A pair of reflection peaks/transmission dips, at twice the Bragg wavelength, were observed in spectra of a Type I fiber Bragg grating written with the standard phase mask technique. The occurrence of two peaks/ dips, rather than one, is attributed to the interleaved refractive index modulations along the fiber core, with the periodicity of the phase mask that has been observed previously in images of gratings that cause destructive interference in a reflected wave at the Bragg condition owing to the π phase difference between the grating phases. Thus the standard phase mask technique produced an alternative type of π-phase-shifted grating at twice the design Bragg wavelength.
Optics Letters, Vol. 34, no. 13 (2009), pp. 2021-2023