A tilted long period grating (LPG) fabricated on a commercial plastic optical fiber (POF) by a simple mechanical die-press-print method was proposed for liquid-level sensing. The liquid level sensing performances of the sensor with different structural parameters were studied theoretically and experimentally. The results show that when the LPG fabricated on a POF with a diameter of 0.25mm and a tilted angle of 30° and the groove depth of 75μm, the highest sensitivity of -0.4631dB/mm was obtained in the level range of 20mm. Additionally, the influences of temperature on the liquid-level sensing performances of sensors were also studied. It shows that the sensitivity of the sensor was decreased with the increase of temperature.
A bent core-offset in-line fiber Mach-Zehnder interferometer (MZIs) is proposed for refractive index (RI) sensing. After simply bending, not only the RI sensitivity but also the temperature sensitivity can be enhanced as the bending radius decreases. To solve the cross-sensitivity problem, a simultaneous measurement of RI and temperature was carried out. When the bending radius is 35.64mm, the highest RI sensitivity of -44.55nm/RIU for the RI range from 1.333 to 1.373, and the highest temperature sensitivity of 0.0799nm/℃ for the temperature range from 25℃ to 60℃ were measured simultaneously.
A cascading core-offset in-line fiber Mach-Zehnder interferometer (MZI) was proposed to improve the RI sensitivity and eliminate the cross sensitivity between RI and temperature. The sensor was fabricated by cascading a small core-offset inline fiber MZI (the offset displacement is 6μm and interference arm length is 30mm) with a large core-offset in-line fiber MZI (the offset displacement is 40μm and interference arm length is 30mm). By enlarging the offset displacement, the RI sensitivity was improved from -9.177 to 108.326nm/RIU. And the MZIs with two offset displacement offer interference dips with different RI response, by utilizing the interference dips, the simultaneously measurement can be realized to eliminate the cross sensitivity.
In this paper, a corrugated surface tilted long period grating (LPG) is proposed for liquid-level sensing. The tilted LPGs are fabricated on commercial plastic optical fiber (POF) by a simple mechanical die-press-print method. The liquid-level sensing performances of the so-obtained non-tilted LPGs (with the tilted angle of 0°) and the tilted LPGs (with the tilted angle of 10-30°) are investigated, respectively. The results show that the tilted LPG exhibits a better sensing performance than that of the non-tilted LPG. When the LPG period is 300μm, the groove depth is 155μm, and the tilted angle is 20°, the highest sensitivity of 0.417dB/mm is obtained by measuring the liquid-level with the refractive index (RI) of 1.33. In addition, the influence of RI on the liquid-level sensing performance of the sensor is also studied. The results show that the sensitivity of sensor is increased as the RI of liquid increased from 1.33-1.43, and the highest sensitivity can reach to 0.454dB/mm when the RI of liquid is 1.43. The proposed sensor is a low-cost solution for liquid-level measurement and with the features of easy fabrication, high sensitivity, and continuous measurement.
In this paper, a helical structure is fabricated on a commercial plastic optical fiber (POF) by heat pressing and twisting method. The helical POF is proposed as a RI sensor. A POF is firstly heated and pressed to form a flat-shape with a thickness of 60% of the diameter of the POF. After that, the flat-shape is twisted into a helical structure. The helical structures fabricated on POFs (with diameters of 0.25, 0.50, and 1.00 mm) and with lengths (of 10, 15, and 20mm) are experimentally evaluated for liquid RI measurement, and the results show that when the helical structure is fabricated on the POF with a diameter of 1.00 mm, a helical pitch of 2mm, and a length of 15mm, the RI sensitivities of 2292%/RIU, 4128%/RIU, and 1750%/RIU are obtained in the RI ranges of 1.33-1.37, 1.37-1.40, and 1.41-1.45, respectively. The experiment results have demonstrated that the proposed sensor is a low-cost solution for RI measurement, and with the features of high sensitivity, simple structure, easy fabrication, compact size and intensity modulation at visible wavelengths.
The temperature dependence of a refractive index (RI) sensor based on a core-offset in-line fiber Mach-Zehnder interferometer was investigated in order to eliminate the temperature and RI cross sensitivity caused by the thermo-optic effect. The RI sensitivity of the sensor was around -20.00 nm/RIU, and the RI correction was given for the sensor in the temperature range of 25-60℃. The experimental results showed that the temperature variation might lead to deviation in the measurement of RI, but hardly affected the RI sensitivity.
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