(Invited) Integration of Functional Coatings with Fiber Optics for Chemical and Medical Sensing Applications
Authors: Yang Minghong , Xiang Feng , Dai Jixiang 
Published: February 2020

The combination of fiber optics with nano-structure technologies and functional coatings offers great potential for the realization of novel sensor concepts. Minitured optical fiber sensors with thin films as sensitive elements could open new fields for chemical and medical sensing applications. Thin films work as sensitive elements and transducer to get response and feedback from environments, optical fiber here are employed to signal carrier. Such fiber-optic component integrated sensitive materials has intrinsic property of small size, light weight and immunity to electric-magnetic interference, and therefore would have promising perspectives in chemical and medical applications.

In chemical industry, hydrogen is a clean energy and an important chemical raw material, however, hydrogen is dangerous because of its smallest molecule and flammable characteristic. Solid-state hydrogen sensors based on piezoelectric, fiber optic, semiconductor and electrochemical devices have been investigated and reported in literatures. Compared to sensing devices based on semiconductor or electrochemical principle of operation, optical fiber sensor is quite immune to an electromagnetic noise and could be safely operated in explosive environment. Novel Pt-loaded WO3 coatings has been developed as hydrogen sensing elements at the National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology (NEL-FOST), and fiber optic hydrogen sensor based on these sensitive coatings have been investigated. Pt-loaded WO3coating with molar ratio of 1:5 has the best hydrogen sensitivity when the annealing temperature is 315°C. The corresponding hydrogen sensor has 536 pm wavelength shift towards 10000 ppm hydrogen under room temperature of 25°C. The hydrogen sensor can detect hydrogen concentration as low as 200 ppm. FBG shows nonlinear wavelength shifts for different hydrogen concentrations. Ambient humidity change has little effect on the performance of hydrogen sensor. The hydrogen sensor has good repeatability, selectivity and continuous monitoring ability during the hydrogen response. The proposed FBG hydrogen sensor may be very promising for hydrogen concentrations detection in air.

In medical applications, relative-humidity (RH) is one of the main indicators for breathing and health monitoring. Fiber optics based RH measurement could be very promising due to its tiny size and low-coat of the sensing head. Minitured optical fiber sensors based on Fabry-Perot thin film structure are also proposed, the transducer deposited on fiber end-face is multilayer coating consisting of a stack of porous dielectric oxide materials. The reversible adsorption and desorption of water molecules in the porous films in dependence on water vapor shifts the reflected interference spectrum fringe, therefore humidity sensing is correlated with the shift of interference fringe. The sensing probes are porous oxide multilayer deposited on fiber tip, which forms thin film Fabry–Perot interferometer (FPI). The coatings are realized by e-beam evaporation without ion-source assistance, this will generate a porous coating stack in the FPI. Fabry–Perot interferometry sensors are extremely sensitive to perturbations that affect the optical path difference between two reflective mirrors and the sensing region can be very compact to ensure miniature size of the sensor.