Using three-layer vector field model,the dispersion characteristics and spectral response of long-period fiber gratings(LPFGs)with different cladding radii are analyzed.Both the numerical and experimental results demonstrate that LPFG’s resonant wavelengths shift towards much longer wavelength and the transmission depth is strengthened significantly by decreasing cladding radius.When the cladding radius is less than 20μm,only a single resonant peak over a wavelength range of 600 nm(from 1200 nm to 1800 nm)is achieved,and the sensitivity of LPFG to external refractive index is enhanced significantly.
An optic-fiber liquid-level sensor based on tapered chirped grating was proposed and demonstrated experimentally.With a properly designed column buoy hung to chain wheel,the liquid-level variation was converted to axial strain,which made the reflection spectrum of the tapered chirped grating narrowed or broadened correspondingly.And a high-sensitive liquid-level sensor was obtained by monitoring the bandwidth variation of the tapered chirped grating.Compared with traditional absolute wavelength or power detection,this special monitoring technique based on bandwidth modulation makes the liquid-level sensor insensitive to spatially uniform temperature variations and power fluctuations from down-lead or light source.
