Numerical Study on Reflection Spectra of an Apodized Fiber Bragg Grating Subjected to Strain Gradients

Fiber Bragg gratings (FBG's) have been used as communication devices and sensors in applications. To extent the FBG's ability for the uniform distributed disturbances, this study computed reflection spectra of an apodized FBG subjected to strain gradients ranging from 0 to 1 strain/m. Based on the coupled-mode theory, in this paper reflection spectra and their corresponding areas were calculated. For apodization functions, the raised-Gaussian and raised-cosine were considered in this study. In parameter study, two parameters of an FBG: the grating length and the effective index change were taken as variables to understand their effects on its reflection spectra. Results show that the larger effective index change increases the range of linear relationship between strain gradients and spectra areas; and the larger grating length increases the slope of the linear relationship. In applications, as a filter, at larger strain gradients those spectra from apodized FBG display almost linear segment over the wavelength ranges 1 or even up to 40 nm, depending on the grating length. Equivalent to the strain gradients, results obtained in this paper also can be applied for the temperature gradients, if the ratio between wavelength shifts from one micro-strain and from one degree Celsius, for example, is suitable assigned. (C) 2014 Elsevier Ltd.