An optical sensor for steel corrosion monitoring based on etched fiber Bragg grating (FBG) sputtered with iron film is developed. The sensor is fabricated by magnetron sputtering an iron film on an etched FBG, and tested in sodium chloride solution. The chemical composition, structure, and morphology of film are investigated by energy dispersive X-ray analysis, X-ray diffraction, scanning electron microscope, and so on. The central wavelength shift and peak power of the FBG are analyzed with a spectral meter. The experimental results show that the wavelength shift and spectral chirp are related to the corrosion of iron film, and the sensor is able to monitor the status of iron in the corrosion process.

We propose and demonstrate Mach-Zehnder interferometer (MZI), which is the refractive index (RI) insensitive and temperature highly sensitive based on etched multi-core fiber (MCF) structure. The MCF and Fiber Bragg Grating (FBG) are used as hybrid sensing elements. The fabrication of the interferometer is provided a new taper-like structure by etching the MCF to further expose the side cores to the surroundings. The interferometer has produced a sensitivity of 103.2pm/°C within the ambient temperature up-to 70°C. Moreover, the superior temperature sensitivity is 89.19pm/°C, 66.64pm/°C, 56.42pm/°C in the range of 24°C to 130°C, and RI-insensitive in the range of 1.34 to 1.38, for different waists of etched seven-core fiber interferometers (E7CFIs) ~ 84.70μm, 93.10μm, 108.67μm, respectively. Compared with the conventional FBGs, the sensitivity of the interferometer is significantly improved by 8 times. E7CFI's novel and advantageous features can easily be distinguished other devices. Besides, the proposed sensing architecture is compact, easy to fabricate, highly sensitive, easy to reproduce, and makes it an inexpensive fiber optic device.

An in-line interferometer based on tapered multicore embedded into a flexible thermo-optical material is proposed and investigated, theoretically and experimentally. The device consists of a tapered multicore fiber spliced between two single-mode fibers covered with PDMS, with high thermo-optic coefficient. The temperature sensitivity improvement obtained from PDMS applied on a tapered multicore fiber (TMCF) interferometer has been fundamentally and experimentally verified. The experimental results show the temperature sensitivity can be improved by reducing the tapered waist diameter of TMCF. The sensor exhibits the high sensitivity of 5-25 nm/°C within the decreasing temperature range from 50 °C down to 10 °C. A sequence of simulations and corresponding experiments are performed to clarify the evolution of the interference fading and consequently build the criteria for sensor design and reachable lower limit of temperature sensing. The proposed sensor can be employed as photonic thermometer with ultra-high sensitivity for biological and deep-sea applications, particularly based on the claimed quantitative criteria.

A novel, to the best of our knowledge, sensor architecture for palladium-coated fiber Bragg gratings is proposed and demonstrated that allows highly accurate multi-parameter sensing and decoupling of hydrogen concentration from temperature. By means of partly Pd-coated Pi-shifted FBGs (PSFBGs), the notch wavelength of the narrow transmission band and the flank wavelength of the broader reflection band experience different hydrogen and temperature sensitivities. PSFBGs were calibrated at hydrogen concentrations between 800 and 10,000 ppm and temperatures from 20 to 40°C, and a decreased hydrogen sensitivity at increased temperatures was found. Nonlinear temperature-dependent hydrogen calibration functions were therefore determined. An iterative matrix algorithm was used to decouple hydrogen concentration and temperature and to account for the nonlinear calibration functions. Achieved improvements and results have great importance for real field applications of FBG-based hydrogen sensing.