High-sensitivity humidity and temperature sensor by cascading a polyimide coated long period fiber grating and a polydimethylsiloxane wrapped Mach-Zehnder interferometer.

Two different structures, long period fiber grating (LPFG) and a Mach-Zehnder interferometer (MZI), were cascaded to fabricate a sensor for sensing humidity and temperature simultaneously. Due to the humidity sensitive characteristic of polyimide (PI) and the temperature sensitive characteristic of polydimethylsiloxane (PDMS), the LPFG was coated with PI and the MZI was wrapped in PDMS to improve the humidity and temperature sensitivities, respectively. Humidity and temperature experiments and the stability and repeatability of the proposed sensors were performed. The sensor shows humidity and temperature sensitivities of ∼164 pm/%RH and 183 pm/°C in the humidity and temperature increasing experiments, respectively. Combining increasing, decreasing, and repeatable experiments of temperature and humidity, the proposed sensor shows stable performance in humidity sensitivity of ∼159 pm/%RH and temperature sensitivity of ∼175 pm/°C, with good linearity of ∼0.994. The structure of the proposed sensor has the advantages of low implementation cost, easy fabrication, and good stability.

Highly sensitive magnetic field sensors based on Fe2O3 nanorods grown on the surface of a tapered few mode fiber.

A magnetic field (MF) sensor with a stable structure and high sensitivity has been proposed and experimentally verified. We used the water bath method to produce a layer of Fe2O3 nanorods on a tapered few mode fiber (FMF) surface to form a Mach-Zehnder interferometer (MZI). The experiment found that the nanostructure produced on the surface of FMF were particularly stable and firm. Under the action of an external MF, the magnetic permeability of a Fe2O3 nanorod will change, leading to a change in its refractive index, resulting in a linear shift in the resonance wavelength of MZI. The experimental results showed that the MF sensitivity of MZI reached -0.5348 nm/mT in 10 mT∼80 mT. In addition, MZI has a certain sensitivity to environmental humidity and temperature. A long-period fiber grating and a fiber Bragg grating are cascaded with MZI to achieve a simultaneous measurement of three quantities and eliminate their cross-sensitivity.

Highly sensitive strain sensor based on tapered few-mode fiber.

A high sensitivity strain sensor using a sandwich structure of "single mode fiber (SMF)-few mode fiber (FMF)-single mode fiber (SMF)" was proposed and experimentally validated. The designed sensor is achieved by splicing a segment of FMF between two segments of SMFs, and then using a fiber optic fusion tapering machine to double the length of FMF. Introducing tapered optical fibers into the structure to excite more evanescent waves improves the sensitivity of the sensor to the surrounding environment. In addition, due to the fact that the FMF is tapered into a very fine shape, the tensile stress applied to the FMF will increase. Therefore, conical FMF has excellent stress concentration ability, which is easily deformed under stress, thus achieving a high strain sensitivity of -23.9 pm/µÎµ. Finally, a cascaded FBG was used to compensate for the temperature cross-sensitivity of the sensor. This strain sensor with an extremely simple structure and high sensitivity has wide application value in the industry.