabricated one direct splicing MZI (sensor 1) and three microcavities-based MZIs with different microcavity sizes to demonstrate their RI sensing properties. We fabricated these three sensors with almost the same discharge location, the same arc time of 500 ms, and the same interference length of 15 mm. Different arc power was used to form different microcavity sizes. The microcavity sizes and related arc power are shown in Table
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When the arc power increases, the size of the microcavity will become larger, and the shape of the microcavity will become rounder, namely, the curvature of ellipse at the longitudinal direction will increase. Because we studied the RI sensing property of the sensor, we first measured the spectra of the sensors in distilled water. The interference spectra of sensors 1, 2, 3, and 4 in distilled water are shown in Fig.
Figure 3.Transmission spectra of the sensors 1, 2, 3, and 4.
As displayed in Fig.
Due to the different effective RI of the PCF core and cladding, there will be an optical path difference in the same transmission distance between two light beams. In general, the interference pattern intensity is the function of the intensities of the fundamental mode and the cladding mode, it can be the simplified as
The free spectral range (FSR) of the fabricated MZI can be expressed as
When the surrounding RI changes, the evanescent field in the MZI will change, and
The RI measurements of the proposed MZIs were carried out in a clean room with an almost constant temperature and humidity in order to eliminate the effects caused by temperature and humidity fluctuations. A detection system was employed to monitor the transmission spectra of the MZIs. The detection system consists of a tunable laser (Agilent 81980A, Agilent Technologies Inc.) and an optical power meter (81636B, Agilent Technologies Inc.). The sensors were connected to the detection system by two SMF jumpers. The tunable laser scans through its wavelength range (1465–1575 nm) at the rate of 5 pm per step. During the measurements, the sensors were straightened and fixed on fiber holders to avoid a bending-induced and force-induced signal change, and the fiber holders were supported by a three-dimensional microtranslation stage to adjust the height of the sensor. The test solution was dropped on a glass slide right under the fiber sensor. The NaCl solutions of different mass percent concentrations (0%, 5%, 10%, 15%, 20%, and 25%) were used in the experiments. The corresponding RIs are 1.3330, 1.3418, 1.3505, 1.3594, 1.3684, and 1.3778, respectively.
In every measurement, we injected the RI solution and made sure the sensing head was totally immersed in the solution, and then measured and saved the spectrum. After each measurement, the NaCl solution was removed. The glass and the fiber sensor head were cleaned with distilled water and alcohol, and then dried with a hairdryer until the spectrum was same as the reference spectrum in the air. Through the above operations, the RI sensing was experimentally demonstrated. From Figs.
Figure 4.Wavelength shifts of interference spectrum of (a) sensor 1, (b) sensor 2, (c) sensor 3, and (d) sensor 4.
In order to demonstrate the sensitivities of the microcavities-based MZIs, wavelength shifts of the corresponding transmission spectra with an external RI change are given, as shown in Fig.
Figure 5.Relations between wavelength shifts and the surrounding RI of different sensors.
The interference spectra of sensor 1 and sensor 4 are fast Fourier transformed to obtain the spatial frequency spectra, as is shown in Fig.
Figure 6.Spatial frequency spectra of sensors 1 and 4.
The temperature property of sensor 4 was also investigated in a temperature range of 33°C–360°C by placing it in a tube furnace. The sensor was first heated to 400°C at the rate of 10°C/min and maintained for 1 h to eliminate the effect of mechanical stress caused by the fusion splicing process, and then cooled down to room temperature. For the second heating process, the temperature was increased from 33°C to 360°C at the rate of 5°C/min. We set the temperature of the tube furnace and calibrated the actual temperature in the tube with a standard thermocouple, then, we recorded the actual temperature and measured the spectrum at each temperature value. The linear fit of the wavelength shifts with the increase of temperature is shown in Fig.
Figure 7.Wavelength shifts with the increase of the temperatures for sensor 4.
We propose an MZI based on two microcavities for RI sensing. The sensor is fabricated by splicing a short of a PCF between two sections of SMFs. By discharging at both sides of the PCF, two microcavities are created at two junctions because of the total collapse of the PCF air holes. The high-order cladding modes are more sensitive to the surrounding RI changes, so the RI sensitivity of the sensor with larger microcavities is higher. In the RI range of 1.3330–1.3778, the RI sensitivity can reach 244.16 nm/RIU. The temperature experiment results show that the proposed sensor has low temperature sensitivity. Furthermore, because of no reduction in the fiber diameter direction, the proposed structure is more robust than taper-based and trench structure fiber sensors.
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Yinfeng Wang, Sumei Wang, Lan Jiang, Hua Huang, Liuchao Zhang, Peng Wang, Lingya Lv, Zhitao Cao. Temperature-insensitive refractive index sensor based on Mach–Zehnder interferometer with two microcavities[J]. Chinese Optics Letters, 2017, 15(2): 020603
Category: Fiber Optics and Optical Communications
Received: Oct. 19, 2016
Accepted: Dec. 6, 2016
Published Online: Jul. 26, 2018
The Author Email: Sumei Wang (wangsumei@bit.edu.cn)