Ammonia Gas Sensor Based on Graphene Oxide-Coated Mach-Zehnder Interferometer with Hybrid Fiber Structure

Polyacrylic Acid/Polyaniline-Coated Multimode Interferometer for Ammonia Detection

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.

Induced Birefringence by Drop Cast in EFBG Ammonia Sensors

In this paper, an induced birefringence was observed on an ammonia sensor based on an etched fiber Bragg grating (EFBG) coated with an organic film. The film was prepared with a solution of the perylene derivative, along with the copolymer PCDTBT in chlorobenzene. The EFBG was coated by drop cast technique, and a split in the reflection band, characteristic of birefringence, was observed after deposition. The response of the two observed peaks as a function of time was modeled by a pair of exponentials that could represent two processes: adsorption followed by diffusion of the gas molecules in the microstructured film. Such phenomena alter the boundary conditions of the optical fiber propagating mode, which allows an indirect ammonia detection by monitoring the EFBG wavelength shift of the reflected peaks. The sensor was experimentally tested in a concentration range from 3 ppm to 41,700 ppm, and the calibrations curves were determined in a range from 27 ppm to 6960 ppm. The two observed peaks showed different sensitivities, which presents an intrinsic potential for multiparameter measurements. The split in the reflection band was hypothesized to be birefringence induced by the formation of an asymmetric film. This hypothesis was numerically evaluated and compared with the experimental results.