To meet the requirements for medium-high frequency vibration monitoring, a new type fiber Bragg grating (FBG) accelerometer with an integrative matrix structure is proposed. Two symmetrical flexible gemels are used as elastic elements, which drive respective inertial mass moving reversely when exciting vibration exists, leading to doubling the wavelength shift of the FBG. The mechanics model and a numerical method are presented in this paper, by which the influence of the structural parameters on the sensitivity and eigenfrequency is discussed. Sensitivity higher than 200  pm/g and an eigenfrequency larger than 3000 Hz can be realized separately, but both cannot be achieved simultaneously. Aiming for a broader measuring frequency range, a prototype accelerometer with an eigenfrequency near 3000 Hz is designed, and results from a shake table test are also demonstrated.

Hydrogen gas sensing properties of Pt-WO₃ films on spiral microstructured fiber Bragg grating (FBG) has been demonstrated. Pt-WO₃ film was prepared by hydrothermal method. The spiral microsturctured FBG was fabricated using femtosecond laser. Spiral microstructure FBG hydrogen sensor can detect hydrogen concentration from 0.02% H₂ to 4% H₂ at room temperature, and the response time is shortened from a few minutes to 10~30 s. Double spiral microstructure at pitch 60 μm and sputtered with 2 μm Pt-WO₃ film recorded hydrogen sensitivity of 522 pm/%(v/v) H₂ responding to hydrogen gas in air. This translated to approximately 2~4 times higher than the unprocessed standard FBG. The humidity has little effect on the sensing property. The sensor has fast response time, good stability, large detection range and has the good prospect of practical application for hydrogen leak detection.

We discuss hydrogen sensors based on fiber Bragg gratings (FBGs) micro-machined by femtosecond laser to form microgrooves and sputtered with Pd/Ag composite film. The atomic ratio of the two metals is controlled at Pd:Ag = 3:1. At room temperature, the hydrogen sensitivity of the sensor probe micro-machined by 75 mW laser power and sputtered with 520 nm of Pd/Ag film is 16.5 pm/%H. Comparably, the standard FBG hydrogen sensitivity becomes 2.5 pm/%H towards the same 4% hydrogen concentration. At an ambient temperature of 35°C, the processed sensor head has a dramatic rise in hydrogen sensitivity. Besides, the sensor shows good response and repeatability during hydrogen concentration test.

This paper proposed and demonstrated fiber Bragg grating (FBG) hydrogen sensor by integrating optical fiber laser micro-machining and thin film technology. The spiral microstructure ablated sensor probe was sputtered with hydrogen sensitive material, palladium-silver (Pd-Ag) composite film, in which the atomic ratio of the two metals is controlled at Pd:Ag = 4:1. Experimental work was conducted at ambient temperature and relative humidity of 25 o C and 32% respectively. Double spiral microstructure at pitch 90 μm and sputtered with 520 nm Pd/Ag film recorded hydrogen sensitivity of 51.5 pm/%H responding to hydrogen gas in air. This translated to approximately 7.5 times higher than the unprocessed standard FBG. The sensor's hydrogen response is reversible, has improved response time and could have potential for hydrogen gas monitoring.

To meet the requirements for low-frequency vibration monitoring, a new type of FBG (fiber Bragg grating) accelerometer with a bended spring plate is proposed. Two symmetrical bended spring plates are used as elastic elements, which drive the FBG to produce axial strains equal in magnitude but opposite in direction when exciting vibrations exist, leading to doubling the wavelength shift of the FBG. The mechanics model and a numerical method are presented in this paper, with which the influence of the structural parameters on the sensitivity and the eigenfrequency are discussed. The test results show that the sensitivity of the accelerometer is more than 1000 pm/g when the frequency is within the 0.7–20 Hz range.