Optical fiber humidity sensors have sparked enormous interests in many fields because of their excellent features. However, it remains a great challenge to balance sensitivity, humidity response, temperature crosstalk, and wet hysteresis for real-world application. To overcome this trade-off, an optical fiber humidity sensor is developed here by coating functional graphene oxide (GO)/polyelectrolyte nanocomposite film on the excessively tilted fiber grating (ex-TFG), in which GO/polyelectrolyte nanocomposite film is employed for enhancing the hydrophilicity and accelerating the adsorption/desorption of water molecule, while the ex-TFG is utilized for improving the sensitivity of refractive index and eliminating the crosstalk of temperature. By this design, optical fiber humidity sensors achieve high sensitivity, rapid response and recovery, low hysteresis, and temperature crosstalk as well as excellent repeatability and stability in large relative humidity (RH) range. Our work provides a promising platform for effective RH monitoring systems that can be widely applied in rapid diagnostics, pharmacy, precision medicine, and so forth.

A coaxial optical fiber interferometer (COFI) is proposed here for ammonia sensing, which comprises two light-carrying single-mode fibers (SMF) fused to a section of no-core fiber (NCF), thus forming an optical interferometer. The outer surface of the COFI is coated with a layer of polyacrylic acid (PAA)/polyaniline (PAni) film. The refractive index (RI) of the sensitive layer varies when PAA/PAni interacts with ammonia, which leads to the resonance wavelength shift. The surface morphology and structure of the PAA/PAni composites were characterized by using a scanning electron microscope (SEM) and Fourier-transform infrared (FTIR) spectroscopy. When the sensor was exposed to an ammonia atmosphere of different concentrations at room temperature, the sensing performance of the PAA/PAni composite film was superior to that of a sensitive film formed by single-component PAA or PAni. According to the experimental results, the composite film formed by 5 wt% PAA mixed with 2 wt% PAni shows better performance when used for ammonia sensing. A maximum sensitivity of 9.8 pm/ppm was obtained under the ammonia concentration of 50 ppm. In addition, the sensor shows good performance in response time (100 s) and recovery time (180 s) and has good stability and selectivity. The proposed optical fiber ammonia sensor is adapted to monitor leakage in its production, storage, transportation, and application.

The measurement of pH has received great attention in diverse fields, such as clinical diagnostics, environmental protection, and food safety. Optical fiber sensors are widely used for pH sensing because of their great advantages. In this work, an optical fiber pH sensor is fabricated, by combining the merits of the multimode interference configuration and pH-sensitive polyaniline/polyacrylic acid (PAni/PAA) coatings, which was successfully in situ deposited on the no-core fiber (NCF) by the layer-by-layer (LBL) self-assembly method. The sensors’ performance was experimentally characterized when used for pH detection. It has a high sensitivity of 0.985 nm/pH and a great linear response in a universal pH range of 2–12. The response time and recovery time is measured to be less than 10 s. In addition, its temperature sensitivity is tested to be about 0.01 nm/°C with a low temperature crosstalk effect, which makes it promising for detecting pH in the liquid phase with temperature variation. The sensors also demonstrated easy fabrication, good stability, and repeatability, which are adapted to pH detection in most practical applications.

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