Simultaneous measurement of gas pressure and temperature with integrated optical fiber FPI sensor based on in-fiber micro-cavity and fiber-tip

In paper, an integrated optical fiber Fabry-Pérot interferometer (FPI) sensor is proposed and fabricated. The all-fiber sensor is composed of an in-fiber micro-cavity machined by 193 nm excimer laser and a tiny segment of single-mode fiber (SMF). Due to the production of reflection mirrors, the composite FPI structure with different interference cavity is formed between every two surfaces of micro-cavity and fiber-tip. Resulting from the influences of thermal expansion effect of fiber and the thermo-optic effect of in-cavity gas, the interference spectrum of FPI sensor can shift with the temperature and pressure change with different rules. Experimental researches indicate, the low-frequency interference fringe of air cavity is sensitive to pressure change and insensitive to temperature change, but the high-frequency interference fringe of micro-fiber cavity is sensitive to temperature change and insensitive to pressure change. Through analyzing with fast Fourier transform (FFT) and Fourier band-pass filtering (FBPF) methods, we obtain the temperature and pressure sensitivities of the two different cavities. Owning to the different responses to gas pressure and temperature change, the fiber sensor can be used in simultaneous measurement of them, and has excellent merits of miniature size, strong tip structure, simple manufacturing and low cost.