Investigation of wavelength variations of fibre Bragg grating features using a chirped phase mask

A simple scheme, using a chirped phase mask, for studying the wavelength dependence of fibre Bragg grating (FBG) responses in the region of 2/3 of the Bragg wavelength, which are due to the use of a non-ideal phase mask, is presented. Comparison of the FBG responses in the regions of 2/3 of the Bragg wavelength (i.e. ~1030 nm) and the usual Bragg wavelength (~1535 nm), as a function of the phase mask periodicity, was achieved by UV laser irradiation at various sections along a chirped phase mask, of centre periodicity 1060 nm. The wavelength of the transmission dips in both regions was proportional to the periodicity of the phase mask and the ratios of these wavelengths showed less than 0.3% variation compared with the theoretical value, supporting the proposition that features at ~1030 nm are the third harmonic of a grating having the phase mask periodicity.