A tapered plastic optical fiber (POF) assisted by a long period grating (LPG) is proposed as a refractive index (RI) sensor. A tapered POF assisted by a corrugated surface LPG is fabricated by the heating and drawing method and simple mechanical die press print method. By changing the structural parameters of the tapered POFs with LPG, the RI sensing performance of the obtained sensor are evaluated. It shows that the structure parameters of sensor affect the RI sensing performance, and the sensor with a smaller diameter of the tapered POF and a deeper groove depth of the LPG exhibits better RI sensing performance. When the POF with a diameter of 1mm is heated and drawn to form a tapered POF with a diameter of 600μm, then an LPG with a period of 300µm, groove depth of 150µm is fabricated on it, the RI sensitivity of 590.7%/RIU is obtained in the measured RI range
An inline fiber Mach-Zehnder interferometer (MZI) that consisted of a standard single-mode fiber sandwiched between a core-offset and a peanut-shape joints is fabricated for refractive Index (RI) sensing purpose. It is found that the straight inline MZI was not sensitive enough with -27.6 nm/RIU for RI measurement. After bending it, the sensitivity was improved markedly, and the maximum RI sensitivity of -153.11 nm/RIU was achieved within the measured RI range of 1.333–1.373. The result showed that bending the inline MZI could enhance its RI sensing performance, and as the curvature radius deceased, the RI sensitivity increased.
An inline fiber Mach-Zehnder Interferometer (MZI) that consisted of a standard single-mode fiber sandwiched between one peanut-shape and a core-offset joints is fabricated by a fusion splicer for RI sensing purpose. After substituted a peanut shape for a core-offset in an inline fiber MZI with the core-offset pair, the influence caused by the misplaced direction of the core-offset is avoided since the peanut structure is symmetrical, and it is found that its RI sensing performance was improved as well. The inline fiber MZIs with the different arm lengths and core-offset displacement were evaluated for refractive index (RI) measurement. The RI sensitivity of -72.4 nm/RIU was achieved within the measured RI range of 1.333–1.373.
A flat-shaped plastic optical fiber (POF) assisted by a corrugated surface long period grating (LPG) is fabricated by a simple heat pressing and mechanical die press print method. By changing the structural parameters of the flat-shaped POF with LPG, the refractive index (RI) sensing performance of the obtained sensor are evaluated. It shows that the structure parameters of sensor affect the RI sensing performance, and the sensor with a thinner flat thickness and a deeper groove depth of the corrugated surface LPG exhibits better RI sensing performance. Moreover, the sensitivity of the sensor after removing the cladding is better than that of the sensor without removing the cladding. When the POF diameter D is 1mm, the LPG period is 300μm, the thickness d of flat-shaped POF without cladding is 600μm, and the LPG groove depth h is 200μm, we obtained the highest sensitivity of 591.3%/RIU with a resolution of 2.872×10-4RIU in the RI range of 1.3330-1.4230. The proposed sensor is a low-cost solution for RI measurement and with the features of easy fabrication, high sensitivity and intensity modulation at visible wavelengths.
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.
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