We investigate the on-line writing identical fiber Bragg grating （FBG） arrays using the phase mask tech- nique. Given the limitation of laser power, the energy density uniformity and the horizontal width of the writing spot cannot be further optimized. The results show that the FBG arrays obtained in the optimal process （drawing speed of 12±0.15 m/min and average tension of 38.2 g） have a central wavelength bandwidth of less than 0.1 nm and an average refiectivity of 0.26%. Thus, the phase mask method is a promising alternative for on-line writing identical FBG arrays.
The fiber Bragg grating （FBG） is a passive optical fiber component with the refractive index modulated along the fiber length and has been widely applied in fiber sensing systems. High-temperature stable fiber gratings are promising for uses at high temperatures and attract extensive attention. In this paper, FBGs were inscribed in hydrogen loaded standard single mode fibers with the 248-nm excimer laser, and regenerated gratings were obtained through heat treatment. The shift of the central wavelength of the regenerated FBG had a good linearity with temperature, and the reflectivity of the regenerated FBG could almost keep unchanged at 800 ℃.
An air-silica microstructure optical fiber based on the anti-resonant reflecting optical waveguide （ARROW） principle was used to develop a spectral absorption gas sensor. The ARROW fiber has an air core and an air cladding layer. An ARROW fiber with a length of 725mm was used to construct a sensing system to detect acetylene gas. The gas was injected into the fiber from one end of the fiber. The transmission spectra were collected using an optical spectrum analyzer. The results indicate that the system can detect the gas of different concentrations and has the good system linearity. The response time of the system is about 200 s.
A novel distributed weak fiber Bragg gratings （FBGs） vibration sensing system has been designed to overcome the disadvantages of the conventional methods for optical fiber sensing networking, which are： low signal intensity in the usually adopted time-division multiplexing （TDM） technology, insufficient quantity of multiplexed FBGs in the wavelength-division multiplexing （WDM） technology, and that the mixed WDM/TDM technology measures only the physical parameters of the FBG locations but cannot perform distributed measurement over the whole optical fiber. This novel system determines vibration events in the optical fiber line according to the intensity variation of the interference signals between the adjacent weak FBG reflected signals and locates the vibration points accurately using the TDM technology. It has been proven by tests that this system performs vibration signal detection and demodulation in a way more convenient than the conventional methods for the optical fiber sensing system. It also measures over the whole optical fiber, therefore, distributed measurement is fulfilled, and the system locating accuracy is up to 20m, capable of detecting any signals of whose drive signals lower limit voltage is 0.2 V while the frequency range is 3 Hz - 1 000 Hz. The system has the great practical significance and application value for perimeter surveillance systems.
